Big Bang

Big Bang models describe a universe that has been expanding and cooling from a very hot and very compact state ¹. Extrapolating the expansion backward with known physical laws yields an extremely hot and dense primordial universe ¹. Britannica describes the model's essential feature as the emergence of the universe from a state of extremely high temperature and density ¹².

Sources differ in how much the term covers. The original model extrapolated back to a singularity ¹. In inflationary models, an early period of accelerated expansion is followed by a reheating event, which initiates the hot Big Bang phase ¹. On that usage, the Big Bang describes only the hot, dense thermal state, and it is a misconception that the model fully explains the origin of the universe ¹. Alan Guth, by contrast, said in March 2014 that he describes inflation as “a theory of the ‘bang’ of the Big Bang”, the propulsion mechanism called the Big Bang ³.

Introductory accounts sometimes describe the universe as beginning from a single point ¹³. A survey of physicists conducted through the American Physical Society's Physics Magazine (the Big Mysteries Survey) found that 68.4% of respondents characterised the Big Bang as a theory of evolution from a hot, dense state that is silent on whether time itself had a beginning ⁸. That figure rests on a single preprint ⁸.

The name has been argued to be a misnomer because it evokes an explosion into surrounding space, whereas the model describes an expansion of the universe's contents that occurs everywhere rather than from a single point ¹.

According to one account, Big Bang models rest on three major assumptions: the universality of physical laws, the cosmological principle, and the treatment of matter as a perfect fluid ¹. Britannica instead names two: that general relativity correctly describes gravity, and the cosmological principle ¹². The cosmological principle states that on large scales the universe is homogeneous and isotropic ¹¹².

These assumptions have been tested. Possible variation of the fine-structure constant over much of cosmic history is limited to about one part in 100,000 ¹. Observations of CMB temperature confirm the cosmological principle to a similar level ¹.

The cosmological principle simplifies general relativity to the Friedmann–Lemaître–Robertson–Walker metric, and the Friedmann equations then describe how that geometry changes with time ¹. The standard model built on this, ΛCDM, combines a cosmological constant (or more general dark energy) with cold dark matter, and has been the dominant paradigm for more than 25 years ⁵.

Estimates of the present composition differ by source and data set. According to Wikipedia, under 5% of the mass-energy is luminous matter, 27% dark matter and 68% dark energy; the classification of the first figure is disputed, and it could not be independently confirmed ¹. WMAP results from 2008 corresponded to 73% dark energy, 23% dark matter, 4.6% ordinary matter and under 1% neutrinos ¹. A Nature Astronomy assessment describes dark energy as roughly 70% of the present-epoch universe ⁵.

In inflationary models, inflation is an extremely brief period in which the universe doubled in size more than 80 times ¹. During inflation, microscopic quantum fluctuations were “frozen in”, seeding later large-scale structure ¹. Reheating then converted the energy driving inflation into a hot, dense plasma ¹. Its mechanism is unknown, and the temperature after reheating is constrained only to between 10^11 and 10^29 K ¹.

An unknown process called baryogenesis produced a small excess of matter over antimatter ¹. The electromagnetic and weak forces separated at about 10^-12 seconds ¹. Quarks and gluons formed baryons such as protons and neutrons at about 10^-6 seconds ¹. Accounts of times later than about 10^-11 seconds are less speculative, because the particle energies involved can be reached in accelerators ¹.

A few minutes into the expansion, at about a billion kelvin, neutrons and protons combined into deuterium and helium nuclei in Big Bang nucleosynthesis (BBN) ¹. Matter came to dominate radiation at about 50,000 years ¹. At about 380,000 years, electrons and nuclei combined into neutral atoms, making the universe transparent ¹. The photons that last scattered at that time make up the CMB ¹.

Denser regions then drew in surrounding matter, eventually forming gas clouds, stars and galaxies ¹. The first quasars and galaxies are thought to have formed within a billion years ¹.

The earliest and most direct evidence comprises three lines: expansion according to Hubble's law, the CMB, and the relative abundances of light elements from BBN ¹. Observations of galaxy formation and large-scale structure are more recent evidence; the four together are sometimes called the “four pillars” ¹. A Particle Data Group review describes BBN work as having further confirmed the necessity of a hot and dense past ¹⁴¹⁵.

Hubble's law: Distant galaxies are redshifted, and their recession velocities increase linearly with distance ¹. WMAP measured the Hubble constant as 70.4 (+1.3, −1.4) km/s/Mpc ¹.

