An Upper Limit on the Stochastic Gravitational-Wave Background of Cosmological Origin

TL;DR

This paper reports the first direct experimental limits on the amplitude of the stochastic gravitational-wave background from the early universe, constraining cosmological models and string theories using LIGO data.

Contribution

It provides the first direct measurement limits on the stochastic gravitational-wave background, improving upon previous indirect constraints and testing early universe models.

Findings

Energy density limit of 6.9 x 10^{-6} at 100 Hz

Rules out certain early universe and string models

Improves upon previous indirect limits

Abstract

A stochastic background of gravitational waves is expected to arise from a superposition of a large number of unresolved gravitational-wave sources of astrophysical and cosmological origin. It is expected to carry unique signatures from the earliest epochs in the evolution of the universe, inaccessible to the standard astrophysical observations. Direct measurements of the amplitude of this background therefore are of fundamental importance for understanding the evolution of the universe when it was younger than one minute. Here we report direct limits on the amplitude of the stochastic gravitational-wave background using the data from a two-year science run of the Laser Interferometer Gravitational-wave Observatory (LIGO). Our result constrains the energy density of the stochastic gravitational-wave background normalized by the critical energy density of the universe, in the frequency band around 100 Hz, to be less than 6.9 x 10^{-6} at 95% confidence. The data rule out models of early universe evolution with relatively large equation-of-state parameter, as well as cosmic (super)string models with relatively small string tension that are favoured in some string theory models. This search for the stochastic background improves upon the indirect limits from the Big Bang Nucleosynthesis and cosmic microwave background at 100 Hz.