Philosophy of Baruch Spinoza

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Spinoza lived from 1632 to 1677.

Baruch Spinoza's philosophy encompasses nearly every area of philosophical discourse, including metaphysics, epistemology, political philosophy, ethics, philosophy of mind, and philosophy of science. It earned Spinoza an enduring reputation as one of the most important and original thinkers of the seventeenth century.

Samuel Shirley, who translated Spinoza's complete works into English, summed up the significance of Spinoza's philosophy as follows:

To my mind, although Spinoza lived and thought long before Darwin, Freud, Einstein, and the startling implications of quantum theory, he had a vision of truth beyond what is normally granted to human beings.

Spinoza's philosophy is largely contained in two books: the Theologico-Political Treatise, and the Ethics. The former was published during his lifetime, but the latter, which contains the entirety of his philosophical system in its most rigorous form, was not published until after his death in 1677. The rest of the writings we have from Spinoza are either earlier, or incomplete, works expressing thoughts that were crystallized in the two aforementioned books (e.g., the Short Treatise and the Treatise on the Emendation of the Intellect), or else they are not directly concerned with Spinoza's own philosophy (e.g., The Principles of Cartesian Philosophy and The Hebrew Grammar). He also left behind many letters that help to illuminate his ideas and provide some insight into what may have been motivating his views.

Philosophy of religion

Spinoza's philosophy of religion is largely contained in the Theologico-Political Treatise. In that work he argues for the view that we should interpret scripture solely on its own terms by carefully studying it, not with any concepts or doctrines that cannot themselves be derived from the text. If we do this, he thought, it would turn out that many things we believe or are told by religious authorities about God and the universe could be shown to be false (e.g., miracles). Spinoza's view is exemplified in the following sentence from the Preface to the Theological Political Treatise:

[It] is further evident from the fact that most of them assume as a basic principle for the understanding of Scripture and for extracting its true meaning that it is throughout truthful and divine--a conclusion which ought to be the end result of study and strict examination; and they lay down at the outset as a principle of interpretation that which would be far more properly derived from Scripture itself, which stands in no need of human fabrications.

Ontological argument

In Spinoza’s Ethics, he wrote a section titled “Treating of God and What Pertains to Him,” in which he discusses God’s existence and what God is. He starts off by saying: “whether there is a God, this, we say, can be proved”. His proof for God follows a similar structure as Descartes’ ontological argument. Descartes attempts to prove God’s existence by arguing that there “must be some one thing that is supremely good, through which all good things have their goodness”. Spinoza’s argument differs in that he does not move straight from the conceivability of the greatest being to the existence of God, but rather uses a deductive argument from the idea of God. Spinoza says that man’s ideas do not come from himself, but from some sort of external cause. Thus the things whose characteristics a man knows must have come from some prior source. So, if man has the idea of God, then God must exist before this thought, because man cannot create an idea of his own imagination.

Substance of God

After stating his proof for God’s existence, Spinoza addresses who “God” is. Spinoza believed that God is “the sum of the natural and physical laws of the universe and certainly not an individual entity or creator”. Spinoza attempts to prove that God is just the substance of the universe by first stating that substances do not share attributes or essences, and then demonstrating that God is a “substance” with an infinite number of attributes, thus the attributes possessed by any other substances must also be possessed by God. Therefore, God is just the sum of all the substances of the universe. God is the only substance in the universe, and everything is a part of God. “Whatever is, is in God, and nothing can be or be conceived without God”. This concept of God is very similar to the Advaita Vedanta of Hinduism. This view was described by Charles Hartshorne as Classical Pantheism. Spinoza has also been described as an "Epicurean materialist", specifically in reference to his opposition to Cartesian mind-body dualism. This view was held by Epicureans before him, as they believed that atoms with their probabilistic paths were the only substance that existed fundamentally. Spinoza, however, deviated significantly from Epicureans by adhering to strict determinism, much like the Stoics before him, in contrast to the Epicurean belief in the probabilistic path of atoms, which is more in line with contemporary thought on quantum mechanics.

Political philosophy

Tractatus Theologico-Politicus was published anonymously.

Spinoza's political philosophy is deeply influenced by both the turbulent time period in which he lived, and by the fact that he happened to live in a comparatively liberal place in Europe, which allowed him freedoms he wished to preserve and defend, as he says in the Preface to the Theological Political Treatise:

Now since we have the rare good fortune to live in a commonwealth where freedom of judgment is fully granted to the individual citizen and he may worship God as he pleases, and where nothing is esteemed dearer and more precious than freedom, I think I am undertaking no ungrateful or unprofitable task in demonstrating that not only can this freedom be granted without endangering piety and the peace of the commonwealth, but also the peace of the commonwealth and piety depend on this freedom.

Spinoza's political philosophy is scattered in three books, the Theologico-political Treatise, the Ethics and the Political Treatise. A first look at its main principles could bring the uninformed reader to believe that it is the same as Hobbes's. Yet both theories differ in their conclusions. Spinoza's political philosophy is also a philosophy of the conatus, the individual tendency to exist, which cannot be brought to extinction even in the most powerful Leviathan, even in the worst of authoritarian regimes. Every individual, in Spinoza's opinion, has a natural right. This right includes everything that he desires and he is able to obtain. As a result, my own natural right is the equivalent of my individual strength or power. Hence, in Spinoza's political philosophy subjective rights (e.g. human rights) do not exist by nature, they are an institution of society, they only exist in the civil state. Moreover, according to Spinoza the notions of right and wrong have no meaning before society, since in the natural state there are no common norms, only individual desires (desires which can bring some people to dominate other weaker people).

