Cosmological imprints of Dark matter and BSM particles.

Relativistic degrees of freedom or N_eff is one of the crucial
cosmological parameters and is sensitive to extra radiation energy
density at the time of neutrino decoupling. The precise measurement
of $N_eff$ at the time of CMB formation by Planck 2018 can be used
to understand fundamental interactions and to shed light on beyond
standard model (BSM) scenarios. On the other hand, widely studied
freeze in Dark matter scenarios are challenging to detect in current
direct detection experiments due to its tiny coupling (~10^{-9}).
In this talk I will explain how N_eff can probe light (MeV) freeze
in dark matter models which contain additional neutrino injection
at late time. We propose a scenario where a long lived scalar decays
to a dark matter and active neutrinos after BBN. Despite the feeble
coupling of DM the parameter space can be probed via N_eff. I will
also talk about how N_eff at CMB can constrain light Z' gauge boson
realized in BSM  U(1)_X scenarios. Our detailed analysis shows N_eff
puts the most stringent bound on MeV scale Z' with small couplings.