The Big Bang theory is our best scientific explanation for how the universe began. About 13.8 billion years ago, all matter and energy in the universe was concentrated in an extremely hot, dense point called a singularity. From this singularity, space itself began expanding rapidly. Key evidence supporting this theory includes the cosmic microwave background radiation, which is the afterglow of the Big Bang, Hubble's observations showing that distant galaxies are moving away from us, and the observed abundance of light elements like hydrogen and helium throughout the universe.
The first moments after the Big Bang were characterized by extreme conditions and rapid changes. During the Planck epoch, lasting until 10 to the minus 43 seconds, the fundamental forces were unified and quantum effects dominated spacetime itself. The inflation period, from 10 to the minus 36 to 10 to the minus 32 seconds, saw the universe expand exponentially, smoothing out irregularities. Nucleosynthesis occurred between 3 and 20 minutes, when temperatures dropped enough for protons and neutrons to form the first atomic nuclei. Finally, recombination happened around 380,000 years later, when electrons combined with nuclei to form the first neutral atoms, making the universe transparent to light.
The formation of cosmic structures began with tiny quantum fluctuations in the early universe. These small density variations were amplified by cosmic expansion. Dark matter, which doesn't interact electromagnetically, formed the scaffolding of the universe by creating gravitational wells. Ordinary matter, primarily hydrogen and helium gas, was drawn into these dark matter halos. As gas accumulated and cooled, it reached densities high enough for nuclear fusion to begin, forming the first stars. This process continued hierarchically, with smaller structures merging to form larger ones, eventually creating the galaxies and galaxy clusters we observe today.
The Big Bang theory is supported by multiple independent lines of evidence that all point to the same conclusion. The cosmic microwave background radiation shows the predicted temperature fluctuations from the early universe. Type Ia supernovae observations reveal that distant galaxies are not only moving away from us, but accelerating, indicating dark energy. Primordial nucleosynthesis predictions match the observed abundance of light elements like hydrogen and helium throughout the universe. Hubble's law demonstrates that galaxy recession velocities increase with distance, exactly as expected from uniform expansion. The remarkable convergence of these diverse observations provides overwhelming support for the Big Bang model.