The unmoved mover (Ancient Greek: ὃ οὐ κινούμενον κινεῖ, romanized: ho ou kinoúmenon kineî, lit. 'that which moves without being moved') or prime mover (Latin: primum movens) is a concept advanced by Aristotle as a primary cause (or first uncaused cause) or "mover" of all the motion in the universe. As is implicit in the name, the unmoved mover moves other things, but is not itself moved by any prior action. In Book 12 (Greek: Λ) of his Metaphysics, Aristotle describes the unmoved mover as being perfectly beautiful, indivisible, and contemplating only the perfect contemplation: self-contemplation. He equates this concept also with the active intellect. This Aristotelian concept had its roots in cosmological speculations of the earliest Greek pre-Socratic philosophers and became highly influential and widely drawn upon in medieval philosophy and theology. St. Thomas Aquinas, for example, elaborated on the unmoved mover in the Quinque viae.
First philosophy
Aristotle argues, in Book 8 of the Physics and Book 12 of the Metaphysics,
"that there must be an immortal, unchanging being, ultimately
responsible for all wholeness and orderliness in the sensible world".
In the Physics (VIII 4–6) Aristotle finds "surprising
difficulties" explaining even commonplace change, and in support of his
approach of explanation by four causes, he required "a fair bit of technical machinery". This "machinery" includes potentiality and actuality, hylomorphism,
the theory of categories, and "an audacious and intriguing argument,
that the bare existence of change requires the postulation of a first cause, an unmoved mover whose necessary existence underpins the ceaseless activity of the world of motion". Aristotle's "first philosophy", or Metaphysics ("after the Physics"), develops his peculiar theology of the prime mover, as πρῶτον κινοῦν ἀκίνητον: an independent divine eternal unchanging immaterial substance.
Celestial spheres
Aristotle adopted the geometrical model of Eudoxus of Cnidus, to provide a general explanation of the apparent wandering of the classical planets arising from uniform circular motions of celestial spheres. While the number of spheres in the model itself was subject to change (47 or 55), Aristotle's account of aether, and of potentiality and actuality, required an individual unmoved mover for each sphere.
Final cause and efficient cause
Simplicius
argues that the first unmoved mover is a cause not only in the sense of
being a final cause—which everyone in his day, as in ours, would
accept—but also in the sense of being an efficient cause (1360. 24ff.),
and his master Ammonius wrote a whole book defending the thesis (ibid.
1363. 8–10). Simplicius's arguments include citations of Plato's views
in the Timaeus—evidence not relevant to the debate unless one
happens to believe in the essential harmony of Plato and Aristotle—and
inferences from approving remarks which Aristotle makes about the role
of Nous in Anaxagoras, which require a good deal of reading
between the lines. But he does point out rightly that the unmoved mover
fits the definition of an efficient cause—"whence the first source of
change or rest" (Phys. II. 3, 194b29–30; Simpl. 1361. 12ff.). The
examples which Aristotle adduces do not obviously suggest an
application to the first unmoved mover, and it is at least possible that
Aristotle originated his fourfold distinction without reference to such
an entity. But the real question is whether, given his definition of
the efficient cause, it includes the unmoved mover willy-nilly. One
curious fact remains: that Aristotle never acknowledges the alleged fact
that the unmoved mover is an efficient cause (a problem of which
Simplicius is well aware: 1363. 12–14)...
— D. W. Graham, Physics
Despite their apparent function in the celestial model, the unmoved movers were a final cause, not an efficient cause for the movement of the spheres; they were solely a constant inspiration, and even if taken for an efficient cause precisely due to being a final cause, the nature of the explanation is purely teleological.
Aristotle's theology
The unmoved movers, if they were anywhere, were said to fill the outer void, beyond the sphere of fixed stars:
It is clear then that there is
neither place, nor void, nor time, outside the heaven. Hence whatever is
there, is of such a nature as not to occupy any place, nor does time
age it; nor is there any change in any of the things which lie beyond
the outermost motion; they continue through their entire duration
unalterable and unmodified, living the best and most self sufficient of
lives… From [the fulfilment of the whole heaven] derive the being and
life which other things, some more or less articulately but other
feebly, enjoy."
— Aristotle, De Caelo, I.9, 279 a17–30
The unmoved movers are, themselves, immaterial substance, (separate
and individual beings), having neither parts nor magnitude. As such, it
would be physically impossible for them to move material objects of any
size by pushing, pulling or collision. Because matter is, for Aristotle,
a substratum in which a potential to change can be actualized, any and
all potentiality must be actualized in a being that is eternal but it
must not be still, because continuous activity is essential for all
forms of life. This immaterial form of activity must be intellectual in
nature and it cannot be contingent upon sensory perception if it is to
remain uniform; therefore eternal substance must think only of thinking
itself and exist outside the starry sphere, where even the notion of
place is undefined for Aristotle. Their influence on lesser beings is
purely the result of an "aspiration or desire", and each aetheric celestial sphere emulates one of the unmoved movers, as best it can, by uniform circular motion. The first heaven, the outmost sphere of fixed stars, is moved by a desire to emulate the prime mover (first cause), in relation to whom, the subordinate movers suffer an accidental dependency.
Many of Aristotle's contemporaries complained that oblivious, powerless gods are unsatisfactory.
Nonetheless, it was a life which Aristotle enthusiastically endorsed as
one most enviable and perfect, the unembellished basis of theology. As
the whole of nature depends on the inspiration of the eternal unmoved
movers, Aristotle was concerned to establish the metaphysical necessity
of the perpetual motions of the heavens. It is through the seasonal
action of the Sun upon the terrestrial spheres, that the cycles of
generation and corruption give rise to all natural motion as efficient cause. The intellect, nous,
"or whatever else it be that is thought to rule and lead us by nature,
and to have cognizance of what is noble and divine" is the highest
activity, according to Aristotle (contemplation or speculative thinking,
theōrētikē). It is also the most sustainable, pleasant, self-sufficient activity;
something which is aimed at for its own sake. (In contrast to politics
and warfare, it does not involve doing things we'd rather not do, but
rather something we do at our leisure.) This aim is not strictly human,
to achieve it means to live in accordance not with mortal thoughts, but
something immortal and divine which is within humans. According to
Aristotle, contemplation is the only type of happy activity which it
would not be ridiculous to imagine the gods having. In Aristotle's
psychology and biology, the intellect is the soul.
First cause
In book VIII of his Physics,
Aristotle examines the notions of change or motion, and attempts to
show by a challenging argument, that the mere supposition of a 'before'
and an 'after', requires a first principle. He argues that in the beginning, if the cosmos had come to be, its first motion would lack an antecedent state, and as Parmenides said, "nothing comes from nothing". The cosmological argument,
later attributed to Aristotle, thereby draws the conclusion that God
exists. However, if the cosmos had a beginning, Aristotle argued, it
would require an efficient first cause, a notion that Aristotle took to demonstrate a critical flaw.
But it is a wrong assumption to suppose universally that
we have an adequate first principle in virtue of the fact that something
always is so … Thus Democritus
reduces the causes that explain nature to the fact that things happened
in the past in the same way as they happen now: but he does not think
fit to seek for a first principle to explain this 'always' … Let this
conclude what we have to say in support of our contention that there
never was a time when there was not motion, and never will be a time
when there will not be motion. (Physics VIII, 2)
The purpose of Aristotle's cosmological argument, that at least one eternal unmoved mover must exist, is to support everyday change.
Of things that exist, substances are the first. But if substances
can, then all things can perish... and yet, time and change cannot. Now,
the only continuous change is that of place, and the only continuous
change of place is circular motion. Therefore, there must be an eternal
circular motion and this is confirmed by the fixed stars which are moved
by the eternal actual substance that's purely actual.
In Aristotle's estimation, an explanation without the temporal actuality and potentiality
of an infinite locomotive chain is required for an eternal cosmos with
neither beginning nor end: an unmoved eternal substance for whom the Primum Mobile
turns diurnally and whereby all terrestrial cycles are driven: day and
night, the seasons of the year, the transformation of the elements, and
the nature of plants and animals.
Substance and change
Aristotle
begins by describing substance, of which he says there are three types:
the sensible, which is subdivided into the perishable, which belongs to
physics, and the eternal, which belongs to "another science". He notes
that sensible substance is changeable and that there are several types
of change, including quality and quantity, generation and destruction,
increase and diminution, alteration, and motion. Change occurs when one
given state becomes something contrary to it: that is to say, what
exists potentially comes to exist actually.
Therefore, "a thing [can come to be], incidentally, out of that which
is not, [and] also all things come to be out of that which is, but is potentially,
and is not actually." That by which something is changed is the mover,
that which is changed is the matter, and that into which it is changed
is the form.
Substance is necessarily composed of different elements. The
proof for this is that there are things which are different from each
other and that all things are composed of elements. Since elements
combine to form composite substances, and because these substances
differ from each other, there must be different elements: in other
words, "b or a cannot be the same as ba".
Number of movers
Near the end of Metaphysics, Book Λ,
Aristotle introduces a surprising question, asking "whether we have to
suppose one such [mover] or more than one, and if the latter, how many".
Aristotle concludes that the number of all the movers equals the
number of separate movements, and we can determine these by considering
the mathematical science most akin to philosophy, i.e., astronomy.
Although the mathematicians differ on the number of movements, Aristotle
considers that the number of celestial spheres would be 47 or 55. Nonetheless, he concludes his Metaphysics, Book Λ, with a quotation from the Iliad: "The rule of many is not good; one ruler let there be."
A cosmological argument, in natural theology and natural philosophy (not cosmology), is an argument in which the existence of God is inferred from alleged facts concerning causation, explanation, change, motion, contingency, dependency, or finitude with respect to the universe or some totality of objects. It is traditionally known as an argument from universal causation, an argument from first cause, or the causal argument. (about the origin).
Whichever term is employed, there are three basic variants of the
argument, each with subtle yet important distinctions: the arguments
from in causa (causality), in esse (essentiality), and in fieri (becoming).