Cosmic microwave background: Arno Penzias and Robert Wilson discovered the radiation in 1964 ¹. It confirmed predictions made around 1950 by Ralph Alpher, Robert Herman and George Gamow, and Penzias and Wilson received the 1978 Nobel Prize in Physics ¹. In 1990, NASA's COBE satellite showed the spectrum to be an almost perfect blackbody, and in 1992 it detected temperature fluctuations of about one part in 100,000 ¹. John C. Mather and George Smoot received the 2006 Nobel Prize in Physics for these results ¹. The present temperature is about 2.725 K ¹.

Light elements: BBN predicts helium-4 at about 25% of hydrogen by mass, with far smaller amounts of deuterium, helium-3 and lithium-7, all set by a single baryon-to-photon ratio ¹. Agreement is excellent for deuterium, close but formally discrepant for helium-4, and off by a factor of two for lithium-7, a discrepancy known as the cosmological lithium problem ¹.

A Big Think article reports that the LBT Yp Project released papers in the latter half of 2026 giving a primordial helium mass fraction of 24.58% ± 0.13%, which it describes as the most precise such value yet ¹⁰. The same article calls BBN historically the lowest-precision and most often disputed of the four pillars ¹⁰. This account rests on a single popular-science article ¹⁰.

Other evidence: In 2011, astronomers reported gas clouds with no detectable heavy elements, which they interpreted as likely primordial ¹. Ages of the oldest stars agree with the age derived from expansion and the CMB ¹. Some early objects, notably the quasar APM 08279+5255, raise the question of whether they had time to form under ΛCDM ¹.

In 1912, Vesto Slipher measured Doppler shifts of spiral nebulae and found that almost all were receding ¹. In 1922, Alexander Friedmann derived equations from general relativity showing that the universe might be expanding ¹¹⁶. Lemaître independently rederived these solutions in 1927 and concluded that the universe was in fact expanding ¹¹⁶. According to the historian Helge Kragh, Friedmann himself did not draw that conclusion ¹⁶.

Edwin Hubble published the distance–velocity correlation now called Hubble's law in 1929 ¹. According to Wikipedia, Milton Humason assisted with the work; whether he should be counted as a co-author could not be confirmed from the sources used here ¹.

Sources differ on when Lemaître first proposed a beginning. NASA's children's site says that in 1927 he proposed that the universe began as a single point ¹³. Kragh writes that Lemaître's 1927 model evolved asymptotically from a pre-existing static universe and had no definite age ¹⁶. Kragh and Wikipedia both date the finite-age “primeval atom” idea to 1931 ¹¹⁶.

In the 1920s and 1930s many cosmologists preferred an eternal universe, and some objected that a beginning in time imported religious concepts into physics ¹. After the Second World War, Fred Hoyle's steady-state model competed with Lemaître's expanding-universe theory, which George Gamow developed ¹. Britannica credits Gamow and colleagues with developing the modern version in the 1940s ¹². Radio source counts and then the CMB discovery shifted support to the Big Bang ¹. By the late 1960s, most cosmologists considered the steady-state model incorrect ¹.

Hoyle coined the term “Big Bang” in a BBC Third Programme broadcast ¹¹⁸. Most sources date it to March 1949, one specifying 28 March ¹¹⁶¹⁸, while one paper gives 28 March 1948 ¹⁷. It is popularly reported that Hoyle meant the term pejoratively ¹. Hoyle denied this, and Kragh calls the evidence for that claim “unconvincing” ¹. According to Kragh, the term became a household word in the scientific community only more than twenty years later ¹⁶.

In 1968 and 1970, Roger Penrose, Stephen Hawking and George F. R. Ellis showed that singularities were an inevitable initial condition of relativistic Big Bang models ¹. In 1981, Alan Guth introduced an early epoch of rapid expansion that he called “inflation” ¹. An MIT account of the history of inflation also mentions work in 1980, so the dating of the idea varies between sources ³.

Problems acknowledged within the standard account include baryon asymmetry, the nature of dark matter, and the origin of dark energy ¹. The Standard Model of particle physics contains all the Sakharov conditions needed for baryogenesis, but their effects are too weak to explain the observed asymmetry ¹. No dark matter particles have been observed in laboratories ¹. The cold dark matter model also faces the dwarf galaxy and cuspy halo problems ¹. Modified-gravity alternatives exist, but according to Wikipedia none has been as successful as cold dark matter in explaining all observations ¹.

Inflation is the most common resolution of the horizon, flatness and magnetic monopole problems ¹. Its details remain unresolved ¹. Wikipedia also reports that some people say inflation has been disproven; the passage cited does not identify the critics, the models they criticise or the responses of proponents, and the claim could not be independently confirmed ¹.

