Revealing 13 Billion Years of Thermal Evolution of the Universe

The ordinary matter content of the Universe is ten thousand times hotter today than naive expectations from the otherwise well-established Lambda-CDM cosmological model.  It is thought that this is because of an epoch of reionization and reheating at around redshift z ~ 6 to 10 when the Universe was about a Gyr old. Understanding when reionization took place and what caused it is one of the most important open problems in cosmology today, and has relevance for a wider array of other problems in physics.  In this talk, I will discuss attempts at revealing the complete thermal evolution of the Universe.  I will begin by showing that, contrary to the classical understanding, the post-reionization Universe does hold clues about reionization: the small-scale structure of the intergalactic medium (IGM) preserves a record of the cosmological thermal history.  I will show how classical descriptions of this small-scale structure fail at redshifts z = 2--5, and after presenting a generalised theoretical characterisation that is valid at these redshifts, I will present the first measurement of this structure at redshifts 2 < z < 3 based on an unprecedented data-set of hundreds of close quasar pair spectra.  I will then introduce the technique of 21 cm cosmology, which in the upcoming decades aims to use large radio telescopes, such as the India-supported SKA, to provide a complete three-dimensional tomographic map of the as-yet-unexplored epoch of reionization.  In this direction, I will discuss my work that has produced the highest dynamic range cosmological simulations of the 21 cm signal from reionization.  Finally, I will discuss attempts at tracking down the cause of reionization by focussing on the possible role played by accreting supermassive black holes.