The basic premises of all of these are the concept of causality. The conclusion of these arguments is first cause (for whichever group of things it is being argued must have a cause or explanation), subsequently deemed to be God. The history of this argument goes back to Aristotle or earlier, was developed in Neoplatonism and early Christianity and later in medieval Islamic theology during the 9th to 12th centuries, and re-introduced to medieval Christian theology in the 13th century by Thomas Aquinas. The cosmological argument is closely related to the principle of sufficient reason as addressed by Gottfried Leibniz and Samuel Clarke, itself a modern exposition of the claim that "nothing comes from nothing" attributed to Parmenides.
Plato (c. 427–347 BC) and Aristotle (c. 384–322 BC) both posited first cause arguments, though each had certain notable caveats. In The Laws (Book X), Plato posited that all movement in the world and the Cosmos was "imparted motion". This required a "self-originated motion" to set it in motion and to maintain it. In Timaeus, Plato posited a "demiurge" of supreme wisdom and intelligence as the creator of the Cosmos.
Aristotle argued against the idea of a first cause, often confused with the idea of a "prime mover" or "unmoved mover" (πρῶτον κινοῦν ἀκίνητον or primus motor) in his Physics and Metaphysics. Aristotle argued in favor of the idea of several unmoved movers, one powering each celestial sphere,
which he believed lived beyond the sphere of the fixed stars, and
explained why motion in the universe (which he believed was eternal) had
continued for an infinite period of time. Aristotle argued the atomist's assertion of a non-eternal universe would require a first uncaused cause – in his terminology, an efficient first cause – an idea he considered a nonsensical flaw in the reasoning of the atomists.
Like Plato, Aristotle believed in an eternal cosmos with no beginning and no end (which in turn follows Parmenides' famous statement that "nothing comes from nothing"). In what he called "first philosophy" or metaphysics, Aristotle did intend a theological correspondence between the prime mover and deity (presumably Zeus); functionally, however, he provided an explanation for the apparent motion of the "fixed stars"
(now understood as the daily rotation of the Earth). According to his
theses, immaterial unmoved movers are eternal unchangeable beings that
constantly think about thinking, but being immaterial, they are
incapable of interacting with the cosmos and have no knowledge of what
transpires therein. From an "aspiration or desire", the celestial spheres, imitate that purely intellectual activity as best they can, by uniform circular motion. The unmoved movers inspiring the planetary
spheres are no different in kind from the prime mover, they merely
suffer a dependency of relation to the prime mover. Correspondingly, the
motions of the planets are subordinate to the motion inspired by the
prime mover in the sphere of fixed stars. Aristotle's natural theology
admitted no creation or capriciousness from the immortal pantheon, but maintained a defense against dangerous charges of impiety.
Plotinus,
a third-century Platonist, taught that the One transcendent absolute
caused the universe to exist simply as a consequence of its existence (creatio ex deo). His disciple Proclus stated "The One is God".
Centuries later, the IslamicphilosopherAvicenna (c. 980–1037) inquired into the question of being, in which he distinguished between essence (Mahiat) and existence (Wujud).
He argued that the fact of existence could not be inferred from or
accounted for by the essence of existing things, and that form and
matter by themselves could not originate and interact with the movement
of the Universe or the progressive actualization of existing things.
Thus, he reasoned that existence must be due to an agent cause
that necessitates, imparts, gives, or adds existence to an essence. To
do so, the cause must coexist with its effect and be an existing thing.
Steven Duncan writes that it "was first formulated by a Greek-speaking Syriac Christian neo-Platonist, John Philoponus,
who claims to find a contradiction between the Greek pagan insistence
on the eternity of the world and the Aristotelian rejection of the
existence of any actual infinite". Referring to the argument as the "'Kalam'
cosmological argument", Duncan asserts that it "received its fullest
articulation at the hands of [medieval] Muslim and Jewish exponents of Kalam ("the use of reason by believers to justify the basic metaphysical presuppositions of the faith").
Thomas Aquinas
(c. 1225–1274) adapted and enhanced the argument he found in his
reading of Aristotle and Avicenna to form one of the most influential
versions of the cosmological argument.
His conception of First Cause was the idea that the Universe must be
caused by something that is itself uncaused, which he claimed is that
which we call God:
The second way is from the nature
of the efficient cause. In the world of sense we find there is an order
of efficient causes. There is no case known (neither is it, indeed,
possible) in which a thing is found to be the efficient cause of itself;
for so it would be prior to itself, which is impossible. Now in
efficient causes it is not possible to go on to infinity, because in all
efficient causes following in order, the first is the cause of the
intermediate cause, and the intermediate is the cause of the ultimate
cause, whether the intermediate cause be several, or only one. Now to
take away the cause is to take away the effect. Therefore, if there be
no first cause among efficient causes, there will be no ultimate, nor
any intermediate cause. But if in efficient causes it is possible to go
on to infinity, there will be no first efficient cause, neither will
there be an ultimate effect, nor any intermediate efficient causes; all
of which is plainly false. Therefore it is necessary to admit a first
efficient cause, to which everyone gives the name of God.
Importantly, Aquinas' Five Ways, given the second question of his
Summa Theologica, are not the entirety of Aquinas' demonstration that
the Christian God exists. The Five Ways form only the beginning of
Aquinas' Treatise on the Divine Nature.
Versions of the argument
Argument from contingency
In the scholastic era, Aquinas formulated the "argument from contingency", following Aristotle in claiming that there must be something to explain why the Universe exists. Since the Universe could, under different circumstances, conceivably not exist (contingency), its existence must have a cause – not merely another contingent thing, but something that exists by necessity (something that must exist in order for anything else to exist). In other words, even if the Universe has always existed, it still owes its existence to an uncaused cause, Aquinas further said: "... and this we understand to be God."
Aquinas's argument from contingency allows for the possibility of
a Universe that has no beginning in time. It is a form of argument from
universal causation.
Aquinas observed that, in nature, there were things with contingent
existences. Since it is possible for such things not to exist, there
must be some time at which these things did not in fact exist. Thus,
according to Aquinas, there must have been a time when nothing existed.
If this is so, there would exist nothing that could bring anything into
existence. Contingent beings, therefore, are insufficient to account for
the existence of contingent beings: there must exist a necessary being whose non-existence is an impossibility, and from which the existence of all contingent beings is derived.
The German philosopher Gottfried Leibniz made a similar argument with his principle of sufficient reason
in 1714. "There can be found no fact that is true or existent, or any
true proposition," he wrote, "without there being a sufficient reason
for its being so and not otherwise, although we cannot know these
reasons in most cases." He formulated the cosmological argument
succinctly: "Why is there something rather than nothing? The sufficient
reason ... is found in a substance which ... is a necessary being
bearing the reason for its existence within itself."
Leibniz's argument from contingency is one of the most popular
cosmological arguments in philosophy of religion. It attempts to prove
the existence of a necessary being and infer that this being is God. Alexander Pruss formulates the argument as follows:
Every contingent fact has an explanation.
There is a contingent fact that includes all other contingent facts.
Therefore, there is an explanation of this fact.
This explanation must involve a necessary being.
This necessary being is God.
Premise 1 is a form of the principle of sufficient reason
stating that all contingently true propositions are explained. This is
one of the several variants of the PSR which differ in strength, scope,
and modal implications. Premise 2 refers to what is known as the Big
Conjunctive Contingent Fact (abbreviated BCCF) in philosophy of
religion. The BCCF is generally taken to be the totality of all
contingent beings or the logical conjunction of all contingent facts.
The approach of the argument is that since a contingent fact cannot
explain the BCCF, a fact involving a necessary object must be its
explanation. Statement 5, which is either seen as a premise or a
conclusion, infers that the necessary being which explains the totality
of contingent facts is God. In academic literature, several philosophers
of religion such as Joshua Rasmussen and T. Ryan Byerly have argued for
the inference from (4) to (5).
In esse and in fieri
The difference between the arguments from causation in fieri and in esse is a fairly important one. In fieri is generally translated as "becoming", while in esse is generally translated as "in essence". In fieri,
the process of becoming, is similar to building a house. Once it is
built, the builder walks away, and it stands on its own accord; compare
the watchmaker analogy. (It may require occasional maintenance, but that is beyond the scope of the first cause argument.)
In esse (essence) is more akin to the light from a candle or the liquid in a vessel. George Hayward Joyce, SJ,
explained that, "where the light of the candle is dependent on the
candle's continued existence, not only does a candle produce light in a
room in the first instance, but its continued presence is necessary if
the illumination is to continue. If it is removed, the light ceases.
Again, a liquid receives its shape from the vessel in which it is
contained; but were the pressure of the containing sides withdrawn, it
would not retain its form for an instant." This form of the argument is
far more difficult to separate from a purely first cause argument than
is the example of the house's maintenance above, because here the First
Cause is insufficient without the candle's or vessel's continued
existence.
Thus, Leibniz's argument is in fieri, while Aquinas' argument is both in fieri and in esse. This distinction is an excellent example of the difference between a deistic view (Leibniz) and a theistic view (Aquinas). As a general trend, the modern slants on the cosmological argument, including the Kalam cosmological argument, tend to lean very strongly towards an in fieri argument.
The philosopher Robert Koons
has stated a new variant on the cosmological argument. He says that to
deny causation is to deny all empirical ideas – for example, if we know
our own hand, we know it because of the chain of causes including light
being reflected upon one's eyes, stimulating the retina and sending a
message through the optic nerve into your brain. He summarised the
purpose of the argument as "that if you don't buy into theistic
metaphysics, you're undermining empirical science. The two grew up
together historically and are culturally and philosophically
inter-dependent ... If you say I just don't buy this causality principle
– that's going to be a big big problem for empirical science." This in fieri
version of the argument therefore does not intend to prove God, but
only to disprove objections involving science, and the idea that
contemporary knowledge disproves the cosmological argument.
William Lane Craig gives this argument in the following general form:
Whatever begins to exist has a cause.
The Universe began to exist.
Therefore, the Universe has a cause.
