What is the Big Bang theory? How the universe began

The Big Bang theory is the scientific account of how the universe has changed over time: it was once extremely hot and dense, and it has been expanding and cooling for about 13.8 billion years. It was not an explosion of matter into empty space. Space itself has been stretching everywhere at once, carrying galaxies apart. (This is about the cosmology, not the TV comedy named after it.)

In short

• The universe began in a hot, dense state about 13.8 billion years ago and has been expanding ever since; there is no center it expanded from.
• Three main lines of evidence support it: galaxies moving apart, the leftover glow called the cosmic microwave background, and the amounts of hydrogen and helium in the universe.
• The theory describes what happened after the first instant. What came before, if "before" means anything, is unknown.

What does the Big Bang theory actually say?

The theory says the universe has been expanding from a hotter, denser state, not that it burst out from a point. Rewind the expansion and everything gets closer, hotter and denser. Run it forward and you get what happened next, in an order physicists can test:

• The first millionths of a second: the universe is a soup of particles too hot for anything to hold together. A few millionths of a second in, according to CERN, quarks join into protons and neutrons.
• The first three minutes: protons and neutrons fuse into the nuclei of the lightest elements, mostly helium, with traces of deuterium and lithium.
• About 380,000 years: the universe cools to a few thousand degrees, cool enough for electrons to settle around nuclei and form atoms. Light can now travel freely. That light is still arriving today.
• Within the first few hundred million years: gas clumps into the first stars, and galaxies, including the ancestors of the Milky Way, begin to form.

A common misconception pictures the Big Bang as a bomb going off at one spot. There is no spot. Every distant galaxy sees every other galaxy moving away from it, the way every raisin in a rising loaf moves away from every other raisin. The expansion happens everywhere at once, so there is no center and no edge it is expanding into.

What is the evidence for the Big Bang?

Galaxies are moving apart

In the 1920s, Georges Lemaître worked out from Einstein's general relativity that the universe could be expanding, and Edwin Hubble measured it in 1929: the farther away a galaxy is, the faster it recedes. Its light is stretched toward red, called redshift, in proportion to its distance. The rate is called the Hubble constant.

The universe's leftover glow

The light released at 380,000 years has been stretched by expansion into microwaves. In 1964 Arno Penzias and Robert Wilson found it by accident as a hiss their radio antenna couldn't get rid of, and in 1978 they shared the Nobel Prize in Physics for discovering this cosmic microwave background. It has a temperature of about 2.7 degrees above absolute zero, almost the same in every direction. The European Space Agency's Planck satellite mapped its tiny ripples, and its 2013 results fit the standard Big Bang model closely, with a few unexplained oddities.

The right amount of helium

Only a hot, dense early universe would have cooked hydrogen into helium in the right proportion, and astronomers measure about a quarter of all ordinary matter's mass as helium, matching the prediction.

How do you get 13.8 billion years and 25% helium?

Both headline numbers can be checked with back-of-the-envelope sums.

Worked example 1: the age from the expansion rate

If galaxies have always moved apart at the same speed, then running time backward, they were all together at a time equal to distance divided by speed. With Hubble's law, that is simply 1 divided by the Hubble constant.

• Take a Hubble constant of 70 kilometers per second per megaparsec. A megaparsec is about 3.26 million light-years, or 3.086 × 10¹⁹ kilometers.
• A galaxy 100 megaparsecs away recedes at 100 × 70 = 7,000 kilometers per second.
• Time to cover that distance at that speed: 3.086 × 10²¹ km ÷ 7,000 km/s = about 4.4 × 10¹⁷ seconds.
• Divide by the 31.6 million seconds in a year: about 14 billion years.

The careful answer, from the Planck collaboration's 2018 analysis, is about 13.8 billion years. The rough sum comes close because the expansion first slowed and then sped up, and the two effects nearly cancel.

Worked example 2: why a quarter of the matter is helium

When nuclei began to form, about three minutes in, there was roughly 1 neutron for every 7 protons, according to the Particle Data Group's review of Big Bang nucleosynthesis.

• Take 16 particles: 2 neutrons and 14 protons.
• A helium-4 nucleus is 2 neutrons and 2 protons. Almost every neutron ends up in helium, so the 2 neutrons make 1 helium nucleus, using 2 protons.
• That leaves 12 protons as hydrogen nuclei.
• Helium's share of the mass: 4 of the 16 units, or 25%.

The observed figure is about 24.5%. A universe that had never been hot and dense would have no reason to have this much helium everywhere.

What came before the Big Bang?

Nobody knows, and it may not be a meaningful question. The theory works back to a tiny fraction of a second after the start. Before that, the known laws of physics, general relativity and quantum mechanics, give contradictory answers, and a theory that unites them does not exist yet. Running the equations all the way back gives a point of infinite density, a singularity, which most physicists read as a sign the equations have broken down, not as a real thing. The same breakdown appears at the center of a black hole.

Still argued

Cosmic inflation, the idea that the universe grew enormously in a tiny fraction of its first second, explains why the cosmic microwave background is so even, and many cosmologists accept it, but the gravitational-wave signal it predicts has not been detected, and some physicists question whether it can be tested at all. The expansion rate is also in dispute: Planck's data imply about 67.4 kilometers per second per megaparsec, while measurements of nearby exploding stars by Adam Riess's team in 2022 give about 73. This gap, the Hubble tension, may be an error in one method or a sign of new physics. Neither dispute threatens the core idea that the universe was once hot and dense and has been expanding.

Where does the Big Bang leave us?

The early universe was extremely smooth and orderly, and everything since, from galaxies to stars to life, has formed as it spread out and cooled. That low starting disorder is why time seems to run one way, a connection that the note on entropy takes further. The Big Bang is the starting point for the story told in what the universe is: how big it is, what it is made of, and how it may end.

Check yourself

• Without looking back: why is the Big Bang not an explosion from a single point in space?
• Name the three main lines of evidence for the Big Bang and say what each one shows.
• Using the helium sum, explain why 1 neutron for every 7 protons leads to about 25% helium.

Sources

• Planck 2018 results. VI. Cosmological parameters — Planck Collaboration, Astronomy and Astrophysics, 2020
• Planck reveals an almost perfect universe — European Space Agency, 2013
• The early universe — CERN
• Big Bang nucleosynthesis — Particle Data Group, 2024
• The Nobel Prize in Physics 1978 — Nobel Prize Outreach
• A comprehensive measurement of the local value of the Hubble constant — Riess and others, The Astrophysical Journal Letters, 2022