New physics from the polarised light of the cosmic microwave background

Abstract

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 3σ . 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.

Cosmic Birefringence from Planck Public Release 4

“…This rotation is what we call "cosmic birefringence" because it is as if space itself acted like a birefringent material (see Ref. 7 for a review).…”

Is cosmic birefringence due to dark energy or dark matter? A tomographic approach

“…Ground-based experiments can constrain the value of m φ , especially at 10 −28 eV, almost independently of α. Together they can discover new physics and provide new scientific opportunities for CMB experiments [9].…”

“…We define the cosine between q and the photon propagation direction as µ ≡ q • k/(qk). We then write the Boltzmann equation for the Fourier coefficients of Q ± iU , ±2 ∆ P (η, q, µ), as [9]…”

“…This effect is often referred to as "cosmic birefringence," as it resembles birefringence in a material (see Ref. [9] for a review).…”

Cosmic Birefringence from Monodromic Axion Dark Energy

“…In the presence of axion-photon coupling, the displacement of the axion field along the light path induces a small difference in the phase velocity between the left-and right-handed photon that might contribute to what is known as cosmic birefringence (CB) [20][21][22][23]. This effect rotates the polarization plane of linearly polarized light by an amount β, called birefringence angle (see [24] for a recent review on this topic).…”