Cosmological Quasiparticles and the Cosmological Collider

TL;DR

This paper explores how strongly coupled conformal field theories during inflation can produce distinctive spectral features, including sharp peaks and oscillations, which could be observable in future cosmological experiments.

Contribution

It demonstrates the existence of particle-like peaks in the spectral density within inflationary backgrounds, linking holographic models to potential observable signatures in primordial fluctuations.

Findings

Identification of sharp, particle-like peaks in the spectral density.

Prediction of displaced oscillatory features in the bi-spectrum.

Potential observability of these features in future cosmological data.

Abstract

The interplay between cosmology and strongly coupled dynamics can yield transient spectral features that vanish at late times, but which may leave behind phenomenological signatures in the spectrum of primordial fluctuations. Of particular interest are strongly coupled extensions of the standard model featuring approximate conformal invariance. In flat space, the spectral density for a scalar operator in a conformal field theory is characterized by a continuum with scaling law governed by the dimension of the operator, and is otherwise featureless. AdS/CFT arguments suggest that for large $N$, in an inflationary background with Hubble rate $H$, this continuum is gapped. We demonstrate that there can be additional peak structures that become sharp and particle-like at phenomenologically interesting regions in parameter space, and we estimate their contribution to cosmological observables. We find phenomena that are potentially observable in future experiments that are unique to these models, including displaced oscillatory features in the squeezed limit of the bi-spectrum. These particles can be either fundamental, and localized to a UV brane, or composite at the Hubble scale, $H$, and bound to a horizon in the bulk of the 5D geometry. We comment on how stabilization of conformal symmetry breaking vacua can be correlated with these spectral features and their phenomenology.