  We study the application of AdS/CFT duality to longitudinal boost invariant
Bjorken expansion of QCD matter produced in ultrarelativistic heavy ion
collisions. As the exact (1+4)-dimensional bulk solutions for the
(1+3)-dimensional boundary theory are not known, we investigate in detail the
(1+1)-dimensional boundary theory, where the bulk is AdS_3 gravity. We find an
exact bulk solution, show that this solution describes part of the spinless
Banados-Teitelboim-Zanelli (BTZ) black hole with the angular dimension
unwrapped, and use the thermodynamics of the BTZ hole to recover the
time-dependent temperature and entropy density on the boundary. After
separating from the holographic energy-momentum tensor a vacuum contribution,
given by the extremal black hole limit in the bulk, we find that the boundary
fluid is an ideal gas in local thermal equilibrium. Including angular momentum
in the bulk gives a boundary flow that is boost invariant but has a nonzero
longitudinal velocity with respect to the Bjorken expansion.