Craig explains, by nature of the event (the Universe coming into
existence), attributes unique to (the concept of) God must also be
attributed to the cause of this event, including but not limited to:
enormous power (if not omnipotence), being the creator of the Heavens
and the Earth (as God is according to the Christian understanding of
God), being eternal and being absolutely self-sufficient. Since these
attributes are unique to God, anything with these attributes must be
God. Something does have these attributes: the cause; hence, the cause
is God, the cause exists; hence, God exists.
Craig defends the second premise, that the Universe had a beginning starting with Al-Ghazali's proof that an actual infinite
is impossible. However, If the universe never had a beginning then
there would be an actual infinite, an infinite amount of cause and
effect events. Hence, the Universe had a beginning.
Metaphysical argument for the existence of God
Duns Scotus,
the influential Medieval Christian theologian, created a metaphysical
argument for the existence of God. Though it was inspired by Aquinas'
argument from motion, he, like other philosophers and theologians,
believed that his statement for God's existence could be considered
separate to Aquinas'. His explanation for God's existence is long, and
can be summarised as follows:
Something can be produced.
It is produced by itself, something or another.
Not by nothing, because nothing causes nothing.
Not by itself, because an effect never causes itself.
Therefore, by another A.
If A is first then we have reached the conclusion.
If A is not first, then we return to 2).
From 3) and 4), we produce another- B. The ascending series is either infinite or finite.
An infinite series is not possible.
Therefore, God exists.
Scotus deals immediately with two objections he can see: first, that
there cannot be a first, and second, that the argument falls apart when
1) is questioned. He states that infinite regress
is impossible, because it provokes unanswerable questions, like, in
modern English, "What is infinity minus infinity?" The second he states
can be answered if the question is rephrased using modal logic, meaning that the first statement is instead "It is possible that something can be produced."
Objections and counterarguments
What caused the First Cause?
One objection to the argument is that it leaves open the question of
why the First Cause is unique in that it does not require any causes.
Proponents argue that the First Cause is exempt from having a cause,
while opponents argue that this is special pleading or otherwise untrue. Critics often press that arguing for the First Cause's exemption raises the question of why the First Cause is indeed exempt,
whereas defenders maintain that this question has been answered by the
various arguments, emphasizing that none of its major forms rests on the
premise that everything has a cause.
William Lane Craig, who famously uses the Kalam cosmological argument,
argues that the infinite is impossible, whichever perspective the
viewer takes, and so there must always have been one unmoved thing to
begin the universe. He uses Hilbert's paradox of the Grand Hotel
and the question 'What is infinity minus infinity?' to illustrate the
idea that the infinite is metaphysically, mathematically, and even
conceptually, impossible. Other reasons include the fact that it is
impossible to count down from infinity, and that, had the universe
existed for an infinite amount of time, every possible event, including
the final end of the universe, would already have occurred. He therefore
states his argument in three points- firstly, everything that begins to
exist has a cause of its existence; secondly, the universe began to
exist; so, thirdly, therefore, the universe has a cause of its
existence.
A response to this argument would be that the cause of the universe's
existence (God) would need a cause for its existence, which, in turn,
could be responded to as being logically inconsistent with the evidence
already presented- even if God did have a cause, there would still
necessarily be a cause which began everything, owing to the
impossibility of the infinite stated by Craig.
Secondly, it is argued that the premise of causality has been arrived at via a posteriori (inductive) reasoning, which is dependent on experience. David Hume highlighted this problem of induction and argued that causal relations were not true a priori. However, as to whether inductive or deductive reasoning is more valuable still remains a matter of debate, with the general conclusion being that neither is prominent.
Opponents of the argument tend to argue that it is unwise to draw
conclusions from an extrapolation of causality beyond experience. Andrew Loke replies that, according to the Kalam Cosmological Argument,
only things which begin to exist require a cause. On the other hand,
something that is without beginning has always existed and therefore
does not require a cause. The Cosmological Argument posits that there
cannot be an actual infinite regress of causes, therefore there must be
an uncaused First Cause that is beginningless and does not require a
cause.
Not evidence for a theist God
The basic cosmological argument merely establishes that a First Cause exists, not that it has the attributes of a theistic god, such as omniscience, omnipotence, and omnibenevolence.
This is why the argument is often expanded to show that at least some
of these attributes are necessarily true, for instance in the modern
Kalam argument given above.
Existence of causal loops
A causal loop is a form of predestination paradox
arising where traveling backwards in time is deemed a possibility. A
sufficiently powerful entity in such a world would have the capacity to
travel backwards in time to a point before its own existence, and to
then create itself, thereby initiating everything which follows from it.
The usual reason which is given to refute the possibility of a
causal loop is it requires that the loop as a whole be its own cause. Richard Hanley
argues that causal loops are not logically, physically, or
epistemically impossible: "[In timed systems,] the only possibly
objectionable feature that all causal loops share is that coincidence is
required to explain them."
However, Andrew Loke argues that causal loop of the type that is
supposed to avoid a First Cause suffers from the problem of vicious
circularity and thus it would not work.
Existence of infinite causal chains
David Hume and later Paul Edwards
have invoked a similar principle in their criticisms of the
cosmological argument. Rowe has called the principle the Hume-Edwards
principle:
If the existence of every member of a set is explained, the existence of that set is thereby explained.
Nevertheless, David White argues that the notion of an infinite causal regress providing a proper explanation is fallacious. Furthermore, Demea states that even if the succession of causes is infinite, the whole chain still requires a cause.
To explain this, suppose there exists a causal chain of infinite
contingent beings. If one asks the question, "Why are there any
contingent beings at all?", it does not help to be told that "There are
contingent beings because other contingent beings caused them." That
answer would just presuppose additional contingent beings. An adequate
explanation of why some contingent beings exist would invoke a different
sort of being, a necessary being that is not contingent.
A response might suppose each individual is contingent but the infinite
chain as a whole is not; or the whole infinite causal chain to be its
own cause.
Severinsen argues that there is an "infinite" and complex causal structure.
White tried to introduce an argument "without appeal to the principle
of sufficient reason and without denying the possibility of an infinite
causal regress".
A number of other arguments have been offered to demonstrate that an
actual infinite regress cannot exist, viz. the argument for the
impossibility of concrete actual infinities, the argument for the
impossibility of traversing an actual infinite, the argument from the
lack of capacity to begin to exist, and various arguments from
paradoxes.
Big Bang cosmology
Some
cosmologists and physicists argue that a challenge to the cosmological
argument is the nature of time: "One finds that time just disappears
from the Wheeler–DeWitt equation" (Carlo Rovelli). The Big Bang theory states that it is the point in which all dimensions came into existence, the start of both space and time.
Then, the question "What was there before the Universe?" makes no
sense; the concept of "before" becomes meaningless when considering a
situation without time. This has been put forward by J. Richard Gott III, James E. Gunn, David N. Schramm, and Beatrice Tinsley, who said that asking what occurred before the Big Bang is like asking what is north of the North Pole.
However, some cosmologists and physicists do attempt to investigate
causes for the Big Bang, using such scenarios as the collision of membranes.
Philosopher Edward Feser
states that classical philosophers' arguments for the existence of God
do not care about the Big Bang or whether the universe had a beginning.
The question is not about what got things started or how long they have
been going, but rather what keeps them going.
There is also a Big Bang Argument, which is a variation of the
Cosmological Argument using the Big Bang Theory to validate the premise
that the Universe had a beginning.
The circled dot was used by the Pythagoreans and later Greeks to represent the first metaphysical being, the Monad or The Absolute.
On causation
Ancient Greek philosopher Aristotle argued that everything must have a cause, culminating in an ultimate uncaused cause.
David Hume
argued that, while we expect everything to have a cause because of our
experience of the necessity of causes, a cause may not be necessary in
the case of the formation of the universe, which is outside our
experience.
Bertrand Russell took a "brute fact" position when he said "I should say that the universe is just there, and that's all."
Philosopher Brian Leftow
has argued that the question cannot have a causal explanation (as any
cause must itself have a cause) or a contingent explanation (as the
factors giving the contingency must pre-exist), and that if there is an
answer it must be something that exists necessarily (i.e., something
that just exists, rather than is caused).
Why is there something rather than
nothing? The sufficient reason [...] is found in a substance
which [...] is a necessary being bearing the reason for its existence
within itself.
Philosopher of physics Dean Rickles has argued that numbers and mathematics (or their underlying laws) may necessarily exist.
Criticism of the question
Philosopher Stephen Law
has said the question may not need answering, as it is attempting to
answer a question that is outside a spatio-temporal setting, from within
a spatio-temporal setting. He compares the question to asking "what is
north of the North Pole?" Noted philosophical wit Sidney Morgenbesser reportedly answered the question with an apothegm: "If there were nothing you'd still be complaining!", or "Even if there was nothing, you still wouldn't be satisfied!"
Physics is not enough
Physicists such as Stephen Hawking and Lawrence Krauss have offered explanations that rely on quantum mechanics, saying that in a quantum vacuum state particles will spontaneously come into existence. Nobel Laureate Frank Wilczek is credited with the aphorism that "nothing is unstable." However, this answer has not satisfied physicist Sean Carroll
who argues that Wilczek's aphorism accounts merely for the existence of
matter, but not the existence of quantum states, space-time or the
universe as a whole.
God is not enough
Philosopher
Roy Sorensen writes in the Stanford Encyclopedia that to many
philosophers the question is intrinsically impossible to answer, like squaring a circle, and even God does not sufficiently answer it:
"To explain why something
exists, we standardly appeal to the existence of something else... For
instance, if we answer 'There is something because the Universal
Designer wanted there to be something', then our explanation takes for
granted the existence of the Universal Designer. Someone who poses the
question in a comprehensive way will not grant the existence of the
Universal Designer as a starting point. If the explanation cannot begin
with some entity, then it is hard to see how any explanation is
feasible. Some philosophers conclude 'Why is there something rather than
nothing?' is unanswerable. They think the question stumps us by
imposing an impossible explanatory demand, namely, 'Deduce the existence
of something without using any existential premises'. Logicians should
feel no more ashamed of their inability to perform this deduction than
geometers should feel ashamed at being unable to square the circle."
