Stochastic Gravitational Wave Background from Light Cosmic Strings

Abstract

Spectra of the stochastic gravitational wave backgrounds from cosmic strings are calculated and compared with present and future experimental limits. Background spectra are calculated numerically for dimensionless string tensions G mu/c^2 between 10^{-7} and 10^{-18}, and initial loop sizes as a fraction of the Hubble radius, alpha, from 0.1 to 10^{-6}. The spectra of the cosmic string backgrounds are compared with current millisecond pulsar limits and LISA sensitivity curves. For models with large stable loops (alpha=0.1), current pulsar-timing limits exclude G mu/c^2>10^{-9} and within the range of current models based on brane inflation. LISA may detect a background from strings as light as G mu/c^2 10^{-16}, corresponding to field-theory strings formed at roughly 10^{11} GeV.