TESTING THE INFLATIONARY NULL ENERGY CONDITION WITH CURRENT AND FUTURE COSMIC MICROWAVE BACKGROUND DATA

In some classes of inflationary models, a violation of the null energy condition (NEC) is expected during the phase of accelerated expansion. The most striking observational effect would be a "blue" spectrum of primordial gravitational waves with a spectral index n(T) larger than zero. In this brief paper, we first update the current constraints on the amplitude of a blue GW spectrum. We then forecast the ability of the ongoing Planck satellite mission to detect a violation of the NEC during inflation, showing that the detection of the GW background is substantially improved if the NEC is violated during inflation.