Quantum effects, soft singularities and the fate of the universe in a braneworld cosmology

TL;DR

This paper investigates quantum effects near a quiescent singularity in braneworld cosmology, showing they can alter the singularity's nature or cause recollapse, with implications for the universe's future evolution.

Contribution

It introduces the impact of quantum vacuum polarization and particle production on quiescent singularities in braneworld models, highlighting possible softer singularities or recollapse scenarios.

Findings

Quantum vacuum polarization can soften or eliminate the singularity.

Quantum particle production may induce oscillations in the Hubble parameter.

Quantum effects are significant near low-density regions like voids.

Abstract

We examine a class of braneworld models in which the expanding universe encounters a "quiescent" future singularity. At a quiescent singularity, the energy density and pressure of the cosmic fluid as well as the Hubble parameter remain finite while all derivatives of the Hubble parameter diverge (i.e., ${\dot H}$, ${\ddot H}$, etc. $\to \infty$). Since the Kretschmann invariant diverges ($R_{iklm}R^{iklm} \to \infty$) at the singularity, one expects quantum effects to play an important role as the quiescent singularity is approached. We explore the effects of vacuum polarization due to massless conformally coupled fields near the singularity and show that these can either cause the universe to recollapse or, else, lead to a softer singularity at which $H$, ${\dot H}$, and ${\ddot H}$ remain finite while ${\dddot H}$ and higher derivatives of the Hubble parameter diverge. An important aspect of the quiescent singularity is that it is encountered in regions of low density, which has obvious implications for a universe consisting of a cosmic web of high and low density regions -- superclusters and voids. In addition to vacuum polarization, the effects of quantum particle production of non-conformal fields are also likely to be important. A preliminary examination shows that intense particle production can lead to an accelerating universe whose Hubble parameter shows oscillations about a constant value.