The oldest light in the universe is that of the cosmic microwave background (CMB). This remnant of the Big Bang has traveled for more than 13 billion years. Along the way, it picked up some stories about the history and evolution of the cosmos. We just have to listen to what it has to say.
The CMB tells a story through its polarization, among other things. If you think of light as an oscillating wave, this wave motion can have different orientations. The alignment of the oscillation of a light wave is called polarization. Often times, light is a random jumble of orientations, making it unpolarized, but the light from the CMB is light that was scattered from the hot gas of the early universe and has an orientation known as E-mode polarization.
E-mode and B-mode polarization in the CMB. Photo credit: WMAP Science Team
If there was just an empty, flat space between us and the cosmic microwave background, all of the light from the CMB would be polarized in E-mode. However, space is not empty. It is filled not only with diffuse gas and dust, but also with dark matter and dark energy. As the light from the Big Bang travels through it, its polarization changes slightly and rotates through an angle. This shifts the alignment of the CMB light towards the B-mode polarization.
The orientation of polarized light changes as it moves through the cosmos. Photo credit: Y. Minami / KEK
Studying this effect is very difficult, partly because the displacement is so small, and partly because the source of the displacement is difficult to determine. Recently, a team set out to learn more about it. (^ 1) Using data from the Planck satellite survey, they analyzed the B-mode polarization with a new method that reduces background noise. They were able to measure a small shift in polarization known as cosmic birefringence.
It is possible that the observed shift is only due to systematic measurement errors. However, the team found that there was a 99.2% chance that the effect was real. That's not high enough to confirm the effect, but it's high enough to warrant further investigation. If the effect is confirmed, it could indicate a new understanding of dark matter.
Is that a left hand or a reflection of a right hand? Photo credit: Siora Photography (@ siora18)
Cosmic birefringence would indicate a lack of symmetry known as parity. When matter has parity, an object and its reflection behave the same. For example, a person's left and right hands are mirror images and can move in the same way so they have parity. Our hearts are left and not right, so they violate parity. Normal matter is symmetrical in the way it interacts with CMB light, so it doesn't cause cosmic birefringence. But maybe dark matter does. This would be a new effect that cannot be seen in dark matter and would indicate physics beyond the Standard Model.
It is too early to say if this is a new dark matter clue. Other phenomena such as early cosmic inflation could also induce B-mode polarization in CMB light. However, this study shows that the effect can be measured and that gives us hope that one day we will understand it.
Reference: Minami, Yuto and Eiichiro Komatsu. "New extraction of cosmic birefringence from Planck 2018 polarization data." Physical Review Letters 125.22 (2020): 221301.