Dark energy

From Wikipedia, the free encyclopedia

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

In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovae, which showed that the universe does not expand at a constant rate; rather, the expansion of the universe is accelerating. Understanding the evolution of the universe requires knowledge of its starting conditions and its composition. Prior to these observations, it was thought that all forms of matter and energy in the universe would only cause the expansion to slow down over time. Measurements of the cosmic microwave background suggest the universe began in a hot Big Bang, from which general relativity explains its evolution and the subsequent large-scale motion. Without introducing a new form of energy, there was no way to explain how an accelerating universe could be measured. Since the 1990s, dark energy has been the most accepted premise to account for the accelerated expansion. As of 2021, there are active areas of cosmology research aimed at understanding the fundamental nature of dark energy.

Assuming that the lambda-CDM model of cosmology is correct, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary (baryonic) matter contributes 26% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount. The density of dark energy is very low (~ 7 × 10⁻³⁰ g/cm³), much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the mass–energy of the universe because it is uniform across space.

Two proposed forms of dark energy are the cosmological constant, representing a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities having energy densities that can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to the zero-point radiation of space i.e. the vacuum energy. Scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.

Due to the toy model nature of concordance cosmology, some experts believe that a more accurate general relativistic treatment of the structures that exist on all scales in the real universe may do away with the need to invoke dark energy. Inhomogeneous cosmologies, which attempt to account for the back-reaction of structure formation on the metric, generally do not acknowledge any dark energy contribution to the energy density of the Universe.

History of discovery and previous speculation

Einstein's cosmological constant

The "cosmological constant" is a constant term that can be added to Einstein's field equation of general relativity. If considered as a "source term" in the field equation, it can be viewed as equivalent to the mass of empty space (which conceptually could be either positive or negative), or "vacuum energy".

The cosmological constant was first proposed by Einstein as a mechanism to obtain a solution of the gravitational field equation that would lead to a static universe, effectively using dark energy to balance gravity. Einstein gave the cosmological constant the symbol Λ (capital lambda). Einstein stated that the cosmological constant required that 'empty space takes the role of gravitating negative masses which are distributed all over the interstellar space'.

The mechanism was an example of fine-tuning, and it was later realized that Einstein's static universe would not be stable: local inhomogeneities would ultimately lead to either the runaway expansion or contraction of the universe. The equilibrium is unstable: if the universe expands slightly, then the expansion releases vacuum energy, which causes yet more expansion. Likewise, a universe which contracts slightly will continue contracting. These sorts of disturbances are inevitable, due to the uneven distribution of matter throughout the universe. Further, observations made by Edwin Hubble in 1929 showed that the universe appears to be expanding and not static at all. Einstein reportedly referred to his failure to predict the idea of a dynamic universe, in contrast to a static universe, as his greatest blunder.

Inflationary dark energy

Alan Guth and Alexei Starobinsky proposed in 1980 that a negative pressure field, similar in concept to dark energy, could drive cosmic inflation in the very early universe. Inflation postulates that some repulsive force, qualitatively similar to dark energy, resulted in an enormous and exponential expansion of the universe slightly after the Big Bang. Such expansion is an essential feature of most current models of the Big Bang. However, inflation must have occurred at a much higher energy density than the dark energy we observe today and is thought to have completely ended when the universe was just a fraction of a second old. It is unclear what relation, if any, exists between dark energy and inflation. Even after inflationary models became accepted, the cosmological constant was thought to be irrelevant to the current universe.

Nearly all inflation models predict that the total (matter+energy) density of the universe should be very close to the critical density. During the 1980s, most cosmological research focused on models with critical density in matter only, usually 95% cold dark matter (CDM) and 5% ordinary matter (baryons). These models were found to be successful at forming realistic galaxies and clusters, but some problems appeared in the late 1980s: in particular, the model required a value for the Hubble constant lower than preferred by observations, and the model under-predicted observations of large-scale galaxy clustering. These difficulties became stronger after the discovery of anisotropy in the cosmic microwave background by the COBE spacecraft in 1992, and several modified CDM models came under active study through the mid-1990s: these included the Lambda-CDM model and a mixed cold/hot dark matter model. The first direct evidence for dark energy came from supernova observations in 1998 of accelerated expansion in Riess et al. and in Perlmutter et al., and the Lambda-CDM model then became the leading model. Soon after, dark energy was supported by independent observations: in 2000, the BOOMERanG and Maxima cosmic microwave background (CMB) experiments observed the first acoustic peak in the CMB, showing that the total (matter+energy) density is close to 100% of critical density. Then in 2001, the 2dF Galaxy Redshift Survey gave strong evidence that the matter density is around 30% of critical. The large difference between these two supports a smooth component of dark energy making up the difference. Much more precise measurements from WMAP in 2003–2010 have continued to support the standard model and give more accurate measurements of the key parameters.

