  In the AdS/CFT description of four-dimensional QCD matter undergoing Bjorken
expansion, does the holographic energy-momentum tensor contain a Casimir-type
contribution that should not be attributed to thermal matter? When the bulk
isometry ansatz that yielded such a Casimir term for (1+1)-dimensional boundary
matter is generalised to a four-dimensional boundary, we show that a Casimir
term does not arise, owing to singularities in the five-dimensional bulk
solution. The geometric reasons are traced to a difference between the
isometries of AdS_3 and AdS_{d+1} for d>=3.
