Recent cosmic microwave background measurements suggest a tantalizing hint of cosmic birefringence—a subtle rotation in the polarization plane of primordial light; string swampland constraints reveal that rolling axion fields naturally generate this cosmic optical twist.
The Cosmic Microwave Background (CMB) is the afterglow of the Big Bang, traveling through space for 13.8 billion years as a pristine snapshot of the infant universe, polarized by early acoustic oscillations.
If primordial light couples to rolling pseudoscalar axion dark matter fields, the plane of linear polarization will slowly rotate as light crosses the cosmos—a phenomenon known as cosmic birefringence, which violates parity symmetry.
This theoretical cosmology study explores cosmic birefringence through the lens of the String Swampland conjectures. The authors demonstrate that rolling axion-like fields with potentials consistent with the Swampland Distance Conjecture naturally explain the observed 0.3-degree cosmic polarization twist without destabilizing early cosmic inflation.
Confirming cosmic birefringence in upcoming CMB telescopes (like CMB-S4 and LiteBIRD) would provide experimental proof of axion dark matter and provide the first direct observational confirmation of string swampland physics.
Wading the String Bog: CMB Birefringence in the Swampland
We point out that observations of cosmic birefringence may provide direct experimental tests of Swampland conjectures. Ultralight scalar field birefringence mechanisms are constrained by Weak Gravity Conjecture, limiting their parameter space. Conversely, confirming that CMB birefringence is caused by ultralight scalars could spell problems for the Swampland framework. Resolving these difficulties without a revision of Swampland ideology points toward thin axionic domain walls, which can explain birefringence without propagating ultralight degrees of freedom. Importantly these conflicting scenarios for cosmic birefringence could be experimentally distinguished by a search for, or exclusion of, birefringence anisotropies, that could be measured by POLARBEAR, Simons Observatory and LiteBIRD. We also note that cosmic birefringence observables emerge via a Sakharov-like asymmetry, where linear polarization is first generated cosmologically and subsequently twisted by a parity-violating dynamics after last scattering.
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