Argument that "Nothing" is impossible
The pre-Socratic philosopher Parmenides was one of the first Western thinkers to question the possibility of nothing. Many other thinkers, such as Bede Rundle, have questioned whether nothing is an ontological possibility. Nothing might be a human concept that is only a construct and inappropriate for a description of a possible state, or absence of state.
Diagram of evolution of the (observable part) of the universe from the Big Bang (left), the CMB-reference afterglow, to the present.
The chronology of the universe describes the history and future of the universe according to Big Bang cosmology. The earliest stages of the universe's existence are estimated as taking place 13.8 billion years ago, with an uncertainty of around 21 million years at the 68% confidence level.
Outline
Chronology in five stages
For the purposes of this summary, it is convenient to divide the chronology of the universe since it originated, into five parts. It is generally considered meaningless or unclear whether time existed before this chronology:
Tiny ripples in the universe at this stage are believed to be the
basis of large-scale structures that formed much later. Different
stages of the very early universe are understood to different extents.
The earlier parts are beyond the grasp of practical experiments in particle physics but can be explored through other means.
Lasting around 370,000 years. Initially, various kinds of subatomic particles are formed in stages. These particles include almost equal amounts of matter and antimatter, so most of it quickly annihilates, leaving a small excess of matter in the universe.
By 20 minutes, the universe is no longer hot enough for nuclear fusion, but far too hot for neutral atoms to exist or photons to travel far. It is therefore an opaqueplasma. At around 47,000 years, as the universe cools, its behaviour begins to be dominated by matter rather than radiation. At about 100,000 years, helium hydride is the first molecule. (Much later, hydrogen and helium hydride react to form molecular hydrogen, the fuel needed for the first stars.)
At about 370,000 years, the universe finally becomes cool enough for neutral atoms to form ("recombination"), and as a result it also became transparent for the first time. The newly formed atoms—mainly hydrogen and helium with traces of lithium—quickly reach their lowest energy state (ground state) by releasing photons ("photon decoupling"), and these photons can still be detected today as the cosmic microwave background (CMB). This is currently the oldest observation we have of the universe.
From 370,000 years until about 1 billion years. After recombination and decoupling, the universe was transparent but the clouds of hydrogen only collapsed very slowly to form stars and galaxies,
so there were no new sources of light. The only photons
(electromagnetic radiation, or "light") in the universe were those
released during decoupling (visible today as the cosmic microwave
background) and 21 cm radio emissions
occasionally emitted by hydrogen atoms. The decoupled photons would
have filled the universe with a brilliant pale orange glow at first,
gradually redshifting to non-visible wavelengths after about 3 million years, leaving it without visible light. This period is known as the cosmic Dark Ages.
Between about 10 and 17 million years the universe's average
temperature was suitable for liquid water 273–373 K (0–100 °C) and there
has been speculation whether rocky planets or indeed life could have
arisen briefly, since statistically a tiny part of the universe could
have had different conditions from the rest as a result of a very
unlikely statistical fluctuation, and gained warmth from the universe as
a whole.
At some point around 200 to 500 million years, the earliest
generations of stars and galaxies form (exact timings are still being
researched), and early large structures gradually emerge, drawn to the
foam-like dark matterfilaments which have already begun to draw together throughout the universe. The earliest generations of stars have not yet been observed astronomically. They may have been huge (100-300 solar masses) and non-metallic, with very short lifetimes compared to most stars we see today, so they commonly finish burning their hydrogen fuel and explode as highly energetic pair-instabilitysupernovae after mere millions of years.
Other theories suggest that they may have included small stars, some
perhaps still burning today. In either case, these early generations of
supernovae created most of the everyday elements we see around us today, and seeded the universe with them.
Galaxy clusters and superclusters emerge over time. At some point, high energy photons from the earliest stars, dwarf galaxies and perhaps quasars leads to a period of reionization
that commences gradually between about 250-500 million years, is
complete by about 700-900 million years, and diminishes by about 1
billion years (exact timings still being researched). The universe
gradually transitioned into the universe we see around us today, and the
Dark Ages only fully came to an end at about 1 billion years.
From 1 billion years, and for about 12.8 billion years, the universe
has looked much as it does today. It will continue to appear very
similar for many billions of years into the future. The thin disk of our galaxy began to form at about 5 billion years (8.8 Gya), and our Solar System formed at about 9.2 billion years (4.6 Gya), with the earliest traces of life on Earth emerging by about 10.3 billion years (3.5 Gya).
From about 9.8 billion years of cosmic time, the slowing expansion of space gradually begins to accelerate under the influence of dark energy, which may be a scalar field
throughout our universe. The present-day universe is understood quite
well, but beyond about 100 billion years of cosmic time (about 86
billion years in the future), uncertainties in current knowledge mean
that we are less sure which path our universe will take.
At some time the Stelliferous Era will end as stars are no longer being born, and the expansion of the universe will mean that the observable universe becomes limited to local galaxies. There are various scenarios for the far future and ultimate fate of the universe. More exact knowledge of our current universe will allow these to be better understood.
The Planck scale
is the physical scale beyond which current physical theories may not
apply, and cannot be used to calculate what happened. During the Planck
epoch, cosmology and physics are assumed to have been dominated by the quantum effects of gravity.
The forces of the Standard Model have separated, but energies are too high for quarks to coalesce into hadrons, instead forming a quark–gluon plasma. These are the highest energies directly observable in the Large Hadron Collider.
Quarks are bound into hadrons. A slight matter-antimatter-asymmetry from the earlier phases (baryon asymmetry) results in an elimination of anti-hadrons.
Neutrinos cease interacting with baryonic matter. The sphere of space that will become the observable universe is approximately 10 light-years in radius at this time.
Protons and neutrons are bound into primordial atomic nuclei, hydrogen and helium-4. Small amounts of deuterium, helium-3, and lithium-7
are also synthesized. At the end of this epoch, the spherical volume of
space which will become the observable universe is about 300
light-years in radius, baryonic matter density is on the order of 4
grams per m3 (about 0.3% of sea level air density)—however, most energy at this time is in electromagnetic radiation.
Electrons and atomic nuclei first become bound to form neutral atoms.
Photons are no longer in thermal equilibrium with matter and the
universe first becomes transparent. Recombination lasts for about 100
ka, during which universe is becoming more and more transparent to
photons. The photons of the cosmic microwave background
radiation originate at this time. The spherical volume of space which
will become the observable universe is 42 million light-years in radius
at this time. The baryonic matter density at this time is about 500
million hydrogen and helium atoms per m3, approximately a billion times higher than today. This density corresponds to pressure on the order of 10−17 atm.
The time between recombination and the formation of the first stars. During this time, the only source of photons was hydrogen emitting radio waves at hydrogen line. Freely propagating CMB photons quickly (within about 3 million years) red-shifted to infrared, and universe was devoid of visible light.
Onset 250 Ma ~ 500 Ma Complete: 700 Ma ~ 900 Ma Ends: 1 Ga (All timings approximate)
20 ~ 6
60 K ~ 19 K
The most distant astronomical objects observable with telescopes date to this period; as of 2016, the most remote galaxy observed is GN-z11, at a redshift of 11.09. The earliest "modern" Population III stars stars are formed in this period.
Farthest observable photons at this moment are CMB photons. They
arrive from a sphere with the radius of 46 billion light-years. The
spherical volume inside it is commonly referred to as the observable
universe.
Alternative subdivisions of the chronology (overlapping several of the above periods)
The time between the first formation of Population III stars until the cessation of star formation, leaving all stars in the form of degenerate remnants.
The Stelliferous Era
will end as stars eventually die and fewer are born to replace them,
leading to a darkening universe. Various theories suggest a number of
subsequent possibilities. Assuming proton decay, matter may eventually evaporate into a Dark Era (heat death). Alternatively the universe may collapse in a Big Crunch. Alternative suggestions include a false vacuum catastrophe or a Big Rip as possible ends to the universe.
If we assume that the FLRW metric equations are valid all the way
back to the beginning of our universe, then we can follow them back in
time, to a point where the equations suggest all distances between
objects in the universe were zero or infinitesimally small. (This does not
necessarily mean the universe was physically small at the Big Bang,
although that is indeed one of the possibilities.) Going forward, this
provides a model of our universe which matches all current physical
observations extremely closely. This initial period of the universe's
chronology is called the "Big Bang". The Standard Model of cosmology does not attempt to explain why the universe began to exist; it explains only how the universe physically developed once that moment happened.
We interpret the singularity
from the FLRW metric as meaning that current theories are inadequate to
describe what actually happened at the start of the Big Bang itself. It
is widely believed that a correct theory of quantum gravity
may allow a more correct description of that event, but no such theory
has yet been developed. After that moment, all distances throughout the
universe began to increase from (perhaps) zero because the FLRW metric
itself changed over time, affecting distances between all non-bound
objects everywhere. For this reason we say that the Big Bang "happened
everywhere".
The very early universe
During
the very earliest moments of cosmic time, the energies and conditions
were so extreme that our current knowledge can only suggest
possibilities, so our current knowledge may turn out to be incorrect. To
give one example, eternal inflation
theories propose that inflation lasts forever throughout most of the
universe, making the notion of "N seconds since Big Bang" ill-defined.
Therefore the earliest stages are an active area of research and based
on ideas which are still speculative and subject to modification as
scientific knowledge improves.
Although a specific "inflationary epoch" is highlighted at around 10−32
seconds, observations and theories both suggest that distances between
objects in space have been increasing at all times since the moment of
the Big Bang, and is still increasing today (with the exception of
gravitationally bound objects such as galaxies and most clusters,
once the rate of expansion had greatly slowed). The inflationary period
marks a specific period when a very rapid change in scale occurred, but
does not mean that it stayed the same at other times. More
precisely, during inflation, the expansion accelerated; then, after
inflation and for about 9.8 billion years, the expansion was much slower
and became an even slower expansion over time (although it never
reversed); and then since about 4 billion years ago it has been slightly
speeding up again.
