Ultracompact horizonless objects in order-reduced semiclassical gravity

Understand

The backreaction of quantum fields in their vacuum state results in equilibrium structures that surpass the Buchdahl compactness limit.

• Such backreaction is encapsulated in the vacuum expectation value of the renormalized stress-energy tensor (RSET).
• In previous works we presented analytic approximations to the RSET, obtained by dimensional reduction, available in spherical symmetry, and showed that the backreaction-generated solutions described ultracompact fluid spheres with a negative mass interior.
• Here, we derive a novel approximation to the RSET that does not rely on dimensional reduction, but rather on a reduction of the differential order.