The Big Bang

The Big Bang is the prevailing account of how the universe began: the cosmos emerged from a state of extremely high temperature and density about 13.8 billion years ago.¹ The model rests on two assumptions — that general relativity correctly describes gravity, and the cosmological principle, which holds that an observer's view of the universe depends neither on the direction in which he looks nor on his location.¹ Those two assumptions make it possible to calculate the history of the cosmos after an epoch called the Planck time; scientists have yet to determine what prevailed before it.¹

It was not an explosion inside pre-existing space; it was the expansion of space itself.² The cosmological principle applies to the large-scale universe, and it implies that the universe has no edge: the big-bang origin occurred not at a particular point in space but throughout space at the same time.¹ We know the universe has no center because galaxies rush away from each other rather than from a central point, and because the heat left over from early times uniformly fills the cosmos.² The Big Bang, on this reading, was a process happening in time, not a point in time.²

In 1929 the American astronomer Edwin Hubble discovered that the distances to far-away galaxies were proportional to their redshifts.³ COBE, WMAP and Planck all saw further back than JWST, mapping the heat that dates from roughly 380,000 years after the beginning.² What follows traces that evidence chain, then the cosmic timeline and the numbers, and closes with the questions that remain open.

The first pillar of the case is expansion: in 1929 the American astronomer Edwin Hubble discovered that the distances to far-away galaxies were proportional to their redshifts.³ The farther a galaxy, the faster it recedes.⁴ The relationship is known today as the Hubble–Lemaître law.⁴ Hubble's discovery was the first observational support for Georges Lemaître's Big Bang theory of the universe, proposed in 1927.³

The groundwork had been laid over the preceding decade: in 1922 the Russian scientist and mathematician Alexander Friedmann published a paper detailing multiple possibilities for the history of the universe, including the possibility that the universe is expanding.⁵ In 1927 the Belgian astronomer Georges Lemaître showed that the equations of Einstein's theory support the idea that the universe is not static but is actually expanding.⁵ Behind both stood earlier work — Slipher's galaxy redshifts and Leavitt's period–luminosity relation for Cepheids, the rungs of the cosmic distance ladder.³

Expansion also explains what redshift means: as space stretches, the light travelling through it is stretched with it — the same effect that has carried the early universe's glow down to a cold 2.7 degrees above absolute zero.⁶ The expansion rate is expressed as the Hubble constant, in kilometres per second per megaparsec: 67.4 from the CMB and 73.5 from local measurements.⁷ The local figure carries an uncertainty of only two percent, the CMB figure less than a percent — which is precisely why their disagreement matters.⁷

About 380,000 years after the Big Bang, the universe had cooled enough that atomic nuclei could capture electrons — a period astronomers call the epoch of recombination.⁹ The universe became transparent, and light could travel freely over great distances for the first time.⁹ That glow, still detectable today, is the cosmic microwave background: the oldest light we can observe in the universe.⁹ It originated when the universe was dense, hot and opaque, and the expansion of space has since stretched it to a cold 2.7 K.¹² At recombination the temperature of the universe was around 2,700 degrees Celsius.⁶

The detection was an accident: Penzias and Wilson spent 1964–1965 fighting an unexplained hiss in their Holmdel antenna — a hunt that famously involved even the New Jersey pigeons — before concluding that it was microwave radiation present throughout the universe at about three degrees above absolute zero.¹⁰ The result matched a Princeton prediction that just such a relic glow should exist.¹³ The noise became known as cosmic microwave background radiation — the missing evidence that could back up the Big Bang theory and led to its acceptance as the standard model of cosmology.¹³ Penzias and Wilson shared half of the 1978 Nobel Prize in Physics for their discovery of cosmic microwave background radiation.¹⁴

Satellites then turned the afterglow into precision data. In 1989 NASA's COBE satellite was launched under John Mather's leadership to study the radiation from orbit.¹⁵ COBE confirmed that the diffuse radiation displayed precisely the expected frequency–wavelength relationships — the perfect blackbody spectrum predicted to result from first light in the universe.¹⁵ In 1992, detectors under George Smoot's direction measured minute variations, or anisotropies, in the background radiation — traces left behind by developing clusters in the expanding universe.¹⁵ Later came WMAP, which fixed the universe's age at 13.77 billion years and the curvature of space to within 0.4% of flat Euclidean geometry⁸, and Planck, which released the most detailed map ever created of the CMB and found ordinary matter at 4.9% and dark matter at 26.8%.⁶

One second after the Big Bang, the universe consisted of an extremely hot primordial soup of light and particles at 18 billion degrees Fahrenheit, or 10 billion degrees Celsius.⁹ In the following minutes, an era called nucleosynthesis, protons and neutrons collided and produced the earliest elements: hydrogen, helium, and traces of lithium and beryllium.⁹ Protons and neutrons formed the nuclei of hydrogen and helium, the basic building blocks of stars.¹¹

After five minutes, most of today's helium had formed, and the universe had expanded and cooled enough that further element formation stopped.⁹ Helium nuclei created in the Big Bang contain two protons and two neutrons each, and helium is the second most abundant element.¹¹ It comprises roughly one quarter of the mass of the universe, a proportion set in the first minutes.¹¹

The heavier elements came later: the chemical elements of life were first produced in the first generation of stars after the Big Bang.² By the time the universe was 1 billion years old, stars and galaxies had transformed nearly all this primordial gas, making the universe transparent to light as we see it today.⁹

The name is a relic of a scientific quarrel: the idea that the universe began in an explosive burst was sarcastically dubbed the 'Big Bang' theory by Fred Hoyle, a critic of the theory, in 1949 — and the nickname stuck.¹⁵ It is, as the Nobel laureate John Mather puts it, a really misleading name for the expanding universe that we see.²