Initially, the universe was inconceivably hot and dense.
It has cooled over time, which eventually allowed the forces, particles
and structures we see around us to manifest as they do today.
The Planck epoch
is an era in traditional (non-inflationary) Big Bang cosmology
immediately after the event which began our known universe. During this
epoch, the temperature and average energies within the universe were so
high that everyday subatomic particles could not form, and even the four
fundamental forces that shape our universe—gravitation, electromagnetism, the weak nuclear force, and the strong nuclear force—were
combined and formed one fundamental force. Little is understood about
physics at this temperature; different hypotheses propose different
scenarios. Traditional big bang cosmology predicts a gravitational singularity before this time, but this theory relies on the theory of general relativity, which is thought to break down for this epoch due to quantum effects.
In inflationary models of cosmology, times before the end of inflation (roughly 10−32
seconds after the Big Bang) do not follow the same timeline as in
traditional big bang cosmology. Models that aim to describe the universe
and physics during the Planck epoch are generally speculative and fall
under the umbrella of "New Physics". Examples include the Hartle–Hawking initial state, string theory landscape, string gas cosmology, and the ekpyrotic universe.
Grand unification epoch
Between 10−43 seconds and 10−36 seconds after the Big Bang
As the universe expanded and cooled, it crossed transition temperatures at which forces separated from each other. These phase transitions can be visualized as similar to condensation and freezing
phase transitions of ordinary matter. At certain temperatures/energies,
water molecules change their behaviour and structure, and they will
behave completely differently. Like steam turning to water, the fields
which define our universe's fundamental forces and particles also
completely change their behaviours and structures when the
temperature/energy falls below a certain point. This is not apparent in
everyday life, because it only happens at far higher temperatures than
we usually see in our present universe.
These phase transitions in the universe's fundamental forces are believed to be caused by a phenomenon of quantum fields called "symmetry breaking".
In everyday terms, as the universe cools, it becomes possible for
the quantum fields that create the forces and particles around us, to
settle at lower energy levels and with higher levels of stability. In
doing so, they completely shift how they interact. Forces and
interactions arise due to these fields, so the universe can behave very
differently above and below a phase transition. For example, in a later
epoch, a side effect of one phase transition is that suddenly, many
particles that had no mass at all acquire a mass (they begin to interact
differently with the Higgs field), and a single force begins to manifest as two separate forces.
Assuming that nature is described by a so-called Grand Unified Theory
(GUT), the grand unification epoch began with a phase transitions of
this kind, when gravitation separated from the universal combined gauge force. This caused two forces to now exist: gravity, and an electrostrong interaction.
There is no hard evidence yet, that such a combined force existed, but
many physicists believe it did. The physics of this electrostrong
interaction would be described by a Grand Unified Theory.
The grand unification epoch ended with a second phase transition,
as the electrostrong interaction in turn separated, and began to
manifest as two separate interactions, called the strong and the electroweak interactions.
Electroweak epoch
Between 10−36 seconds (or the end of inflation) and 10−32 seconds after the Big Bang
Depending on how epochs are defined, and the model being followed, the electroweak epoch
may be considered to start before or after the inflationary epoch. In
some models it is described as including the inflationary epoch. In
other models, the electroweak epoch is said to begin after the
inflationary epoch ended, at roughly 10−32 seconds.
According to traditional big bang cosmology, the electroweak epoch began 10−36 seconds after the Big Bang, when the temperature of the universe was low enough (1028 K) for the electronuclear force
to begin to manifest as two separate interactions, called the strong
and the electroweak interactions. (The electroweak interaction will also
separate later, dividing into the electromagnetic and weak
interactions.) The exact point where electrostrong symmetry was broken
is not certain, because of the very high energies of this event.
Inflationary epoch and the rapid expansion of space
Before c. 10−32 seconds after the Big Bang
At this point of the very early universe, the metric that defines distance within space suddenly and very rapidly changed in scale, leaving the early universe at least 1078 times its previous volume (and possibly much more). This is equivalent to a linear increase of at least 1026 times in every spatial dimension—equivalent to an object 1 nanometre (10−9m, about half the width of a molecule of DNA)
in length, expanding to one approximately 10.6 light-years
(100 trillion kilometres) long in a tiny fraction of a second. This
change is known as inflation.
Although light and objects within spacetime cannot travel faster than the speed of light, in this case it was the metric
governing the size and geometry of spacetime itself that changed in
scale. Changes to the metric are not limited by the speed of light.
There is good evidence that this happened, and it is widely accepted that it did take place. But the exact reasons why
it happened are still being explored. So a range of models exist that
explain why and how it took place—it is not yet clear which explanation
is correct.
In several of the more prominent models, it is thought to have been triggered by the separation
of the strong and electroweak interactions which ended the grand
unification epoch. One of the theoretical products of this phase
transition was a scalar field called the inflaton field.
As this field settled into its lowest energy state throughout the
universe, it generated an enormous repulsive force that led to a rapid
expansion of the metric that defines space itself. Inflation explains
several observed properties of the current universe that are otherwise
difficult to account for, including explaining how today's universe has
ended up so exceedingly homogeneous (similar) on a very large scale, even though it was highly disordered in its earliest stages.
It is not known exactly when the inflationary epoch ended, but it is thought to have been between 10−33 and 10−32 seconds after the Big Bang. The rapid expansion of space meant that elementary particles
remaining from the grand unification epoch were now distributed very
thinly across the universe. However, the huge potential energy of the
inflation field was released at the end of the inflationary epoch, as
the inflaton field decayed into other particles, known as "reheating".
This heating effect led to the universe being repopulated with a dense,
hot mixture of quarks, anti-quarks and gluons. In other models, reheating is often considered to mark the start of the electroweak epoch, and some theories, such as warm inflation, avoid a reheating phase entirely.
In non-traditional versions of Big Bang theory (known as
"inflationary" models), inflation ended at a temperature corresponding
to roughly 10−32 seconds after the Big Bang, but this does not imply that the inflationary era lasted less than 10−32 seconds. To explain the observed homogeneity of the universe, the duration in these models must be longer than 10−32 seconds. Therefore, in inflationary cosmology, the earliest meaningful time "after the Big Bang" is the time of the end of inflation.
After inflation ended, the universe continued to expand, but at a
much slower rate. About 4 billion years ago the expansion gradually
began to speed up again. This is believed to be due to dark energy
becoming dominant in the universe's large-scale behaviour. It is still
expanding today.
On 17 March 2014, astrophysicists of the BICEP2 collaboration announced the detection of inflationary gravitational waves in the B-modespower spectrum which was interpreted as clear experimental evidence for the theory of inflation. However, on 19 June 2014, lowered confidence in confirming the cosmic inflation findings was reported and finally, on 2 February 2015, a joint analysis of data from BICEP2/Keck and the European Space Agency'sPlanck
microwave space telescope concluded that the statistical "significance
[of the data] is too low to be interpreted as a detection of primordial
B-modes" and can be attributed mainly to polarized dust in the Milky
Way.
Electroweak symmetry breaking
10−12 seconds after the Big Bang
As the universe's temperature continued to fall below a certain very
high energy level, a third symmetry breaking occurs. So far as we
currently know, it was the penultimate symmetry breaking event in the
formation of our universe, the final one being chiral symmetry breaking in the quark sector. In the Standard Model of particle physics, electroweak symmetry breaking happens at a temperature of 159.5±1.5 GeV. When this happens, it breaks electroweak gauge symmetry. This has two related effects:
Via the Higgs mechanism, all elementary particles interacting with the Higgs field become massive, having been massless at higher energy levels.
As a side-effect, the weak nuclear force and electromagnetic force, and their respective bosons (the W and Z bosons
and photon) now begin to manifest differently in the present universe.
Before electroweak symmetry breaking these bosons were all massless
particles and interacted over long distances, but at this point the W
and Z bosons abruptly become massive particles only interacting over
distances smaller than the size of an atom, while the photon remains
massless and remains a long-distance interaction.
After electroweak symmetry breaking, the fundamental interactions we
know of—gravitation, electromagnetic, weak and strong interactions—have
all taken their present forms, and fundamental particles have their
expected masses, but the temperature of the universe is still too high
to allow the stable formation of many particles we now see in the
universe, so there are no protons or neutrons, and therefore no atoms, atomic nuclei,
or molecules. (More exactly, any composite particles that form by
chance, almost immediately break up again due to the extreme energies.)
Supersymmetry breaking (speculative)
If supersymmetry is a property of our universe, then it must be broken at an energy that is no lower than 1 TeV, the electroweak scale. The masses of particles and their superpartners
would then no longer be equal. This very high energy could explain why
no superpartners of known particles have ever been observed.
The early universe
After cosmic inflation ends, the universe is filled with a hot quark–gluon plasma,
the remains of reheating. From this point onwards the physics of the
early universe is much better understood, and the energies involved in
the Quark epoch are directly accessible in particle physics experiments and other detectors.
The quark epoch
Between 10−12 seconds and 10−6 seconds after the Big Bang
The quark epoch began approximately 10−12
seconds after the Big Bang. This was the period in the evolution of the
early universe immediately after electroweak symmetry breaking, when
the fundamental interactions of gravitation, electromagnetism, the
strong interaction and the weak interaction had taken their present
forms, but the temperature of the universe was still too high to allow quarks to bind together to form hadrons.
During the quark epoch the universe was filled with a dense, hot quark–gluon plasma, containing quarks, leptons and their antiparticles. Collisions between particles were too energetic to allow quarks to combine into mesons or baryons.
The quark epoch ended when the universe was about 10−6 seconds old, when the average energy of particle interactions had fallen below the binding energy of hadrons.
Baryogenesis
Perhaps by 10−11 seconds
Baryons are subatomic particles such as protons and neutrons, that are composed of three quarks. It would be expected that both baryons, and particles known as antibaryons
would have formed in equal numbers. However, this does not seem to be
what happened—as far as we know, the universe was left with far more
baryons than antibaryons. In fact, almost no antibaryons are observed in
nature. It is not clear how this came about. Any explanation for this
phenomenon must allow the Sakharov conditions related to baryogenesis to have been satisfied at some time after the end of cosmological inflation.
