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

Dark matter and the sniff test

[bradley13]

I'm just a lowly engineer, but for me "dark matter" has never passed the sniff test. It's a kludge factor thrown in to make equations balance. And a kludge factor so huge that "dark matter" is supposed to outweigh all of the observable matter in the entire universe. The only reason this doesn't sound ridiculous is because we've been hearing it for so long.

If you need a kludge factor that big, it is far more likely that the equations are wrong.

There are other possible explanations. For example, if the speed of light were a function of space and time, then the situation changes completely. All observations of the distant/ancient universe are suddenly thrown into question; the interactions within that distant/ancient universe were also different from what we see locally, today. This particular theory (variability of C) is one that crops up periodically, most recently in 2013. 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.

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

[Dutch Gun]

Indeed there is probably something going on at large scales, where gravity doesn't work as it does on small scales.

I've often wondered about that. It's pretty well known that classical Newtonian physics tends to break down at the quantum level. I wonder if the same thing happens at the largest scales - galaxy-sized measurements. I'd never go so far as to argue against the prevailing theory with people that study these things their entire lives, but it does make for interesting reading and/or discussion to consider some of the proposed alternatives.

Re:Dark matter and the sniff test

[Dutch Gun]

It just doesn't add up to me.

"Not adding up" was the reason dark matter was invented. ;)

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

[wonkey_monkey]

Compared to the galaxy as a whole, the solar system is very dense. That would (or so something I read said) made it harder to detect because the gravity of all the regular matter in the immediate neighbourhood swamps the signal.

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.

Re:Dark matter and the sniff test

[EdgePenguin]

Although you make the common error of thinking that the only requirement for dark matter is galaxy/cluster dynamics, you have stumbled on an interesting question; why is the Milky Way dynamically dominated by dark matter but the solar system is not. Fortunately, its easily answered.

Dark matter makes up most of the mass of galaxies, including the Milky Way. One of the best bits of evidence our own galaxy has dark matter is the rate at which M31 (Andromeda) is approaching us. The expansion of the universe drives galaxies apart, so the combined mass of the local group (basically just us and M31, M33 is next in mass but its much smaller than M31) has to be enough to pull the two galaxies together such that you would see M31 at the present distance and velocity. This is called the local group timing argument, and it shows that there is much more mass than can be accounted for with visible matter in the local group. This is not the only evidence for dark matter, but it along with the milk way rotation curve makes us confident that there is substantial dark mass in our galaxy.

As to the reason you don't see much dark matter impacting on the gravity of the solar system: that comes down to geometry. dark matter is arranged in a spheroidal halo whereas most of the visible matter is in a thin disk. The dark matter halo is much less dense in our galaxy than visible matter (and especially so in our solar system as for obvious reasons its got a higher density than the galaxy as a whole) but for large enough $r$, proportionality to $r^3$ always wins out over proportionality to $r^2$, which is why dark matter dominates the outer part of the rotation curve whilst at the same time being irrelevant for the internal kinematics of our solar system.

Re: Dark matter and the sniff test

[EdgePenguin]

Another good question. Gas can lose energy because it interacts with other radiation and matter, but dark matter cannot. The total angular momentum of the system must always be conserved, and a disk is the lowest energy configuration that does this. Both dark matter and gas start off in the spheroidal arrangement, and then the gas the cools to form a disk.

Check out this unusual pattern of gamma rays!

[RevWaldo]

~ What do you think professor? Does it prove the existence of dark matter?

~ This? Oh, bless your heart, no, my young assistant. This is just chatter from some ancient interstellar civilization. Trade negotiations if I'm not mistaken.

~ Gosh, you're right, professor, I should have realized that myself.

~ Yes, completely useless I'm afraid. Fear not, we'll find the evidence someday.

.

Re:I'm also an engineer

[EdgePenguin]

You do not have a better theory. You have a hypothesis, posted on a webpage. This is timecube territory, sorry. Why is it that engineers make such laughably bad attempts to second guess scientists? And why do they insist on bothering actual scientists with mass emails about their crazy ideas?

Dark Matter is not a tweak to gravity

[EdgePenguin]

A lot of engineers and computer programmers seem to think that dark matter is just a fudge to make rotation curves fit, and that they being smarter than astrophysicists can see through this obvious error. This is profoundly irritating

Dark matter is required to explain the ratio of elements produced during big bang nucleosynthesis, the acoustic peaks of the cosmic microwave background, gravitational lensing, cluster dynamics, the Local Group timing and finally, yes, rotation curves. In the last application (which is bizarrely considered to be the only place dark matter is invoked), the most popular alternative hypothesis MOND, which has no theoretical basis and exists purely to fit rotation curve data, doesn't actually do that well on modern rotation curves.

You cannot offering any critical comment on dark matter that won't make you sound like a terminal case of Dunning-Kruger to an astrophysicist unless you understand all of the things I mentioned above.