New research explores the possibility that dark matter consists of theoretical particles called axions, focusing on detecting them through the extra light emitted by pulsars. Preliminary observations have yet to confirm axions, but the research is crucial to understanding dark matter.

The core question in the current search for dark matter is: What is dark matter made of? One possible answer is that dark matter consists of particles called axions. Recent research by astrophysicists at the University of Amsterdam and Princeton University proposes that if dark matter is indeed made of axions, it might manifest itself in the form of a faint extra glow from pulsating stars.

Dark matter may be the hottest ingredient in our universe. Surprisingly, this mysterious form of matter, which physicists and astronomers have so far eluded to detect, is thought to make up a large portion of the matter in the universe. No less than 85% of the matter in the universe is suspected to be "dark matter", which can currently only be detected through the gravitational pull it exerts on other astronomical objects. Scientists understandably want more. They want to actually see dark matter—or at least detect its presence directly, rather than just infer it from gravitational effects. Of course, they also want to know what dark matter is.

Solve two problems

One thing is clear: Dark matter cannot be the same stuff you and I are made of. If that were the case, dark matter would behave like ordinary matter - it would form objects like stars, glow and no longer be "dark". So scientists are looking for something new - a yet-to-be-detected particle that likely only interacts very weakly with the types of particles we know, which explains why this component of our world has remained elusive so far.

We have many clues to look for. One popular hypothesis is that dark matter may be made of axions. This hypothetical particle type first appeared in the 1970s to solve a problem unrelated to dark matter. As one of the building blocks of ordinary atoms, the separation of positive and negative charges within the neutron is surprisingly small. Scientists certainly want to know why. It turns out that a hitherto undiscovered particle interacts very weakly with a component of the neutron to produce exactly this effect. Later Nobel Prize winner Frank Wilczek gave the new particle a name: the axion - not only similar in name to other particles such as protons, neutrons, electrons and photons, but also inspired by a washing powder of the same name. Axions emerged to solve a problem.

In fact, although it has never been detected, it may solve two problems. Several fundamental particle theories, including string theory (one of the leading candidates to unify all forces in nature), appear to predict the possible existence of axion-like particles. If axions do exist, could they also make up some or even all of the missing dark matter? Maybe so, but another question that plagues all dark matter research applies equally to axions: If so, how can we see them? How to make "dark" things visible?

Illuminating dark matter

Fortunately for axions, there seems to be a way around this conundrum. If the theories predicting axions are correct, then not only are they expected to be produced in large numbers in the universe, but some axions may also be converted into light under the influence of strong electromagnetic fields. Once there is light, we can see. Could this be the key to detecting axions - and therefore dark matter?

To answer this question, scientists first have to ask themselves, where do the strongest known electric and magnetic fields in the universe occur? The answer: in the region around a rotating neutron star, also known as a pulsar. These pulsars - short for "pulsar" - are compact objects with roughly the same mass as the Sun but with a radius about 100,000 times smaller, only about 10 kilometers. Pulsars are so small but spin at extremely high frequencies, emitting bright, narrow radio beams along their axis of rotation. The pulsar's beam acts like a lighthouse and can sweep across the Earth, making it easy to observe the pulsar.

However, there's more to the pulsar's massive spin. It turns the neutron star into an extremely powerful electromagnet. This in turn could mean that pulsars are very efficient axion factories. An ordinary pulsar can produce 50 digits of axions per second. Due to the strong electromagnetic fields surrounding the pulsar, some of these axions can be converted into observable light. That is: if axions existed - but now this mechanism can be used to answer that question. Just look at pulsars to see if they emit extra light, and if so, determine whether that extra light might be coming from axions.

Simulate a subtle glow

In the scientific field, it is certainly not that simple to actually make such an observation. The light emitted by axions can be detected as radio waves - just a small fraction of the total light these bright cosmic beacons emit to us. We would need to know very precisely what a pulsar without axions and a pulsar with axions looks like to be able to see the difference - let alone quantify the difference and translate it into a measure of the amount of dark matter.

That's exactly what a team of physicists and astronomers have now done. Working together in the Netherlands, Portugal and the United States, the research team has built a comprehensive theoretical framework that provides a detailed understanding of how axions are created, how they escape the gravity of a neutron star, and how they are converted into low-energy radio radiation during the escape process.

These theoretical results were then put onto a computer to simulate the production of axions around pulsars using state-of-the-art numerical simulations of plasmas, which were originally developed to understand the physics behind how pulsars emit radio waves. Once virtually generated, the propagation of axions in the electromagnetic field of the neutron star is simulated. This allowed the researchers to quantitatively understand the subsequent production of radio waves and model how this process provides additional radio signals on top of the inherent emissions produced by the pulsar itself.

Test the axial model

The results of the theory and simulations were then put to the first observational test. Using observations of 27 nearby pulsars, the researchers compared the observed radio waves to models to determine whether any measured excess could provide evidence for the presence of axions. Unfortunately, the answer is "no" -- or, more optimistically, "not yet." Axions didn't appear immediately to us, but maybe that's not what we expected. If dark matter gave away its secrets so easily, it would have been observed long ago.

So now we can only hope to find axions in future observations. At the same time, the fact that no radio signals from axions have been observed so far is an interesting result in itself. The first comparison between simulated and real pulsars sets the tightest limits yet on the interaction of axions with light.

Of course, our ultimate goal is not just to set limits, but to prove that axions do exist, or to ensure that axions cannot be a component of dark matter at all. The new results are just a first step in this direction; they are just the beginning of a new, highly interdisciplinary field that has the potential to significantly advance the study of axions.

References "Dion Noordhuis, Anirudh Prabhu, Samuel J. Witte, Alexander Y.C hen), Fábio Cruz and Christoph Weniger, "New constraints on axions produced in pulsar polar cap cascades", September 15, 2023, "Physical Review Letters".

DOI:10.1103/PhysRevLett.131.111004

Compiled source: ScitechDaily