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

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

The laws of physics that govern the universe are thought not to change when they are shot in a mirror. For example, electromagnetism works the same regardless of whether you are in the original system or in a mirror system where all spatial coordinates have been flipped. If this symmetry, called “parity,” is violated, it could be the key to understanding the elusive nature of dark matter and dark energy, which today occupy 25 and 70 percent of the universe’s energy balance, respectively. 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 expand faster and faster.

A new study, which includes researchers from the Institute of Particle and Nuclear Studies (IPNS) at the High Energy Accelerator Research Organization (KEK), the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) of the University of Tokyo , and the Max Planck Institute for Astrophysics (MPA), reports a tantalizing suggestion of new physics, with a 99.2% confidence level, which violates parity symmetry. Their findings were published in the journal Physical Review Letters on November 23, 2020; the document was selected as an “Editors’ Tip”, deemed important, interesting and well written by the editors of the magazine.

The clue to a violation of parity symmetry was found in the cosmic microwave background radiation, the residual light from the Big Bang. The key is polarized light from the cosmic microwave background. Light is an electromagnetic wave that propagates. When it is made up of waves oscillating in a preferred 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; therefore, it is not polarized. The light from a rainbow, meanwhile, is polarized because sunlight is scattered by water droplets in the atmosphere. Similarly, cosmic microwave background light initially became polarized when scattered by electrons 400,000 years after the Big Bang. Since this light has traveled across the universe for 13.8 billion years, the interaction of the cosmic microwave background with dark matter or dark energy could cause the polarization plane to rotate by an angle β (Figure).

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

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

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

The research team applied the new method to measure β from polarization data taken from the Planck satellite. They found a suggestion for parity symmetry violation with a 99.2% confidence level. To claim a discovery of new physics, much greater statistical significance or a confidence level of 99.99995% is required. Eiichiro Komatsu, director of the MPA and principal investigator at the IPMU in Kavli, said: “It is clear that we have not yet found definitive evidence for the new physics; more statistical significance is needed to confirm this signal. But we are excited because our new method has finally allowed us to make this “impossible” measurement, which could indicate new physics. “

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