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New physics from the polarized light of the cosmic microwave background

Abstract

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.

Key points

  • The current cosmological model includes at least three elements (the nature of dark matter and of dark energy, and the origin of all structures in the Universe) whose explanation requires new physics beyond the standard model (SM) of elementary particles and fields. The polarization of the cosmic microwave background (CMB), the afterglow of the primordial fireball Universe, is sensitive to new physics that violates parity symmetry and may shed new light on these three elements.

  • Dark matter and dark energy might be a new pseudoscalar field (like a pion in the SM or an axion in the extension of the SM) that couples to photons in a parity-violating manner and that has rotated the plane of linear polarization of CMB photons as they have been travelling for more than 13 billion years, an effect dubbed ‘cosmic birefringence’.

  • A tantalizing hint of cosmic birefringence has been found in the CMB polarization data of the Planck mission with a statistical significance of 3σ.

  • Quantum-mechanical vacuum fluctuations in spacetime in the early Universe might have produced a stochastic background of primordial gravitational waves (GWs) that are scale-invariant, Gaussian and parity-symmetric.

  • Matter fields, such as non-Abelian gauge fields, might have produced non-scale-invariant, non-Gaussian and parity-violating GWs that could be measured in the CMB polarization, with profound implications for the fundamental physics behind ‘cosmic inflation’, the period of accelerated expansion in the very early Universe.

  • The goal of measuring cosmic birefringence and testing the statistical properties of primordial GWs will require new observational and calibration strategies for future CMB experiments.

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Fig. 1: Observed cosmic microwave background polarization.
Fig. 2: Observed CMB polarization power spectra.
Fig. 3: Rotation of linear polarization by cosmic birefringence.
Fig. 4: Present-day energy density of primordial GWs across 21 decades in frequency.
Fig. 5: B-mode polarization power spectra.

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Acknowledgements

This article is dedicated to the memory of Steven Weinberg. I thank P. Campeti for sharing his work on the axion-SU(2) model and J. Chluba, G. Domènech, G. Dvali, J. R. Eskilt, K. Lozanov, A. Maleknejad, Y. Minami, I. Obata and M. Shiraishi for comments on the draft. The materials in this Review are based partly on the Van der Waals Lecture delivered at the University of Amsterdam in 2020. I thank the institutes of Physics and Astronomy at the University of Amsterdam and the Vrije Universiteit Amsterdam for their hospitality, and the Stichting Van der Waals Fonds for the Johannes Diderik van der Waals rotating chair, which enabled the visit. This work was also supported in part by JSPS KAKENHI grants no. JP20H05850 and no. JP20H05859, and the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy EXC-2094 390783311. This work has also received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 101007633. The Kavli IPMU is supported by World Premier International Research Center Initiative (WPI), MEXT, Japan.

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Nature Reviews Physics thanks Arthur Kosowsky, M. M. Sheikh-Jabbari, Ryo Namba and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Komatsu, E. New physics from the polarized light of the cosmic microwave background. Nat Rev Phys 4, 452–469 (2022). https://doi.org/10.1038/s42254-022-00452-4

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