Gravitational-wave cosmology across 29 decades in frequency

TL;DR

This paper combines diverse experiments across 29 frequency decades to constrain primordial gravitational waves, providing new limits on inflationary models and early Universe theories.

Contribution

It integrates multiple observational data sets to set the most stringent limits on the gravitational-wave spectrum and inflationary parameters across a broad frequency range.

Findings

Most stringent nanohertz limit from Parkes Pulsar Timing Array

CMB observations limit spectral index to $n_t\lesssim5$ for $r=0.11$

Combined data constrains $n_t<0.36$, with future LIGO data expected to tighten this further.

Abstract

Quantum fluctuations of the gravitational field in the early Universe, amplified by inflation, produce a primordial gravitational-wave background across a broad frequency band. We derive constraints on the spectrum of this gravitational radiation, and hence on theories of the early Universe, by combining experiments that cover 29 orders of magnitude in frequency. These include Planck observations of cosmic microwave background temperature and polarization power spectra and lensing, together with baryon acoustic oscillations and big bang nucleosynthesis measurements, as well as new pulsar timing array and ground-based interferometer limits. While individual experiments constrain the gravitational-wave energy density in specific frequency bands, the combination of experiments allows us to constrain cosmological parameters, including the inflationary spectral index, $n_t$, and the tensor-to-scalar ratio, $r$. Results from individual experiments include the most stringent nanohertz limit of the primordial background to date from the Parkes Pulsar Timing Array, $\Omega_{\rm gw}(f)<2.3\times10^{-10}$. Observations of the cosmic microwave background alone limit the gravitational-wave spectral index at 95\% confidence to $n_t\lesssim5$ for a tensor-to-scalar ratio of $r = 0.11$. However, the combination of all the above experiments limits $n_t<0.36$. Future Advanced LIGO observations are expected to further constrain $n_t<0.34$ by 2020. When cosmic microwave background experiments detect a non-zero $r$, our results will imply even more stringent constraints on $n_t$ and hence theories of the early Universe.