How can civil society exist if people are only dominated by their own impulse to live? Through many ways. First, through the action of affections, the same ones that are described in the Ethics. Those affections, my feelings, will bring me to cluster, to gather with people similar to myself: this similarity reinforces the feeling or representation of my own existence. In a similar fashion, human needs will also play a role: society, through distribution and specialisation of each task, can provide more goods than I can generate myself and with less effort. This is why the sciences and the arts can only develop in societies, where there is time to attend to things other than one's own survival. This fear, the need to constantly look after danger and threats and to live in constant tension, is the third cause or root phenomenon of society. Society brings me protection and security. We see hence that Spinoza, while incorporating in his work Hobbesian arguments (the argument of fear), develops a distinct analysis that will bring him to different conclusions: the need of a free society.

Here individuals never entirely renounce their individual right of nature. If in the Theologico-Political Treatise Spinoza refers to the notion of a pact that would be at the root of civil society, this notion disappears in the Political Treatise. People are not brought to form a society by their free will, but rather by their affections, or domination (a great number of individuals gathered through the authority of an unusually strong or charismatic man could also be a way to explain the birth of civil society). They are not passive subjects under the power of an absolute sovereign, but rather citizens that bring their own strength to the State. The power of the state exists in Spinoza's opinion only through the gathering of individual powers, powers which the society incorporates and can even develop if its political institutions are well designed. "Well designed" means that they must induce political leaders to act according to the rules, by their own will. In Spinoza's political philosophy, state is not opposed to the society but it is the apparatus that gives a certain form or existence to the society, to a gathering of human beings. It is not transcendent to it, as it is in Hobbes's philosophy.

These affirmations have some political implications. Here, individual rights exist only because we, as individuals, benefit from the power of our entire group. Members' rights are guaranteed by the strength of their political group (=State or imperium). Individual or subjective rights do not exist outside of a state, out of an organised society. But that doesn't mean that the government should have absolute power over us. To understand that well, we have to remember that according to Spinoza the government or society (there is no difference between them) are nothing else and do not exist without the individual conatuses of the individuals that are gathered in social entities. Individuals hold a part of their natural right in the civil state. They cannot restrain themselves from judging about the state of things as they wish, and any action that would go against this tendency can induce social unrest. It follows that the state must restrain itself from any action that could jeopardise its own integrity, as condemning determinate opinions can. In a broader perspective, a state that relies on fearsome and inhuman ways to preserve its power cannot survive for long, since those ways impede the development of its own strength, and reinforce the tendency of the multitudo, the masses, to unrest or to disobedience: obedience is necessary to preserve social order and peace.

Thus, we can distinguish Hobbes and Spinoza through the way they see the normal operation of the state. For Hobbes, the object of the state is to preserve peace through security and fear if needed. According to Spinoza, that kind of peace would not be a true peace but only the absence of unrest. True peace implies a state of things where individuals can accomplish and realise their potentialities, where there is a minimum peace of mind. This is why Spinoza favors states that are organised so that citizens can participate in the elaboration of laws, as a way to improve their quality, and in the operation of the state.

The vocabulary of Spinoza shows a modification of the way philosophers see politics compared to the Antiquity. In Plato's and Aristotle's works good politics imply good government (defined as the way decisions are taken in a certain political community), in the sense that the different types of government can be ranked according to their virtues (aristocracy is better than democracy, which is better than oligarchy and tyranny according to Plato, and so on). Spinoza goes beyond this way of seeing things. There is not a better government in this sense: the better government is the government that the people of a certain country have been accustomed to, and there is no good in changing it: such a change alters the balance of power already in place and can bring unrest, conflict between opposed or entrenched interests. According to him, one should rather aim to design better institutions: for type of regime or government (Monarchy, Aristocracy, Democracy) Spinoza implements the outlines of what should be the good institutions for this regime. For example, in Monarchy there should be an official Council of the king, whose members are chosen formally, and whose opinions form a set of possible decisions for the king. This is a way of avoiding the issue of the king's secret counselors or ministers, who have a lot of influence on the king and often are the true decision takers. This system makes public and transparent through a formal process a matter of fact, the existence of a circle of advisers around the king.

For further reference, see Spinoza's Political Philosophy.

Philosophy of mind/psychology

The human mind

Spinoza argues for a distinct conception of the human mind in Part Two of The Ethics. He says the following:

The first thing that constitutes the actual being of a human Mind is nothing but the idea of a singular thing which actually exists.(E2P11)

He then argues that it follows that "the human Mind is a part of the infinite intellect of God."(E2P11c)  Further, Spinoza says: "Whatever happens in the object of the idea constituting the human Mind must be perceived by the human Mind"(E2P12) From this we get a clear rejection of Descartes' mind/body dualism: "The object of the idea constituting the human Mind is the Body, or a certain mode of Extension which actually exists, and nothing else."(E2P13)

The emotions

One thing which seems, on the surface, to distinguish Spinoza's view of the emotions from both Descartes' and Hume's pictures of them is that he takes the emotions to be cognitive in some important respect. Jonathan Bennett claims that "Spinoza mainly saw emotions as caused by cognitions. [However] he did not say this clearly enough and sometimes lost sight of it entirely." Spinoza provides several demonstrations which purport to show truths about how human emotions work. The picture presented is, according to Bennett, "unflattering, coloured as it is by universal egoism" Spinoza's treatment of the emotions in Part Three of The Ethics, "On the Origin and Nature of the Affects", utilizes a broad set of terminology, clearly intended to cover the whole of human experience. He tells us in the Preface:

The Affects, therefore, of hate, anger, envy, etc., considered in themselves, follow from the same necessity and force of nature as any other singular things. And therefore they acknowledge certain causes, through which they are understood, and have certain properties, as worthy of our knowledge as the properties of any other thing, by the mere contemplation of which we are pleased.

