  The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a radio
interferometer array currently in development, with an initial 256-element
array to be deployed at the South African Radio Astronomy Observatory (SARAO)
Square Kilometer Array (SKA) site in South Africa. Each of the 6m, $f/0.23$
dishes will be instrumented with dual-polarisation feeds operating over a
frequency range of 400-800 MHz. Through intensity mapping of the 21 cm emission
line of neutral hydrogen, HIRAX will provide a cosmological survey of the
distribution of large-scale structure over the redshift range of $0.775 < z <
2.55$ over $\sim$15,000 square degrees of the southern sky. The statistical
power of such a survey is sufficient to produce $\sim$7 percent constraints on
the dark energy equation of state parameter when combined with measurements
from the Planck satellite. Additionally, HIRAX will provide a highly
competitive platform for radio transient and HI absorber science while enabling
a multitude of cross-correlation studies. In this paper, we describe the
science goals of the experiment, overview of the design and status of the
sub-components of the telescope system, and describe the expected performance
of the initial 256-element array as well as the planned future expansion to the
final, 1024-element array.