The term "dark energy", echoing Fritz Zwicky's "dark matter" from the 1930s, was coined by Michael Turner in 1998.

Change in expansion over time

Diagram representing the accelerated expansion of the universe due to dark energy.

High-precision measurements of the expansion of the universe are required to understand how the expansion rate changes over time and space. In general relativity, the evolution of the expansion rate is estimated from the curvature of the universe and the cosmological equation of state (the relationship between temperature, pressure, and combined matter, energy, and vacuum energy density for any region of space). Measuring the equation of state for dark energy is one of the biggest efforts in observational cosmology today. Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the "standard model of cosmology" because of its precise agreement with observations.

As of 2013, the Lambda-CDM model is consistent with a series of increasingly rigorous cosmological observations, including the Planck spacecraft and the Supernova Legacy Survey. First results from the SNLS reveal that the average behavior (i.e., equation of state) of dark energy behaves like Einstein's cosmological constant to a precision of 10%. Recent results from the Hubble Space Telescope Higher-Z Team indicate that dark energy has been present for at least 9 billion years and during the period preceding cosmic acceleration.

Nature

The nature of dark energy is more hypothetical than that of dark matter, and many things about it remain in the realm of speculation. Dark energy is thought to be very homogeneous and not very dense, and is not known to interact through any of the fundamental forces other than gravity. Since it is quite rarefied and un-massive—roughly 10⁻²⁷ kg/m³—it is unlikely to be detectable in laboratory experiments. The reason dark energy can have such a profound effect on the universe, making up 68% of universal density in spite of being so dilute, is that it uniformly fills otherwise empty space.

Independently of its actual nature, dark energy would need to have a strong negative pressure to explain the observed acceleration of the expansion of the universe. According to general relativity, the pressure within a substance contributes to its gravitational attraction for other objects just as its mass density does. This happens because the physical quantity that causes matter to generate gravitational effects is the stress–energy tensor, which contains both the energy (or matter) density of a substance and its pressure. In the Friedmann–Lemaître–Robertson–Walker metric, it can be shown that a strong constant negative pressure (i.e., tension) in all the universe causes an acceleration in the expansion if the universe is already expanding, or a deceleration in contraction if the universe is already contracting. This accelerating expansion effect is sometimes labeled "gravitational repulsion".

Technical definition

See also: Friedmann equations

In standard cosmology, there are three components of the universe: matter, radiation, and dark energy. Matter is anything whose energy density scales with the inverse cube of the scale factor, i.e., ρ ∝ a⁻³, while radiation is anything which scales to the inverse fourth power of the scale factor (ρ ∝ a⁻⁴). This can be understood intuitively: for an ordinary particle in a cube-shaped box, doubling the length of an edge of the box decreases the density (and hence energy density) by a factor of eight (2³). For radiation, the decrease in energy density is greater, because an increase in spatial distance also causes a redshift.

The final component is dark energy; "dark energy" is anything that is, in its effect, an intrinsic property of space: That has a constant energy density, regardless of the dimensions of the volume under consideration (ρ ∝ a⁰). Thus, unlike ordinary matter, it is not diluted by the expansion of space.

Evidence of existence

The evidence for dark energy is indirect but comes from three independent sources:

• Distance measurements and their relation to redshift, which suggest the universe has expanded more in the latter half of its life.
• The theoretical need for a type of additional energy that is not matter or dark matter to form the observationally flat universe (absence of any detectable global curvature).
• Measures of large-scale wave patterns of mass density in the universe.

