Thermal Tachyacoustic Cosmology
  An intriguing possibility that can address pathologies in both early universe
cosmology (i.e. the horizon problem) and quantum gravity (i.e.
non-renormalizability), is that particles at very high energies and/or
temperatures could propagate arbitrarily fast. A concrete realization of this
possibility for the early universe is the Tachyacoustic (or Speedy Sound)
cosmology, which could also produce a scale-invariant spectrum for scalar
cosmological perturbations. Here, we study Thermal Tachyacoustic Cosmology
(TTC), i.e. this scenario with thermal initial conditions. We find that a phase
transition in the early universe, around the scale of Grand Unified Theories
(GUT scale; $T\sim 10^{15}$ GeV), during which the speed of sound drops by $25$
orders of magnitude within a Hubble time, can fit current CMB observations. We
further discuss how production of primordial black holes constrains the
cosmological acoustic history, while coupling TTC to Horava-Lifshitz gravity
leads to a lower limit on the amplitude of tensor modes ($r \gtrsim 10^{-3}$),
that are detectable by CMBPol (and might have already been seen by the
BICEP-Keck collaboration).
