Deflating Claims That ESA Craft Has Spotted Dark Matter

Yesterday, we posted news that data from the European Space Agency's XMM-Newton spacecraft had been interpreted as a possible sign of dark matter; researchers noted that a spike in X-ray emissions from two different celestial objects, the Andromeda galaxy and the Perseus galaxy cluster, matched just what they "were expecting with dark matter — that is, concentrated and intense in the center of objects and weaker and diffuse on the edges." StartsWithABang writes with a skeptical rejoinder: There seems to be a formula for this very specific extraordinary claim: point your high-energy telescope at the center of a galaxy or cluster of galaxies, discover an X-ray or gamma ray signal that you can't account for through conventional, known astrophysics, and claim you've detected dark matter! Only, these results never pan out; they've turned out either to be due to conventional sources or simply non-detections every time. There's a claim going around the news based on this paper recently that we've really done it this time, and yet that's not even physically possible, as our astrophysical constraints already rule out a particle with this property as being the dark matter!

Comments

Re:Dark matter and the sniff test

[Anonymous Coward]

There's a few scenarios were evidence of dark matter has been observed. All you have to do is smash two galaxies together and the non-interacting dark matter separates from ordinary matter. The separated dark matter then causes a gravitational lensing effect, which is displaced from the ordinary/visible matter in the galaxies.

It's possible that a modified theory of gravity (e.g. MOND) could still account for the behavior, but it puts requirements on the theory that (I am told) are difficult to accomodate. Sort of like how the Higgs boson discovery at 125 GeV puts requirements on supersymmetry that are hard to accomodate--it's still possible, but much less appealing.

This particular theory (variability of C) is one that crops up periodically, most recently in 2013 [livescience.com]. It is difficult to prove, but really, it's no more unlikely than the existence of huge amounts of dark matter that stubbornly refuse to interact with the known universe.

Considering it's 100% likely that there are particles which don't interact electromagnetically or via the strong force (i.e. neutrinos), dark matter isn't a stretch at all. It's strictly required to exist in most beyond standard model theories. And since the standard model sucks at explaining some observations (e.g. the maginitude of CP violation), we have reason to believe there's more physics going on than what we can currently observe.

Re: Dark matter and the sniff test

[Anonymous Coward]

You're already been given one clear proven kind of "dark matter"; the neutrino. This is incredibly difficult to spot; interacts very little; is almost absent from normal ("small scale") physics and yet it's existence is clear and well evidenced. It's really not that big a stretch that there is something else.

The thing is that if there isn't someone has to come up with really clever expansions for a whole load of other stuff. This would not be nearly the first time a physicist was wrong. In fact a truly dedicated physicist should try took be wrong several times a day. However strange and contrary to instinct would rule out relativity and quantum physics; in fact most of what we know to be true about the world. You have to find something more than gut instinct to oppose this with.

Re:Dark matter and the sniff test

[wonkey_monkey]

I'm just a lowly engineer, but for me "dark matter" has never passed the sniff test.

And yet it seems like most physicists - of whom I am not one - seem to think it is the simplest explanation for what we see.

The quote in the summary sums up, for me, the somewhat churlish attitude some people adopt when faced with dark matter:

There seems to be a formula for this very specific extraordinary claim: point your high-energy telescope at the center of a galaxy or cluster of galaxies, discover an X-ray or gamma ray signal that you can't account for through conventional, known astrophysics, and claim you've detected dark matter! Only, these results never pan out;

Of course they have never panned out - so far. If one of them had panned out, we would have stopped looking. Your keys are always in the last place you look.

Photons started out their theoretical life as a kludge factor to solve the ultraviolet catastrophe (great band), and people were appalled by the idea.

Re:Dark matter and the sniff test

[Baloroth]

However, for some reason unknown to me, the visible matter in our solar system perfectly describes how the planets orbit the sun, how the moon orbits the earth, and how hard I hit the ground when I try to fly. So where is this dark matter, all this extra gravity? Shouldn't I hit the ground a lot harder than we can explain just based on the mass of our planet?

It's because dark matter only interacts gravitationally. See, normal matter clumps up into planets and stars because it sticks to other particles, and loses energy from collisions, causing it to collapse over time into locally dense spheres (planets, stars, black holes, etc.). But dark matter doesn't: it just passes through itself (mostly: it may interact through the weak force, but only very very very rarely if so, not enough to clump up). That means it doesn't form local regions of high density. On the other hand, an object immersed in a more or less uniform sea of matter (of any kind) won't notice any gravitational effects, because it's being pulled in all directions equally (for example: you'd be weightless at the center of the Earth. Dead from the pressure/heat/lack of air, but weightless). So, we can float through a sea, even a fairly dense one, of dark matter and notice nothing at all. Now, there is an non-uniformity in this dark matter "sea": there is more on the side of us towards the center of the galaxy than there is on the other side, but that pulls the entire solar system uniformly, accelerating it in it's galactic orbit, and that effect we do in fact see.