[Paper Review] New physics from the polarised light of the cosmic microwave background

Eiichiro Komatsu|arXiv (Cornell University)|Feb 28, 2022

Cosmology and Gravitation Theories4 citations

TL;DR

This paper explores how cosmic microwave background (CMB) polarisation can reveal new physics beyond the Standard Model, focusing on cosmic birefringence—indicating parity-violating physics from dark matter and dark energy—and primordial gravitational waves from matter fields in the early universe. A 3σ hint for cosmic birefringence and distinct statistical properties of matter-sourced gravitational waves are highlighted as key signals for future CMB experiments.

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.

Motivation & Objective

• To investigate whether parity-violating physics in dark matter and dark energy induces cosmic birefringence, detectable via rotation of CMB polarisation planes.
• To explore the origin of primordial gravitational waves beyond vacuum fluctuations, particularly those sourced by matter fields in the early universe.
• To assess how these new physics signals will influence the design, calibration, and data analysis strategies of next-generation CMB experiments.
• To evaluate the potential of CMB polarisation as a probe of quantum vacuum fluctuations and their role in seeding cosmic structure.
• To determine the systematic challenges in detecting these signals, especially as instrumental and astrophysical uncertainties become dominant.

Proposed method

• Analyzing CMB polarisation data from Planck to search for a rotation of the polarisation angle, which would indicate cosmic birefringence due to parity-violating couplings of photons to dark matter and dark energy.
• Using the formalism of linearised cosmological perturbation theory to model how tensor metric perturbations (gravitational waves) generate a quadrupolar photon intensity distribution, leading to CMB polarisation.
• Applying the Stokes parameters and the radiation transfer equation to compute the observed polarisation patterns on the sky, distinguishing between E-mode and B-mode signals.
• Comparing the statistical properties of gravitational waves sourced by matter fields with those from vacuum fluctuations, particularly in their power spectra and correlation with CMB polarisation.
• Evaluating the impact of systematics—both instrumental and astrophysical—on the detection of cosmic birefringence and primordial gravitational waves in upcoming CMB experiments.
• Assessing the implications of a 3σ signal for cosmic birefringence in Planck data for theories of dark energy, dark matter, and quantum gravity.

Experimental results

Research questions

• RQ1 Does the observed 3σ signal in Planck CMB polarisation data for cosmic birefringence indicate a fundamental parity-violating interaction involving dark matter and dark energy?
• RQ2 How do gravitational waves sourced by matter fields in the early universe differ statistically from those generated by vacuum fluctuations in spacetime?
• RQ3 What are the distinct signatures of matter-sourced primordial gravitational waves in CMB polarisation, particularly in B-mode power spectra?
• RQ4 How will the detection of cosmic birefringence or matter-sourced gravitational waves constrain models of dark energy and dark matter?
• RQ5 What new calibration and analysis strategies are required in next-generation CMB experiments to detect these subtle new physics signals?

Key findings

• A 3σ statistical hint for cosmic birefringence has been observed in Planck CMB polarisation data, suggesting a rotation of the polarisation plane due to parity-violating couplings of photons to dark matter and dark energy.
• The observed polarisation pattern on the sky is generated by the scattering of photons with a quadrupolar intensity distribution around electrons at the surface of last scattering, with the direction of polarisation tied to the local photon intensity gradient.
• Tensor-mode gravitational waves (gravitational waves) generate a quadrupolar distortion of spacetime that imprints a characteristic polarisation pattern on the CMB, with E-modes and B-modes arising from different polarisation orientations.
• B-mode polarisation is a clean probe of primordial gravitational waves at linear order, as scalar perturbations do not generate B-modes, making them ideal for detecting tensor modes.
• Primordial gravitational waves sourced by matter fields in the early universe produce statistical properties—such as non-Gaussianity and specific power spectra—distinct from those of vacuum-fluctuation-induced gravitational waves.
• Future CMB experiments like Simons Observatory, CMB-S4, and LiteBIRD will reduce noise by another order of magnitude, necessitating improved control of systematic uncertainties to detect these new physics signals.