Supernovae

A Type Ia supernova (bright spot on the bottom-left) near a galaxy

In 1998, the High-Z Supernova Search Team published observations of Type Ia ("one-A") supernovae. In 1999, the Supernova Cosmology Project followed by suggesting that the expansion of the universe is accelerating. The 2011 Nobel Prize in Physics was awarded to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess for their leadership in the discovery.

Since then, these observations have been corroborated by several independent sources. Measurements of the cosmic microwave background, gravitational lensing, and the large-scale structure of the cosmos, as well as improved measurements of supernovae, have been consistent with the Lambda-CDM model. Some people argue that the only indications for the existence of dark energy are observations of distance measurements and their associated redshifts. Cosmic microwave background anisotropies and baryon acoustic oscillations serve only to demonstrate that distances to a given redshift are larger than would be expected from a "dusty" Friedmann–Lemaître universe and the local measured Hubble constant.

Supernovae are useful for cosmology because they are excellent standard candles across cosmological distances. They allow researchers to measure the expansion history of the universe by looking at the relationship between the distance to an object and its redshift, which gives how fast it is receding from us. The relationship is roughly linear, according to Hubble's law. It is relatively easy to measure redshift, but finding the distance to an object is more difficult. Usually, astronomers use standard candles: objects for which the intrinsic brightness, or absolute magnitude, is known. This allows the object's distance to be measured from its actual observed brightness, or apparent magnitude. Type Ia supernovae are the best-known standard candles across cosmological distances because of their extreme and consistent luminosity.

Recent observations of supernovae are consistent with a universe made up 71.3% of dark energy and 27.4% of a combination of dark matter and baryonic matter.

Cosmic microwave background

Estimated division of total energy in the universe into matter, dark matter and dark energy based on five years of WMAP data.

The existence of dark energy, in whatever form, is needed to reconcile the measured geometry of space with the total amount of matter in the universe. Measurements of cosmic microwave background (CMB) anisotropies indicate that the universe is close to flat. For the shape of the universe to be flat, the mass–energy density of the universe must be equal to the critical density. The total amount of matter in the universe (including baryons and dark matter), as measured from the CMB spectrum, accounts for only about 30% of the critical density. This implies the existence of an additional form of energy to account for the remaining 70%. The Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft seven-year analysis estimated a universe made up of 72.8% dark energy, 22.7% dark matter, and 4.5% ordinary matter. Work done in 2013 based on the Planck spacecraft observations of the CMB gave a more accurate estimate of 68.3% dark energy, 26.8% dark matter, and 4.9% ordinary matter.

Large-scale structure

The theory of large-scale structure, which governs the formation of structures in the universe (stars, quasars, galaxies and galaxy groups and clusters), also suggests that the density of matter in the universe is only 30% of the critical density.

A 2011 survey, the WiggleZ galaxy survey of more than 200,000 galaxies, provided further evidence towards the existence of dark energy, although the exact physics behind it remains unknown. The WiggleZ survey from the Australian Astronomical Observatory scanned the galaxies to determine their redshift. Then, by exploiting the fact that baryon acoustic oscillations have left voids regularly of ≈150 Mpc diameter, surrounded by the galaxies, the voids were used as standard rulers to estimate distances to galaxies as far as 2,000 Mpc (redshift 0.6), allowing for accurate estimate of the speeds of galaxies from their redshift and distance. The data confirmed cosmic acceleration up to half of the age of the universe (7 billion years) and constrain its inhomogeneity to 1 part in 10. This provides a confirmation to cosmic acceleration independent of supernovae.

Late-time integrated Sachs–Wolfe effect

Accelerated cosmic expansion causes gravitational potential wells and hills to flatten as photons pass through them, producing cold spots and hot spots on the CMB aligned with vast supervoids and superclusters. This so-called late-time Integrated Sachs–Wolfe effect (ISW) is a direct signal of dark energy in a flat universe. It was reported at high significance in 2008 by Ho et al. and Giannantonio et al.

Observational Hubble constant data

A new approach to test evidence of dark energy through observational Hubble constant data (OHD) has gained significant attention in recent years.

