Bias in the tensor-to-scalar ratio from self-interacting dark radiation

We investigate the cosmological imprint of self-interacting dark radiation (DR) on the primordial B-mode angular power spectrum and its impact on the estimation of the tensor-to-scalar ratio r. We consider a minimal model in which DR is described as an effectively massless axion-like particle with quartic self-interactions. These interactions are incorporated into the Einstein-Boltzmann equations using the relaxation time approximation and implemented in the CLASS code. We show that increasing the strength of DR self-interactions suppresses anisotropic stress, thereby reducing the damping of gravitational waves and leading to an enhancement of the primordial B-mode signal relative to the free-streaming case. Using mock CMB data and Markov Chain Monte Carlo analyses, we show that neglecting DR self-interactions may bias the inferred value of r by an amount comparable to the uncertainty expected in forthcoming CMB polarization experiments, such as the ground-based Simons Observatory and the satellite missions LiteBIRD and PICO. Our results emphasize the importance of properly modeling DR interactions in future precision searches for primordial B-modes in order to obtain unbiased constraints on inflationary gravitational waves.