The “Billion Galaxy” Cosmological HI Large Deep Survey (BiG-CHILDS)

We outline the case for a comprehensive wide and deep survey ultimately targeted at obtaining 21-cm HI line emission spectroscopic observations of more than a billion galaxies to redshift z > 1.5 over half the sky. This survey provides a database of galaxy redshifts, HI gas masses, and galaxy rotation curves that would enable a wide range of science, including fundamental cosmology and studies of Dark Energy. This science requires the next generation of radio arrays, which are being designed under the umbrella of the “Square Kilometer Array” (SKA) project. We present a science roadmap, extending to 2020 and beyond, that would enable this ambitious survey. We also place this survey in the context of other multi-wavelength surveys. The first decade of the 21st century has seen cosmology build upon the successes of the pioneering cosmic microwave background and galaxy surveys and build a cohesive “standard model” for the Universe. In this model, Dark Energy and Dark Matter are the primary constituents, and their densities and properties govern the evolution of the expansion rate (Hubble constant) and the growth of structure in the cosmic web. As we head into the next decade, the focus is on probing the physics of the components of the model and testing the limits of our understanding. Is dark energy a quintessence fluid or is it a failure of general relativity? Did inflation really set up the initial conditions? What is the mass of the neutrino? Current data is insufficient to answer fundamental questions such as these, but next-generation cosmological surveys across the observable electromagnetic spectrum are the necessary next steps in this enterprise as set forth in nationally recognized documents such as [1] and [2]. This Science White Paper focuses on the exploitation of the 21-cm wavelength HI line emission from gas in galaxies as a probe of the Universe from the present epoch back to a redshift z > 1.5 through a “billion galaxy” Cosmological HI Large Deep Survey (BiG-CHILDS) as a key science project for future radio astronomical facilities. 1 Science with a Billion Galaxy HI Survey To enable the precise and accurate determination of the fundamental cosmological parameters, observations must cover a large volume of the Universe (to defeat sample variance). A number of complementary approaches have been proposed to meet this challenge. For example, the exploitation of the Baryon Acoustic Oscillation (BAO) signature in the galaxy angular power spectrum is a particularly promising line of attack for dark energy studies (e.g. [3, 2]). Other approaches include the measurement of weak gravitational lensing distortions in galaxy shapes, the growth rate of structure in the Universe, and the use of standard candles (SNe) and rulers (maser disks) in the determination of the distance scale. Big Questions + Compelling Opportunities: A CHILD Survey would enable astronomical observations to address key questions in cosmology such as — How does rate of cosmic expansion evolve, and what physical phenomena control this expansion? What is Dark Matter? What is the nature of Dark Energy? What are the masses of the neutrinos, and how have they shaped the evolution of the Universe? A suite