Current particle physics suggests asymmetries under which these
conditions would be met, but these asymmetries appear to be too small to
account for the observed baryon-antibaryon asymmetry of the universe.
Hadron epoch
Between 10−6 second and 1 second after the Big Bang
The quark–gluon plasma that composes the universe cools until hadrons, including baryons such as protons and neutrons, can form.
Initially, hadron/anti-hadron pairs could form, so matter and antimatter were in thermal equilibrium.
However, as the temperature of the universe continued to fall, new
hadron/anti-hadron pairs were no longer produced, and most of the newly
formed hadrons and anti-hadrons annihilated
each other, giving rise to pairs of high-energy photons. A
comparatively small residue of hadrons remained at about 1 second of
cosmic time, when this epoch ended.
Theory predicts that about 1 neutron remained for every 7
protons. We believe this to be correct because, at a later stage, all
the neutrons and some of the protons fused, leaving hydrogen, a hydrogen isotope
called deuterium, helium and other elements, which we can measure. A
1:7 ratio of hadrons at the end of this epoch would indeed produce the
observed element ratios in the early as well as current universe.
Neutrino decoupling and cosmic neutrino background (CνB)
Around 1 second after the Big Bang
At approximately 1 second after the Big Bang neutrinos decouple and
begin travelling freely through space. As neutrinos rarely interact with
matter, these neutrinos still exist today, analogous to the much later
cosmic microwave background emitted during recombination, around 370,000
years after the Big Bang. The neutrinos from this event have a very low
energy, around 10−10 times smaller than is possible with present-day direct detection. Even high energy neutrinos are notoriously difficult to detect, so this cosmic neutrino background (CνB) may not be directly observed in detail for many years, if at all.
However, Big Bang cosmology makes many predictions about the CνB,
and there is very strong indirect evidence that the CνB exists, both
from Big Bang nucleosynthesis
predictions of the helium abundance, and from anisotropies in the
cosmic microwave background (CMB). One of these predictions is that
neutrinos will have left a subtle imprint on the CMB. It is well known
that the CMB has irregularities. Some of the CMB fluctuations were
roughly regularly spaced, because of the effect of baryonic acoustic oscillations. In theory, the decoupled neutrinos should have had a very slight effect on the phase of the various CMB fluctuations.
In 2015, it was reported that such shifts had been detected in
the CMB. Moreover, the fluctuations corresponded to neutrinos of almost
exactly the temperature predicted by Big Bang theory (1.96 +/-0.02K compared to a prediction of 1.95K), and exactly three types of neutrino, the same number of neutrino flavors currently predicted by the Standard Model.
Possible formation of primordial black holes
May have occurred within about 1 second after the Big Bang
Primordial black holes are a hypothetical type of black hole proposed in 1966, that may have formed during the so-called radiation-dominated era,
due to the high densities and inhomogeneous conditions within the first
second of cosmic time. Random fluctuations could lead to some regions
becoming dense enough to undergo gravitational collapse, forming black
holes. Current understandings and theories place tight limits on the
abundance and mass of these objects.
Typically, primordial black hole formation requires density contrasts (regional variations in the universe's density) of around (10%), where is the average density of the universe. Several mechanisms could produce dense regions meeting this criterion
during the early universe, including reheating, cosmological phase
transitions and (in so-called "hybrid inflation models") axion
inflation. Since primordial black holes didn't form from stellar gravitational collapse, their masses can be far below stellar mass (~2×1033 g). Stephen Hawking calculated in 1971 that primordial black holes could have a mass as low as 10−5 g. But they can have any size, so they could also be large, and may have contributed to the formation of galaxies.
Lepton epoch
Between 1 second and 10 seconds after the Big Bang
The majority of hadrons and anti-hadrons annihilate each other at the end of the hadron epoch, leaving leptons (such as the electron, muons and certain neutrinos) and antileptons, dominating the mass of the universe.
The lepton epoch follows a similar path to the earlier hadron
epoch. Initially leptons and antileptons are produced in pairs. About 10
seconds after the Big Bang the temperature of the universe falls to the
point at which new lepton–antilepton pairs are no longer created and
most remaining leptons and antileptons quickly annihilated each other,
giving rise to pairs of high energy photons, and leaving a small residue
of non-annihilated leptons.
Photon epoch
Between 10 seconds and 370,000 years after the Big Bang
After most leptons and antileptons are annihilated at the end of the
lepton epoch, most of the mass-energy in the universe is left in the
form of photons. (Much of the rest of its mass-energy is in the form of neutrinos and other relativistic particles).
Therefore, the energy of the universe, and its overall behaviour, is
dominated by its photons. These photons continue to interact frequently
with charged particles, i.e., electrons, protons and (eventually)
nuclei. They continue to do so for about the next 370,000 years.
Nucleosynthesis of light elements
Between 2 minutes and 20 minutes after the Big Bang
Between about 2 and 20 minutes after the Big Bang, the temperature
and pressure of the universe allowed nuclear fusion to occur, giving
rise to nuclei of a few light elements beyond hydrogen ("Big Bang nucleosynthesis"). About 25% of the protons, and all the neutrons fuse to form deuterium, a hydrogen isotope, and most of the deuterium quickly fuses to form helium-4.
Atomic nuclei will easily unbind (break apart) above a certain
temperature, related to their binding energy. From about 2 minutes, the
falling temperature means that deuterium no longer unbinds, and is
stable, and starting from about 3 minutes, helium and other elements
formed by the fusion of deuterium also no longer unbind and are stable.
The short duration and falling temperature means that only the
simplest and fastest fusion processes can occur. Only tiny amounts of
nuclei beyond helium are formed, because nucleosynthesis of heavier
elements is difficult and requires thousands of years even in stars. Small amounts of tritium (another hydrogen isotope) and beryllium-7 and -8 are formed, but these are unstable and are quickly lost again. A small amount of deuterium is left unfused because of the very short duration.
Therefore, the only stable nuclides created by the end of Big
Bang nucleosynthesis are protium (single proton/hydrogen nucleus),
deuterium, helium-3, helium-4, and lithium-7. By mass, the resulting matter is about 75% hydrogen nuclei, 25% helium nuclei, and perhaps 10−10 by mass of lithium-7. The next most common stable isotopes produced are lithium-6, beryllium-9, boron-11, carbon, nitrogen and oxygen ("CNO"), but these have predicted abundances of between 5 and 30 parts in 1015 by mass, making them essentially undetectable and negligible.
The amounts of each light element in the early universe can be
estimated from old galaxies, and is strong evidence for the Big Bang.
For example, the Big Bang should produce about 1 neutron for every 7
protons, allowing for 25% of all nucleons to be fused into helium-4 (2
protons and 2 neutrons out of every 16 nucleons), and this is the amount
we find today, and far more than can be easily explained by other
processes.
Similarly, deuterium fuses extremely easily; any alternative
explanation must also explain how conditions existed for deuterium to
form, but also left some of that deuterium unfused and not immediately
fused again into helium.
Any alternative must also explain the proportions of the various light
elements and their isotopes. A few isotopes, such as lithium-7, were
found to be present in amounts that differed from theory, but over time,
these differences have been resolved by better observations.
Matter domination
47,000 years after the Big Bang
Until now, the universe's large scale dynamics and behaviour have
been determined mainly by radiation—meaning, those constituents that
move relativistically (at or near the speed of light), such as photons
and neutrinos. As the universe cools, from around 47,000 years (redshift z = 3600),
the universe's large scale behaviour becomes dominated by matter
instead. This occurs because the energy density of matter begins to
exceed both the energy density of radiation and the vacuum energy
density.
Around or shortly after 47,000 years, the densities of non-relativistic
matter (atomic nuclei) and relativistic radiation (photons) become
equal, the Jeans length,
which determines the smallest structures that can form (due to
competition between gravitational attraction and pressure effects),
begins to fall and perturbations, instead of being wiped out by free streamingradiation, can begin to grow in amplitude.
According to the Lambda-CDM model, by this stage, the matter in the universe is around 84.5% cold dark matter
and 15.5% "ordinary" matter. (However the total matter in the universe
is only 31.7%, much smaller than the 68.3% of dark energy.) There is
overwhelming evidence that dark matter
exists and dominates our universe, but since the exact nature of dark
matter is still not understood, the Big Bang theory does not presently
cover any stages in its formation.
From this point on, and for several billion years to come, the presence of dark matter accelerates the formation of structure
in our universe. In the early universe, dark matter gradually gathers
in huge filaments under the effects of gravity, collapsing faster than
ordinary (baryonic) matter because its collapse is not slowed by radiation pressure.
This amplifies the tiny inhomogeneities (irregularities) in the density
of the universe which was left by cosmic inflation. Over time, slightly
denser regions become denser and slightly rarefied (emptier) regions
become more rarefied. Ordinary matter eventually gathers together faster
than it would otherwise do, because of the presence of these
concentrations of dark matter.
The properties of dark matter that allow it to collapse quickly without radiation pressure, also mean that it cannot lose
energy by radiation either. Losing energy is necessary for particles to
collapse into dense structures beyond a certain point. Therefore dark
matter collapses into huge but diffuse filaments and haloes, and not
into stars or planets. Ordinary matter, which can lose energy by radiation, forms dense objects and also gas clouds when it collapses.
First molecules
100,000 years after the Big Bang
At around 100,000 years, the universe has cooled enough for helium hydride, the first molecule, to form. In April 2019, this molecule was first announced to have been discovered in interstellar space. (Much later, atomic hydrogen reacts with helium hydride to create molecular hydrogen, the fuel required for star formation.)
Recombination, photon decoupling, and the cosmic microwave background (CMB)
About 370,000 years after the Big Bang, two connected events occurred: recombination and photon decoupling.
Recombination describes the ionized particles combining to form the
first neutral atoms, and decoupling refers to the photons released
("decoupled") as the newly formed atoms settle into more stable energy
states.
