HIRAX: A Probe of Dark Energy and Radio Transients
  The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a new
400-800MHz radio interferometer under development for deployment in South
Africa. HIRAX will comprise 1024 six meter parabolic dishes on a compact grid
and will map most of the southern sky over the course of four years. HIRAX has
two primary science goals: to constrain Dark Energy and measure structure at
high redshift, and to study radio transients and pulsars. HIRAX will observe
unresolved sources of neutral hydrogen via their redshifted 21-cm emission line
(`hydrogen intensity mapping'). The resulting maps of large-scale structure at
redshifts 0.8-2.5 will be used to measure Baryon Acoustic Oscillations (BAO).
HIRAX will improve upon current BAO measurements from galaxy surveys by
observing a larger cosmological volume (larger in both survey area and redshift
range) and by measuring BAO at higher redshift when the expansion of the
universe transitioned to Dark Energy domination. HIRAX will complement CHIME, a
hydrogen intensity mapping experiment in the Northern Hemisphere, by completing
the sky coverage in the same redshift range. HIRAX's location in the Southern
Hemisphere also allows a variety of cross-correlation measurements with
large-scale structure surveys at many wavelengths. Daily maps of a few thousand
square degrees of the Southern Hemisphere, encompassing much of the Milky Way
galaxy, will also open new opportunities for discovering and monitoring radio
transients. The HIRAX correlator will have the ability to rapidly and
eXperimentciently detect transient events. This new data will shed light on the
poorly understood nature of fast radio bursts (FRBs), enable pulsar monitoring
to enhance long-wavelength gravitational wave searches, and provide a rich data
set for new radio transient phenomena searches. This paper discusses the HIRAX
instrument, science goals, and current status.
