The Big Bang and cosmic history: evidence, cosmic epochs and the Hubble tension

At a glance

The Big Bang theory is the scientific account of how the Universe evolved from an extremely hot, dense early state into the cool, structured cosmos we see today. It is not a description of an explosion at a point in space, but of space itself expanding everywhere at once. It rests on four independent pillars: the expansion of the Universe, the cosmic microwave background (CMB), the abundances of the lightest elements and the growth of large-scale structure. The table gives reference values from the standard cosmological model (ΛCDM) fitted to Planck data and from the measurements behind each pillar.

Sources and dates: Planck Collaboration, “Planck 2018 results VI”, Astronomy & Astrophysics 641, A6 (2020); Fixsen, Astrophysical Journal (2009); Particle Data Group, 2025 review of Big-Bang nucleosynthesis; ESA Planck pages; NASA Webb reionisation infographic. The age is the rounded NASA value; ΛCDM parameters assume the base six-parameter model.

Sources: [4] [5] [7] [8] [9]

Pillar one: an expanding Universe

In 1929 Edwin Hubble published “A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae” in the Proceedings of the National Academy of Sciences. Galaxies farther away were receding faster, in proportion to their distance. Georges Lemaître had arrived at the same relation from Einstein’s general relativity, interpreting it as the expansion of space. In October 2018, members of the International Astronomical Union voted (78% in favour) to recommend that the law be called the Hubble–Lemaître law.

The constant of proportionality, the Hubble constant H₀, sets the present expansion rate. Run the expansion backwards and everything was once closer together, denser and hotter. Distances to galaxies (see the cosmic distance ladder) and their redshifts together trace not only today’s rate but its history: observations of distant supernovae showed that the expansion is accelerating, which is attributed to dark energy.

Sources: [1] [2]

Pillar two: the cosmic microwave background

If the early Universe was hot and dense, it should have been filled with radiation. While the cosmos was still an opaque plasma, light was constantly scattered by free electrons. About 380,000 years after the Big Bang, when the temperature had fallen to around 3,000 K, electrons and protons combined into neutral hydrogen (an epoch called recombination), and the light was released. Stretched by cosmic expansion ever since, that light now arrives as microwaves from every direction: the cosmic microwave background.

Arno Penzias and Robert Wilson discovered it, a discovery recognised with the 1978 Nobel Prize in Physics. NASA’s COBE satellite, launched in 1989, showed that its spectrum is an almost perfect blackbody and found temperature fluctuations of about one part in 100,000; John Mather and George Smoot shared the 2006 Nobel Prize for these discoveries. WMAP (launched 2001) and ESA’s Planck (launched 2009) mapped the fluctuations in ever greater detail; Planck can distinguish temperature differences of a few millionths of a degree. The best determination of the mean temperature today is 2.72548 ± 0.00057 K.

The pattern of hot and cold spots encodes the Universe’s ingredients. Fitting it with the six-parameter ΛCDM model, the Planck collaboration obtained H₀ = 67.4 ± 0.5 km/s/Mpc and a matter density Ωm = 0.315 ± 0.007, with ordinary matter only a small part of the total.

These tiny fluctuations matter far beyond the CMB itself: they are the seeds of all later structure. Regions that were slightly denser than average pulled in more matter over billions of years and became the filaments and clusters of the cosmic web, while slightly underdense regions emptied into voids. The characteristic angular size of the hot and cold spots, set by how far sound could travel in the plasma before recombination, also depends on the geometry of space, which is why the CMB is one of the most sensitive tests of whether the Universe is spatially flat.

Sources: [3] [4] [5] [6] [7]

Pillar three: the lightest elements

In the first minutes the whole Universe was a nuclear reactor. According to the Particle Data Group’s review, the outcome of Big Bang nucleosynthesis (BBN) is sensitive to conditions at a temperature of about 1 MeV, when the Universe was about one second old; deuterium starts to survive once the temperature drops to roughly 0.1 MeV, and the light-element abundances are essentially fixed by about 180 seconds. Almost all neutrons end up in helium-4, with traces of deuterium, helium-3 and lithium-7.

The predictions depend mainly on one number, the ratio of baryons to photons, η. Measurements in pristine gas give a primordial helium mass fraction of 0.245 ± 0.003 and a deuterium-to-hydrogen ratio of (25.08 ± 0.29) × 10⁻⁶. The value of η inferred from BBN and deuterium, 6.040 ± 0.118 × 10⁻¹⁰, agrees with the value read independently from the CMB, 6.12 ± 0.04 × 10⁻¹⁰: physics of the first three minutes and of 380,000 years later point to the same baryon density. One discrepancy persists: observed lithium is lower than predicted by a factor of about 3.3, the long-standing “lithium problem”.

Sources: [8]

A timeline of cosmic epochs

Pieced together, the evidence gives a history in distinct chapters. Before BBN, many cosmologists favour a brief period of extremely rapid expansion called inflation, which would explain why the Universe is so uniform and spatially flat and would have produced the seed fluctuations. Inflation is a well-motivated hypothesis rather than an established fact: its predicted tilt of the fluctuation spectrum matches Planck (n_s = 0.965 ± 0.004), but the primordial gravitational waves it would also create have not been detected. The BICEP/Keck collaboration’s 2021 analysis limits their strength to a tensor-to-scalar ratio r < 0.036 (95% confidence).

After recombination came the dark ages, when there were no stars yet. NASA places their end at roughly 200 million years after the Big Bang, when the first stars and galaxies began to shine (cosmic dawn). Their ultraviolet light gradually ionised the neutral hydrogen between galaxies; this era of reionisation ended about 1 billion years after the Big Bang. The CMB records it too, through the optical depth τ = 0.054 ± 0.007 measured by Planck.

