This talk is divided in two parts. In the first part, I will describe the ramifications to hidden sector model building if future CMB surveys find no evidence of dark radiation. In particular, using the Millicharged particle model as an example, I will show how CMB measurements can provide leading constraints on BSM portal interactions and how these constraints are largely independent of detailed hidden sector model.
In the second part of the talk, I consider the alternative possibility where we might detect dark radiation in future surveys. Then to find if the dark radiation ever had considerable interaction with Standard Model particles, we need to look for signatures of isocurvature. I show how we can use the distribution of Helium and Deuterium to search for such an isocurvature.