New physics from the polarized light of the cosmic microwave background

The current cosmological model requires new physics beyond the standard model of elementary particles and fields, such as dark matter and dark energy. Their nature is unknown and so is that of the initial fluctuations in the early Universe that led to the creation of the cosmic structure we see today. Polarized light of the cosmic microwave background (CMB) may hold the answer to these fundamental questions. Here, I discuss two phenomena that could be uncovered in CMB observations. First, if the physics behind dark matter and dark energy violates parity symmetry, their coupling to photons should have rotated the plane of linear polarization as the CMB photons have been travelling for more than 13 billion years. This effect is known as ‘cosmic birefringence’. A tantalizing hint of such a signal has been found with a statistical significance of 3σ. Second, the period of accelerated expansion in the very early Universe, called ‘cosmic inflation’, might have produced a stochastic background of primordial gravitational waves (as yet unobserved). These might have been generated by vacuum fluctuations in spacetime or by matter fields and could be measurable in the CMB polarization. The goal of observing these two phenomena will influence how data from future CMB experiments are collected, calibrated and analysed