A hint of new physics in the polarized radiation of the first universe

Using Planck’s data from microwave cosmic background radiation, an international team of researchers observed a hint of new physics. The team developed a new method to measure the angle of polarization of ancient light by calibrating it with dust emission from our own Milky Way. Although the signal is not detected with sufficient precision to draw definitive conclusions, it may suggest that dark matter or dark energy is causing a violation of the so-called “symmetry of parity.”

It is believed that the laws of physics governing the universe do not change when returned to a mirror. For example, electromagnetism works the same whether you are in the original system or in a mirrored system in which all spatial coordinates have been reversed. If this symmetry, called “parity”, is violated, it may hold the key to understanding the elusive nature of dark matter and dark energy, which respectively occupy 25 and 70% of the energy budget of the universe today. . Although both dark, these two components have opposite effects on the evolution of the universe: dark matter attracts, while dark energy causes the universe to grow ever faster.

A new study, including researchers from the Institute for Particle and Nuclear Studies (IPNS) of the High Energy Accelerator Research Organization (KEK), the Kavli Institute for Physics and Mathematics of the Universe (Kavli IPMU ) at the University of Tokyo, and the Max Planck Institute for Astrophysics (MPA), report a tantalizing hint of new physics – with a 99.2% confidence level – that violates parity symmetry. Their results were published in the journal Physical examination letters November 23, 2020; the article was chosen as “Suggestion from the editors”, judged by the editors of the journal as important, interesting and well written.

The indication of a violation of parity symmetry was found in microwave cosmic background radiation, the residual light of the Big Bang. The key is polarized light from the cosmic microwave background. Light is an electromagnetic wave that propagates. When it comes to waves oscillating in a privileged direction, physicists call it “polarized”. Polarization occurs when light is scattered. Sunlight, for example, is made up of waves with all possible directions of oscillation; thus, it is not polarized. The light of a rainbow, on the other hand, is polarized because sunlight is diffused by water droplets in the atmosphere. Likewise, cosmic microwave background light first became polarized when it was scattered by electrons 400,000 years after the Big Bang. While this light has passed through the universe for 13.8 billion years, the interaction of the microwave cosmic background with dark matter or dark energy could rotate the plane of polarization by an angle β (Figure ).

“If dark matter or dark energy interacts with microwave cosmic background light in a way that violates parity symmetry, we can find its signature in the polarization data,” emphasizes Yuto Minami, postdoctoral fellow at IPNS, KEK.

To measure the angle of rotation β, scientists needed detectors sensitive to polarization, such as those on board the Planck satellite of the European Space Agency (ESA). And they needed to know how the polarization sensitive detectors are oriented relative to the sky. If this information was not known with sufficient precision, the measured plane of polarization would appear to be artificially rotated, creating a false signal. In the past, the uncertainties on the artificial rotation introduced by the detectors themselves limited the measurement accuracy of the cosmic polarization angle β.

“We have developed a new method to determine artificial rotation using polarized light emitted by dust in our Milky Way,” Minami said. “With this method, we have achieved twice the accuracy of previous work, and are finally able to measure β.” The distance that dust light travels in the Milky Way is much shorter than that of the microwave cosmic background. This means that the dust emission is not affected by dark matter or dark energy, i.e. β is only present in cosmic microwave background light, while artificial rotation affects both. The difference in the polarization angle measured between the two light sources can thus be used to measure β.

The research team applied the new method to measure β from polarization data taken by the Planck satellite. They found an index of violation of parity symmetry with a confidence level of 99.2%. To claim a discovery of new physics, much greater statistical significance, or a 99.99995% confidence level, is needed. Eiichiro Komatsu, Director of AMP and Principal Investigator at IPMU Kavli, said: “It is clear that we have yet to find definitive evidence for new physics; higher statistical significance is needed to confirm this signal. But we are excited because our new This method finally allowed us to make this “impossible” measurement, which may indicate new physics. “

To confirm this signal, the new method can be applied to any of the existing – and future – experiments measuring the polarization of the microwave cosmic background, such as the Simons Array and LiteBIRD, in which KEK and Kavli IPMU are involved.