Sources: [4] [7] [8] [9] [10] [11]

Redshift, scale factor and temperature

Cosmologists describe expansion with the scale factor a, the size of the Universe relative to today (a = 1 now). Light emitted when the scale factor was a arrives stretched by a factor 1/a, so its redshift obeys 1 + z = 1/a. A galaxy at z = 1 emitted its light when cosmic distances were half their present size. Radiation cools as it stretches, so the CMB temperature scales as T = T₀(1 + z). With T₀ ≈ 2.7255 K today and about 3,000 K at recombination, the CMB comes from z ≈ 1,100: when it was released, distances were about a thousand times smaller than now.

Sources: [4] [5]

Lab: travel back in time

Move the redshift slider to see the age of the Universe, the temperature of the background radiation and the scale factor at that moment. Start with the presets for nucleosynthesis, matter–radiation equality and recombination, then jump to the most distant known galaxies and to the formation of the Solar System.

The lab uses a simplified flat ΛCDM model with H₀ = 67.4 km/s/Mpc (see its method note). Ages at very high redshift depend on the particle content of the early Universe, so earliest-epoch values are approximate.

Pillar four, and a tension: structure and the expansion rate

The fourth pillar is the cosmic web itself (see galaxies and the cosmic web). The sound waves that rippled through the early plasma left a preferred separation, the baryon acoustic oscillation scale, imprinted both on the CMB and on the distribution of galaxies. Measuring the same ruler at different epochs tests the whole model.

That test has produced the field’s best-known disagreement. Values of H₀ inferred from the CMB within ΛCDM cluster around 67–68 km/s/Mpc, while many local distance-ladder measurements give about 73 (details on our distance ladder page). New CMB data have tightened the early-Universe side. The Atacama Cosmology Telescope’s sixth data release (ACT DR6, March 2025), combined with Planck, CMB lensing and DESI BAO, gives H₀ = 68.22 ± 0.36 km/s/Mpc, and 68.43 ± 0.27 with DESI DR2. The South Pole Telescope’s SPT-3G analysis (June 2025) finds 66.66 ± 0.60 on its own, which it reports as 6.2σ away from the SH0ES local value; combining SPT, ACT and Planck gives 67.19 ± 0.38.

So far, no single change to the standard model resolves the tension cleanly. Proposed fixes range from new physics before recombination (for example “early dark energy” or a modified recombination history) to unrecognised systematic errors in either method.

Sources: [12] [13] [14]

What does “before the Big Bang” mean?

The Big Bang theory describes the evolution of the Universe from a hot, dense state; it does not describe a moment of creation. If general relativity is extrapolated all the way back, density and temperature become infinite, a “singularity”, but that is generally read as a sign that the theory has reached the edge of its validity. Describing those conditions would require a quantum theory of gravity, which does not yet exist in tested form.

Observationally, there is also a wall. Light cannot reach us from before recombination, because the Universe was opaque; the CMB is the earliest light there is. Earlier epochs are probed indirectly: the light elements test the first minutes, and the statistics of the CMB fluctuations test inflation. Primordial gravitational waves, if detected, would open a direct window onto inflation. Whether “before” even has a meaning, and whether our Universe is one region of a larger whole, are questions that physics cannot yet test. The scientific “age of the Universe” is therefore the time since the hot dense phase, not necessarily the age of everything that exists.

Sources: [4] [10]

Current research (2024–2026)

• ACT DR6 (March 2025) and SPT-3G D1 (June 2025, revised April 2026) independently confirm Planck’s picture with ground-based data and keep the CMB value of H₀ near 67–68 km/s/Mpc.
• DESI DR2 (March 2025). BAO from more than 14 million galaxies and quasars. Combined with the CMB, a time-varying dark energy is preferred over a cosmological constant at 3.1σ, and at 2.8–4.2σ when supernovae are added. This falls short of the 5σ discovery convention and depends on which supernova sample is used, so it should be read as an intriguing hint. SPT-3G also reports a 2.8σ difference between CMB and DESI DR2 BAO within ΛCDM.
• JWST at cosmic dawn. MoM-z14, confirmed spectroscopically at z = 14.44, is seen about 280 million years after the Big Bang. Luminous early galaxies like it are more than 100 times more abundant than pre-JWST consensus models predicted. This challenges models of early star formation, not the Big Bang framework itself; see our James Webb page.

Sources: [11] [12] [13] [14]

Going further: data archives

Planck maps, power spectra and likelihoods are public in the ESA Planck Legacy Archive. NASA’s LAMBDA archive hosts COBE and WMAP data and many ground-based CMB products. DESI catalogues are on the DESI data portal. Most papers cited here are freely available on arXiv; reading the abstract and conclusions of the Planck 2018 parameters paper is a good entry point to how cosmological numbers are actually derived.

Known vs open questions

Well established

• The Universe is expanding and was once much hotter and denser.
• The CMB is a near-perfect blackbody at 2.7255 K, released about 380,000 years after the Big Bang.
• The abundances of deuterium and helium agree with BBN for the baryon density measured in the CMB.
• The standard ΛCDM model fits the CMB and large-scale structure with six parameters.

Open questions

• Why do early- and late-Universe values of H₀ disagree?
• Is dark energy a cosmological constant or does it evolve?
• Did inflation happen, and what drove it?
• Why is primordial lithium lower than predicted?
• How did galaxies form so quickly after cosmic dawn?

Related on this site

• Research dossier: cosmology
• Dark matter and dark energy
• Galaxies and the cosmic web
• The cosmic distance ladder
• Gravitational waves
• James Webb Space Telescope
• Cosmic scales tool