Gazing into the Dark Sector through the Lens of Neff

Dr. Dibyendu Nanda

In this talk, I will explore how cosmological observables, in particular the effective number of additional relativistic species (ΔNeff), provide a window to constrain the dark sector physics. The parameter ΔNeff captures deviations from the Standard Model expectation for the effective number of neutrino species and is highly sensitive to new light degrees of freedom in the early universe. I will discuss how precision measurements of ΔNeff can be used to constrain a variety of dark sector scenarios, including models with light hidden particles or feebly interacting species. Furthermore, I will highlight the possible interplay between ΔNeff and the dark matter relic abundance, demonstrating how these seemingly independent observables can provide complementary constraints on the nature and interactions of dark matter. This framework offers a promising avenue for connecting cosmological data with the properties of the dark sector.