Singularities In Scalar-Tensor Cosmologies

TL;DR

This paper investigates scalar-tensor cosmologies and finds that while most probes do not detect singularities, gravity waves always do, indicating that singularities cannot be fully eliminated but only hidden from certain observations.

Contribution

The study demonstrates that in scalar-tensor theories, gravity waves reveal singularities despite other probes not detecting them, highlighting limitations in removing cosmological singularities.

Findings

Gravity waves always detect singularities in these models.

Most probes do not see the singularity, making it effectively invisible.

Singularities cannot be fully removed, only hidden from some probes.

Abstract

In this article, we examine the possibility that there exist special scalar-tensor theories of gravity with completely nonsingular FRW solutions. Our investigation in fact shows that while most probes living in such a Universe never see the singularity, gravity waves always do. This is because they couple to both the metric and the scalar field, in a way which effectively forces them to move along null geodesics of the Einstein conformal frame. Since the metric of the Einstein conformal frame is always singular for configurations where matter satisfies the energy conditions, the gravity wave world lines are past inextendable beyond the Einstein frame singularity, and hence the geometry is still incomplete, and thus singular. We conclude that the singularity cannot be entirely removed, but only be made invisible to most, but not all, probes in the theory.