Casimir energy in the gauge/gravity description of Bjorken flow?

TL;DR

This paper investigates whether a Casimir energy contribution appears in the holographic energy-momentum tensor during Bjorken flow in AdS/CFT, finding that such a term does not arise in four dimensions due to geometric singularities.

Contribution

It extends previous (1+1)-dimensional results to four dimensions, showing that the Casimir term is absent in the holographic description of Bjorken flow.

Findings

No Casimir term in four-dimensional holographic energy-momentum tensor.

Singularities in the bulk prevent the emergence of the Casimir contribution.

Differences in isometry groups explain the geometric reasons for the absence.

Abstract

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.