Quasar variability limits on cosmological density of cosmic strings
  We put robust upper limits on the average cosmological density \Omega_s of
cosmic strings based on the variability properties of a large homogeneous
sample of SDSS quasars. We search for an excess of characteristic variations of
quasar brightness that are associated with string lensing and use the observed
distribution of this variation to constrain the density of strings. The limits
obtained do not invoke any clustering of strings, apply to both open segments
and closed loops of strings, usefully extend over a wide range of tensions
10^{-13} < G\mu/c^2 < 10^{-9} and reach down the level of \Omega_s=0.01 and
below. Further progress in this direction will depend on better understanding
of quasar intrinsic variability rather than a mere increase in the volume of
data.
