Observational Constraints on Theories with a Blue Spectrum of Tensor Modes

TL;DR

This paper investigates observational constraints on models predicting a blue tilt in the primordial gravitational wave spectrum, analyzing current data from CMB, pulsar timing, and nucleosynthesis.

Contribution

It provides the first comprehensive analysis of observational bounds on a blue tensor spectral index within string gas cosmology.

Findings

Current bounds constrain the tensor spectral index to less than 0.79 from CMB data.

Pulsar timing data restricts the index to below 0.53.

Nucleosynthesis bounds limit the index to under 0.15.

Abstract

Motivated by the string gas cosmological model, which predicts a blue tilt of the primordial gravitational wave spectrum, we examine the constraints imposed by current and planned observations on a blue tilted tensor spectrum. Starting from an expression for the primordial gravitational wave spectrum normalized using cosmic microwave background observations, pulsar timing, direct detection and nucleosynthesis bounds are examined. If we assume a tensor to scalar ratio on scales of the CMB which equals the current observational upper bound, we obtain from these current observations constraints on the tensor spectral index of $n_{T} \lesssim 0.79$, $n_{T} \lesssim 0.53$, and $n_{T} \lesssim 0.15$ respectively.