Human freedom

Whether there is any meaningful kind of freedom which humans may genuinely have is, in Spinoza's picture, at least contentious. He certainly claims that there is a kind of freedom, namely, that which is arrived at through adequate knowledge of God, or, what is the same: the universe. But in the last two propositions of Part Two of The Ethics, P48 and P49, he explicitly rejects the traditional notion of free will. In E2P48, he claims:

In the Mind there is no absolute, or free, will, but the Mind is determined to will this or that by a cause which is also determined by another, and this again by another, and so to infinity.

So from this we get a strong sense of Spinoza's metaphysical naturalism, that is, that the natural and human orders are contiguous. With that being the case, human freedom of a kind which would extricate us from the order of physical causes is impossible. However, Spinoza argues, we still ought to strive to understand the world around us, and in doing so, gain a greater degree of power, which will allow us to be more active than passive, and there is a sense in which this is a kind of freedom. For more, see: Stanford.edu

Metaphysics

Spinoza's metaphysics consists of one thing, substance, and its modifications (modes). Early in The Ethics Spinoza argues that there is only one substance, which is absolutely infinite, self-caused, and eternal. He calls this substance "God", or "Nature". In fact, he takes these two terms to be synonymous (in the Latin the phrase he uses is "Deus sive Natura"). For Spinoza the whole of the natural universe is made of one substance, God, or, what's the same, Nature, and its modifications (modes).

It cannot be overemphasized how the rest of Spinoza's philosophy—his philosophy of mind, his epistemology, his psychology, his moral philosophy, his political philosophy, and his philosophy of religion—flows more or less directly from the metaphysical underpinnings in Part I of the Ethics.

Substance

Spinoza defines "substance" as follows:

By substance I understand what is in itself and is conceived through itself, i.e., that whose concept does not require the concept of another thing, from which it must be formed.(E1D3)

This means, essentially, that substance is just whatever can be thought of without relating it to any other idea or thing. For example, if one thinks of a particular object, one thinks of it as a kind of thing, e.g., x is a cat. Substance, on the other hand, is to be conceived of by itself, without understanding it as a particular kind of thing (because it isn't a particular thing at all).

Attributes

Spinoza defines "attribute" as follows:

By attribute I understand what the intellect perceives of a substance, as constituting its essence.(E1D4)

From this it can be seen that attributes are related to substance in some way. It is not clear, however, even from Spinoza's direct definition, whether, a) attributes are really the way(s) substance is, or b) attributes are simply ways to understand substance, but not necessarily the ways it really is. Spinoza thinks that there are an infinite number of attributes, but there are two attributes for which Spinoza thinks we can have knowledge. Namely, thought and extension.

Thought

The attribute of thought is how substance can be understood to give rise to thoughts, or thinking things. When we understand a particular thing in the universe through the attribute of thought, we are understanding the mode as an idea of something (either another idea, or an object).

Extension

The attribute of extension is how substance can be understood to be physically extended in space. Particular things which have breadth and depth (that is, occupy space) are what is meant by extended. It follows from this that if substance and God are identical, on Spinoza's view, and contrary to the traditional conception, God has extension as one of His attributes.

Modes

Modes are particular modifications of substance, i.e., particular things in the world. Spinoza gives the following definition:

By mode I understand the affections of a substance, or that which is in another through which it is also conceived.(E1D5)

Substance monism

The argument for there only being one substance in the universe occurs in the first fourteen propositions of The Ethics. The following proposition expresses Spinoza's commitment to substance monism:

Except God, no substance can be or be conceived.(E1P14)

Spinoza takes this proposition to follow directly from everything he says prior to it. Spinoza's monism is contrasted with Descartes' dualism and Leibniz's pluralism. It allows Spinoza to avoid the problem of interaction between mind and body, which troubled Descartes in his Meditations on First Philosophy.

Causality and modality

The issue of causality and modality (possibility and necessity) in Spinoza's philosophy is contentious. Spinoza's philosophy is, in one sense, thoroughly deterministic (or necessitarian). This can be seen directly from Axiom 3 of The Ethics:

From a given determinate cause the effect follows necessarily; and conversely, if there is no determinate cause, it is impossible for an effect to follow.(E1A3)

Yet Spinoza seems to make room for a kind of freedom, especially in the fifth and final section of The Ethics, "On the Power of the Intellect, or on Human Freedom":

I pass, finally, to the remaining Part of the Ethics, which concerns the means or way, leading to Freedom. Here, then, I shall treat of the power of reason, showing what it can do against the affects, and what Freedom of Mind, or blessedness, is.(E5, Preface)

So Spinoza certainly has a use for the word 'freedom', but he equates "Freedom of Mind" with "blessedness", a notion which is not traditionally associated with freedom of the will at all.

