The Role of Quantum Mechanics in Modern Cosmology

I will briefly review, in simple terms, the crucial role of quantum mechanics in modern inflationary cosmology. Quantum processes throughout the inflationary epoch determine the subsequent evolution of our Universe, including the formation of its large-scale structures (galaxies, clusters, voids, ...). In the inflationary scenario the role of the hot big bang of pre-inflationary cosmology is completely overturned: it is played by the instant at which the Universe, after having been cooled down to practically zero temperature, suddenly “reheats” through an irreversible quantum process. Therefore, an unavoidable consequence of inflation is that the hot big bang is neither associated with a singularity nor with the beginning of time. At the end I will offer some more speculative ideas, mostly based on string theory, on what may have preceded inflation and about whether time had a beginning.