The standard cosmological model (ΛCDM) relies on six free parameters, five of which have direct cosmological origins, while the sixth, the optical depth (τ), is an ad hoc parameter used to model the scattering effects experienced by cosmic microwave background (CMB) photons. CMB photons decoupled from the primordial plasma at z ~ 1100 when neutral hydrogen formed, reducing the abundance of free electrons, and ultimately halting Thomson scattering. However, ultraviolet photons emitted by the first stars and galaxies later (z ~ 10) reionized the intergalactic medium, creating free electrons that free streaming CMB photons will then scatter off again.
In this talk, I will demonstrate that τ can be transformed from free parameter to a derived one, unlocking several benefits. This approach not only tightens cosmological constraints from CMB data — achieving gains equivalent to a CMB satellite— but also simultaneously provides the first detection of key astrophysical parameters governing reionization. Our methodology promises to shed new light into fundamental questions that have so far been limited by degeneracies with τ, such as the hierarchy of the neutrino masses.
BIO
Paulo got his bachelor in University of Costa Rica (his home country) in 2013. He obtained his PhD at The Ohio State University under the supervision of Chris Hirata in 2019 and later moved to a postdoctoral position / shuimu fellowship at Tsinghua University in Yi Mao’s group. Paulo joined Pengcheng laboratory as an assistant researcher in 2022. His research focuses on cosmic reionization, the Lyman-alpha forest, 21 cm cosmology, and the Cosmic Microwave Background. Paulo is also interested in dark matter research being best known for establishing the asteroid mass range for primordial black holes to be the totality of dark matter.
