Dark Matter Not So Dark Anymore?

A paper published on arXiv.org has been released that might just show the first hints of dark matter detection, giving us an unprecedented chance at figuring out just what the fuck the stuff is.

I haven’t had the time to go through the paper so I can’t offer many insights (and my favourite physics bloggers have yet to cover it…that’s right, I’m looking you dead in the eye Sean Carroll) but it seems as though an anomalous x-ray spectral emission is causing some eyebrows to be raised in the lab.

The line is weak, but detectable, sitting at around the 3.5 keV range. Basically spectroscopy is the science of taking light (including the infra-red and ultra-violet spectrum within this) and splitting it into it’s component colours, measuring them and then using that data to figure out what it was that emitted that light. Depending on what materials were present in the object that emitted the light, the narrow bands of solidly coloured light you are able to see after the splitting process will tell you what the object is made out of. That’s the laymans version, at least.

So they measured the spectral emissions, in x-rays, of the Andromeda galaxy and the Perseus galaxy and, after post-processing to remove emission lines that we know the cause of, they were left with a weak x-ray at the aforementioned 3.5 keV that was unexplained. That doesn’t definitely mean it’s dark matter, but it’s a hopeful start (the fact that it was found in two separate objects AND that they are both dark matter heavy objects lends a little weight to the finding).

So, it seems as though we just might have our first detection of the mysterious substance (or substances) known as dark matter. I’ve scanned the paper, but I can’t find a p-value yet, so I’m not entirely sure how sure they are about it. Let us wait and see what the beauty of peer-review will uncover.

EDIT: Ethan Seigel disagrees! You can read his post about this here, but to quote some of the relevant bits:

The news would love you to believe that the second possibility is worth considering, because, well, how awesome would it be to find dark matter? But not only is the evidence that this is even a real signal not at all compelling (under a 4σ significant detection even for the combined data set, when 5σ is the “gold standard” for discovery), but there is no way this could account for the dark matter in our Universe!


Well, what is that threshold? It has to be, at this point in time, heavier than about 10 keV, based on the strength of the observed absorption lines. In other words, about a factor of 3 heavier (or 50% heavier, for a decaying particle) that this supposed “dark matter signal” is!

Don’t get me wrong, the discovery of a potential new X-ray line is very interesting, and could be a window either into new astrophysics or, potentially (if a bit fantastically and unlikely), a new type of particle. It’s just that even if it turns out to be a new particle, that particle cannot be the dark matter, since it would screw up structure formation in the Universe (particularly on small scales), and our observations of those structures simply rules that scenario out.

So, as I said, Ethan disagrees. And I would trust his suitably doctorated (that’s a new word I made up to describe learnedness (which, surprisingly, isn’t a made up word)) arse over my pale, spotty and doctorateless cheeks any day.


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