Harmonic Gravitational Wave Spectra of Cosmic String Loops in the Galaxy
  A new candidate source of gravitational radiation is described: the
nearly-perfect harmonic series from individual loops of cosmic string. It is
argued that theories with light cosmic strings give rise to a population of
numerous long-lived stable loops, many of which cluster gravitationally in
galaxy halos along with the dark matter. Each cosmic string loop produces a
spectrum of discrete frequencies in a nearly perfect harmonic series, a
fundamental mode and its integer multiples. The gravitational wave signal from
cosmic string loops in our Galactic halo is analyzed numerically and it is
found that the for light strings, the nearest loops typically produce strong
signals which stand out above the confusion noise from Galactic binaries. The
total population of cosmic string loops in the Milky Way also produces a broad
signal that acts as a confusion noise. Both signals are enhanced by the
clustering of loops gravitationally bound to the Galaxy, which significantly
decreases the average distance from the solar system to the nearest loop.
Numerical estimates indicate that for dimensionless string tension G\mu<
10^{-11}, many loops are likely to be found in the Galactic halo. Lighter
strings, down to G\mu=10^{-19}, are detectable by the Laser Interferometer
Space Antenna (LISA). For these light strings, the fundamental and low-order
harmonics of typical loops often lie in the band where LISA is sensitive, 0.1
to 100 mHz. The harmonic nature of the cosmic string loop modes leaves a
distinct spectral signature different from any other known source of
gravitational waves.
