Hydrogen is the most abundant element in the universe and as such, mapping the evolution of neutral hydrogen (hereafter, HI) across cosmic time promises deep insights into galaxy evolution, as well as theories of gravity and fundamental physics.
There are several ongoing and planned measurements which will give us clues to the distribution and evolution of HI over the last 12 billion years (redshifts 0 to 5). Prominent among these are 21-cm emission line experiments probing the local universe and UV/optical observations of Damped Lyman Alpha systems (DLAs) at early times. The 21-cm line emission of HI is expected to act as a good tracer of the underlying dark matter distribution, due to the absence of the complicated reionization astrophysics. Traditionally, galaxy surveys have been used as probes of the neutral hydrogen distribution at late times. The limits of current radio facilities, however, hamper the detection of 21-cm in emission from normal galaxies at early times.
At intervening times, a promising technique to map out the evolution of hydrogen (and in general, several baryonic species) across cosmic time is called intensity mapping. In this technique, the unresolved emission of the tracer is mapped without the need to separate individual systems hosting the tracer. Being faster and less expensive than galaxy surveys, intensity mapping has been shown to have the potential to provide unprecedented constraints on cosmological parameters.
Thus, the three main probes of HI over the last 12 billion years are galaxy emission surveys, intensity mapping experiments and DLA observations. Although there have been several studies that model these observations separately, there were no detailed attempts to unify the data into a self-consistent framework that addresses both high- and low-redshift observations. Such a framework, which encapsulates the astrophysical information into a set of key physical parameters, is crucial to properly take into account the astrophysical uncertainties in constraining models of Fundamental Physics and cosmology.
I have been leading a research program aimed at filling this gap.
I have developed a data-driven halo model framework for the distribution and evolution of HI over the past 12 billion years. This built upon my previous work analyzing the different datasets available and bringing together the models available in the literature.
In a map of unresolved HI emission over a large area in the sky, the main quantity of interest is known as the power spectrum, which is sensitive to both the underlying cosmology as well as the astrophysics of HI in the galaxies. Incorporating all the available data into a consistent framework, as I did for the first time, allows us to precisely separate both these aspects and make predictions for future surveys, especially the Square Kilometre Array (SKA). Being highly versatile, this framework is being extensively used to model and interpret 21 cm observations. My novel approach allows us to construct large statistical samples of mock galaxies, include the effects of many models, and impose the most realistic priors conveniently within a Fisher matrix analysis.
I gave an invited talk on some of these ideas and future prospects at the IAU symposium 333 held at Dubrovnik, Croatia in October 2017.
I am a member of the SKA Cosmology Science Working Group. Recently, I co-led the SKA white paper on Fundamental Physics with the Square Kilometre Array, which examines how the SKA will deliver unprecedented constraints that can transform our understanding of theoretical physics. A summary of the tremendous potential of the SKA to provide unique tests of cosmology is provided in The SKA Cosmology Red Book, 2018.My halo model framework for HI is available as part of the public code Axion21cmIM developed by Jurek Bauer, who led a study demonstrating the potential of the SKA to constrain models of ultralight axion dark matter.
I am a member of the Cosmic Visions 21 cm Collaboration, proposing a revolutionary experiment over the next decade, which aims to map out the three-dimensional integrated intensity of the redshifted 21 cm line of neutral hydrogen. As a specific example of such an experiment, the Packed Ultra-wideband Mapping Array (PUMA) concept will enable stringent constraints on cosmology and theories of gravity, dark energy and the transient radio sky.
Recently, the MeerKAT collaboration released its first ever autocorrelation results of the 21 cm signal of neutral hydrogen, covering redshifts 0.32 and 0.44. These results extend the previous analysis down to an order of magnitude smaller scales, thanks to the power of interferometry. In our new paper, we describe the analytical framework to address small-scale effects in the HI power spectrum, and provide comparisons to the MeerKAT results. A recent talk summarizing this at the PUMA meeting is available here: PDF, Keynote, video recording