Cosmic history begins with inflation: around 13.8 billion years ago the universe expanded faster than the speed of light for a fraction of a second.⁹ When cosmic inflation stopped, the energy driving it transferred to matter and light — the big bang.⁹ In the standard model of cosmology, the fluctuations we see in the CMB arose immediately after the Big Bang and were stretched to cosmologically large scales during that brief period of accelerated expansion.⁶ Cosmologists think inflation explains many aspects of the universe we observe today, such as its flatness, or lack of curvature, on the largest scales.⁹ What came before inflation, and what powered it, scientists are not sure.⁹

After nucleosynthesis the universe entered a long dark age: for the next 200 million years it remained dark, with no stars to shine.⁹ Present observations suggest that the first stars formed from clouds of gas around 150–200 million years after the Big Bang.³ They were 30 to 300 times more massive than the Sun and millions of times brighter.⁹ By the time the universe was 1 billion years old, stars and galaxies had transformed nearly all this gas, making the universe transparent to light as we see it today.⁹

A single NASA/WMAP illustration — the timeline graphic included with this article — traces the full 13.8-billion-year journey from inflation through the first stars and galaxies to the present.⁸ The story has not stopped: more recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate it.¹¹ Cosmologists suggest it is likely that the universe will continue to expand forever.⁹

In 1998, astronomers found that certain supernovae — bright stellar explosions — were fainter than expected, evidence that cosmic expansion is speeding up.⁹ Two teams working independently at Berkeley observed that supernovae were moving away from Earth at an accelerating rate.³ Nine billion years after the universe began, its expansion started to speed up, driven by an unknown force that scientists have named dark energy.⁵

Dark energy is the name for that unknown force, and it accounts for about 70% of the mass-energy content of the universe.³ In 1984 James Peebles contributed to reviving Einstein's cosmological constant, the energy of empty space; named dark energy, it fills 69 per cent of the cosmos.¹⁶ The standard model of cosmology includes cold dark matter, which unlike ordinary matter does not interact with light, and dark energy, which drives the currently accelerated expansion.⁷

Candidate explanations remain speculative: vacuum energy, whose discrepancy with observation is the cosmological constant problem, along with quintessence fields, cosmic defects, or a modification of general relativity; Euclid, Roman, Rubin and DESI are among the next tests.⁵ That discrepancy between the observed and the theoretically predicted vacuum energy has yet to be solved.⁵ DESI Data Release 2, based on three years of operation, presents baryon acoustic oscillation measurements from more than 14 million galaxies and quasars.¹⁷

The most-quoted discrepancy in modern cosmology is the Hubble tension. The value astronomers derive from local observations, including the Hubble Space Telescope and Gaia, is 73.5 km/s/Mpc, with an uncertainty of only two percent.⁷ The value predicted from the Planck CMB data is 67.4 km/s/Mpc, with a tiny uncertainty of less than a percent.⁷

No one has been able to come up with a satisfactory explanation for the differences between the two measurements, and the question remains to be resolved.⁷ The tension sharpened in 2016, when astronomers using NASA's Hubble Space Telescope discovered that the universe is expanding 5 percent to 9 percent faster than expected.¹⁸ Candidate resolutions — new physics, early dark energy, or a local under-density — are all under discussion.⁷

Inflation remains unexplained in physical detail: the process behind it is unknown, and it lies at energies and densities beyond standard particle physics and beyond terrestrial particle accelerators.¹² One of the most surprising Planck findings is that the fluctuations in the CMB temperatures at large angular scales do not match those predicted by the standard model.⁶ And dark energy may not be constant: DESI DR2's mild tension with the CMB at 2.3σ is alleviated by a time-evolving dark energy, preferred over the standard picture at up to 4.2σ when supernovae are included.¹⁷

Some things, though, are settled: the big-bang model is the widely held theory of the evolution of the universe.¹ Known matter comprises just five per cent of all the matter and energy contained in the universe; the remaining 95 per cent is hidden from us.¹⁶ Yet that small share of known matter has proved enough to trace the history of the cosmos from the first minutes to the present day.¹⁶

The theory won its place the hard way. A nickname coined in mockery in 1949 became the standard model once the microwave background supplied the missing evidence.¹³ The Nobel Prizes mark the milestones: 1978 for the discovery of the cosmic microwave background¹⁴, 2006 for COBE's blackbody spectrum and its anisotropies¹⁵, and 2019 for James Peebles and the theoretical framework that is the foundation of our modern understanding of the universe's history, from the Big Bang to the present day.¹⁶

Precision cosmology grew out of that framework. WMAP determined the universe to be 13.77 billion years old to within a half percent, nailed down the curvature of space to within 0.4% of flat Euclidean geometry, and reduced the allowed volume of cosmological parameters by a factor in excess of 68,000.⁸ Planck's data set a new value for the Hubble constant — 67.15 kilometres per second per megaparsec, significantly less than the standard value in astronomy at the time.⁶ The mission's first all-sky picture of the oldest light was based on the initial 15.5 months of data, imprinted on the sky when the universe was just 380,000 years old.⁶ In 2018 the final Planck release confirmed the 'almost perfect Universe' while quantifying the most important tension: 73.5 versus 67.4 km/s/Mpc.⁷

So far the standard model of cosmology has survived all the tests it has faced, with Planck supplying the measurements that show it.⁷ At the same time, known matter comprises just five per cent of all the matter and energy in the universe, and the remaining 95 per cent is hidden from us.¹⁶ The dark matter is detected so far only indirectly, through its gravitational influence.⁶ The blueprint, in other words, is far from complete.