DR. GIRISH KULKARNI

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, the speaker will discuss attempts at revealing the complete thermal evolution of the Universe. He 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. He will show how classical descriptions of this small-scale strucuture fail at redshifts z = 2--5, and after presenting a generalised theoretical characterisation that is valid at these redshifts, he 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. He 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, he will discuss his work that has produced the highest dynamic range cosmological simulations of the 21 cm signal from reionization. Finally, the speaker will discuss attempts at tracking down the cause of reionization by focussing on the possible role played by accreting supermassive black holes.