New constraints on primordial gravitational waves from Planck 2015

TL;DR

This paper uses Planck 2015 data to set new, tighter constraints on the energy density of primordial gravitational waves, surpassing previous bounds and highlighting prospects for future observations.

Contribution

It provides the most stringent limits to date on the cosmic gravitational wave background using Planck data and combines multiple observations for improved constraints.

Findings

CGWB energy density constrained to $oxed{ ext{Ω}_{ ext{gw}} h^2 < 1.7 imes 10^{-6}}$ at 95% CL

Combined data reduces the limit to $oxed{ ext{Ω}_{ ext{gw}} h^2 < 1.2 imes 10^{-6}}$

Future missions could improve bounds by over an order of magnitude.

Abstract

We show that the new precise measurements of Cosmic Microwave Background (CMB) temperature and polarization anisotropies made by the Planck satellite significantly improves previous constraints on the cosmic gravitational waves background (CGWB) at frequencies $f>10^{-15}$ Hz. On scales smaller than the horizon at the time of decoupling, primordial gravitational waves contribute to the total radiation content of the Universe. Considering adiabatic perturbations, CGWB affects temperature and polarization CMB power spectra and matter power spectrum in a manner identical to relativistic particles. Considering the latest Planck results we constrain the CGWB energy density to $\Omega_{\rm gw} h^2 <1.7\times 10^{-6} $ at 95\% CL. Combining CMB power spectra with lensing, BAO and primordial Deuterium abundance observations, we obtain $\Omega_{\rm gw} h^2 <1.2\times 10^{-6} $ at 95\% CL, improving previous Planck bounds by a factor 3 and the recent direct upper limit from the LIGO and VIRGO experiments a factor 2. A combined analysis of future satellite missions as COrE and EUCLID could improve current bound by more than an order of magnitude.