  Cosmology requires new physics beyond the Standard Model of elementary
particles and fields. What is the fundamental physics behind dark matter and
dark energy? What generated the initial fluctuations in the early Universe?
Polarised light of the cosmic microwave background (CMB) may hold the key to
answers. In this article, we discuss two new developments in this research
area. First, if the physics behind dark matter and dark energy violates parity
symmetry, their coupling to photons rotates the plane of linear polarisation as
the CMB photons travel more than 13 billion years. This effect is known as
`cosmic birefringence': space filled with dark matter and dark energy behaves
as if it were a birefringent material, like a crystal. A tantalising hint for
such a signal has been found with the statistical significance of $3\sigma$.
Next, the period of accelerated expansion in the very early Universe, called
`cosmic inflation', produced a stochastic background of primordial
gravitational waves (GW). What generated GW? The leading idea is vacuum
fluctuations in spacetime, but matter fields could also produce a significant
amplitude of primordial GW. Finding its origin using CMB polarisation opens a
new window into the physics behind inflation. These new scientific targets may
influence how data from future CMB experiments are collected, calibrated, and
analysed.
