Gravitational Waves and Light Cosmic Strings

Abstract

Gravitational wave signatures from cosmic strings are analyzed numerically. Cosmic string networks form during phase transistions in the early universe and these networks of long cosmic strings break into loops that radiate energy in the form of gravitational waves until they decay. The gravitational waves come in the form of harmonic modes from individual string loops, a "confusion noise" from galactic loops, and a stochastic background of gravitational waves from a network of loops. In this study string loops of larger size $α$ and lower string tensions $Gμ$ (where $μ$ is the mass per unit length of the string) are investigated than in previous studies. Several detectors are currently searching for gravitational waves and a space based satellite, the Laser Interferometer Space Antenna (LISA), is in the final stages of pre-flight. The results for large loop sizes ($α=0.1$) put an upper limit of about $Gμ<10^{-9}$ and indicate that gravitational waves from string loops down to $Gμ\approx 10^{-20}$ could be detectabe by LISA. The string tension is related to the energy scale of the phase transition and the Planck mass via $Gμ= Λ_s^2 / m_{pl}^2$, so the limits on $Gμ$ set the energy scale of any phase transition to $Λ_s < 10^{-4.5} m_{pl}$. Our results indicate that loops may form a significant gravitational wave signal, even for string tensions too low to have larger cosmological effects.