# 28th Texas Symposium on Relativistic Astrophysics

13-18 December 2015
International Conference Centre Geneva
Europe/Zurich timezone

## Current constraints and forecasts on the tilt and running of the primordial tensor spectrum

14 Dec 2015, 16:55
20m
Level 0, Room 4 (International Conference Centre Geneva)

### Level 0, Room 4

#### International Conference Centre Geneva

17 Rue de Varembé, 1211 Geneva

### Speaker

Mr Giovanni Cabass (Physics Department and INFN, “La Sapienza” University of Rome)

### Description

Gravitational waves can be produced by a wide range of astrophysical phenomena, such as inspiral and merging of neutron stars and black holes, supernova of massive stars, accreting neutron stars, etc. This talk is focused, instead, on a stochastic background of gravitational waves (GWs) of cosmological origin, like the one predicted by inflation. I start by considering a power law parametrization of the frequency spectrum of primordial tensor modes, with tilt $n_T$ and tensor-to-scalar ratio $r$. I discuss the constraints that can be placed on these parameters by Cosmic Microwave Background (CMB) temperature and polarization anisotropies alone, and then show how such constraints become stronger if one takes into account the contribution of gravitational waves to the radiation energy density. GWs add to the effective number of relativistic degrees of freedom $N_\text{eff} = 3.046 + N_\text{eff}^\text{GW}$, and then have an effect on the CMB angular spectra and the primordial abundances. More precisely, we find that the $95\%$ CL limits on the tilt $n_T$ at a pivot scale of $0.01\,\text{Mpc}^{-1}$ go from $n_T = 1.7^{+2.1}_{-2.0}$ (Planck + BKP dataset, no $N_\text{eff}^\text{GW}$) to $n_T = 0.06^{+0.78}_{-0.56}$ (with $N_\text{eff}^\text{GW}$). In the second part of the talk I discuss the forecasts on a spectrum described by a spectral index $n_T$ and its running $n_{T,\text{run}}$: our analysis considers a future COrE-like satellite mission combined with direct-detection experiments like AdvLIGO. When the contribution of primordial tensor modes to CMB spectral distortions of the $\mu$-type is taken into account, we add the future measurements of the CMB spectrum by a PIXIE-like experiment. For these forecasts two fiducial cosmologies are considered: at first a cosmology with no primordial gravitational waves, in order to see how well future experiments will be able to measure the tensor parameters. The second forecast takes as fiducial model one where the tensor-to-scalar ratio is of order $10^{-2}$ (a value that can be reached by futuristic ground-based experiments such as AdvACT), and the tilt and running are fixed by the consistency relations of single-field slow-roll inflation. In the final part of the talk, I discuss the implications of our results for models of single-field slow-roll inflation.

### Primary author

Mr Giovanni Cabass (Physics Department and INFN, “La Sapienza” University of Rome)

### Co-authors

Mr Alessandro Melchiorri (Physics Department and INFN, “La Sapienza” University of Rome) Ms Elena Giusarma (Physics Department and INFN, “La Sapienza” University of Rome) Ms Laura Salvati (Physics Department and INFN, “La Sapienza” University of Rome) Mr Luca Pagano (Physics Department and INFN, “La Sapienza” University of Rome) Ms Martina Gerbino (Physics Department and INFN, “La Sapienza” University of Rome)