The Hubble constant, H(z), is measured as a function of cosmological redshift. OHD directly tracks the expansion history of the universe by taking passively evolving early-type galaxies as “cosmic chronometers”. From this point, this approach provides standard clocks in the universe. The core of this idea is the measurement of the differential age evolution as a function of redshift of these cosmic chronometers. Thus, it provides a direct estimate of the Hubble parameter

${\displaystyle H(z)=-{\frac {1}{1+z}}{\frac {dz}{dt}}\approx -{\frac {1}{1+z}}{\frac {\Delta z}{\Delta t}}.}$

The reliance on a differential quantity, Δz/Δt, brings more information and is appealing for computation: It can minimize many common issues and systematic effects. Analyses of supernovae and baryon acoustic oscillations (BAO) are based on integrals of the Hubble parameter, whereas Δz/Δt measures it directly. For these reasons, this method has been widely used to examine the accelerated cosmic expansion and study properties of dark energy.

Direct observation

An attempt to directly observe dark energy in a laboratory failed to detect a new force. Recently, it has been speculated that the currently unexplained excess observed in the XENON1T detector in Italy may have been caused by a chameleon model of dark energy.

Theories of dark energy

Dark energy's status as a hypothetical force with unknown properties makes it a very active target of research. The problem is attacked from a great variety of angles, such as modifying the prevailing theory of gravity (general relativity), attempting to pin down the properties of dark energy, and finding alternative ways to explain the observational data.

The equation of state of Dark Energy for 4 common models by Redshift.
A: CPL Model,
B: Jassal Model,
C: Barboza & Alcaniz Model,
D: Wetterich Model

Cosmological constant

Main article: Cosmological constant

Further information: Equation of state (cosmology)

Estimated distribution of matter and energy in the universe

The simplest explanation for dark energy is that it is an intrinsic, fundamental energy of space. This is the cosmological constant, usually represented by the Greek letter Λ (Lambda, hence Lambda-CDM model). Since energy and mass are related according to the equation E = mc² , Einstein's theory of general relativity predicts that this energy will have a gravitational effect. It is sometimes called a vacuum energy because it is the energy density of empty space – the vacuum.

A major outstanding problem is that the same quantum field theories predict a huge cosmological constant, about 120 orders of magnitude too large. This would need to be almost, but not exactly, cancelled by an equally large term of the opposite sign.

Some supersymmetric theories require a cosmological constant that is exactly zero. Also, it is unknown if there is a metastable vacuum state in string theory with a positive cosmological constant, and it has been conjectured by Ulf Danielsson et al. that no such state exists. This conjecture would not rule out other models of dark energy, such as quintessence, that could be compatible with string theory.

Quintessence

Main article: Quintessence (physics)

In quintessence models of dark energy, the observed acceleration of the scale factor is caused by the potential energy of a dynamical field, referred to as quintessence field. Quintessence differs from the cosmological constant in that it can vary in space and time. In order for it not to clump and form structure like matter, the field must be very light so that it has a large Compton wavelength.

No evidence of quintessence is yet available, but it has not been ruled out either. It generally predicts a slightly slower acceleration of the expansion of the universe than the cosmological constant. Some scientists think that the best evidence for quintessence would come from violations of Einstein's equivalence principle and variation of the fundamental constants in space or time. Scalar fields are predicted by the Standard Model of particle physics and string theory, but an analogous problem to the cosmological constant problem (or the problem of constructing models of cosmological inflation) occurs: renormalization theory predicts that scalar fields should acquire large masses.

The coincidence problem asks why the acceleration of the Universe began when it did. If acceleration began earlier in the universe, structures such as galaxies would never have had time to form, and life, at least as we know it, would never have had a chance to exist. Proponents of the anthropic principle view this as support for their arguments. However, many models of quintessence have a so-called "tracker" behavior, which solves this problem. In these models, the quintessence field has a density which closely tracks (but is less than) the radiation density until matter–radiation equality, which triggers quintessence to start behaving as dark energy, eventually dominating the universe. This naturally sets the low energy scale of the dark energy.

In 2004, when scientists fit the evolution of dark energy with the cosmological data, they found that the equation of state had possibly crossed the cosmological constant boundary (w = −1) from above to below. A no-go theorem has been proved that this scenario requires models with at least two types of quintessence. This scenario is the so-called Quintom scenario.

