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Chiral Photons from Chiral Gravitational Waves
Keisuke Inomata1,2, Marc Kamionkowski3
1ICRR, University of Tokyo, Kashiwa 277-8582, Japan.
Physical Review Letters
|August 7, 2019
Summary
A chiral gravitational-wave background can induce circular polarization in cosmic microwave background photons. However, even non-chiral backgrounds can mimic this effect, requiring specific conditions to confirm chirality.
Area of Science:
- Cosmology
- Astrophysics
- Particle Physics
Background:
- Gravitational waves (GWs) are ripples in spacetime predicted by general relativity.
- The cosmic microwave background (CMB) is the afterglow of the Big Bang.
- Parity symmetry is a fundamental concept in physics, dictating that physical laws remain unchanged under spatial inversion.
Purpose of the Study:
- To investigate the potential for a chiral gravitational-wave background to induce a detectable circular polarization in the CMB.
- To determine the conditions under which such a signal could be distinguished from parity-conserving phenomena.
Main Methods:
- Theoretical modeling of chiral and non-chiral gravitational-wave backgrounds.
- Calculation of induced circular polarization in the CMB.
- Analysis of the magnitude and statistical properties of the polarization signal.
Main Results:
- A chiral GW background can induce a uniform circular polarization (V₀₀) in the CMB, proportional to its chirality parameter (Δχ) and tensor-to-scalar ratio (r).
- A non-chiral GW background, through spontaneous parity breaking, can also generate uniform circular polarization, but with a smaller magnitude.
- Distinguishing a truly chiral GW background requires Δχ ≳ 0.12(r/0.06)⁻¹/².
Conclusions:
- The study serves as a proof of principle that cosmological parity breaking via chiral GWs can imprint chirality onto CMB photons.
- It highlights that seemingly parity-breaking cosmological signals may arise from underlying parity-conserving physics.
- Current detectable limits are beyond foreseeable experimental capabilities, but the theoretical framework is established.
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