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Throughout known space, between stars and galaxies, an extremely faint glow pervades, a relic left by the dawn of the Universe. This is the cosmic microwave background (CMB), the first light that could travel across the Universe when it cooled down enough around 380,000 years after the Big Bang to allow ions and electrons to combine into atoms.
But now scientists have discovered something peculiar about the CMB. A new measurement technique has revealed hints of a turn in the light, something that could be a sign of a violation of parity symmetry, suggesting physics outside the Standard Model.
According to the standard model of physics, if we were to flip the Universe as if it were a mirror reflection of itself, the laws of physics would have to remain firm. Subatomic interactions should happen in the mirror in exactly the same way they do in the real Universe. This is called parity symmetry.
As far as we have been able to measure so far, there is only one fundamental interaction that breaks the parity symmetry; this is the weak interaction between the subatomic particles which is responsible for the radioactive decay. But finding another point where parity symmetry breaks could potentially lead us to new physics beyond the standard model.
And two physicists: Yuto Minami of the Japan High Energy Accelerator Research Organization; and Eiichiro Komatsu of the Max Planck Institute for Astrophysics in Germany and Kavli Institute for the Physics and Mathematics of the Universe in Japan – believe they have found clues in the polarization angle of the CMB.
Polarization occurs when light is scattered, causing its waves to propagate in a certain orientation.
Reflective surfaces such as glass and water polarize the light. You’re probably familiar with polarized sunglasses, which are designed to block certain orientations to reduce the amount of light reaching the eye.
Even water and particles in the atmosphere can scatter and polarize light; a rainbow is a good example of this.
The early Universe, for about the first 380,000 years, was so hot and dense that atoms could not exist. Protons and electrons flew around like ionized plasma and the Universe was opaque, like a thick smoky fog.
Only once the Universe cooled enough to allow those protons and electrons to combine into a neutral gas did the hydrogen atoms of space become clear, allowing photons to travel freely.
When the ionized plasma switched to a neutral gas, the photons scattered the electrons, causing the CMB to polarize. The polarization of the CMB can tell us a lot about the Universe. Especially if it is rotated by an angle.
This angle, described as β, could indicate a CMB interaction with dark matter or dark energy, the mysterious internal and external forces that appear to dominate the Universe, but which we cannot detect directly.
(Y. Minami / KEK)
“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,” Minami explained.
The problem with identifying β with some certainty is in the technology we use to detect the polarization of the CMB. The European Space Agency’s Planck satellite, which released its most up-to-date CMB observations in 2018, is equipped with polarization-sensitive detectors.
But unless you know exactly how these detectors are oriented relative to the sky, it is impossible to tell if what you are looking at is actually β, or a rotation in the detector that looks like β.
The team’s technique is based on studying a different source of polarized light and comparing the two to extract the false signal.
“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 Milky Way’s radiation sources come from much closer than the CMB, so they are not affected by dark matter or dark energy. Any rotation in polarization should, therefore, only be the result of a rotation in the detector.
The CMB is affected by both β and artificial rotation, so if you subtract the observed artificial rotation in the Milky Way sources from the CMB observations, you should be left with β alone.
Using this technique, the team determined that β is non-zero, with a certainty of 99.2%. It sounds pretty tall, but it’s still not high enough to claim a new physics discovery. For this, a confidence level of 99.99995% is required.
But the finding certainly proves that CMB is worth studying more closely.
“It is clear that we have not yet found definitive evidence for the new physics; greater statistical significance is needed to confirm this signal,” said astrophysicist Eiichiro Komatsu of the Kavli Institute for the Physics and Mathematics of the Universe.
“But we’re thrilled because our new method has finally allowed us to make this ‘impossible’ measurement, which could indicate new physics.”
The research was published in Physical Review Letters.
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