Observing left-right symmetry in the cosmic microwave background

We consider the possibility of probing the left-right symmetric model (LRSM) via the cosmic microwave background (CMB). We adopt the minimal LRSM with Higgs doublets, also known as the doublet left-right model (DLRM), where all fermions including the neutrinos acquire masses only via their couplings to the Higgs bidoublet. Because of the Dirac nature of light neutrinos, there exist additional relativistic degrees of freedom which can thermalize in the early Universe by virtue of their gauge interactions corresponding to the right sector. We constrain the model from Planck 2018 bound on the effective relativistic degrees of freedom and also estimate the prospects for planned CMB stage IV experiments to constrain the model further. We find that the W-R boson mass below 4.06 TeV can be ruled out from the Planck 2018 bound at 2 sigma C.L. in the exact left-right symmetric limit which is equally competitive as the LHC bounds from dijet resonance searches. On the other hand, the Planck 2018 bound at 1 sigma C.L. can rule out a much larger parameter space out of reach of present direct search experiments, even in the presence of additional relativistic degrees of freedom around the TeV corner. We also study the consequence of these constraints on dark matter in the DLRM by considering a right-handed real fermion quintuplet to be the dominant dark matter component in the Universe.