  The large radii of many hot Jupiters can only be matched by models that have
hot interior adiabats, and recent theoretical work has shown that the interior
evolution of hot Jupiters has a significant impact on their atmospheric
structure. Due to its inflated radius, low gravity, and ultra-hot equilibrium
temperature, WASP-76b is an ideal case study for the impact of internal
evolution on observable properties. Hot interiors should most strongly affect
the non-irradiated side of the planet, and thus full phase curve observations
are critical to ascertain the effect of the interior on the atmospheres of hot
Jupiters. In this work, we present the first Spitzer phase curve observations
of WASP-76b. We find that WASP-76b has an ultra-hot day side and relatively
cold nightside with brightness temperatures of $2471 \pm 27~\mathrm{K}$/$1518
\pm 61~\mathrm{K}$ at $3.6~\micron$ and $2699 \pm 32~\mathrm{K}$/$1259 \pm
44~\mathrm{K}$ at $4.5~\micron$, respectively. These results provide evidence
for a dayside thermal inversion. Both channels exhibit small phase offsets of
$0.68 \pm 0.48^{\circ}$ at $3.6~\micron$ and $0.67 \pm 0.2^{\circ}$ at
$4.5~\mu\mathrm{m}$. We compare our observations to a suite of general
circulation models that consider two end-members of interior temperature along
with a broad range of frictional drag strengths. Strong frictional drag is
necessary to match the small phase offsets and cold nightside temperatures
observed. From our suite of cloud-free GCMs, we find that only cases with a
cold interior can reproduce the cold nightsides and large phase curve amplitude
at $4.5~\micron$, hinting that the hot interior adiabat of WASP-76b does not
significantly impact its atmospheric dynamics or that clouds blanket its
nightside.
