Speaker
Description
The number of extra relativistic degrees of freedom, $\Delta N_{\rm eff}$ , has recently received attention as a possible way to alleviate the Hubble tension. Non-standard values, i.e. $\Delta N_{\rm eff} \neq 0$, can arise from different physical origins, such as the presence of additional ultra-relativistic species or non-standard values of the temperature ratio between photons and standard model neutrinos. In this talk I will show how these distinct origins of $\Delta N_{\rm eff}$ yield different observable features by altering predictions for the CMB and matter power spectra as well as primordial element abundances from BBN in different ways. To obtain meaningful results a consistent treatment and implementation of BSM likelihoods and models is essential. I will discuss how these issues can be addressed with the new CosmoBit module of the Global and Modular Beyond-the-StandardModel Inference Tool (GAMBIT).