by Ari Cukierman, KIPAC postdoctoral fellow
The cosmic microwave background (CMB), the afterglow of the Big Bang, has been a treasure trove of information about the cosmos since its discovery in the 1960s. The CMB is detectable as a faint background of microwaves, which we measure with specialized telescopes in remote locations like the high Andes and at the South Pole. (To read about the adventures of previous KIPAC alums working on CMB instrumentation at the Pole, see e.g. this 2015 interview with Val Monticue, this 2016 interview with Albert Wandui, or this 2017 research highlight about Kyle Story).
In this research highlight, I will describe a new method by which the CMB may help solve the mystery of dark matter.
One of the central mysteries in modern cosmology is that of dark matter. We see galaxies moving in ways that suggest an invisible source of gravity, and we find that the distribution of matter in the Universe suggests the same. We call this mysterious source of gravity dark matter for the simple reason that we can’t see it directly—in other words, it doesn’t emit light that we can detect with telescopes. We don’t know what dark matter is at a fundamental level, but we do know roughly where it is and roughly how much there is. It turns out there is about five times as much dark matter in the Universe as there is ordinary visible matter, so we’re talking about a rather large blind spot in our understanding of the cosmos.
Many ideas have been proposed for what this dark matter could possibly be. This conversation involves many different subdisciplines of physics, since the consequences are relevant simultaneously in astrophysics and in particle physics. At the same time, the experimental techniques required to test these ideas bring in even more fields, such as condensed-matter physics, electrical engineering, cryogenics, optics, nuclear physics, high-performance computing, and more.
The subtle interplay of axions and light
One of the most popular candidates for dark matter is a particle called the axion. If axions (or axion-like particles) exist, they interact with the electromagnetic field in subtle ways, including the optical wavelengths of the electromagnetic field we see as visible light. These effects are small enough that it is still fair to use the label dark matter, but many experiments have been optimized to be sensitive enough to look for these subtle effects. In the case of axions, we can search for how they affect the polarization of light.
You may be familiar with the phenomenon of polarization from everyday objects like sunglasses. Some sunglasses have polarized lenses, which means that they block electromagnetic waves vibrating along certain directions more than along other directions. (If you have polarized sunglasses, you can do a simple test to see this phenomenon directly. If you’re outside on a sunny day, simply tilt your head 90 degrees to the left or right. What you will notice is that everything appears a little brighter. That’s because the light that reflects off the ground and into your eyes has a little more of one polarization than the other.)
When light travels through a region occupied by a substantial population of axions, because of subtle properties of how the electromagnetic field interacts with the axion field, the polarization of the light rotates. And because the phase of the axion field is oscillating quantum mechanically in time, this causes the polarization of the light to appear to oscillate back and forth over time.
What this means for the CMB
As a propagating electromagnetic signal travelling to us from the early Universe, the microwaves of the CMB come to us with an intensity and temperature which have been explored in depth for several decades. They also have a specific polarization signal which has been analyzed more carefully only in recent years, due to the information that polarization carries about the density structure of the early Universe, and possibly even about inflation.
If dark matter is composed of axions, we must be living in a sea of axions right now. We should be able to see the polarized light of the CMB oscillating over time as it travels through those axions to our CMB experiments. And the advantage of CMB experiments is that they have been making very sensitive measurements over the course of many years of intensity, and more recently, polarization, so they turn out serendipitously to be very well matched to this search.
Note that while we don’t know the original polarization of the CMB microwaves, the rotating polarization effect is a time-dependent phenomenon, since the axion field itself is oscillating in time, causing the orientation of the polarization to change with it. The time period the polarization takes to rotate through a certain angle depends on details of the coupling between axions and photons, but in the generally expected domain, this can happen over minutes to days.
Ultimately, if we detect the polarization of the CMB across the entire sky rotate over a certain amount of time, the finding will constitute actual evidence that axions are among us. Or, rather, that we are among axions.
Searching for oscillations
Collecting data on the polarization of the CMB is not new, but the reason we haven't seen any indication of rotation is that the data are never processed in a way that would reveal it. The assumption in CMB experiments has been that the CMB is static, so the data are accumulated over time in much the same way that a photograph accumulates light over the course of an exposure. If the subject is changing during the exposure, the photograph will only show a slight blurring. If you instead take many short exposures and form a kind of motion picture, then you will see the changes more clearly.
I am currently implementing such a search for oscillations using nearly a decade of data collected by the BICEP series of experiments at the South Pole. At the same time, the South Pole Telescope and BICEP have formed a partnership called the South Pole Observatory. By combining our datasets, we can achieve even better sensitivity.
There are multiple types of experiments searching for axions in a variety of ways. The search of oscillations in the polarization of the CMB is just one method among several. Currently operating CMB experiments are roughly as sensitive as other probes, and next-generation experiments will do even better.
The 2020s will see the design and construction of the next generation of CMB experiments, called CMB Stage 4. This will bring together the entire US CMB community to build telescopes of unprecedented sensitivity in both the Atacama Desert in Chile and at the South Pole.
There is plenty of work left to do…!
------------ Related reading
Axion Dark Matter Detection with CMB Polarization (arXiv link for Fedderke et al. (2019) )
Detecting black hole gravitation atoms in the sky (with half-diamonds) --- previous 2015 KIPAC research highlight about another potential way to see the effects of axion dark matter in gravitational wave signals received at the Earth