Throughout all known space, between the stars and the galaxies, an extremely faint glow suffuses, a relic left over from the dawn of the Universe. This is the cosmic microwave background (CMB), the first light that could travel through the Universe when it cooled enough around 380,000 years after the Big Bang for 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 twist in the light - something that could be a sign of a violation of parity symmetry, hinting at physics outside the Standard Model.
According to the Standard Model of physics, if we were to flip the Universe as though it were a mirror reflection of itself, the laws of physics should hold firm. Subatomic interactions should occur in exactly the same way in the mirror as they do in the real Universe. This is called parity symmetry.
As far as we have been able to measure so far, there's only one fundamental interaction that breaks parity symmetry; that's the weak interaction between subatomic particles that is responsible for radioactive decay. But finding another place where parity symmetry breaks down could potentially lead us to new physics beyond the Standard Model.
And two physicists - Yuto Minami of the High Energy Accelerator Research Organisation in Japan; 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 hints of it in the polarisation angle of the CMB.
Polarisation occurs when light is scattered, causing its waves to propagate on a certain orientation.
Reflective surfaces such as glass and water polarise light. You're probably familiar with polarised sunglasses, designed to block certain orientations to lessen the amount of light reaching the eye.
Even water and particles in the atmosphere can scatter and polarise light; a rainbow is a good example of this.
The early Universe, for around the first 380,000 years, was so hot and dense that atoms couldn't exist. Protons and electrons were flying around as an ionised plasma, and the Universe was opaque, like a thick smoky fog.
Only once the Universe cooled enough for those protons and electrons to combine into a neutral gas hydrogen atoms did space become clear, allowing photons to travel freely.
As the ionised plasma transitioned into a neutral gas, the photons scattered off electrons, causing the CMB to become polarised. The polarisation of the CMB can tell us a lot about the Universe. Especially if it's rotated at an angle.
This angle, described as β, could indicate a CMB interaction with dark matter or dark energy, the mysterious inward and outward forces that seem to dominate the Universe, but which we're unable to directly detect.
(Y. Minami/KEK)
"If dark matter or dark energy interact with the light of the cosmic microwave background in a way that violates parity symmetry, we can find its signature in the polarisation data," Minami explained.
The problem with identifying β with any certainty is in the technology we use to detect the polarisation of the CMB. The European Space Agency's Planck satellite, which released its most up-to-date observations of the CMB in 2018, is equipped with polarisation-sensitive detectors.
But unless you know exactly how these detectors are oriented relative to the sky, it's impossible to tell whether what you're looking at is actually β, or a rotation in the detector that just looks like β.
The team's technique relies on studying a different source of polarised light, and comparing the two to extract the false signal.
"We developed a new method to determine the artificial rotation using the polarised light emitted by dust in our Milky Way," Minami said. "With this method, we have achieved a precision that is twice that of the previous work, and are finally able to measure β."
Milky Way sources of radiation are from much closer than the CMB, so they are not affected by dark matter or dark energy. Any rotation in the polarisation should, therefore, only be a result of a rotation in the detector.
The CMB is affected by both β and the artificial rotation - so if you subtract the artificial rotation observed in the Milky Way sources from the CMB observations, you should be left only with β.
Using this technique, the team determined that β is non-zero, with a 99.2 percent certainty. That seems pretty high, but it's still not quite enough to claim a discovery of new physics. For that, a confidence level of 99.99995 percent is required.
But the finding certainly demonstrates that the CMB is worth studying more closely.
"It is clear that we have not found definitive evidence for new physics yet; higher 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 are excited because our new method finally allowed us to make this 'impossible' measurement, which may point to new physics."
The research has been published in Physical Review Letters.