  This work describes a new instrument optimized for a detection of the neutral
hydrogen 21cm power spectrum between redshifts of 0.5-1.5: the Baryon Acoustic
Oscillation Broadband and Broad-beam (BAOBAB) Array. BAOBAB will build on the
efforts of a first generation of 21cm experiments which are targeting a
detection of the signal from the Epoch of Reionization at z ~ 10. At z ~ 1, the
emission from neutral hydrogen in self-shielded overdense halos also presents
an accessible signal, since the dominant, synchrotron foreground emission is
considerably fainter than at redshift 10. The principle science driver for
these observations are Baryon Acoustic Oscillations in the matter power
spectrum which have the potential to act as a standard ruler and constrain the
nature of dark energy. BAOBAB will fully correlate dual-polarization antenna
tiles over the 600-900MHz band with a frequency resolution of 300 kHz and a
system temperature of 50K. The number of antennas will grow in staged
deployments, and reconfigurations of the array will allow for both traditional
imaging and high power spectrum sensitivity operations. We present calculations
of the power spectrum sensitivity for various array sizes, with a 35-element
array measuring the cosmic neutral hydrogen fraction as a function of redshift,
and a 132-element system detecting the BAO features in the power spectrum,
yielding a 1.8% error on the z ~ 1 distance scale, and, in turn, significant
improvements to constraints on the dark energy equation of state over an
unprecedented range of redshifts from ~0.5-1.5.