Some special cases of quintessence are phantom energy, in which the energy density of quintessence actually increases with time, and k-essence (short for kinetic quintessence) which has a non-standard form of kinetic energy such as a negative kinetic energy. They can have unusual properties: phantom energy, for example, can cause a Big Rip.

Interacting dark energy

This class of theories attempts to come up with an all-encompassing theory of both dark matter and dark energy as a single phenomenon that modifies the laws of gravity at various scales. This could, for example, treat dark energy and dark matter as different facets of the same unknown substance, or postulate that cold dark matter decays into dark energy. Another class of theories that unifies dark matter and dark energy are suggested to be covariant theories of modified gravities. These theories alter the dynamics of the spacetime such that the modified dynamics stems to what have been assigned to the presence of dark energy and dark matter. Dark energy could in principle interact not only with the rest of the dark sector, but also with ordinary matter. However, cosmology alone is not sufficient to effectively constrain the strength of the coupling between dark energy and baryons, so that other indirect techniques or laboratory searches have to be adopted. A recent proposal speculates that the currently unexplained excess observed in the XENON1T detector in Italy may have been caused by a chameleon model of dark energy.

Variable dark energy models

The density of the dark energy might have varied in time during the history of the universe. Modern observational data allow us to estimate the present density of the dark energy. Using baryon acoustic oscillations, it is possible to investigate the effect of dark energy in the history of the Universe, and constrain parameters of the equation of state of dark energy. To that end, several models have been proposed. One of the most popular models is the Chevallier–Polarski–Linder model (CPL). Some other common models are, (Barboza & Alcaniz. 2008), (Jassal et al. 2005), (Wetterich. 2004), (Oztas et al. 2018).

Observational skepticism

Some alternatives to dark energy, such as inhomogeneous cosmology, aim to explain the observational data by a more refined use of established theories. In this scenario, dark energy doesn't actually exist, and is merely a measurement artifact. For example, if we are located in an emptier-than-average region of space, the observed cosmic expansion rate could be mistaken for a variation in time, or acceleration. A different approach uses a cosmological extension of the equivalence principle to show how space might appear to be expanding more rapidly in the voids surrounding our local cluster. While weak, such effects considered cumulatively over billions of years could become significant, creating the illusion of cosmic acceleration, and making it appear as if we live in a Hubble bubble. Yet other possibilities are that the accelerated expansion of the universe is an illusion caused by the relative motion of us to the rest of the universe, or that the statistical methods employed were flawed. It has also been suggested that the anisotropy of the local Universe has been misrepresented as dark energy. This claim was quickly countered by others, including a paper by physicists D. Rubin and J. Heitlauf. A laboratory direct detection attempt failed to detect any force associated with dark energy.

A study published in 2020 questioned the validity of the essential assumption that the luminosity of Type Ia supernovae does not vary with stellar population age, and suggests that dark energy may not actually exist. Lead researcher of the new study, Young-Wook Lee of Yonsei University, said "Our result illustrates that dark energy from SN cosmology, which led to the 2011 Nobel Prize in Physics, might be an artifact of a fragile and false assumption." Multiple issues with this paper were raised by other cosmologists, including Adam Riess, who won the 2011 Nobel Prize for the discovery of dark energy.

Other mechanism driving acceleration

Modified gravity

See also: Massive gravity

The evidence for dark energy is heavily dependent on the theory of general relativity. Therefore, it is conceivable that a modification to general relativity also eliminates the need for dark energy. There are very many such theories, and research is ongoing. The measurement of the speed of gravity in the first gravitational wave measured by non-gravitational means (GW170817) ruled out many modified gravity theories as explanations to dark energy.

Astrophysicist Ethan Siegel states that, while such alternatives gain a lot of mainstream press coverage, almost all professional astrophysicists are confident that dark energy exists, and that none of the competing theories successfully explain observations to the same level of precision as standard dark energy.

Implications for the fate of the universe

Cosmologists estimate that the acceleration began roughly 5 billion years ago. Before that, it is thought that the expansion was decelerating, due to the attractive influence of matter. The density of dark matter in an expanding universe decreases more quickly than dark energy, and eventually the dark energy dominates. Specifically, when the volume of the universe doubles, the density of dark matter is halved, but the density of dark energy is nearly unchanged (it is exactly constant in the case of a cosmological constant).

