Inflation and alternatives with blue tensor spectra

Abstract

We study the tilt of the primordial gravitational waves spectrum. A hint of blue tilt is shown from analyzing the BICEP2 and POLARBEAR data. Motivated by this, we explore the possibilities of blue tensor spectra from the very early universe cosmology models, including null energy condition violating inflation, inflation with general initial conditions, and string gas cosmology, etc. For the simplest G-inflation, blue tensor spectrum also implies blue scalar spectrum. In general, the inflation models with blue tensor spectra indicate large non-Gaussianities. On the other hand, string gas cosmology predicts blue tensor spectrum with highly Gaussian fluctuations. If further experiments do confirm the blue tensor spectrum, non-Gaussianity becomes a distinguishing test between inflation and alternatives. * yw366@cam.ac.uk † wei.xue@sissa.it 1 There is a debate about whether the observed polarisation signal comes from primordial gravitational waves or dust contamination [5,6].

Propagating speed of primordial gravitational waves and inflation

“…Implementing the superinflation without the ghost instability is still a significant issue, e.g. [20][57]. Here, we actually suggest such a superinflation scenario.…”

“…The GWs with P T ∼ 1 will induce the same-order scalar perturbation at nonlinear order, e.g. [20], which will result in the overproduction of primordial black holes at the corresponding scale, which is inconsistent with their abundance.…”

“…Obtaining a blue GWs spectrum n T > 0.1 without the ghost instability while reserving a scale-invariant scalar spectrum with slightly red tilt is still a challenge for the inflation scenario 1 , see e.g. [20] for comments.…”

Tensors, BICEP2 results, prior dependence, and dust

“…Finally, we note that while the above analysis implies that if the BICEP2 signal has a detectably large primordial component then n T > 0, short-scale observations such as big bang nucleosynthesis impose a fairly strict upper limit from avoiding having too large a density in subhorizon gravitational waves at key epochs [12,15,29]. This limit is typically around 0.5 with some dependence on cosmological modeling assumptions, and also depending on the amplitude of perturbations.…”

“…There were various attempts at addressing the discrepancy by invoking a cosmological origin. These branched mainly into investigating modifications of the scalar sector of the perturbations as well as parameters which are degenerate with it [8][9][10][11][12][13][14], and into analyses considering a positive value for the tilt of the tensor perturbations n T [11,[15][16][17][18][19][20][21] which would be in contradiction to normal models of inflation. However, these analyses make prior assumptions in the modeling of the tensor perturbations which we shall show may be inappropriate.…”