Tensor Modes from a Primordial Hagedorn Phase of String Cosmology

TL;DR

This paper demonstrates that a Hagedorn phase in string gas cosmology produces a nearly scale-invariant spectrum of gravitational waves with a small blue tilt, offering a potential observational distinction from slow-roll inflation.

Contribution

It calculates the tensor mode spectrum in a Hagedorn phase, revealing a small blue tilt, unlike the scalar modes and traditional inflation predictions.

Findings

Tensor spectrum is nearly scale-invariant with a small blue tilt.

Scalar spectrum has a small red tilt.

Provides a potential observational test to distinguish cosmological models.

Abstract

It has recently been shown that a Hagedorn phase of string gas cosmology can provide a causal mechanism for generating a nearly scale-invariant spectrum of scalar metric fluctuations, without the need for an intervening period of de Sitter expansion. In this paper we compute the spectrum of tensor metric fluctuations (gravitational waves) in this scenario, and show that it is also nearly scale-invariant. However, whereas the spectrum of scalar modes has a small red-tilt, the spectrum of tensor modes has a small blue tilt, unlike what occurs in slow-roll inflation. This provides a possible observational way to distinguish between our cosmological scenario and conventional slow-roll inflation.