The study of cosmology at the time very soon after the big bang. Theories of the early universe have led to a mutually beneficial interaction between cosmology and the theory of elementary particles, particularly grand unified theories.

Because there were very high temperatures in the early universe many of the broken symmetries in gauge theories were unbroken symmetries at these temperatures. As the universe cools after the big bang there is thought to be a sequence of transitions to broken symmetry states.

Combining cosmology with grand unified theories helps to explain why the observed universe appears to consist of matter with no antimatter (see antiparticle). This means that one has a nonzero baryon number for the universe. This solution relies on the fact that there were nonequilibrium conditions in the early universe due to its rapid expansion after the big bang.

An important idea in the theory of the early universe is that of inflation—the idea that the nature of the vacuum state gave rise, after the big bang, to an exponential expansion of the universe. The hypothesis of the inflationary universe solves several long-standing problems in cosmology, such as the flatness and homogeneity of the universe. In particular, it is thought that quantum fluctuations in the early universe were responsible for the emergence of large-scale structure in the universe, such as galaxies. The early universe is still a topic of great interest, particularly because precise measurements of cosmological quantities provide severe challenges to specific theories of elementary particles that attempt to calculate these quantities. See also ekpyrotic universe.