The History of the Universe

This chapter charts the history of the Universe from the Big Bang to the present day. As the Universe has aged, so its temperature has fallen, the energy per particle has dropped, and the four fundamental interactions have successively become distinct. In the earliest moments of the Universe, superunification was in evidence up until the Planck time, when the gravitational interaction became distinct. Just after the strong and electroweak interactions became distinct, the Universe underwent a period of accelerated expansion, known as inflation, which smoothed out any irregularities in the Universe. In the quark-lepton era, massive quarks and leptons decayed, but there was a slight imbalance of matter over antimatter, leading to 10 billion photons per baryon after annihilation. In the subsequent hadron era, protons and neutrons formed, and free neutrons subsequently decayed. Electron–positron annihilation occurred when the Universe was about 10 seconds old. Primordial nucleosynthesis occurred in the first few minutes, forming helium and lithium nuclei. When the Universe had cooled to about 3000 K about 300,000 years after the Big Bang, background photons interacted for the last time with neutral atoms in the Universe. This radiation, redshifted by a factor of a thousand, is seen today as the cosmic microwave background. Galaxies subsequently formed via a bottom-up process of mergers, seeded by irregularities in the matter density at this time.