Graviton creation by small scale factor oscillations in an expanding universe

What is it about?

We consider the 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. Cosmological constraints on the present graviton energy density and the dimensionless amplitude of the oscillations are discussed.

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Why is it important?

A time-dependent spacetime metric can result in quantum particle creation, as was first discussed by Parker in the context of the expansion of the universe. The process of quantum particle creation has been mostly studied in the context of inflation and more recently in bouncing cosmology. We here focus on a novel scenario involving graviton production due to rapid oscillations around a mean expansion rate in a spatially flat Friedmann-Robertson-Walker background. Surprisingly, two different cosmological scenarios can produce such kind of oscillations: (1) The usual matter fields in standard general relativity plus a minimally coupled scalar field (GRSF) in a harmonic potential; (2) F(R) gravity, when a term proportional to the square of the Ricci scalar is added to the Einstein-Hilbert action. We determine the two different graviton creation rates and discuss cosmological constraints on the present graviton energy density.