Using SquiRels to Find LiMRs: A Model-Insensitive Approach to Cosmic Microwave Background Studies of Light but Massive Relics

Abstract

Light but Massive Relics (LiMRs), cosmological relics that are relativistic at some point in the observable early universe but are non-relativistic today, are a generic prediction of many theories of physics beyond the Standard Model, and are a target of high interest for current and upcoming cosmological surveys. The enormous variety of scenarios that can give rise to LiMRs also gives rise to a wide range of possible phase space distributions for these relics, making it valuable to have a model-agnostic quantification of LiMR signatures. Toward that end we identify three independent physical quantities that govern LiMRs&#39; dominant effects on cosmological observables, and introduce a phenomenological family of &#34;Squished Relics&#34; (SquiRels) that allow us to efficiently and flexibly search for the imprint of LiMRs with different phase space distributions in data. Our primary focus here is on LiMRs that transition from radiation to matter during the Cosmic Microwave Background epoch. Using this framework we explicitly demonstrate that current Planck observations are not sensitive to the shape of the LiMR&#39;s phase space distribution function, which allows us to provide a model-insensitive and readily reinterpretable limit on LiMRs that become non-relativistic at redshifts z_{NR}<10^5. We further demonstrate that upcoming and future ground-based observatories will be able to distinguish LiMRs from massless dark radiation species, and could begin to provide information about the shape of its distribution.