Is Cosmology Compatible with Blue Gravity Waves ?

Understand

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.