Just before recombination, the baryonic matter
in the universe was at a temperature where it formed a hot ionized
plasma. Most of the photons in the universe interacted with electrons
and protons, and could not travel significant distances without
interacting with ionized particles. As a result, the universe was opaque
or "foggy". Although there was light, it was not possible to see, nor
can we observe that light through telescopes.
At around 370,000 years, the universe has cooled to a point where free electrons can combine with the hydrogen and helium nuclei to form neutral atoms. This process is relatively fast (and faster for the helium than for the hydrogen), and is known as recombination.
The name is slightly inaccurate and is given for historical reasons: in
fact the electrons and atomic nuclei were combining for the first time.
Directly combining in a low energy state (ground state) is less
efficient, so these hydrogen atoms generally form with the electrons
still in a high energy state, and once combined, the electrons quickly
release energy in the form of one or more photons as they transition to a
low energy state. This release of photons is known as photon
decoupling. Some of these decoupled photons are captured by other
hydrogen atoms, the remainder remain free. By the end of recombination,
most of the protons in the universe have formed neutral atoms. This
change from charged to neutral particles means that the mean free path
photons can travel before capture in effect becomes infinite, so any
decoupled photons that have not been captured can travel freely over
long distances. The universe has become transparent to visible light, radio waves and other electromagnetic radiation for the first time in its history.
The photons released by these newly formed hydrogen atoms initially had a temperature/energy of around ~ 4000 K. This would have been visible to the eye as a pale yellow/orange tinted, or "soft", white color. Over billions of years since decoupling, as the universe has expanded, the photons have been red-shifted
from visible light to radio waves (microwave radiation corresponding to
a temperature of about 2.7 K). Red shifting describes the photons
acquiring longer wavelengths and lower frequencies
as the universe expanded over billions of years, so that they gradually
changed from visible light to radio waves. These same photons can still
be detected as radio waves today. They form the cosmic microwave
background, and they provide crucial evidence of the early universe and
how it developed.
Around the same time as recombination, existing pressure waves within the electron-baryon plasma—known as baryon acoustic oscillations—became
embedded in the distribution of matter as it condensed, giving rise to a
very slight preference in distribution of large-scale objects.
Therefore, the cosmic microwave background is a picture of the universe
at the end of this epoch including the tiny fluctuations generated
during inflation,
and the spread of objects such as galaxies in the universe is an
indication of the scale and size of the universe as it developed over
time.
The Dark Ages and large-scale structure emergence
370 thousand to about 1 billion years after the Big Bang
Dark Ages
After recombination and decoupling, the universe was transparent and
had cooled enough to allow light to travel long distances, but there
were no light-producing structures such as stars and galaxies. Stars and
galaxies are formed when dense regions of gas form due to the action of
gravity, and this takes a long time within a near-uniform density of
gas and on the scale required, so it is estimated that stars did not
exist for perhaps hundreds of millions of years after recombination.
This period, known as the Dark Ages, began around 370,000 years
after the Big Bang. During the Dark Ages, the temperature of the
universe cooled from some 4000 K to about 60 K (3727 °C to about
−213 °C), and only two sources of photons existed: the photons released
during recombination/decoupling (as neutral hydrogen atoms formed),
which we can still detect today as the cosmic microwave background
(CMB), and photons occasionally released by neutral hydrogen atoms,
known as the 21 cm spin line of neutral hydrogen. The hydrogen spin line is in the microwave range of frequencies, and within 3 million years, the CMB photons had redshifted out of visible light to infrared;
from that time until the first stars, there were no visible light
photons. Other than perhaps some rare statistical anomalies, the
universe was truly dark.
The first generation of stars, known as Population III stars, formed within a few hundred million years after the Big Bang.
These stars were the first source of visible light in the universe
after recombination. Structures may have begun to emerge from around 150
million years, and early galaxies emerged from around 380 to 700
million years. (We do not have separate observations of very early
individual stars; the earliest observed stars are discovered as
participants in very early galaxies.) As they emerged, the Dark Ages
gradually ended. Because this process was gradual, the Dark Ages only
fully ended around 1 billion years, as the universe took its present
appearance.
There is also currently an observational effort
underway to detect the faint 21 cm spin line radiation, as it is in
principle an even more powerful tool than the cosmic microwave
background for studying the early universe.
Speculative "habitable epoch"
c. 10–17 million years after the Big Bang
For about 6.6 million years, between about 10 to 17 million years
after the Big Bang (redshift 137–100), the background temperature was
between 273–373 K (0–100 °C), a temperature compatible with liquid water and common biologicalchemical reactions. Abraham Loeb (2014) speculated that primitive life might in principle have appeared during this window, which he called the "habitable epoch of the early Universe".
Loeb argues that carbon-based life might have evolved in a hypothetical
pocket of the early universe that was dense enough both to generate at
least one massive star that subsequently releases carbon in a supernova,
and that was also dense enough to generate a planet. (Such dense
pockets, if they existed, would have been extremely rare.) Life would
also have required a heat differential, rather than just uniform
background radiation; this could be provided by naturally-occurring
geothermal energy. Such life would likely have remained primitive; it is
highly unlikely that intelligent life would have had sufficient time to
evolve before the hypothetical oceans freeze over at the end of the
habitable epoch.
Earliest structures and stars emerge
Around 150 million to 1 billion years after the Big Bang
The Hubble Ultra Deep Fields often showcase galaxies from an ancient era that tell us what the early Stelliferous Era was like
Another
Hubble image shows an infant galaxy forming nearby, which means this
happened very recently on the cosmological timescale. This shows that
new galaxy formation in the universe is still occurring.
The matter in the universe is around 84.5% cold dark matter and 15.5%
"ordinary" matter. Since the start of the matter-dominated era, the
dark matter has gradually been gathering in huge spread out (diffuse)
filaments under the effects of gravity. Ordinary matter eventually
gathers together faster than it would otherwise do, because of the
presence of these concentrations of dark matter. It is also slightly
more dense at regular distances due to early baryon acoustic oscillations
(BAO) which became embedded into the distribution of matter when
photons decoupled. Unlike dark matter, ordinary matter can lose energy
by many routes, which means that as it collapses, it can lose the energy
which would otherwise hold it apart, and collapse more quickly, and
into denser forms. Ordinary matter gathers where dark matter is denser,
and in those places it collapses into clouds of mainly hydrogen gas. The
first stars and galaxies form from these clouds. Where numerous
galaxies have formed, galaxy clusters and superclusters will eventually
arise. Large voids with few stars will develop between them, marking where dark matter became less common.
The exact timings of the first stars, galaxies, supermassive black holes, and quasars, and the start and end timings and progression of the period known as reionization,
are still being actively researched, with new findings published
periodically. As of 2019, the earliest confirmed galaxies date from
around 380–400 million years (for example GN-z11), suggesting surprisingly fast gas cloud condensation and stellar birth rates, and observations of the Lyman-alpha forest
and other changes to the light from ancient objects allows the timing
for reionization, and its eventual end, to be narrowed down. But these
are all still areas of active research.
Structure formation in the Big Bang model proceeds
hierarchically, due to gravitational collapse, with smaller structures
forming before larger ones. The earliest structures to form are the
first stars (known as Population III stars), dwarf galaxies, and quasars
(which are thought to be bright, early active galaxies containing a supermassive black hole surrounded by an inward-spiralling accretion disk of gas). Before this epoch, the evolution of the universe could be understood through linear cosmological perturbation theory:
that is, all structures could be understood as small deviations from a
perfect homogeneous universe. This is computationally relatively easy to
study. At this point non-linear structures begin to form, and the computational problem becomes much more difficult, involving, for example, N-body simulations with billions of particles. The Bolshoi Cosmological Simulation is a high precision simulation of this era.
These Population III stars are also responsible for turning the
few light elements that were formed in the Big Bang (hydrogen, helium
and small amounts of lithium) into many heavier elements. They can be
huge as well as perhaps small—and non-metallic (no elements except
hydrogen and helium). The larger stars have very short lifetimes
compared to most Main Sequence stars we see today, so they commonly
finish burning their hydrogen fuel and explode as supernovae
after mere millions of years, seeding the universe with heavier
elements over repeated generations. They mark the start of the
Stelliferous Era.
As yet, no Population III stars have been found, so our understanding of them is based on computational models
of their formation and evolution. Fortunately, observations of the
cosmic microwave background radiation can be used to date when star
formation began in earnest. Analysis of such observations made by the Planck
microwave space telescope in 2016 concluded that the first generation
of stars may have formed from around 300 million years after the Big
Bang.
The October 2010 discovery of UDFy-38135539, the first observed galaxy to have existed during the following reionization epoch, gives us a window into these times. Subsequently, Leiden University's Rychard J. Bouwens and Garth D. Illingworth from UC Observatories/Lick Observatory found the galaxy UDFj-39546284
to be even older, at a time some 480 million years after the Big Bang
or about halfway through the Dark Ages 13.2 billion years ago. In
December 2012 the first candidate galaxies dating to before reionization
were discovered, when UDFy-38135539, EGSY8p7
and GN-z11 galaxies were found to be around 380–550 million years after
the Big Bang, 13.4 billion years ago and at a distance of around
32 billion light-years (9.8 billion parsecs).
Quasars provide some additional evidence of early structure formation. Their light shows evidence of elements such as carbon, magnesium, iron
and oxygen. This is evidence that by the time quasars formed, a massive
phase of star formation had already taken place, including sufficient
generations of Population III stars to give rise to these elements.
Reionization
As the first stars, dwarf galaxies and quasars gradually form, the
intense radiation they emit reionizes much of the surrounding universe;
splitting the neutral hydrogen atoms back into a plasma of free
electrons and protons for the first time since recombination and
decoupling.
Reionization is evidenced from observations of quasars. Quasars
are a form of active galaxy, and the most luminous objects observed in
the universe. Electrons in neutral hydrogen have a specific patterns of
absorbing photons, related to electron energy levels and called the Lyman series.
Ionized hydrogen does not have electron energy levels of this kind.
