Spectra of the stochastic gravitational wave backgrounds from cosmic strings are calculated and compared with present and future experimental limits .
Motivated by theoretical expectations of light cosmic strings in superstring cosmology , improvements in experimental sensitivity , and recent demonstrations of large , stable loop formation from a primordial network , this study explores a new range of string parameters with masses lighter than previously investigated .
A standard “ one-scale ” model for string loop formation is assumed .
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 show a low frequency power-law tail , a broad spectral peak due to loops decaying at the present epoch ( including frequencies higher than their fundamental mode , and radiation associated with cusps ) , and a flat ( constant energy density ) spectrum at high frequencies due to radiation from loops that decayed during the radiation-dominated era .
The string spectrum is distinctive and unlike any other known source .
The peak of the spectrum for light strings appears at high frequencies , significantly affecting predicted signals .
The spectra of the cosmic string backgrounds are compared with current millisecond pulsar limits and Laser Interferometer Space Antenna ( LISA ) sensitivity curves .
For models with large stable loops ( \alpha = 0.1 ) , current pulsar-timing limits exclude G \mu / c ^ { 2 } > 10 ^ { -9 } , a much tighter limit on string tension than achievable with other techniques , 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 } \approx 10 ^ { -16 } , corresponding to field-theory strings formed at roughly 10 ^ { 11 } GeV .