Massless particle creation in a f(R) expanding universe

TL;DR

This paper derives an exact spectrum for massless particle creation in a modified gravity universe with power-law expansion, showing small deviations from general relativity significantly affect particle production.

Contribution

It provides an exact solution for particle creation spectrum in f(R) gravity with power-law expansion, highlighting the impact of small deviations from general relativity.

Findings

Particle spectrum is independent of the parameter β.

Smaller n leads to fewer created particles.

Small deviations from GR cause an order of magnitude change in particle number.

Abstract

In this paper we present an exact solution to the spectrum of massless particle creation for a power law expansion of the scale factor of the form $t^{2n/3}$ for real scalar particles in a flat and matter dominated universe. Such an evolution follows from a modified theory of gravity of the type $f(R)=\beta R^n$, and it is showed that the spectrum of created particles is $\beta$ independent. We find that greater the value of $n$ smaller is the number of created particles. We study in detail the spectrum of the total particle number created for $n=1\pm \varepsilon$, with $\varepsilon = 0.1$, a very small deviation from the standard general relativity case ($n=1$). We find that such a very small deviation causes a great difference in the total particle number, of about one order of magnitude as compared to the standard general relativity. Our calculations are based on the method of instantaneous Hamiltonian diagonalization, where the vacuum states are defined as those which minimizes the energy at a particular instant of time. The spectrum of the total number and total energy of created particles can be determined exactly for any value of the physical time in a matter dominated universe. We also find that the main contribution to the total number of particles and total energy comes from small wavenumbers $k$ and for particles with large values of $k$ the contribution is very small.