Primordial or Post-Inflationary: Distinguishing the Origin of Scale-Dependent Features in the CMB

Abstract

Physics beyond the Standard Model, whether of primordial or post-inflationary origin, can leave scale-dependent signals in the anisotropies of the Cosmic Microwave Background (CMB). In this work, we study how distinguishable these broad classes of signals are and quantify their level of degeneracy. In this initial investigation, for primordial features, we limit ourselves to those of the ``sharp'' and ``resonant'' kinds; for post-inflationary features, we consider two qualitatively distinct models: exotic energy injection, which deposits energy into the primordial plasma and modifies the global thermodynamic history, and early dark energy (EDE), which affects the CMB through the background expansion and gravitational perturbations. By studying representative examples, we find that while primordial features can leave oscillatory signatures in the CMB power spectrum with essentially arbitrary frequency and running, the post-inflationary models considered here produce qualitatively different patterns that cannot generically reproduce the characteristic running of primordial feature oscillations. We verify these findings first with a principal component analysis, and show using only the most significant principal components that there are no significant degeneracies between the primordial and post-inflationary features considered here, and secondly with a full likelihood analysis, which shows that the resulting posteriors do not change significantly when considering individual classes of models or combinations of primordial/post-inflationary features. While the two post-inflationary models probe distinct physical mechanisms and both are implemented in a simplified, phenomenological fashion, the methods used in this work can be generalized to include more realistic signals or other classes of signals.