Exploding stars made of dark matter could heat up universe (Patreon)

[This is a transcript with links to references.]

85% of matter in the universe is dark matter, astrophysicists say. Which might make you wonder what all this stuff is doing. I mean, some of the matter in the universe has learned to walk and talk, so why is dark matter so boring.

In case you’ve been wondering too, astrophysicists now say that some of this dark matter might form stars, sort of, which can explode, sort of. Let’s have a look.

Dark matter is one of the explanations that astrophysicists have come up with to explain numerous anomalous observations they have been making during the past century. It basically seems like the pull of gravity is stronger than it should be though this becomes noticeably only on large scales. Galaxies rotate faster than they should, galaxies in clusters move too fast, gravitational lenses are too strong.

The hypothesis of dark matter has it that this happens because there is more stuff in galaxies and galaxy clusters, it’s just that we can’t see it. The alternative explanation is that the law of gravity doesn’t work as we think it does. Of these two hypotheses, dark matter is currently the more popular one.

We know very little about dark matter, if that exists, which it may not. That’s because to explain observations, dark matter just needs to be dark and matter and not clump too much. Featureless stuff fits the bill just fine.

However, particle physicists like the idea that it’s some kind of particle and they have come up with all kinds of ideas for just what that particle could be. There are WIMPS and MACHOs and Macros and all kinds of supersymmetric particles and massive gravitons and what not. Though there’s no evidence any of those actually exist.

The new paper is about one of those particles called an axion-like particle. Axions were first proposed in the late 1970 to make the standard model of particle physics somewhat prettier.

Yes, they are named after a washing detergent because they were supposed to, you know, wash away what particle physicists considered to be a blemish in the laws of nature, that’s a constant whose value they don’t like because they think it’s too small.

Unfortunately, if axions existed, they would be emitted in large numbers from neutron stars, and the neutron stars would cool very quickly. By 1980 it was clear that observations were just incompatible with the existence of axions.

After that happened, physicists came up with various amendments to the original axion hypothesis that would make the particle more difficult to detect. So not only was the axion invented for an unscientific reason – it was to make a perfectly fine theory prettier. But after it had been ruled out, particle physicists made it even more unscientific by amending a theory that didn’t explain anything.

These new types of axions have now grown to an entire army called “axion-like particles” and there are dozens of experiments looking for those particles. They continue to not find them.

But about the paper. If dark matter exists and if it’s made of these axions and if these axions have the right masses and right interactions, then they can condense to form compact objects called solitons. This is because axions are a type of particle called boson, and bosons can undergo Bose-Einstein condensation. Yes, it’s Albert again.

These means that given the right circumstances, axions just lump onto each other. They clump, basically. The idea has been around for about 10 years and those axion clumps have been called “axion stars”. They don’t look anything like our stars though because they don’t do nuclear fusion. There are no nuclei to fuse there.

Further calculations then showed that if the axion starts get too big they become unstable, and if they become unstable, they could explode. And if they could explode that could release radiation which would heat up the gas in the vicinity. And that might be observable.

And this is what they looked at in the new paper. They asked: If those axion stars were produced in the early universe, and they exploded,
what would this have done to the gas because that could still be observable today. They looked at CMB data and didn’t find any evidence of axion star explosions. As it’s common habit among physicists, this isn’t called a negative result, but an “interesting constraints”. And also in line with physicists’ habits, they then say that a next generation of experiments might be able to find the missing evidence.

This next generation of experiments are various planned radio telescopes that will be looking for old hydrogen emissions which might come from the gas that might have been affected by the axion stars. This type of experiment has become known as 21-centimeter astronomy because this hydrogen emission line is today approximately of the wavelength of 21 centimeters. 21-centimeter astronomy is basically the next big thing in astronomy and that’s why the current game of theorists is to make “predictions” for those experiments.

Ok, let’s sum it up. If dark matter exists, which it may not, and if it’s made of axions, for which there is no reason, and if these axions form stars , and if these stars have the right properties to explode, and if the axions couple strongly enough to releases photons, then that could leave some observable traces in the interstellar gas, that could probably also be caused by many other astrophysical effects. The most interesting part is that people still get paid for this.