Graviton Creation by Small Scale Factor Oscillations in an Expanding Universe

TL;DR

This paper investigates how small oscillations in the universe's scale factor can lead to quantum creation of gravitons, analyzing different gravity models and their cosmological implications.

Contribution

It provides a comparative analysis of graviton creation in standard and modified gravity theories using a perturbative approach.

Findings

Oscillations can produce a significant graviton population.

Different gravity models yield distinct graviton creation rates.

Cosmological constraints limit the amplitude of oscillations.

Abstract

We treat quantum creation of gravitons by small scale factor oscillations around the average of an expanding universe. Such oscillations can arise in standard general relativity due to oscillations of a homogeneous, minimally coupled scalar field. They can also arise in modified gravity theories with a term proportional to the square of the Ricci scalar in the gravitational action. The graviton wave equation is different in the two cases, leading to somewhat different creation rates. Both cases are treated using a perturbative method due to Birrell and Davies, involving an expansion in a conformal coupling parameter to calculate the number density and energy density of the created gravitons. Cosmological constraints on the present graviton energy density and the dimensionless amplitude of the oscillations are discussed. We also discuss decoherence of quantum systems produced by the spacetime geometry fluctuations due to such a graviton bath.