Seeing Higher-Dimensional Grand Unification In Primordial
  Non-Gaussianities
  The observed low-energy values of the $SU(3)\times SU(2)\times U(1)$ gauge
couplings, extrapolated via the minimal Standard Model Renormalization Group
evolution, hint at the exciting possibility of a Grand Unified Theory (GUT) at
$M_U \sim 10^{14}$ GeV --- a scale, however, too high to probe directly via
collider searches. Fortunately, since the Hubble scale H can be as high as $5
\times 10^{13}$ GeV $\sim M_U$ during the inflationary era, such GUT scale
states can be cosmologically produced at that time and leave direct on-shell
signatures such as their masses and spins, via primordial non-Gaussianity (NG).
We explore this possibility in one of its simplest realizations given by the
extra-dimensional framework of orbifold GUTs, in which proton decay can be
straightforwardly suppressed to be within the stringent bounds. Here, along
with the massive GUT states there must also be H-mass spin-2 Kaluza-Klein (KK)
gravitons, collectively giving rise to striking NG signatures. In our set-up we
localize the inflaton on one of the boundaries of an extra dimension. The
inflationary vacuum energy can readily lead to formation of a horizon in the
bulk, where the KK modes then form a continuum above a mass gap of $\sim
\mathcal{O}(H)$. We find that the optimal case for observable NG signals is
when the extra dimension is stabilized close to the onset of this horizon,
ensuring a discrete KK spectrum such that the lightest KK modes can be
cosmologically produced without significant Boltzmann suppressions. Although we
mostly focus on the case where there is no higher-dimensional cosmological
constant, we also obtain considerable holographic insights from the
$\text{AdS}_5/\text{CFT}_4$ correspondence when such a cosmological constant is
included.