The principle of sufficient reason (PSR)

Though the PSR is most commonly associated with Gottfried Leibniz, it is arguably found in its strongest form in Spinoza's philosophy. Within the context of Spinoza's philosophical system, the PSR can be understood to unify causation and explanation. What this means is that for Spinoza, questions regarding the reason why a given phenomenon is the way it is (or exists) are always answerable, and are always answerable in terms of the relevant cause(s). This constitutes a rejection of teleological, or final causation, except possibly in a more restricted sense for human beings. Given this, Spinoza's views regarding causality and modality begin to make much more sense.

Parallelism

Spinoza's philosophy contains as a key proposition the notion that mental and physical (thought and extension) phenomena occur in parallel, but without causal interaction between them. He expresses this proposition as follows:

The order and connection of ideas is the same as the order and connection of things.(E2P7)

His proof of this proposition is that:

The knowledge of an effect depends on, and involves, the knowledge of its cause.(E1A4)

The reason Spinoza thinks the parallelism follows from this axiom is that since the idea we have of each thing requires knowledge of its cause, this cause must be understood under the same attribute. Further, there is only one substance, so whenever we understand some chain of ideas of things, we understand that the way the ideas are causally related must be the same as the way the things themselves are related, since the ideas and the things are the same modes understood under different attributes.

Epistemology

Spinoza's epistemology is deeply rationalist. That is, unlike the empiricists who rejected knowledge of things as they are in themselves (in favour of knowledge merely of what appears to the senses), to think we can have a priori knowledge, knowledge of a world external from our sense perceptions, and, further, that this is tantamount to knowledge of God. The majority of Spinoza's epistemological claims come in Part Two of The Ethics.

Truth and falsity

Spinoza's notions of truth and falsity have to do with the relation between ideas and their objects. He thinks that:

Every idea that in us is absolute, or adequate and perfect, is true. (E2P34)

Falsity consists in the privation of knowledge which inadequate, or mutilated and confused, ideas involve.(E2P35)

Adequate and inadequate ideas

From this it is clear that the notions of adequate and inadequate ideas are important for understanding how Spinoza's view works. This may be explained in the following way. Spinoza argues that "All ideas, insofar as they are related to God, are true."(E2P32) Since by "God", he means the one substance which exists necessarily and absolutely infinitely, it follows that an idea as it is with no reference to knowledge a particular person has, is necessarily true, since it just is a particular instance of God. (E2P32)

On the other hand, Spinoza argues: "All ideas are in God; and, insofar as they are related to God, are true, and adequate. And so there are no inadequate or confused ideas except insofar as they are related to the singular Mind of someone."(E2P36d). That is, even though ideas considered objectively as elements of the universe are always adequate (meaning their relation to their object is total), when a particular individual has an idea of something, such an idea is necessarily incomplete, and therefore, inadequate. This is the source of falsehood.

Three kinds of knowledge

Spinoza discusses the three kinds of knowledge in E2P40s2.

The first kind of knowledge

Spinoza thinks there are two ways we can have the first kind of knowledge:

1. From random experience: "from singular things which have been represented to us through the senses in a way that is mutilated, confused, and without order for the intellect; for that reasons I have been accustomed to call such perceptions knowledge from random experience."
2. From imagination: "from signs, e.g., from the fact that, having heard or read certain words, we recollect things, and form certain ideas of them, which are like them, and through which we imagine the things."

He calls these two ways "knowledge of the first kind, opinion or imagination."

The second kind of knowledge

Spinoza argues that the second kind of knowledge arises:

from the fact that we have common notions and adequate ideas of the properties of things."

He goes on to explain what this means in the propositions which immediately follow.

The third kind of knowledge

This can be referred to as Intuition, but it means something rather technical for Spinoza. The third kind of knowledge is a particularly important part of Spinoza's philosophy because it is what he thinks allows us to have adequate knowledge, and therefore know things absolutely truly. As he says:

there is (as I shall show in what follows) another, third kind, which we shall call intuitive knowledge. And this kind of knowing proceeds from an adequate idea of certain attributes of God to the adequate knowledge of the essence of things.

Ethics

The opening page of Spinoza's magnum opus, Ethics

Spinoza's ethical views are deeply tied to his metaphysical system. This is evident from the following claim:

As far as good and evil are concerned, they also indicate nothing positive in things, considered in themselves, nor are they anything other than modes of thinking, or notions we form because we compare things to one another.(E4, Preface)

It is also apparent from this that he is a kind of subjectivist about moral values. That is, he does not take good and evil to be real properties/facts in the objects we attribute them to, but rather, they are simply thoughts we have about the comparative value of one thing to another for a particular person.

"Good" and "Evil"

Spinoza gives the following definitions of "Good", and "Evil":

By good I shall understand what we certainly know to be useful to us.(E4D1)

By evil, however, I shall understand what we certainly know prevents us from being masters of some good.(E4D2)

From this it is clear that Spinoza's view of moral value is in some sense instrumental. That is, the goodness or badness of a particular object or action is measured not by some essential property. The emphasis on "essential knowledge" is important, given Spinoza's view of what epistemic certainty amounts to, i.e., adequate knowledge of God (a notion which is briefly elaborated on in this article).

