An upper limit on the stochastic gravitational-wave background of cosmological origin

A stochastic background of gravitational waves is expected to arisefroma superposition of a large number of unresolved gravitational-wave sources of astrophysical and cosmological origin. It should carry unique signatures from the earliest epochs in the evolution of the Universe, inaccessible to standard astrophysical observations. Direct measurements of the amplitude of this backgroundare therefore of fundamental importance for understandingthe evolution of the Universe when it was younger than oneminute. Here we report 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 Obser-vatory (LIGO). Our result constrains the energy density of the stochastic gravitational-wave background normalized by the criticalenergy density of the Universe, in the frequency band around 100Hz, to be ,6.931026 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 on the indirect limits from Big Bang nucleosynthesis and cosmic microwave background at 100Hz.