Big Bang

The universe has expanded from a hot, dense state

The Big Bang theory describes how the universe expanded and cooled from a very hot, dense early state. Galaxy motions, radiation still present throughout space, and the abundance of light elements support this account.

The standard cosmological model puts the age of the universe at about 13.8 billion years. The early universe had no stars or galaxies. As it cooled, atoms formed; later, gravity gathered gas into stars and galaxies.

About 13.8 billion years The age inferred from observations and the standard cosmological model
The first few minutes When helium and other light nuclei formed
About 380,000 years later When atoms formed and light could travel long distances
Three main lines of evidence Cosmic expansion, the microwave background, and light-element abundances

Distances between galaxies are increasing

Astronomers measure how much the wavelengths in a distant galaxy’s light have lengthened. This is called redshift. More distant galaxies generally have larger redshifts, consistent with light stretching as the universe expands. This measurement involves more than comparing how red galaxies look.

Tracing expansion backward leads to a universe in which matter was more closely packed and temperatures were higher. Expansion has no particular center. On sufficiently large scales, average distances between galaxies increase whichever galaxy we choose as a reference. Gravitationally bound systems, such as the Solar System or the interior of a galaxy, do not expand in the same way.

Cosmic expansion means that distances between galaxies grow.

Comparing the Big Bang to an explosion can suggest matter flying outward from one location. The theory describes the expansion and changing temperature of the universe as a whole.

Light from the early universe is still observable

In the early universe, light scattered frequently off free electrons and could not travel far. About 380,000 years after the Big Bang, cooling allowed electrons to join nuclei and form atoms. Light that could then travel long distances is observed today as the cosmic microwave background (CMB). Expansion has lengthened its wavelengths into the microwave range.

COBE measured the spectrum of this radiation. It closely matches the distribution of light emitted in thermal equilibrium at about 2.725 K. WMAP and Planck also measured small temperature differences across the sky. These differences carry information about the distribution of matter before galaxies formed. The CMB shows the universe about 380,000 years after the Big Bang.

First few minutes: protons and neutrons combine to form helium and other light nuclei.
About 380,000 years later: electrons join nuclei, and light begins to travel long distances.
Over the following hundreds of millions of years: gravity gathers gas into the first stars and galaxies.
About 13.8 billion years later, today: we observe early radiation as microwaves in an expanded universe.

Helium abundance tests our account of the early universe

A single proton is already the nucleus of the most common form of hydrogen. In the first few minutes, protons and neutrons combined to form helium and other nuclei, a process called Big Bang nucleosynthesis. Calculations predict that ordinary matter was then roughly 75% hydrogen and 25% helium by mass. This agrees well with observations of matter that has undergone little processing in stars.

Deuterium abundance provides another important test. Deuterium is hydrogen whose nucleus contains one proton and one neutron. Lithium-7 remains a problem: calculated amounts do not agree with measurements in old stars. Heavier elements such as carbon and oxygen were produced mainly by nuclear reactions inside later generations of stars.

The origin of the universe remains unknown

Expansion, cooling, and the formation of stars and galaxies eventually led to the Milky Way and the Solar System. The Big Bang theory explains this long evolution, but does not establish why the universe came to exist.

Inflation proposes an extremely brief period of rapid expansion very early on. It helps explain why the universe looks uniform on large scales, but its physical cause has not been established. The expansion rate measured using nearby objects also differs from the value inferred by applying a model to the CMB. Researchers are investigating whether measurement errors or changes to the theory explain the difference.

A singularity is where quantities such as density become infinite in a calculation, reaching the limits of the theory. Singularities in black holes and in calculations tracing the Big Bang backward both raise questions about the limits of current theories. A singularity in an equation does not establish that the real universe began as an infinitely small point. There is also no settled answer to what, if anything, preceded the Big Bang.