About 13.8 billion years ago, everything in the observable universe, all the matter and energy that would become stars, planets, galaxies and ourselves, was packed into an incredibly hot, dense state. Since then, the universe has been expanding and cooling. This is the core idea of the Big Bang theory, the scientific explanation for the origin and evolution of the universe. Despite its name, the Big Bang was not an explosion in space like a bomb going off. It was the rapid expansion of space itself. This explainer describes what the Big Bang theory says, what happened in the early universe and the evidence that supports it.
What the Big Bang theory says
The Big Bang theory describes how the universe evolved from an extremely hot, dense early state to the vast, cool cosmos we see today. Its main points are:
- The universe was once much hotter and denser than it is now.
- Space has been expanding ever since, carrying galaxies farther apart.
- As the universe expanded, it cooled, allowing particles, atoms, stars and galaxies to form.
A common misconception is that the Big Bang happened at a single point in space, from which everything flew outward. In fact, the Big Bang happened everywhere at once. Every point in the universe was part of that hot, dense early state, and space itself has been stretching in all directions since.
The Big Bang theory describes what happened after the universe's earliest moments. It does not, by itself, explain what caused the Big Bang or what, if anything, came before. Those questions remain at the frontier of physics.
The expanding universe
The idea of an expanding universe emerged in the 1920s.
- In 1922, the Russian mathematician Alexander Friedmann found solutions to Einstein's equations of general relativity describing an expanding universe.
- In 1927, the Belgian physicist and priest Georges Lemaître independently proposed that the universe is expanding and suggested that it began from a "primeval atom."
- In 1929, the American astronomer Edwin Hubble showed that distant galaxies are moving away from us, and that the farther away they are, the faster they recede. This relationship is now called the Hubble-Lemaître law.
If galaxies are moving apart today, they must have been closer together in the past. Rewinding the expansion leads back to a time when everything was extremely close together and hot.
The name "Big Bang" was coined in 1949 by the British astronomer Fred Hoyle, who supported a rival idea called the steady-state theory, which held that the universe has always existed in roughly the same state.
A timeline of the early universe
Physicists can describe the history of the universe from a tiny fraction of a second after its beginning.
| Time after the Big Bang | Event |
|---|---|
| tiny fraction of a second | possible period of cosmic inflation, rapid exponential expansion |
| about 1 microsecond | quarks combine into protons and neutrons |
| about 1 second | neutrinos decouple; universe dominated by radiation |
| about 3 to 20 minutes | Big Bang nucleosynthesis forms helium and traces of other light elements |
| about 380,000 years | atoms form; light travels freely, creating the cosmic microwave background |
| first few hundred million years | first stars and galaxies form, ending the cosmic "dark ages" |
| about 9 billion years | the Sun and solar system form, about 4.6 billion years ago |
| 13.8 billion years | today |
Cosmic inflation
Many cosmologists think that, in the first tiny fraction of a second, the universe underwent a brief period of extremely rapid expansion called inflation. Inflation would explain why the universe looks so uniform on large scales and why space appears geometrically flat. It would also have stretched tiny quantum fluctuations into the seeds of later structure, such as galaxies. Inflation is supported by some observations but is not yet confirmed, and scientists are still testing it.
The first particles and elements
As the universe expanded and cooled, fundamental particles called quarks combined into protons and neutrons. Within a few minutes, protons and neutrons fused to form the nuclei of light elements, a process called Big Bang nucleosynthesis. The universe ended up with about 75 percent hydrogen and 25 percent helium by mass, with tiny traces of deuterium and lithium. Almost all heavier elements were made much later in stars.
The first light
For about 380,000 years, the universe was so hot that atoms could not form. It was filled with a hot, glowing fog of charged particles, and light could not travel far before bouncing off free electrons. When the universe cooled to about 3,000 kelvins, electrons combined with nuclei to form neutral atoms. Light could finally travel freely. This event is called recombination, and the light released then still fills the universe today.
Stars and galaxies
After recombination came a long period with no stars, called the cosmic dark ages. Gravity slowly pulled matter together into denser regions. Within the first few hundred million years, the first stars ignited, and galaxies began to form and merge. The James Webb Space Telescope has observed galaxies that existed only a few hundred million years after the Big Bang.
The evidence for the Big Bang
The Big Bang theory is supported by several independent lines of evidence.
1. The expansion of the universe
Light from distant galaxies is redshifted: its wavelengths are stretched toward the red end of the spectrum because the space it travels through is expanding. The more distant the galaxy, the greater the redshift. This is exactly what an expanding universe predicts.
2. The cosmic microwave background
In 1964, the American radio astronomers Arno Penzias and Robert Wilson detected a faint, steady microwave signal coming from every direction in the sky. They had discovered the cosmic microwave background (CMB), the afterglow of the hot early universe released at recombination, now stretched by expansion into microwaves with a temperature of about 2.7 kelvins, just under −270 °C. They received the 1978 Nobel Prize in Physics.
Satellites such as COBE, WMAP and Planck have mapped the CMB in great detail. It is remarkably uniform, with tiny temperature variations of about one part in 100,000. These small fluctuations match the predictions of Big Bang models and reveal the seeds of later galaxies.
3. The abundance of light elements
The Big Bang theory predicts the amounts of hydrogen, helium, deuterium and lithium produced in the first minutes. Measurements of these elements in the oldest stars and gas clouds closely match the predictions, especially for helium and deuterium.
4. The evolution of galaxies
Because light takes time to travel, looking at distant galaxies means looking back in time. Distant galaxies look younger and different from nearby galaxies, consistent with a universe that has changed over time rather than one that has always looked the same.
Dark matter and dark energy
Observations show that ordinary matter, the atoms that make up stars, planets and people, makes up only about 5 percent of the universe's total energy content. About 27 percent is dark matter, an invisible form of matter detected by its gravitational effects on galaxies and the CMB. About 68 percent is dark energy, a mysterious form of energy causing the expansion of the universe to accelerate, discovered in 1998 through observations of distant supernovae. Understanding dark matter and dark energy is one of the great open challenges in cosmology.
Open questions
The Big Bang theory is extremely successful, but important questions remain:
- What happened at the very beginning? Our current theories break down at the earliest moments, where a theory of quantum gravity is needed.
- Did inflation really happen, and what caused it?
- What are dark matter and dark energy?
- Why is there more matter than antimatter? The Big Bang should have produced equal amounts, yet the universe is made almost entirely of matter.
- How fast is the universe expanding? Different measurement methods give slightly different values for the expansion rate, a puzzle called the "Hubble tension."
Questions readers ask
What was the Big Bang? The rapid expansion of the universe from an extremely hot, dense state about 13.8 billion years ago, which continues today.
Was the Big Bang an explosion? Not in the usual sense. It was an expansion of space itself, happening everywhere at once.
How do we know the Big Bang happened? From the expansion of the universe, the cosmic microwave background, the abundance of light elements and the changing appearance of galaxies over time.
What came before the Big Bang? Scientists do not yet know. Current physics cannot describe the very earliest moments.
How old is the universe? About 13.8 billion years, based on measurements of the cosmic microwave background and the expansion of the universe.
In short
The Big Bang theory describes how the universe expanded and cooled from an extremely hot, dense state about 13.8 billion years ago. In the first minutes, light elements formed; after 380,000 years, atoms formed and released the light we now see as the cosmic microwave background; later, stars and galaxies emerged. Redshifted galaxies, the microwave background, the abundance of light elements and the evolution of galaxies all support the theory. Questions remain about the earliest moments, dark matter and dark energy, but the Big Bang is our best scientific account of the history of the cosmos.