### Citation:

### Date Published:

September 1, 200### Abstract:

We explore the requirements for a Lyman-α forest survey designedto measure the angular diameter distance and Hubble parameter at2≲z≲4 using the standard ruler provided by baryonic acousticoscillations (BAO). The goal would be to obtain a high enough density ofsources to probe the three-dimensional density field on the scale of theBAO feature. A percent-level measurement in this redshift range canalmost double the Dark Energy Task Force figure of merit, relative tothe case with only a similar precision measurement at z˜1, if theUniverse is not assumed to be flat. This improvement is greater than theone obtained by doubling the size of the z˜1 survey, with Planckand a weak Sloan Digital Sky Survey-like z=0.3 BAO measurement assumedin each case. Galaxy BAO surveys at z˜1 may be able to make aneffective Lyα forest measurement simultaneously at minimal addedcost, because the required number density of quasars is relativelysmall. We discuss the constraining power as a function of area,magnitude limit (density of quasars), resolution, and signal-to-noise ofthe spectra. For example, a survey covering 2000 sq. deg. and achievingS/N=1.8 per Å at g=23 (˜40quasars per sq. deg.) with anR≳250 spectrograph is sufficient to measure both the radial and