Blessedness

Spinoza's notion of blessedness figures centrally in his ethical philosophy. Blessedness (or salvation or freedom), Spinoza thinks,

consists...in a constant and eternal love of God, or in God's love for men.(E5P36s)

And this means, as Jonathan Bennett explains, that "Spinoza wants "blessedness" to stand for the most elevated and desirable state one could possibly be in." Here, understanding what is meant by 'most elevated and desirable state' requires understanding Spinoza's notion of conatus (read: striving, but not necessarily with any teleological baggage) and that "perfection" refers not to (moral) value, but to completeness. Given that individuals are identified as mere modifications of the infinite Substance, it follows that no individual can ever be fully complete, i.e., perfect, or blessed. Absolute perfection, is, as noted above, reserved solely for Substance. Nevertheless, mere modes can attain a lesser form of blessedness, namely, that of pure understanding of oneself as one really is, i.e., as a definite modification of Substance in a certain set of relationships with everything else in the universe. That this is what Spinoza has in mind can be seen at the end of the Ethics, in E5P24 and E5P25, wherein Spinoza makes two final key moves, unifying the metaphysical, epistemological, and ethical propositions he has developed over the course of the work. In E5P24, he links the understanding of particular things to the understanding of God, or Substance; in E5P25, the conatus of the mind is linked to the third kind of knowledge (Intuition). From here, it is a short step to the connection of Blessedness with the amor dei intellectualis ("intellectual love of God").

Observable universe

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Observable_universe 

 

Observable universe

Visualization of the whole observable universe. The scale is such that the fine grains represent collections of large numbers of superclusters. The Virgo Supercluster—home of Milky Way—is marked at the center, but is too small to be seen.
Diameter	8.8×1026 m or 880 Ym (28.5 Gpc or 93 Gly)
Volume	3.566×1080 m3
Mass (ordinary matter)	1.5×1053 kg
Density (of total energy)	9.9×10−27 kg/m3 (equivalent to 6 protons per cubic meter of space)
Age	13.799±0.021 billion years
Average temperature	2.72548 K
Contents	Ordinary (baryonic) matter (4.9%) Dark matter (26.8%) Dark energy (68.3%)

The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion. There may be 2 trillion galaxies in the observable universe, although that number has recently been estimated at only several hundred billion based on new data from New Horizons. Assuming the universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction. That is, the observable universe has a spherical volume (a ball) centered on the observer. Every location in the universe has its own observable universe, which may or may not overlap with the one centered on Earth.

The word observable in this sense does not refer to the capability of modern technology to detect light or other information from an object, or whether there is anything to be detected. It refers to the physical limit created by the speed of light itself. No signal can travel faster than light, hence there is a maximum distance (called the particle horizon) beyond which nothing can be detected, as the signals could not have reached us yet. Sometimes astrophysicists distinguish between the visible universe, which includes only signals emitted since recombination (when hydrogen atoms were formed from protons and electrons and photons were emitted)—and the observable universe, which includes signals since the beginning of the cosmological expansion (the Big Bang in traditional physical cosmology, the end of the inflationary epoch in modern cosmology).

According to calculations, the current comoving distance—proper distance, which takes into account that the universe has expanded since the light was emitted—to particles from which the cosmic microwave background radiation (CMBR) was emitted, which represents the radius of the visible universe, is about 14.0 billion parsecs (about 45.7 billion light-years), while the comoving distance to the edge of the observable universe is about 14.3 billion parsecs (about 46.6 billion light-years), about 2% larger. The radius of the observable universe is therefore estimated to be about 46.5 billion light-years and its diameter about 28.5 gigaparsecs (93 billion light-years, or 8.8×10²⁶ metres or 2.89×10²⁷ feet), which equals 880 yottametres. The total mass of ordinary matter in the universe can be calculated using the critical density and the diameter of the observable universe to be about 1.5 × 10⁵³ kg. In November 2018, astronomers reported that the extragalactic background light (EBL) amounted to 4 × 10⁸⁴ photons.

As the universe's expansion is accelerating, all currently observable objects, outside our local supercluster, will eventually appear to freeze in time, while emitting progressively redder and fainter light. For instance, objects with the current redshift z from 5 to 10 will remain observable for no more than 4–6 billion years. In addition, light emitted by objects currently situated beyond a certain comoving distance (currently about 19 billion parsecs) will never reach Earth.

The universe versus the observable universe

The size of the whole universe is unknown, and it might be infinite in extent. Some parts of the universe are too far away for the light emitted since the Big Bang to have had enough time to reach Earth or space-based instruments, and therefore lie outside the observable universe. In the future, light from distant galaxies will have had more time to travel, so additional regions will become observable. However, owing to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light (special relativity prevents nearby objects in the same local region from moving faster than the speed of light with respect to each other, but there is no such constraint for distant objects when the space between them is expanding; see uses of the proper distance for a discussion) and furthermore the expansion rate appears to be accelerating owing to dark energy.

Assuming dark energy remains constant (an unchanging cosmological constant), so that the expansion rate of the universe continues to accelerate, there is a "future visibility limit" beyond which objects will never enter our observable universe at any time in the infinite future, because light emitted by objects outside that limit could never reach the Earth. (A subtlety is that, because the Hubble parameter is decreasing with time, there can be cases where a galaxy that is receding from the Earth just a bit faster than light does emit a signal that reaches the Earth eventually.) This future visibility limit is calculated at a comoving distance of 19 billion parsecs (62 billion light-years), assuming the universe will keep expanding forever, which implies the number of galaxies that we can ever theoretically observe in the infinite future (leaving aside the issue that some may be impossible to observe in practice due to redshift, as discussed in the following paragraph) is only larger than the number currently observable by a factor of 2.36.

Artist's logarithmic scale conception of the observable universe with the Solar System at the center, inner and outer planets, Kuiper belt, Oort cloud, Alpha Centauri, Perseus Arm, Milky Way galaxy, Andromeda Galaxy, nearby galaxies, Cosmic web, Cosmic microwave radiation and the Big Bang's invisible plasma on the edge. Celestial bodies appear enlarged to appreciate their shapes.

