Theoretical Physics Colloquium
Preparing for 21 cm Cosmology: Building Accurate Models for Upcoming Data (Theoretical Physics Seminar)
by Dr. Girish Kulkarni (Institute of Astronomy and Kavli Institute.of Cosmology, Cambridge, U.K.)
Wednesday, February 22, 2017 from to (Asia/Kolkata)
Detection of the 21 cm hyperfine line of hydrogen on cosmological scales has the potential to provide us with a three-dimensional tomographic view of the Universe during the first billion years of its existence. This will not only probe an unexplored regime in cosmology, but will also open a new window on the early Universe. A new generation of radio observatories (such as LOFAR, India-supported SKA, HERA) aim to make a 21 cm detection in the coming decade. In this talk, I will discuss the physics of the 21 cm line and present the most accurate models yet for the 21 cm signal from redshifts z = 6 to 10. These models use cosmological simulations of the epoch of reionization with the highest dynamic range currently possible and will help retrieve physics from upcoming 21 cm data. I will show that models consistent with the available Lyman alpha data and CMB measurement of the Thomson optical depth predict typical values of 10--20 mK2 for the variance of the 21 cm brightness temperature at redshifts z=7 -- 10 at scales accessible to ongoing and upcoming experiments (k < 1 h/cMpc). I will then present results from a deep quantitative Bayesian analysis of every quasar observed from redshift 0 to 7.1 and argue that reionization can be dominated by quasars. I will argue that this bodes well for 21 cm experiments quasar-dominated reionization histories increase the variance of the 21 cm emission by a factor of up to ten compared to similar reionization histories dominated by galaxies, to values close to 100 mK2 at scales accessible to experiments. This is lower than the sensitivity claimed to have been already reached by experiments by only a factor of about two or less. I will end by touching upon the relevance of 21 cm cosmology for other outstanding problems such as the nature of dark matter and the very large scale structure of the Universe.