Welcome to the new look Three Alpha! I’ve been tweaking things a bit and I’d love to hear what you think. Since last time: In the Solar System, a new interstellar object has been detected, zooming towards the inner solar system; in the Galaxy, a star has been found that has exploded in two supernovas; and in the Universe, it turns out there are a lot more supermassive black holes than we thought.

Meanwhile, in this edition of the newsletter we’re focusing on two papers both claiming they’ve found the Universe’s missing matter. Read on for more…

Have two different teams solved the mystery of the missing baryons?

Much of the matter in the Universe is missing. 85% of the matter in the Universe is dark matter, all of which is famously missing. The rest is the everyday matter that we’re all familiar with, almost all in the form of protons and neutrons (plus a tiny amount from electrons and neutrinos, which are much less massive). Protons and neutrons are what’s called baryonic matter, and about 40% of that matter is also missing, based on comparisons between estimates from the early Universe and the present day. This is called the missing baryon problem.

Last month, two teams of astronomers claimed to have found the Universe’s missing baryonic matter using two completely different methods. One used radio observations of fast radio bursts to measure the diffuse gas in and around galaxies, while the other mapped a single filament of hot gas in detail using X-ray observations. So, do they contradict each other? Or have they found the same thing? Let’s have a look at both of them and see what’s going on.

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The missing baryon problem is one of those ongoing mysteries in astronomy that has numerous researchers clambering to be the ones to solve it, and many people have claimed to have done that over the years. Here are a few examples from recent years: 2024, 2020, 2018. I could go on. So it is worth being skeptical when you read any claims about it being solved. With that in mind, what have the two papers from last month actually claimed?

They’ve both claimed to have found new matter in the cosmic web (the long, hard-to-measure filaments of matter stretching across the Universe between clusters of galaxies), closely matching the predictions of models of the Universe which potentially account for the missing baryon problem. In other words, established theories predicted that the missing matter should be in the cosmic web, and these new observations seem to back that up.

One of the papers, published in Astronomy & Astrophysics, makes detailed measurements of the matter in one filament, using observations from two X-ray observatories. They used archived wide-field observations from Japan’s Suzaku satellite (which stopped operating back in 2015) and narrow observations from the European Space Agency’s XMM-Newton. The wide observations provide the map of the matter in the filament (which is very hot, and hence visible in X-rays) while XMM-Newton allows contamination from X-ray sources such as black holes to be precisely removed. After producing the map of the filament (the first time that’s been done), they compared the result to the predictions from cosmological models, and found they look more or less the same.

The other paper, published in Nature Astronomy, looked at things from the other end of the scale. Rather than looking at one specific filament, they looked at the distribution of matter across the entire sky. They did this by analysing how radio waves emitted in fast radio bursts (extremely short, bright flashes of radio waves) were dispersed by the matter they travelled through (meaning long wavelengths arrive later than short wavelengths, depending on the amount of matter). The results showed that 76% of the Universe’s matter is in the intergalactic medium, which includes the cosmic web. And as with the other paper, this matches expectations based on theory.

So, is the missing baryon problem now solved? I don’t think we’re quite there yet (even if the researchers’ press releases imply otherwise), but cosmological models seem to be holding up to scrutiny, and we’re starting to be able to detect and map the missing matter. The missing baryons probably won’t be missing for too much longer.

Finally

Check out this time lapse of the interstellar object 3I/ATLAS:

What is Three Alpha? Other than being the name of the newsletter you’re reading now, the name “three alpha” comes from the triple-alpha process, a nuclear chain reaction in stars which turns helium into carbon. Read more here.

Who writes this? My name is Dr. Adam McMaster. I’m an astronomer in the UK, where I mainly work on finding black holes. You can find me on BlueSky, @adammc.space.

Let me know what you think! You can send comments and feedback by hitting reply or by emailing [email protected].