Though, in principle, more galaxies will become observable in the future, in practice, an increasing number of galaxies will become extremely redshifted due to ongoing expansion; so much so that they will seem to disappear from view and become invisible. An additional subtlety is that a galaxy at a given comoving distance is defined to lie within the "observable universe" if we can receive signals emitted by the galaxy at any age in its past history (say, a signal sent from the galaxy only 500 million years after the Big Bang), but because of the universe's expansion, there may be some later age at which a signal sent from the same galaxy can never reach the Earth at any point in the infinite future (so, for example, we might never see what the galaxy looked like 10 billion years after the Big Bang), even though it remains at the same comoving distance (comoving distance is defined to be constant with time—unlike proper distance, which is used to define recession velocity due to the expansion of space), which is less than the comoving radius of the observable universe. This fact can be used to define a type of cosmic event horizon whose distance from the Earth changes over time. For example, the current distance to this horizon is about 16 billion light-years, meaning that a signal from an event happening at present can eventually reach the Earth in the future if the event is less than 16 billion light-years away, but the signal will never reach the Earth if the event is more than 16 billion light-years away.

Both popular and professional research articles in cosmology often use the term "universe" to mean "observable universe". This can be justified on the grounds that we can never know anything by direct experimentation about any part of the universe that is causally disconnected from the Earth, although many credible theories require a total universe much larger than the observable universe.No evidence exists to suggest that the boundary of the observable universe constitutes a boundary on the universe as a whole, nor do any of the mainstream cosmological models propose that the universe has any physical boundary in the first place, though some models propose it could be finite but unbounded, like a higher-dimensional analogue of the 2D surface of a sphere that is finite in area but has no edge.

It is plausible that the galaxies within our observable universe represent only a minuscule fraction of the galaxies in the universe. According to the theory of cosmic inflation initially introduced by its founders, Alan Guth and D. Kazanas, if it is assumed that inflation began about 10⁻³⁷ seconds after the Big Bang, then with the plausible assumption that the size of the universe before the inflation occurred was approximately equal to the speed of light times its age, that would suggest that at present the entire universe's size is at least 3 × 10²³ (1.5 × 10³⁴ light-years) times the radius of the observable universe.

If the universe is finite but unbounded, it is also possible that the universe is smaller than the observable universe. In this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies, formed by light that has circumnavigated the universe. It is difficult to test this hypothesis experimentally because different images of a galaxy would show different eras in its history, and consequently might appear quite different. Bielewicz et al. claim to establish a lower bound of 27.9 gigaparsecs (91 billion light-years) on the diameter of the last scattering surface (since this is only a lower bound, since the whole universe is possibly much larger, even infinite). This value is based on matching-circle analysis of the WMAP 7 year data. This approach has been disputed.

Size

Hubble Ultra-Deep Field image of a region of the observable universe (equivalent sky area size shown in bottom left corner), near the constellation Fornax. Each spot is a galaxy, consisting of billions of stars. The light from the smallest, most redshifted galaxies originated nearly 14 billion years ago.

The comoving distance from Earth to the edge of the observable universe is about 14.26 gigaparsecs (46.5 billion light-years or 4.40×10²⁶ m) in any direction. The observable universe is thus a sphere with a diameter of about 28.5 gigaparsecs (93 billion light-years or 8.8×10²⁶ m). Assuming that space is roughly flat (in the sense of being a Euclidean space), this size corresponds to a comoving volume of about 1.22×10⁴ Gpc³ (4.22×10⁵ Gly³ or 3.57×10⁸⁰ m³).

The figures quoted above are distances now (in cosmological time), not distances at the time the light was emitted. For example, the cosmic microwave background radiation that we see right now was emitted at the time of photon decoupling, estimated to have occurred about 380,000 years after the Big Bang, which occurred around 13.8 billion years ago. This radiation was emitted by matter that has, in the intervening time, mostly condensed into galaxies, and those galaxies are now calculated to be about 46 billion light-years from us. To estimate the distance to that matter at the time the light was emitted, we may first note that according to the Friedmann–Lemaître–Robertson–Walker metric, which is used to model the expanding universe, if at the present time we receive light with a redshift of z, then the scale factor at the time the light was originally emitted is given by

${\displaystyle \!a(t)={\frac {1}{1+z}}}$.

WMAP nine-year results combined with other measurements give the redshift of photon decoupling as z = 1091.64±0.47, which implies that the scale factor at the time of photon decoupling would be 1⁄1092.64. So if the matter that originally emitted the oldest cosmic microwave background (CMBR) photons has a present distance of 46 billion light-years, then at the time of decoupling when the photons were originally emitted, the distance would have been only about 42 million light-years.

The light-travel distance to the edge of the observable universe is the age of the Universe divided by the speed of light, 13.8 billion light years. This is the distance that a photon emitted shortly after the Big Bang, such as one from the cosmic microwave background, has travelled to reach observers on Earth. Because spacetime is curved, corresponding to the expansion of space, this distance does not correspond to the true distance at any moment in time.

Large-scale structure

Galaxy clusters, like RXC J0142.9+4438, are the nodes of the cosmic web that permeates the entire Universe.

