Pre-Big-Bang Cosmology and Circles in the Cosmic Microwave Background
  We examine the possibility that circles in the cosmic microwave background
could be formed by the interaction of a gravitational wave pulse emitted in
some pre-big-bang phase of the universe with the last scattering surface. We
derive the expected size distribution of such circles, as well as their typical
ring width and (for concentric circles) angular separation. We apply these
results in particular to conformal cyclic cosmology, ekpyrotic cosmology as
well as loop quantum cosmology with and without inflation in order to determine
how the predicted geometric properties of these circles would vary from one
model to the other, and thus, if detected, could allow us to differentiate
between various pre-big-bang cosmological models. We also obtain a relation
between the angular ring width and the angular radius of such circles that can
be used in order to determine whether or not circles observed in the cosmic
microwave background are due to energetic pre-big-bang events.
