The Big Bang theory says the universe began about 13.8 billion years ago in an extremely hot, dense state and has been expanding and cooling ever since. It does not describe an explosion into empty space: space itself is expanding. Several independent lines of evidence support it.
1. The universe is expanding
In the late 1920s, Georges Lemaître and Edwin Hubble showed that distant galaxies are moving away from us, and the farther they are, the faster they recede. Their light is stretched to longer, redder wavelengths, a redshift. Running the expansion backward points to a time when everything was packed together, see how spectroscopy measures redshift.
2. The cosmic microwave background
If the early universe was hot, its leftover glow should fill space today, cooled to a few degrees above absolute zero. In 1965, Arno Penzias and Robert Wilson accidentally detected this faint microwave radiation coming from every direction. Satellites such as COBE, WMAP and Planck later measured its temperature, about 2.7 K, and its tiny variations with great precision. These variations are the seeds of later galaxies.
3. The abundance of light elements
In the first few minutes, the universe was hot enough to fuse protons and neutrons into the lightest nuclei. The theory predicts that roughly a quarter of ordinary matter by mass should be helium, with small amounts of deuterium and lithium. Observations of the oldest stars and pristine gas clouds match these predictions closely.
4. Galaxies evolve over cosmic time
Because light takes time to travel, distant galaxies are seen as they were long ago. They look younger, smaller and more irregular, and quasars were more common in the past. A universe that has always been the same, as the old “steady state” theory proposed, would not show such changes, see how galaxies form and evolve.
5. Large-scale structure
The distribution of galaxies in a cosmic web of filaments and voids matches simulations that start from the small fluctuations seen in the microwave background, with dark matter included, see dark matter.
What the theory does not explain
The Big Bang model describes how the universe evolved from a fraction of a second after the beginning. What happened at time zero, or “before”, remains unknown. Open puzzles include the nature of dark matter and dark energy and the “Hubble tension”, a disagreement between methods measuring the expansion rate. See how astronomers know the age of the universe and the future of the universe.
