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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!
It just doesn't add up to me.
"Not adding up" was the reason dark matter was invented. ;)
Irony: Agile development has too much intertia to be abandoned now.
There are entire research groups dedicated to alternative gravity theories, but even their talks are often prefaced with the idea that dark matter does a better job of explaining the evidence in general. It would take several modifications to gravity to explain what could be explained by some dark matter theories. And for people who complain about dark matter being too arbitrary or just a fudge factor, several of the alternative gravity theories are literally fitted, arbitrary scales to just make it match the data. Yet most internet "scientists" don't seem to care in that case, because they spend so much time depending on a sniff test, going with their gut preferences instead of learning about the topic.
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.
systemd is Roko's Basilisk.
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.
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.
"None can love freedom heartily, but good men; the rest love not freedom, but license." --John Milton
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.
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.