Altering the Pre-recombination Expansion History with Hidden Sector Decays

Many cosmological models introduce a new energy density contribution to the universe before the
epoch of recombination in an attempt to alter the expansion history and address the Hubble tension:
a 5σ discrepancy between the present-day expansion rate inferred from local measurements and that
inferred from observations of the Cosmic Microwave Background (CMB). I propose to quantify the
level that one can alter the expansion history between Big Bang Nucleosynthesis (BBN) and recom-
bination by considering the decay of a massive hidden sector particle, dubbed the Y particle. I have
shown that if the Y particle decays into a mixture of photons and dark radiation (DR), observations
of the CMB and measurements of the abundance of deuterium limit the particle to contribute at
most 2.35% (95% CL) of the energy density of the universe before decaying. Furthermore, prelim-
inary results show that the Y particle is allowed to contribute up to 3.3% (95% CL) of the energy
density of the universe if it decays solely into DR. I propose to investigate how much these limits on
energy injection may be relaxed by considering decay products that are massive and transition from
being relativistic to non-relativistic before matter-radiation equality. Additionally, photon injection
from a Y decay has the potential to relax the cosmology-inferred bound on the sum of neutrino
masses by making the neutrinos colder than in the standard cosmology. I propose to quantify how
robust the standard expansion history and cosmology-inferred bound on the sum of neutrino masses
are in the context of such decay scenarios.

Alex Sobotka

Physics PhD Student

UNC Department of Physics and Astronomy

Location: Phillips Hall 277

Zoom link: https://unc.zoom.us/j/95448872335?pwd=RDJSOXFSTER4dlNQemFCYkxhYjhZQT09

Meeting ID: 954 4887 2335

Passcode: 360796