based on arXiv 1802.06589
Precise measurements of the Cosmic Microwave Background (CMB) offer a powerful tool to probe physics beyond the most simple LCDM scenario. However, a meaningful comparison requires precise theory predictions. In this work we consider a scenario where LCDM is augmented by elastic scattering between dark matter and photons. This new interaction necessitates modifications to the tight coupling approximation, an approximation scheme commonly used in Boltzmann codes. We explicitly derive these changes, which have been neglected in previous works, and show that their effect on the final CMB spectra is small. Using our results and the latest 2015 Planck data release we set new limits on the allowed cross section to dark matter mass ratio. Our most conservative constraint improves existing limits obtained from CMB measurements by about 20%.