Projections into the future can differ radically for different models of dark energy. For a cosmological constant, or any other model that predicts that the acceleration will continue indefinitely, the ultimate result will be that galaxies outside the Local Group will have a line-of-sight velocity that continually increases with time, eventually far exceeding the speed of light. This is not a violation of special relativity because the notion of "velocity" used here is different from that of velocity in a local inertial frame of reference, which is still constrained to be less than the speed of light for any massive object (see Uses of the proper distance for a discussion of the subtleties of defining any notion of relative velocity in cosmology). Because the Hubble parameter is decreasing with time, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit a signal which reaches us eventually.

However, because of the accelerating expansion, it is projected that most galaxies will eventually cross a type of cosmological event horizon where any light they emit past that point will never be able to reach us at any time in the infinite future because the light never reaches a point where its "peculiar velocity" toward us exceeds the expansion velocity away from us (these two notions of velocity are also discussed in Uses of the proper distance). Assuming the dark energy is constant (a cosmological constant), the current distance to this cosmological event horizon is about 16 billion light years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event were less than 16 billion light years away, but the signal would never reach us if the event were more than 16 billion light years away.

As galaxies approach the point of crossing this cosmological event horizon, the light from them will become more and more redshifted, to the point where the wavelength becomes too large to detect in practice and the galaxies appear to vanish completely. Planet Earth, the Milky Way, and the Local Group of which the Milky Way is a part, would all remain virtually undisturbed as the rest of the universe recedes and disappears from view. In this scenario, the Local Group would ultimately suffer heat death, just as was hypothesized for the flat, matter-dominated universe before measurements of cosmic acceleration.

There are other, more speculative ideas about the future of the universe. The phantom energy model of dark energy results in divergent expansion, which would imply that the effective force of dark energy continues growing until it dominates all other forces in the universe. Under this scenario, dark energy would ultimately tear apart all gravitationally bound structures, including galaxies and solar systems, and eventually overcome the electrical and nuclear forces to tear apart atoms themselves, ending the universe in a "Big Rip". On the other hand, dark energy might dissipate with time or even become attractive. Such uncertainties leave open the possibility of gravity eventually prevailing and lead to a universe that contracts in on itself in a "Big Crunch", or that there may even be a dark energy cycle, which implies a cyclic model of the universe in which every iteration (Big Bang then eventually a Big Crunch) takes about a trillion (10¹²) years. While none of these are supported by observations, they are not ruled out.

In philosophy of science

In philosophy of science, dark energy is an example of an "auxiliary hypothesis", an ad hoc postulate that is added to a theory in response to observations that falsify it. It has been argued that the dark energy hypothesis is a conventionalist hypothesis, that is, a hypothesis that adds no empirical content and hence is unfalsifiable in the sense defined by Karl Popper.

 

Cosmological argument

From Wikipedia, the free encyclopedia

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

A cosmological argument, in natural theology, is an argument which claims that the existence of God can be inferred from facts concerning causation, explanation, change, motion, contingency, dependency, or finitude with respect to the universe or some totality of objects. A cosmological argument can also sometimes be referred to as an argument from universal causation, an argument from first cause, the causal argument, or prime mover argument. Whichever term is employed, there are two basic variants of the argument, each with subtle yet important distinctions: in esse (essentiality), and in fieri (becoming).

The basic premises of all of these arguments involve the concept of causation. The conclusion of these arguments is that there exists a first cause (for whichever group of things it is being argued has a cause), 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 was 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.

Contemporary defenders of cosmological arguments include William Lane Craig, Robert Koons, and Alexander Pruss.

History

Plato and Aristotle, depicted here in Raphael's The School of Athens, both developed first cause arguments.

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 Islamic philosopher Avicenna (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, Avicenna, and Maimonides 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:

{{quote|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 ultimately 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:

1. Every contingent fact has an explanation.
2. There is a contingent fact that includes all other contingent facts.
3. Therefore, there is an explanation of this fact.
4. This explanation must involve a necessary being.
5. This necessary being is God.