In the Big Mysteries Survey, 50.8% of respondents favoured inflation as the explanation of early-universe uniformity and flatness ⁸. Some respondents favoured bouncing, cyclic or quantum-gravity models ⁸. The preprint reports that several positions often described as field-wide consensus are supported by narrower majorities or by pluralities ⁸. The excerpt cited here does not give the sample size, how respondents were recruited or their representativeness, so the percentages should not be read as a measure of scientific consensus ⁸.

The Hubble tension is an unexplained discrepancy in measurements of the Hubble constant: CMB-based techniques give a lower value than the cosmic distance ladder ¹. In the survey, the most common response on its cause was “no opinion” at 24.4%, followed by early dark energy at 22.1% ⁸.

Whether dark energy is a constant is under debate. A Nature Astronomy article evaluates the evidence for a constant Λ against dynamical dark energy in light of recent galaxy-clustering, supernova and CMB measurements ⁵. In the survey, 25.9% favoured a time-varying dark energy field and 24.0% the cosmological constant ⁸. On dark matter, no candidate led clearly: a hybrid model drew 20.6%, MOND about 11.5% and quantum-gravity effects about 10.1% ⁸.

A 2026 Phys.org report describes a dispute over whether cosmic expansion is still accelerating ⁹. Researchers at Yonsei University reported that, once a systematic bias related to the ages of Type Ia supernovae is taken into account, the universe may no longer be accelerating, and that dark energy may be weakening with time rather than being a cosmological constant ⁹. They said this closely agrees with baryon acoustic oscillation (BAO)-only measurements from the Dark Energy Spectroscopic Instrument (DESI) ⁹.

A team led by the University of Southampton, including two Nobel laureates, published a rebuttal ⁹. According to the report, its analysis found a much shallower relationship between supernova luminosity and age than the Yonsei team reported, and it argued that the ages of supernova progenitors may evolve much less with redshift than the ages of their host galaxies ⁹. The Yonsei group then argued in the Monthly Notices of the Royal Astronomical Society that both central arguments of the rebuttal have critical analytical problems ⁹. The report mainly presents the Yonsei group's account, and the excerpt does not include a Southampton reply ⁹.

The Yonsei group proposed an “evolution-free” test using only supernovae in young, similarly aged host galaxies, and reported preliminary results consistent with its earlier findings ⁹. Larger samples from the Vera C. Rubin Observatory are expected to allow this test to be carried out with greater precision ⁹. The matter is unresolved in the cited sources ⁹.

Some critics question the evidential basis of particular components of the standard picture. An Aeon essay contends that inflation, dark matter and dark energy are all needed but that, despite 20 to 30 years of effort, no additional empirical evidence for them has been secured beyond the basic need for them ⁶. The essay also calls the theory remarkably successful yet full of explanatory holes ⁶. This is one author's assessment and could not be independently confirmed here.

These criticisms are directed at inflation, dark matter and dark energy, which are distinct from the hot Big Bang itself ⁶. Wikipedia states that a wide range of empirical evidence strongly favours the Big Bang model ¹, and the Particle Data Group review says BBN work further confirmed the necessity of a hot and dense past ¹⁴¹⁵. Observational support and unresolved physical mechanisms therefore need to be assessed separately for each component.

No direct probes exist for what came before inflation, so accounts of the earliest times are more speculative ¹. There are no easily testable models of conditions before about 10^-15 seconds ¹. The initial singularity arises from extrapolating general relativity and may be absent in a theory of quantum gravity ¹.

Speculative proposals, each involving untested hypotheses, include the Hartle–Hawking no-boundary condition, emergent universe models, ekpyrotic and cyclic brane models, and eternal inflation ¹. A 2025 book by the cosmologist Niayesh Afshordi and the science communicator Phil Halper compares bouncing and cyclic universes, time loops, creation from nothing, multiverses, black-hole births, string theories and holograms ⁴. A Physics World review states that the book's focus is comparing models rather than persuading readers that one is correct ⁷.

Before dark energy was observed, cosmologists considered two outcomes. If density exceeded the critical value, the universe would collapse in a Big Crunch; otherwise it would expand forever toward a Big Freeze and eventual heat death ¹. Under ΛCDM, only gravitationally bound systems such as galaxies would remain together ¹. Phantom dark energy theories instead predict a Big Rip that tears matter apart ¹.

According to Wikipedia, the Big Bang has become a prominent topic in discussions between science and religion ¹. It records that some people connect the theory with a creator, while others argue that Big Bang cosmology makes a creator unnecessary ¹. The sources used here do not describe other positions, such as the view that theological questions lie outside the scope of physical cosmology.