Therefore, light travelling through ionized hydrogen and neutral
hydrogen shows different absorption lines. In addition, the light will
have travelled for billions of years to reach us, so any absorption by
neutral hydrogen will have been redshifted by varied amounts, rather
than by one specific amount, indicating when it happened. These features
make it possible to study the state of ionization at many different
times in the past. They show that reionization began as "bubbles" of
ionized hydrogen which became larger over time. They also show that the absorption was due to the general state of the universe (the intergalactic medium) and not due to passing through galaxies or other dense areas. Reionization might have started to happen as early as z = 16 (250 million years of cosmic time) and was complete by around z = 9 or 10 (500 million years)before gradually diminishing and probably coming to an end by around z = 5
or 6 (1 billion years) as the era of Population III stars and
quasars—and their intense radiation—came to an end, and the ionized
hydrogen gradually reverted to neutral atoms.
These observations have narrowed down the period of time during
which reionization took place, but the source of the photons that caused
reionization is still not completely certain. To ionize neutral
hydrogen, an energy larger than 13.6 eV is required, which corresponds to ultraviolet photons with a wavelength of 91.2 nm
or shorter, implying that the sources must have produced significant
amount of ultraviolet and higher energy. Protons and electrons will
recombine if energy is not continuously provided to keep them apart,
which also sets limits on how numerous the sources were and their
longevity. With these constraints, it is expected that quasars and first generation stars and galaxies were the main sources of energy.
The current leading candidates from most to least significant are
currently believed to be Population III stars (the earliest stars)
(possibly 70%), dwarf galaxies (very early small high-energy galaxies) (possibly 30%), and a contribution from quasars (a class of active galactic nuclei).
However, by this time, matter had become far more spread out due
to the ongoing expansion of the universe. Although the neutral hydrogen
atoms were again ionized, the plasma was much more thin and diffuse, and
photons were much less likely to be scattered. Despite being reionized,
the universe remained largely transparent during reionization. As the
universe continued to cool and expand, reionization gradually ended.
Galaxies, clusters and superclusters
Computer simulated view of the large-scale structure of a part of the universe about 50 million light-years across
Matter continues to draw together under the influence of gravity, to form galaxies. The stars from this time period, known as Population II stars, are formed early on in this process, with more recent Population I stars formed later. Gravitational attraction also gradually pulls galaxies towards each other to form groups, clusters and superclusters. Hubble Ultra Deep Field
observations has identified a number of small galaxies merging to form
larger ones, at 800 million years of cosmic time (13 billion years ago). (This age estimate is now believed to be slightly overstated).
Using the 10-metre Keck II telescope on Mauna Kea, Richard Ellis
of the California Institute of Technology at Pasadena and his team
found six star forming galaxies about 13.2 billion light-years away and
therefore created when the universe was only 500 million years old. Only about 10 of these extremely early objects are currently known.
More recent observations have shown these ages to be shorter than
previously indicated. The most distant galaxy observed as of October
2016, GN-z11, has been reported to be 32 billion light-years away, a vast distance made possible through spacetime expansion (z = 11.1; comoving distance of 32 billion light-years; lookback time of 13.4 billion years).
The universe as it appears today
The
universe has appeared much the same as it does now, for many billions
of years. It will continue to look similar for many more billions of
years into the future.
Based upon the emerging science of nucleocosmochronology, the Galactic thin disk of the Milky Way is estimated to have been formed 8.8 ± 1.7 billion years ago.
Dark energy dominated era
From about 9.8 billion years after the Big bang
From about 9.8 billion years of cosmic time,
the universe's large-scale behaviour is believed to have gradually
changed for the third time in its history. Its behaviour had originally
been dominated by radiation (relativistic constituents such as photons
and neutrinos) for the first 47,000 years, and since about 370,000 years
of cosmic time, its behaviour had been dominated by matter. During its
matter-dominated era, the expansion of the universe had begun to slow
down, as gravity reined in the initial outward expansion. But from about
9.8 billion years of cosmic time, observations show that the expansion
of the universe slowly stops decelerating, and gradually begins to
accelerate again, instead.
While the precise cause is not known, the observation is accepted
as correct by the cosmologist community. By far the most accepted
understanding is that this is due to an unknown form of energy which has
been given the name "dark energy".
"Dark" in this context means that it is not directly observed, but can
currently only be studied by examining the effect it has on the
universe. Research is ongoing to understand this dark energy. Dark
energy is now believed to be the single largest component of the
universe, as it constitutes about 68.3% of the entire mass-energy of the physical universe.
Dark energy is believed to act like a cosmological constant—a
scalar field that exists throughout space. Unlike gravity, the effects
of such a field do not diminish (or only diminish slowly) as the
universe grows. While matter and gravity have a greater effect
initially, their effect quickly diminishes as the universe continues to
expand. Objects in the universe, which are initially seen to be moving
apart as the universe expands, continue to move apart, but their outward
motion gradually slows down. This slowing effect becomes smaller as the
universe becomes more spread out. Eventually, the outward and repulsive
effect of dark energy begins to dominate over the inward pull of
gravity. Instead of slowing down and perhaps beginning to move inward
under the influence of gravity, from about 9.8 billion years of cosmic
time, the expansion of space starts to slowly accelerate outward at a gradually increasing rate.
The far future and ultimate fate
The predicted main-sequence lifetime of a red dwarf star plotted against its mass relative to the Sun
The universe has existed for around 13.8 billion years, and we
believe that we understand it well enough to predict its large-scale
development for many billions of years into the future—perhaps as much
as 100 billion years of cosmic time (about 86 billion years from now).
Beyond that, we need to better understand the universe to make any
accurate predictions. Therefore, the universe could follow a variety of
different paths beyond this time.
There are several competing scenarios for the possible long-term
evolution of the universe. Which of them will happen, if any, depends on
the precise values of physical constants such as the cosmological constant, the possibility of proton decay, the energy of the vacuum (meaning, the energy of "empty" space itself), and the natural laws beyond the Standard Model.
If the expansion of the universe continues and it stays in its
present form, eventually all but the nearest galaxies will be carried
away from us by the expansion of space at such a velocity that our
observable universe will be limited to our own gravitationally bound local galactic cluster.
In the very long term (after many trillions—thousands of billions—of
years, cosmic time), the Stelliferous Era will end, as stars cease to be
born and even the longest-lived stars gradually die. Beyond this, all objects in the universe will cool and (with the possible exception of protons)
gradually decompose back to their constituent particles and then into
subatomic particles and very low level photons and other fundamental particles, by a variety of possible processes.
Ultimately, in the extreme future, the following scenarios have been proposed for the ultimate fate of the universe:
As expansion continues, the universe becomes larger, colder, and
more dilute; in time, all structures eventually decompose to subatomic
particles and photons.
In the case of indefinitely continuing metric expansion of space,
the energy density in the universe will decrease until, after an
estimated time of 101000 years, it reaches thermodynamic equilibrium
and no more structure will be possible. This will happen only after an
extremely long time because first, all matter will collapse into black holes, which will then evaporate extremely slowly via Hawking radiation. The universe in this scenario will cease to be able to support life much earlier than this, after some 1014 years or so, when star formation ceases., §IID. In some Grand Unified Theories, proton decay after at least 1034
years will convert the remaining interstellar gas and stellar remnants
into leptons (such as positrons and electrons) and photons. Some
positrons and electrons will then recombine into photons., §IV, §VF. In this case, the universe has reached a high-entropy state consisting of a bath of particles and low-energy radiation. It is not known however whether it eventually achieves thermodynamic equilibrium., §VIB, VID. The hypothesis of a universal heat death stems from the 1850s ideas of William Thomson
(Lord Kelvin), who extrapolated the classical theory of heat and
irreversibility (as embodied in the first two laws of thermodynamics) to
the universe as a whole.
Expansion of space accelerates and at some point becomes so extreme that even subatomic particles and the fabric of spacetime are pulled apart and unable to exist.
For any value of the dark energy content of the universe where the
negative pressure ratio is less than -1, the expansion rate of the
universe will continue to increase without limit. Gravitationally bound
systems, such as clusters of galaxies, galaxies, and ultimately the
Solar System will be torn apart. Eventually the expansion will be so
rapid as to overcome the electromagnetic forces holding molecules and
atoms together. Even atomic nuclei will be torn apart. Finally, forces
and interactions even on the Planck scale—the
smallest size for which the notion of "space" currently has a
meaning—will no longer be able to occur as the fabric of spacetime
itself is pulled apart and the universe as we know it will end in an
unusual kind of singularity.
Expansion eventually slows and halts, then reverses as all matter
accelerates towards its common centre. Not now considered likely.
In the opposite of the "Big Rip" scenario, the metric expansion of
space would at some point be reversed and the universe would contract
towards a hot, dense state. This is a required element of oscillatory universe scenarios, such as the cyclic model,
although a Big Crunch does not necessarily imply an oscillatory
universe. Current observations suggest that this model of the universe
is unlikely to be correct, and the expansion will continue or even
accelerate.
Collapse of the quantum fields that underpin all forces, particles and structures, to a different form.
Cosmology traditionally has assumed a stable or at least metastable universe, but the possibility of a false vacuum in quantum field theory implies that the universe at any point in spacetime might spontaneously collapse into a lower energy state (see Bubble nucleation), a more stable or "true vacuum", which would then expand outward from that point with the speed of light.The effect would be that the quantum fields that underpin all forces,
particles and structures, would undergo a transition to a more stable
form. New forces and particles would replace the present ones we know
of, with the side effect that all current particles, forces and
structures would be destroyed and subsequently (if able) reform into
different particles, forces and structures.
In this kind of extreme timescale, extremely rare quantum phenomena
may also occur that are extremely unlikely to be seen on a timescale
smaller than trillions of years. These may also lead to unpredictable
changes to the state of the universe which would not be likely to be
significant on any smaller timescale. For example, on a timescale of
millions of trillions of years, black holes might appear to evaporate
almost instantly, uncommon quantum tunnelling
phenomena would appear to be common, and quantum (or other) phenomena
so unlikely that they might occur just once in a trillion years may
occur many times.
