WI Seminar- Adrienne Erickcek

One century after the Big Bang, points separated by more than 6 Mpc today were not yet in causal contact. Since scales smaller than 6 Mpc are heavily damped in the cosmic microwave background (CMB), we do not have a clear window into the expansion history during the Universe’s first century. In the standard picture, inflation was shortly followed by a radiation-dominated era that persisted uninterrupted for 51,000 years with no changes in the number of photons after electron-positron annihilation. Relaxing these assumptions could reduce the number density of cosmic neutrinos and change the growth of structure on scales smaller than a few megaparsecs. I will show how diluting the cosmological neutrino abundance by injecting photons after neutrino decoupling has unintended consequences that make the cosmological bound on the neutrino masses surprisingly robust to changes in the Universe’s thermal history. I will also show how deviations from radiation domination during the first 100 years can alter small-scale structure without significantly affecting CMB observations. Finally, I will discuss what the CMB does, and does not, tell us about the sound horizon at recombination, and what that means for proposed solutions to the Hubble tension.