Gravitational Particle Production in Oscillating Background and Its Cosmological Implications

TL;DR

This paper investigates how oscillations in the universe's expansion rate, caused by various scalar fields and gravity models, lead to particle production with significant cosmological consequences like dark radiation.

Contribution

It introduces a new method to estimate gravitational particle production rates in oscillating backgrounds across different gravity theories.

Findings

Oscillating scalar fields induce particle production regardless of their energy contribution.

Extended gravity models also cause scale factor oscillations leading to particle creation.

Potential observable effects include axion dark radiation from Starobinsky inflation.

Abstract

We study production of light particles due to oscillation of the Hubble parameter or the scale factor. Any coherently oscillating scalar field, irrespective of its energy fraction in the universe, imprints such an oscillating feature on them. Not only the Einstein gravity but extended gravity models, such as models with non-minimal (derivative) coupling to gravity and $f(R)$ gravity, lead to oscillation of the scale factor. We present a convenient way to estimate the gravitational particle production rate in these circumstances. Cosmological implications of gravitational particle production, such as dark matter/radiation and moduli problem, are discussed. For example, if the theory is described solely by the standard model plus the Peccei-Quinn sector, the Starobinsky $R^2$ inflation may lead to observable amount of axion dark radiation.