Inflation, String Theory and Cosmology
  At its very beginning, the universe is believed to have grown exponentially
in size via the mechanism of inflation. The almost scale-invariant density
perturbation spectrum predicted by inflation is strongly supported by
cosmological observations, in particular the cosmic microwave background
radiation. However, the universe's precise inflationary scenario remains a
profound problem for cosmology and for fundamental physics. String theory, the
most-studied theory as the final physical theory of nature, should provide an
answer to this question. Some of the proposals on how inflation is realized in
string theory are reviewed. Since everything is made of strings, some string
loops of cosmological sizes are likely to survive in the hot big bang that
followed inflation. They appear as cosmic strings, which can have intricate
properties. Because of the warped geometry in flux compactification of the
extra spatial dimensions in string theory, some of the cosmic strings may have
tensions substantially below the Planck or string scale. Such strings cluster
in a manner similar to dark matter leading to hugely enhanced densities. As a
result, numerous fossil remnants of the low tension cosmic strings may exist
within the galaxy. They can be revealed through the optical lensing of
background stars in the near future and studied in detail through gravitational
wave emission. We anticipate that these cosmic strings will permit us to
address central questions about the properties of string theory as well as the
birth of our universe.
