Non-Gaussian Signatures of a Thermal Big Bang
  What if Big Bang was hot from its very inception? This is possible in a
bimetric theory where the source of fluctuations is thermal, requiring the
model to live on a critical boundary in the space of parameters and can be
realized when an anti-DBI brane moves within an $EAdS_2 \times E_3$ geometry.
This setup renders the model unique, with sharp predictions for the scalar
spectral index and its running. We investigate the non-Gaussian signatures of
this thermal bimetric model, or "bi-thermal" for short. We adapt the standard
calculation of non-Gaussianities for $P(X,\phi)$ models to the thermal nature
of the model, emphasising how the bi-thermal peculiarities affect the
calculation and alter results. This leads to precise predictions for the shape
and amplitude of the three-point function of the bi-thermal model (at
tree-level): $f^{\rm local} _{\rm NL} = -3/2$ and $f^{\rm equil} _{\rm NL} = -2
+ 4 \sqrt{3}\pi/9 \simeq 0.4$. We also discover a new shape of flattened
non-gaussianity $\propto (k_1+k_2-k_3)^{-3/2} +$ permutations, which is
expected due to the excited thermal initial conditions. These results, along
with our earlier predictions for the scalar power spectrum, provide sharp
targets for the future generation of cosmological surveys.
