Ekpyrotic Cosmology with a Zero-Shear S-Brane
  In a recent paper we proposed a mechanism for a continuous transition between
a contracting Ekpyrotic phase and the Standard Big Bang phase of expansion: the
bounce is generated by an S-brane which represents the effects of higher mass
string states in the low energy effective field theory. We showed that
gravitational waves on cosmological scales obtain a nearly scale-invariant
spectrum. Here, we study the cosmological fluctuations in this setup, assuming
that the S-brane has zero shear. We find a nearly scale-invariant spectrum of
cosmological perturbations with a slight red tilt. The scenario yields two
consistency relations for cosmological observations, the first one relating the
tensor to scalar ratio with the scalar spectral tilt, the second relating the
tensor tilt to the scalar tilt. The predicted tensor to scalar ratio is within
the reach of upcoming CMB observations. The tensor tilt is blue.