 

Playmedia

An image and a video of a cosmological simulation of the local universe, showing large-scale structure of clusters of galaxies and dark matter

Sky surveys and mappings of the various wavelength bands of electromagnetic radiation (in particular 21-cm emission) have yielded much information on the content and character of the universe's structure. The organization of structure appears to follow a hierarchical model with organization up to the scale of superclusters and filaments. Larger than this (at scales between 30 and 200 megaparsecs), there seems to be no continued structure, a phenomenon that has been referred to as the End of Greatness.

Walls, filaments, nodes, and voids

Map of the cosmic web generated from a slime mould-inspired algorithm

 

DTFE reconstruction of the inner parts of the 2dF Galaxy Redshift Survey

The organization of structure arguably begins at the stellar level, though most cosmologists rarely address astrophysics on that scale. Stars are organized into galaxies, which in turn form galaxy groups, galaxy clusters, superclusters, sheets, walls and filaments, which are separated by immense voids, creating a vast foam-like structure sometimes called the "cosmic web". Prior to 1989, it was commonly assumed that virialized galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction. However, since the early 1980s, more and more structures have been discovered. In 1983, Adrian Webster identified the Webster LQG, a large quasar group consisting of 5 quasars. The discovery was the first identification of a large-scale structure, and has expanded the information about the known grouping of matter in the universe.

In 1987, Robert Brent Tully identified the Pisces–Cetus Supercluster Complex, the galaxy filament in which the Milky Way resides. It is about 1 billion light-years across. That same year, an unusually large region with a much lower than average distribution of galaxies was discovered, the Giant Void, which measures 1.3 billion light-years across. Based on redshift survey data, in 1989 Margaret Geller and John Huchra discovered the "Great Wall", a sheet of galaxies more than 500 million light-years long and 200 million light-years wide, but only 15 million light-years thick. The existence of this structure escaped notice for so long because it requires locating the position of galaxies in three dimensions, which involves combining location information about the galaxies with distance information from redshifts. Two years later, astronomers Roger G. Clowes and Luis E. Campusano discovered the Clowes–Campusano LQG, a large quasar group measuring two billion light-years at its widest point which was the largest known structure in the universe at the time of its announcement. In April 2003, another large-scale structure was discovered, the Sloan Great Wall. In August 2007, a possible supervoid was detected in the constellation Eridanus. It coincides with the 'CMB cold spot', a cold region in the microwave sky that is highly improbable under the currently favored cosmological model. This supervoid could cause the cold spot, but to do so it would have to be improbably big, possibly a billion light-years across, almost as big as the Giant Void mentioned above.

Unsolved problem in physics:

The largest structures in the universe are larger than expected. Are these actual structures or random density fluctuations?

Computer simulated image of an area of space more than 50 million light-years across, presenting a possible large-scale distribution of light sources in the universe—precise relative contributions of galaxies and quasars are unclear.

Another large-scale structure is the SSA22 Protocluster, a collection of galaxies and enormous gas bubbles that measures about 200 million light-years across.

In 2011, a large quasar group was discovered, U1.11, measuring about 2.5 billion light-years across. On January 11, 2013, another large quasar group, the Huge-LQG, was discovered, which was measured to be four billion light-years across, the largest known structure in the universe at that time. In November 2013, astronomers discovered the Hercules–Corona Borealis Great Wall, an even bigger structure twice as large as the former. It was defined by the mapping of gamma-ray bursts.

End of Greatness

The End of Greatness is an observational scale discovered at roughly 100 Mpc (roughly 300 million light-years) where the lumpiness seen in the large-scale structure of the universe is homogenized and isotropized in accordance with the Cosmological Principle. At this scale, no pseudo-random fractalness is apparent. The superclusters and filaments seen in smaller surveys are randomized to the extent that the smooth distribution of the universe is visually apparent. It was not until the redshift surveys of the 1990s were completed that this scale could accurately be observed.

Observations

"Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. The image is derived from the 2MASS Extended Source Catalog (XSC)—more than 1.5 million galaxies, and the Point Source Catalog (PSC)—nearly 0.5 billion Milky Way stars. The galaxies are color-coded by 'redshift' obtained from the UGC, CfA, Tully NBGC, LCRS, 2dF, 6dFGS, and SDSS surveys (and from various observations compiled by the NASA Extragalactic Database), or photo-metrically deduced from the K band (2.2 μm). Blue are the nearest sources (z < 0.01); green are at moderate distances (0.01 < z < 0.04) and red are the most distant sources that 2MASS resolves (0.04 < z < 0.1). The map is projected with an equal area Aitoff in the Galactic system (Milky Way at center)."

Another indicator of large-scale structure is the 'Lyman-alpha forest'. This is a collection of absorption lines that appear in the spectra of light from quasars, which are interpreted as indicating the existence of huge thin sheets of intergalactic (mostly hydrogen) gas. These sheets appear to be associated with the formation of new galaxies.

Caution is required in describing structures on a cosmic scale because things are often different from how they appear. Gravitational lensing (bending of light by gravitation) can make an image appear to originate in a different direction from its real source. This is caused when foreground objects (such as galaxies) curve surrounding spacetime (as predicted by general relativity), and deflect passing light rays. Rather usefully, strong gravitational lensing can sometimes magnify distant galaxies, making them easier to detect. Weak lensing (gravitational shear) by the intervening universe in general also subtly changes the observed large-scale structure.

The large-scale structure of the universe also looks different if one only uses redshift to measure distances to galaxies. For example, galaxies behind a galaxy cluster are attracted to it, and so fall towards it, and so are slightly blueshifted (compared to how they would be if there were no cluster) On the near side, things are slightly redshifted. Thus, the environment of the cluster looks somewhat squashed if using redshifts to measure distance. An opposite effect works on the galaxies already within a cluster: the galaxies have some random motion around the cluster center, and when these random motions are converted to redshifts, the cluster appears elongated. This creates a "finger of God"—the illusion of a long chain of galaxies pointed at the Earth.

