The plot thickens: independent confirmation of early Universe data hints at new physics
In recent years, a tantalizing hint of new physics was found in polarization data of the cosmic microwave background from the WMAP and Planck space missions. The so called “cosmic birefringence” is violating parity symmetry, however, the validity of the result was questioned, because the analysis method depends on the modeling of Galactic dust emission. Now, the cosmological interpretation of the signal gains strength as MPA scientists find a comparable effect in the newest data release from the Atacama Cosmology Telescope without relying on Galactic emission. If further independent observations confirm this result as a genuine cosmological signal, it would have profound implications for the fundamental laws of physics and shed light on the mysterious nature of dark matter and dark energy.
The exact physical nature of dark energy and dark matter remains elusive. The study of parity symmetry – the symmetry of the laws of physics under an inversion of spatial coordinates – can provide new insights into this puzzle. In the standard model of elementary particles and fields, the violation of parity symmetry is confined to the weak interactions at atomic scales. Therefore, observations of parity violation at cosmological scales will provide stringent constraints on dark matter and dark energy models.
Some theories predict that dark matter and/or dark energy could be made of a parity-violating particle. Axions would be a prime example. If coupled to the standard electromagnetism, this new particle would rotate the plane of linear polarization of electromagnetic waves as they propagate through space. Even though small, this effect would accumulate over time and potentially show up in the cosmic microwave background (CMB) – the afterglow of the primordial Universe. In particular, the “cosmic birefringence” would have a distinct effect on the polarization of the CMB, as it would make the observed signal appear rotated with respect to the standard prediction (see Figure 1).
The use of cosmic birefringence as a probe for new particles is not a new idea. It was originally proposed in the late 1990s. However, its practical implementation has been severely limited by the insufficient calibration of CMB experiments. If the orientation of polarization-sensitive detectors relative to the sky is imprecisely known, this makes polarization measurements appear artificially rotated, creating a false signal that obfuscates cosmic birefringence.
In recent years, the quest for cosmic birefringence has regained momentum after a team of international scientists led by MPA's Eiichiro Komatsu found tantalizing hints of such a signal in the CMB polarization data measured by NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and ESA’s Planck space missions. This series of works (previously highlighted in 2020 and 2022) reports a β=0.34°±0.09° birefringence angle obtained through an analysis technique that calibrates the orientation of detectors against the polarized emission from Galactic dust clouds. Although still under scrutiny for its dependence on Galactic dust modeling, this exciting result has inspired the scientific community to search for matching rotations in other CMB experiments.
The Atacama Cosmology Telescope (ACT; Figure 2) was the first ground-based experiment to provide an independent test. Last March, the ACT Collaboration released the polarization data taken by the advanced ACT receiver at frequencies of 90, 150, and 220 GHz, in what they called the data release 6 (DR6). Compared to previous datasets, ACT DR6 has about three times better sensitivity and five times better spatial resolution than Planck.
In a reanalysis of ACT DR6, Patricia Diego Palazuelos and Eiichiro Komatsu explored the impact of instrumental systematics on the measurement of cosmic birefringence by incorporating the knowledge of the telescope optics to separate the instrumental and cosmological components of the observed rotation. This analysis builds on previous efforts from the ACT team to model and determine the rotation of the incoming light’s polarization created by the telescope’s optics. Using Bayesian statistics to marginalize over the uncertainties in the optics model, they find β=0.215°±0.074° with a 99.6% confidence level.
Although there could still remain systematics in the ACT data that are not understood and would not allow us to draw strong cosmological conclusions, it is suggestive that independent datasets and analyses using different methodologies have yielded the same sign and comparable magnitudes for β (Figure 3).
However, definitive proof of cosmic birefringence will require independent confirmation from experiments such as BICEP3, CLASS, and the Simons Observatory. These experiments are developing artificial polarization sources to calibrate the orientation of their polarization-sensitive detectors. Once fully developed and honed, these calibration procedures will allow them to robustly measure cosmic birefringence without relying on models of optics or Galactic polarization. The discovery of cosmic birefringence is a clear sign of new physics and will have a revolutionary impact on cosmology and fundamental physics.
