Can we detect dark matter using light from pulsars?
Dark matter is called dark for a reason. We can’t see it, not with our eyes, and not with any human detection method so far. The theories of astronomers suggest that dark matter exists. They believe they can measure its gravitational effects. And, on October 6, 2023, researchers at the University of Amsterdam and elsewhere announced a possible new dark matter viewing method. using the small, incredibly dense stars that are the heating hearts of pulsars. The astronomers say we can use pulsars to make direct detections of dark matter … even to “see” it.
Now, remember, astronomers haven’t directly detected dark matter, yet. So we don’t know what it is, exactly. But one as yet unproven idea is that dark matter is composed of axions (a type of subatomic particle). The new dark matter detection method relies on this possibility.
The researchers published their peer-reviewed findings in the journal Physical Review Letters on September 15, 2023.
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Looking for a new type of matter
What is dark matter? Why can’t we see it? Scientists know we don’t know everything about the universe. One endeavor, on the scientific frontier, is the search for new kinds of elementary particles – smaller than atoms, thought to be the basic particles making up atoms – that are much harder to detect then the 61 elementary particles now discussed (including electrons and other leptons, quarks, fundamental bosons and so on).
Axions are hypothetical elementary particles. They’re thought to be very light, electrically neutral particles. Scientists have speculated for some decades that axions might explain the very small separation of positive and negative charges inside neutrons. So it’s possible they do exist. But scientists still haven’t definitively detected axions yet.
That fact doesn’t stop scientists from speculating further about axions. Could dark matter be made of axions? If it is, then – at a time in science when we still can’t detect axions – it makes sense we can’t detect dark matter either.
Using pulsars to detect dark matter
Enter the new pulsar method. If dark matter is composed of axions – still a big if – then could we use distant spinning stars – pulsars – to detect it? Current theories suggest that axions should be mass-produced in the universe. And indeed, scientists now say that 85% of the matter in the universe is dark matter.
Theoretically (and, remember, we’re on the frontiers of science here), electromagnetic fields might be able to convert some axions into light, albeit extremely faint light. Could that be a way to find axions and “see” dark matter? Perhaps.
So where are the strongest electromagnetic fields in the universe? The answer is (drum roll here) pulsars. Pulsars are well-known for the intense beams of radio energy that they emit as they rotate. This makes them kind of like cosmic lighthouses.
The rapid spin of a pulsar can also turn the neutron star into a powerful electromagnet. On average, a pulsar could produce huge numbers of axions, to the order of 50-digit numbers. That’s a lot of zeros! A fraction of those axions would then be converted into visible light particles. That light would be so faint, however, that astronomers could only detect it as radio waves.
Searching for a faint glow
So how can astronomers tell if axions – if they do exist – are what dark matter is made of? They can look for the subtle glow of light they emanate around pulsars. The key is to know what a pulsar with axions looks like, and what a pulsar without axions looks like.
This is where the new study comes in. Since axions haven’t yet been proven to exist, the approach is still a theoretical one. It addresses three basic issues: how axions are produced, how axions escape the gravitational pull of a neutron star and how they are converted into low energy radio radiation (the subtle glow). The researchers used state-of-the-art numerical plasma simulations to model how pulsars would produce axions. Then they simulated how the axions would move through the electromagnetic fields of the neutron stars. With this data, the researchers could model how the faint radio waves are produced. Those radio waves would be in addition to the usual much stronger radio waves that pulsars generate.
No smoking gun yet for dark matter
The researchers conducted their first actual observational tests, using 27 nearby pulsars. Did they detect any excess radio waves that might be from axions? Unfortunately, no. So at least for now, there is no smoking gun to confirm the existence of axions. Or dark matter. The observations did set limits, however, on how much extra radio wave energy might be generated by axions. It’s still possible that future observations will be able to tease out the elusive radio signal. As the paper explains:
The limits derived in this Letter significantly improve upon existing bounds, and unlike axion haloscope experiments (and radio line searches), do not assume axions contribute to the dark matter. In addition, since the radio flux scales, the constraint is largely insensitive to minor mismodeling errors. The mass range covered by our constraints is limited by the frequency of radio observations (higher frequencies could probe higher masses), and the computational expense (computing time increases at both lower and higher masses).
A comprehensive analysis of all pulsars in the ATNF Pulsar Catalogue, as well as more dedicated pulsar observations at high frequencies, could significantly improve upon these results; we reserve this broader analysis for future work.
The search continues
So the search for both axions and dark matter continues. What Dan Hooper wrote in The Conversation in 2017 still seems true today:
The stubborn elusiveness of dark matter has left many scientists both surprised and confused. We had what seemed like very good reasons to expect particles of dark matter to be discovered by now. And yet the hunt continues, and the mystery deepens.
Bottom line: Researchers used pulsars to confirm the existence of long-sought axiom particles, and perhaps dark matter. The results were negative, but are only a first step.
Source: Novel Constraints on Axions Produced in Pulsar Polar-Cap Cascades
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