Cosmography of Earth's cosmic neighborhood

At the centre of the Hydra-Centaurus Supercluster, a gravitational anomaly called the Great Attractor affects the motion of galaxies over a region hundreds of millions of light-years across. These galaxies are all redshifted, in accordance with Hubble's law. This indicates that they are receding from us and from each other, but the variations in their redshift are sufficient to reveal the existence of a concentration of mass equivalent to tens of thousands of galaxies.

The Great Attractor, discovered in 1986, lies at a distance of between 150 million and 250 million light-years (250 million is the most recent estimate), in the direction of the Hydra and Centaurus constellations. In its vicinity there is a preponderance of large old galaxies, many of which are colliding with their neighbours, or radiating large amounts of radio waves.

In 1987, astronomer R. Brent Tully of the University of Hawaii's Institute of Astronomy identified what he called the Pisces–Cetus Supercluster Complex, a structure one billion light-years long and 150 million light-years across in which, he claimed, the Local Supercluster was embedded.

Mass of ordinary matter

The mass of the observable universe is often quoted as 10⁵⁰ tonnes or 10⁵³ kg. In this context, mass refers to ordinary matter and includes the interstellar medium (ISM) and the intergalactic medium (IGM). However, it excludes dark matter and dark energy. This quoted value for the mass of ordinary matter in the universe can be estimated based on critical density. The calculations are for the observable universe only as the volume of the whole is unknown and may be infinite.

Estimates based on critical density

Critical density is the energy density for which the universe is flat. If there is no dark energy, it is also the density for which the expansion of the universe is poised between continued expansion and collapse. From the Friedmann equations, the value for ${\displaystyle \rho _{c}}$ critical density, is:

${\displaystyle \rho _{c}={\frac {3H^{2}}{8\pi G}},}$

where G is the gravitational constant and H = H₀ is the present value of the Hubble constant. The value for H₀, due to the European Space Agency's Planck Telescope, is H₀ = 67.15 kilometres per second per megaparsec. This gives a critical density of 0.85×10⁻²⁶ kg/m³ (commonly quoted as about 5 hydrogen atoms per cubic metre). This density includes four significant types of energy/mass: ordinary matter (4.8%), neutrinos (0.1%), cold dark matter (26.8%), and dark energy (68.3%). Although neutrinos are Standard Model particles, they are listed separately because they are ultra-relativistic and hence behave like radiation rather than like matter. The density of ordinary matter, as measured by Planck, is 4.8% of the total critical density or 4.08×10⁻²⁸ kg/m³. To convert this density to mass we must multiply by volume, a value based on the radius of the "observable universe". Since the universe has been expanding for 13.8 billion years, the comoving distance (radius) is now about 46.6 billion light-years. Thus, volume (4/3πr³) equals 3.58×10⁸⁰ m³ and the mass of ordinary matter equals density (4.08×10⁻²⁸ kg/m³) times volume (3.58×10⁸⁰ m³) or 1.46×10⁵³ kg.

Matter content—number of atoms

Assuming the mass of ordinary matter is about 1.45×10⁵³ kg as discussed above, and assuming all atoms are hydrogen atoms (which are about 74% of all atoms in our galaxy by mass), the estimated total number of atoms in the observable universe is obtained by dividing the mass of ordinary matter by the mass of a hydrogen atom (1.45×10⁵³ kg divided by 1.67×10⁻²⁷ kg). The result is approximately 10⁸⁰ hydrogen atoms, also known as the Eddington number.

Most distant objects

The most distant astronomical object yet announced as of 2016 is a galaxy classified GN-z11. In 2009, a gamma ray burst, GRB 090423, was found to have a redshift of 8.2, which indicates that the collapsing star that caused it exploded when the universe was only 630 million years old. The burst happened approximately 13 billion years ago, so a distance of about 13 billion light-years was widely quoted in the media (or sometimes a more precise figure of 13.035 billion light-years), though this would be the "light travel distance" rather than the "proper distance" used in both Hubble's law and in defining the size of the observable universe (cosmologist Ned Wright argues against the common use of light travel distance in astronomical press releases on this page, and at the bottom of the page offers online calculators that can be used to calculate the current proper distance to a distant object in a flat universe based on either the redshift z or the light travel time). The proper distance for a redshift of 8.2 would be about 9.2 Gpc, or about 30 billion light-years. Another record-holder for most distant object is a galaxy observed through and located beyond Abell 2218, also with a light travel distance of approximately 13 billion light-years from Earth, with observations from the Hubble telescope indicating a redshift between 6.6 and 7.1, and observations from Keck telescopes indicating a redshift towards the upper end of this range, around 7. The galaxy's light now observable on Earth would have begun to emanate from its source about 750 million years after the Big Bang.

Horizons

The limit of observability in our universe is set by a set of cosmological horizons which limit—based on various physical constraints—the extent to which we can obtain information about various events in the universe. The most famous horizon is the particle horizon which sets a limit on the precise distance that can be seen due to the finite age of the universe. Additional horizons are associated with the possible future extent of observations (larger than the particle horizon owing to the expansion of space), an "optical horizon" at the surface of last scattering, and associated horizons with the surface of last scattering for neutrinos and gravitational waves.