Premise 1 is a form of the principle of sufficient reason stating that all contingently true sentences (i.e. contingent facts) have a sufficient explanation as to why they are the case. Premise 2 refers to what is known as the Big Conjunctive Contingent Fact (abbreviated BCCF), and the BCCF is generally taken to be the logical conjunction of all contingent facts. It can be thought about as the sum total of all contingent reality. Premise 3 then concludes that the BCCF has an explanation, as every contingency does (in virtue of the PSR). It follows that this explanation is non-contingent (i.e. necessary); no contingency can explain the BCCF, because every contingent fact is a part of the BCCF. 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. 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.

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.

Kalām cosmological argument

Main article: Kalam cosmological argument

William Lane Craig, who was responsible for re-popularizing this argument in Western philosophy, presents it in the following general form:

1. Whatever begins to exist has a cause of its existence.
2. The universe began to exist.
3. Therefore, the universe has a cause of its existence.

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 infinity is impossible. However, If the universe never had a beginning then there would be an actual infinite, Craig claims, namely 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:

1. Something can be produced.
2. It is produced by itself, something or another.
3. Not by nothing, because nothing causes nothing.
4. Not by itself, because an effect never causes itself.
5. Therefore, by another A.
6. If A is first then we have reached the conclusion.
7. If A is not first, then we return to 2).
8. From 3) and 4), we produce another- B. The ascending series is either infinite or finite.
9. An infinite series is not possible.
10. 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."

Cosmological argument and infinite regress

Depending on its formulation, the cosmological argument is an example of a positive infinite regress argument. An infinite regress is an infinite series of entities governed by a recursive principle that determines how each entity in the series depends on or is produced by its predecessor. An infinite regress argument is an argument against a theory based on the fact that this theory leads to an infinite regress. A positive infinite regress argument employs the regress in question to argue in support of a theory by showing that its alternative involves a vicious regress. The regress relevant for the cosmological argument is the regress of causes: an event occurred because it was caused by another event that occurred before it, which was itself caused by a previous event, and so on. For an infinite regress argument to be successful, it has to demonstrate not just that the theory in question entails an infinite regress but also that this regress is vicious. Once the viciousness of the regress of causes is established, the cosmological argument can proceed to its positive conclusion by holding that it is necessary to posit a first cause in order to avoid it.

A regress can be vicious due to metaphysical impossibility, implausibility or explanatory failure. It is sometimes held that the regress of causes is vicious because it is metaphysically impossible, i.e. that it involves an outright contradiction. But it is difficult to see where this contradiction lies unless an additional assumption is accepted: that actual infinity is impossible. But this position is opposed to infinity in general, not just specifically to the regress of causes. A more promising view is that the regress of causes is to be rejected because it is implausible. Such an argument can be based on empirical observation, e.g. that, to the best of our knowledge, our universe had a beginning in the form of the Big Bang. But it can also be based on more abstract principles, like Ockham's razor, which posits that we should avoid ontological extravagance by not multiplying entities without necessity. A third option is to see the regress of causes as vicious due to explanatory failure, i.e. that it does not solve the problem it was formulated to solve or that it assumes already in disguised form what it was supposed to explain. According to this position, we seek to explain one event in the present by citing an earlier event that caused it. But this explanation is incomplete unless we can come to understand why this earlier event occurred, which is itself explained by its own cause and so on. At each step, the occurrence of an event has to be assumed. So it fails to explain why anything at all occurs, why there is a chain of causes to begin with.

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 rest on the premise that everything has a cause.

William Lane Craig, who popularised and is notable for defending 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. Craig argues in the Blackwell Companion to Natural Theology that there cannot be an infinite regress of causes and thus there must be a first uncaused cause, even if one posits a plurality of causes of the universe. He argues Occam's Razor may be employed to remove unneeded further causes of the universe, to leave a single uncaused cause.

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 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 Kalam and the Thomistic cosmological argument posit 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 theistic God

According to this objection 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 given to refute the possibility of a causal loop is that 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. William L. Rowe has called this 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, in Hume's Dialogues Concerning Natural Religion, the character 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 argues that most of the classical philosophers' cosmological arguments for the existence of God do not depend on 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.