Abstract:
In this work, we shall exhaustively study the effects of modified gravity on the energy
spectrum of the primordial gravitational waves background. S. Weinberg has also produced significant works related to the primordial gravitational waves, with the most important one being
the effects of neutrinos on primordial gravitational waves. With this short review, our main aim is
to gather all the necessary information for studying the effects of modified gravity on primordial
gravitational waves in a concrete and quantitative way and in a single paper. After reviewing all the
necessary techniques for extracting the general relativistic energy spectrum, and how to obtain, in a
WKB way, the modified gravity damping or amplifying factor, we concentrate on specific forms of
modified gravity of interest. The most important parameter involved for the calculation of the effects
of modified gravity on the energy spectrum is the parameter aM, which we calculate for the cases of
f(R, φ) gravity, Chern–Simons-corrected f(R, φ) gravity, Einstein–Gauss–Bonnet-corrected f(R, φ)
gravity, and higher derivative extended Einstein–Gauss–Bonnet-corrected f(R, φ) gravity. The exact
form of aM is presented explicitly for the first time in the literature. With regard to Einstein–Gauss–
Bonnet-corrected f(R, φ) gravity, and higher derivative extended Einstein–Gauss–Bonnet-corrected
f(R, φ) gravity theories, we focus on the case in which the gravitational wave propagating speed is
equal to that of light in a vacuum. We provide expressions for aM expressed in terms of the cosmic
time and in terms of the redshift, which can be used directly for the numerical calculation of the effect
of modified gravity on the primordial gravitational wave energy spectrum.
