# Primordial Gravitational Waves in Nonstandard Cosmologies

## TL;DR

This paper investigates how nonstandard post-inflationary phases with different equations of state affect the primordial gravitational wave spectrum, providing constraints from gravitational wave experiments on early Universe models.

## Contribution

It introduces a framework to evaluate gravitational wave spectra in nonstandard cosmologies with a dominant component before BBN, linking theory with experimental constraints.

## Key findings

- Constraints on the equation of state of the early Universe component.
- Limits on the tensor spectral index and tensor-to-scalar ratio.
- Impact of scale dependence on gravitational wave spectrum.

## Abstract

Assuming that inflation is followed by a phase where the energy density of the Universe is dominated by a component with a general equation of state, we evaluate the spectrum of primordial gravitational waves induced in the post-inflationary Universe. We show that if the energy density of the Universe is dominated by a component $\phi$ before Big Bang nucleosynthesis, its equation of state could be constrained by gravitational wave experiments depending on the ratio of energy densities of $\phi$ and radiation, and also the temperature at the end of the $\phi$ dominated era. Also, we discuss the impact of scale dependence of tensor modes on the primordial gravitational wave spectrum during the $\phi$-domination. These models are motivated by beyond Standard Model physics and scenarios for non-thermal production of dark matter in the early Universe. We also constrain the parameter space of the tensor spectral index and the tensor-to-scalar ratio, using the experimental limits from gravitational wave experiments.