Effects of Finite-time Singularities on Gravitational Waves

TL;DR

This paper investigates how finite-time singularities, specifically Type II and Type IV, influence primordial gravitational waves during inflation in $F(R)$ gravity, revealing that Type II singularities suppress waves while Type IV do not.

Contribution

It identifies which finite-time singularities are compatible with inflation and analyzes their distinct effects on gravitational wave amplitudes in $F(R)$ gravity.

Findings

Type II singularities lead to negligible gravitational wave amplitudes.

Type IV singularities do not affect gravitational wave amplitudes.

The universe can pass smoothly through a Type IV singularity during inflation.

Abstract

We analyze the impact of finite-time singularities on gravitational waves, in the context of $F(R)$ gravity. We investigate which singularities are allowed to occur during the inflationary era, when gravitational waves are considered, and we discuss the quantitative implications of each allowed singularity. As we show, only a pressure singularity, the so-called Type II and also a Type IV singularity are allowed to occur during the inflationary era. In the case of a Type II, the resulting amplitude of the gravitational wave is zero or almost zero, hence this pressure singularity has a significant impact on the primordial gravitational waves. The case of a Type IV singularity is more interesting since as we show, the singularity has no effect on the amplitude of the gravitational waves. Therefore, this result combined with the fact that the Type IV singularity affects only the dynamics of inflation, leads to the conclusion that the Universe passes smoothly through a Type IV singularity.