What is dark matter made of is the central question in the ongoing search for it. One possible explanation is that dark matter is made up of particles known as axions. A team of astrophysicists led by researchers from the universities of Amsterdam and Princeton has now demonstrated that if dark matter is made up of axions, it may manifest itself as a subtle additional glow emitted by pulsating stars.
Dark matter could be the most sought-after component of our universe. Surprisingly, this mysterious form of matter, which physicists and astronomers have yet to detect, is assumed to make up a large portion of what is out there. At least 85% of matter in the universe is thought to be ‘dark,’ visible only through the gravitational pull it exerts on other astronomical objects. Scientists, understandably, want more. They want to see dark matter, or at the very least detect it directly, rather than infer it from gravitational effects. And, naturally, they want to know what it is.
The theory and simulation results were then put to the first observational test. The researchers compared the observed radio waves to the models using observations from 27 nearby pulsars to see if any measured excess could provide evidence for the existence of axions.
Cleaning up two problems
One thing is certain: dark matter cannot be the same material as you and I. If that were the case, dark matter would simply behave like ordinary matter, forming objects like stars, lighting up, and no longer being referred to as “dark.” Scientists are therefore looking for something new — a type of particle that has yet to be discovered and that most likely only interacts weakly with the particles we know, explaining why this component of our world has remained elusive thus far.
There are numerous hints as to where to look. One widely held belief is that dark matter is composed of axions. This hypothetical type of particle was first proposed in the 1970s to solve a problem unrelated to dark matter. The separation of positive and negative charges within the neutron, one of the building blocks of ordinary atoms, was discovered to be unusually small. Scientists naturally wanted to know why. It was discovered that the presence of a previously unknown type of particle, interacting very weakly with the neutron’s constituents, could cause precisely such an effect.
The later Nobel Prize winner Frank Wilczek came up with a name for the new particle: axion — not just similar to other particle names like proton, neutron, electron and photon, but also inspired by a laundry detergent of the same name. The axion was there to clean up a problem.
In fact, despite never being detected, it might clean up two. Several theories for elementary particles, including string theory, one of the leading candidate theories to unify all forces in nature, appeared to predict that axion-like particles could exist. If axions were indeed out there, could they also constitute part or even all of the missing dark matter? Perhaps, but an additional question that haunted all dark matter research was just as valid for axions: if so, then how can we see them? How does one make something ‘dark’ visible?
Shining a light on dark matter
Fortunately, it seems that for axions there may be a way out of this conundrum. If the theories that predict axions are correct, they are not only expected to be mass-produced in the universe but some axions could also be converted into light in the presence of strong electromagnetic fields. Once there is light, we can see. Could this be the key to detecting axions — and therefore to detecting dark matter?
To answer that question, scientists first had to ask themselves where in the universe the strongest known electric and magnetic fields occur. The answer is: in regions surrounding rotating neutron stars also known as pulsars. These pulsars — short for ‘pulsating stars’ — are dense objects, with a mass roughly the same as that of our Sun, but a radius that is around 100,000 times smaller, only about 10 km. Being so small, pulsars spin with enormous frequencies, emitting bright narrow beams of radio emission along their axis of rotation. Similar to a lighthouse, the pulsar’s beams can sweep across the Earth, making the pulsating star easily observable.
However, the pulsar’s enormous spin has a greater impact. It converts the neutron star into a powerful electromagnet. This could imply that pulsars are highly efficient axion factories. A 50-digit number of axions could be produced by an average pulsar every second. Because of the strong electromagnetic field surrounding the pulsar, some of these axions may be converted into visible light. That is, if axions exist at all; however, the mechanism can now be used to answer that question. Simply examine pulsars to see if they emit extra light, and if so, determine whether this extra light is caused by axions.
Simulating a subtle glow
Of course, as is always the case in science, carrying out such an observation is not so simple. The light emitted by axions, detectable as radio waves, would be a small fraction of the total light emitted by these bright cosmic lighthouses. To be able to see the difference between a pulsar without axions and a pulsar with axions, one must first understand how to quantify that difference and convert it into a measurement of the amount of dark matter.
This is exactly what a team of physicists and astronomers have now done. In a collaborative effort between the Netherlands, Portugal, and the USA, the team has constructed a comprehensive theoretical framework that allows for a detailed understanding of how axions are produced, how axions escape the gravitational pull of the neutron star, and how, during their escape, they convert into low energy radio radiation.
The theoretical results were then used on a computer to simulate the formation of axions around pulsars, using cutting-edge numerical plasma simulations originally developed to understand the physics behind how pulsars emit radio waves. The propagation of the axions through the neutron star’s electromagnetic fields was simulated after they were virtually produced. This enabled the researchers to quantify the subsequent production of radio waves and model how this process would provide an additional radio signal in addition to the intrinsic emission generated by the pulsar itself.
Putting axion models to a test
The theory and simulation results were then put to the first observational test. The researchers compared the observed radio waves to the models using observations from 27 nearby pulsars to see if any measured excess could provide evidence for the existence of axions. Unfortunately, the answer was ‘no’ – or, perhaps more optimistically, ‘not yet’. Axions don’t immediately stand out to us, but that’s to be expected. If dark matter were to reveal its secrets so easily, it would have been discovered a long time ago.
As a result, the hope for a smoking-gun detection of axions now rests on future observations. Meanwhile, the current lack of axion radio signal observation is an intriguing result in and of itself. The first comparison of simulations and actual pulsars has placed the most stringent limits on the interaction of axions with light to date.
Of course, the ultimate goal is to do more than just set limits; it is to either demonstrate that axions exist or to demonstrate that axions are extremely unlikely to be a constituent of dark matter at all. The new results are just a first step in that direction; they are only the beginning of what could become an entirely new and highly cross-disciplinary field that has the potential to dramatically advance the search for axions.
