The Pseudo-Conformal Universe: Scale Invariance from Spontaneous
  Breaking of Conformal Symmetry
  We present a novel theory of the very early universe which addresses the
traditional horizon and flatness problems of big bang cosmology and predicts a
scale invariant spectrum of perturbations. Unlike inflation, this scenario
requires no exponential accelerated expansion of space-time. Instead, the early
universe is described by a conformal field theory minimally coupled to gravity.
The conformal fields develop a time-dependent expectation value which breaks
the flat space so(4,2) conformal symmetry down to so(4,1), the symmetries of de
Sitter, giving perturbations a scale invariant spectrum. The solution is an
attractor, at least in the case of a single time-dependent field. Meanwhile,
the metric background remains approximately flat but slowly contracts, which
makes the universe increasingly flat, homogeneous and isotropic, akin to the
smoothing mechanism of ekpyrotic cosmology. Our scenario is very general,
requiring only a conformal field theory capable of developing the appropriate
time-dependent expectation values, and encompasses existing incarnations of
this idea, specifically the U(1) model of Rubakov and the Galileon Genesis
scenario. Its essential features depend only on the symmetry breaking pattern
and not on the details of the underlying lagrangian. It makes generic
observational predictions that make it potentially distinguishable from
standard inflation, in particular significant non-gaussianities and the absence
of primordial gravitational waves.
