Is Cosmology Compatible with Blue Gravity Waves ?

TL;DR

This paper demonstrates that models with a blue spectral index for primordial gravitational waves are compatible with current data, challenging previous assumptions and highlighting the importance of future measurements from Planck.

Contribution

It shows that blue gravitational wave spectra are consistent with existing cosmological data, expanding the parameter space for inflationary models.

Findings

Blue tensor spectra are compatible with current cosmological constraints.

Allowing blue tilts widens the parameter space for scalar spectral index.

Future Planck data could crucially test these models.

Abstract

A primordial gravitational wave background with positive(blue) spectral index is expected in several non-standard inflationary cosmologies where the stress-energy tensor violates the null energy condition. Here we show that a sizable amount of blue gravitational waves is compatible with current cosmological and astrophysical data. So far most of the works on parameter estimation from cosmic microwave background data have assumed a negative or negligible spectral index. The present limits on cosmological parameters, especially on the scalar spectral index, widen up considerably when one allows also for blue tilts of the tensor spectrum. Since the amplitude of the CMB B-mode polarization is larger in these models, future data from Planck are likely to provide crucial measurements.