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Dark Matter Exists
olclops writes "It's a big day for astrophysics. After much speculation, scientists now have conclusive proof of dark matter. This result doesn't rule out alternate gravity theories like MOND, but it does mean those theories will have to account for exotic forms of dark matter."
It may be spread uniformly in the intergallactic space, meaning it's useless with density under a gram per cubic kilometer. Or it may form denser formations at distances that are useless. I mean, we're harnessing power of only one star out of a whole universe of them...
Anagram("United States of America") == "Dine out, taste a Mac, fries"
I had read that if the universe were infinite that the sky would be blindingly white from all the old light from old stars, which is one of the reasons that a Big Bang (or other creation) was assumed to have happened.
But if there are dark clouds that can absorb the light, could there be stars further than 13ish billion light years away, that are simply obscured?
Storm
that the night sky is not the temp of the suns surface is called olbers paradox http://jersey.uoregon.edu/~imamura/123/lecture-5/o lbers.html.
I believe the resoluiton of this paradox is one of hte outstanding successes of the expanding universe idea discoverd by hubble
Well, AFAIK even most of the MOND folks have acknowledged that some dark matter is necessary. E.g., even when a MOND theory looks like it can explain galactic rotation curves, it doesn't fully explain galaxy clusters or come close to explaining cosmological observations.
The MOND people (generally) aren't kooks. They're just pushing in different directions, which is a good thing. But yes, this does make it even harder for them.
We have known for several decades now that large clusters of galaxies are immersed in giant clouds of very hot gas, at temperatures of millions of degrees. The gas emits copious amounts of X-rays.
Do we know yet what keeps that gas at million-degree temperatures? Maybe I'm naive, but I'd expect radiation (especially X-Rays!) to cool the gas, and I can't think of any mechanisms that would heat it back up that quickly.
How exactlty does demonstrating that something cannot explain a phenomina prove that a counter argument is proven? That's like saying the spontaniuos combustion of my dog cannot be proven with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen.
A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort.
From the NASA press release: "These results are being published in an upcoming issue of The Astrophysical Journal Letters."
Two points. First, journals really hate it when press releases are made prior to the publication date. Second, this journal has an "impact factor" of ~5-6, compared to Nature, or Science, which have impact factors of ~25. Why are they publishing in some obscure journal if this is really the rock-solid proof that they claim it is?? Makes me wonder.
We say dark matter, but we don't really mean dark _matter_ right? I mean, this isn't just a lot of peanut butter and jelly sandwiches floating in space? It's just an intangile. Other than having a lot of gravity at this point, it's pretty much undefined.
So what if it's a ripple/tight spot in spacetime? How could we tell?
I imagine it like the universe being a mostly inflated balloon. Everything inside is the universe. All of the super massive things (Black holes, etc) are so large, they cause outward bulges in the ballon. If you were to be standing on the inside, you would feel the effect by having the tendency of being pulled towards the bulge (gravity). So if you took your fingers and pinched the balloon and pulled a bit, you'd cause a depression (gravity from an inside observer's POV).
What if these things are also a precursor of a black hole? They obviously attract a lot of stuff with their gravity, so eventually, they'd attract A LOT of stuff, which would eventually lead to a black hole. Maybe super massive stars nearing death aren't the only mammas to black holes?
"Dark Matter" is not just non-emitting matter. It's also non-interacting matter. ...
Anybody ever think dark matter might be like Niven's 'quantum black holes'? (read "Borderland of Sol").
The idea: a miniscule black hole formed in the high pressures during the creation of the universe. Or in supernovae. Or in some other way. The method of formation doesn't matter for this little intellectual exercise.
They can have event horizon on the atomic or even subatomic scale; as such, they would have very dense gravity gradients, but would easily fail entirely to interact with matter outside of their own gravity. A couple quadrillion of them spread out in a thin hydrogen cloud - far enough apart to not fall into each other (say, an AU^3 (not AMU) holds a couple thousand), but close enough to seem like a very large, very consistent, very weak gravity shift.
Something to think about.
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This appears to be no more a confirmation for dark matter than when the Michelson-Morley experiment (in 1881) "confirmed" the existence of ether. In the immediate aftermath of the Michelson-Morley experiment, theoreticians generated lots of mathematical "proofs" (e.g., The Ether of Space, Sir Oliver Lodge, Harper & Bros, 1909) that showed how a boundary layer in the ether surrounding the Earth accounted for the observed results. A series of subsequent refinements of the Michelson-Morley experiment showed that the speed of light was truly independent of direction, and Einstein's theories, which did not require the existence of ether, provided a better fit for the observed results than was a boundary layer in the ether.
Over time, the Michelson-Morley experiment was recognized to have disproved the existence of ether -- but it wasn't that way initially.
Alternative explanations include "quantum critical phase transitions", and I'm sure that there are other possibilities, that a series of observations of similar cosmological events will provide the range of data needed to select the hypothesis that best describes the observations.
Being able to fudge one theory to fit a single observation falls quite a bit short of a "conclusive proof". Maybe dark matter does exist, but it's going to take a lot more observations for it to be convincing to me.
How precisely does dark matter permit the expansion of the universe to be defined, and how precisely does the observed phenomenon fit those numbers?
Wake me up when someone has a quantum mechanical model that tells how quarks are bound together in dark matter, or when someone manages to tap into dark energy (which is supposedly all around us).
My understanding is that gravitational lensing here suggests that dark matter hangs out around "cold matter" or solid matter, like stars and planets, but not "hot matter" like plasma. Why would the mysterious dark matter only stay with the solid matter? What theory of dark matter predicts that it ignores plasma? Am I missing something?
Table-ized A.I.
it reeks of PR. ... the kind of PR that happens when people are REALLY trying to get others accept a point that is hard to accept.
I've been following the "dark" story on and off since I stopped studying physics seriously after college. The MOND system makes a whole lot of sense. My non-professional-physicist read on the MOND / DARK controversy is that several of the alternate theories (like MOND) that remove the need for dark matter are fairly convincing. Dark matter is not convincing at all - not testable, not observable, and reminds me a lot of Santa Claus. Somebody brought the presents, right? The problem is that a vast majority of cosmologists are all so far down the dark matter band wagon that if dark matter goes away... lots of careers will be lost. Destroyed. These professionals who trade solely in reputation and intellectual-ism will have their rug pulled right out from under them.
A much more plausible explanation is that some people are trying really hard to amp up the PR. Sort of like what happens when you need a distraction from a big debate, so you get all the airline travelers to throw away liquids. Anyone who tells you they have proof for something that by definition can not be observed is selling PR. For those of you who believe it without question, I've got a bridge I'll sell you.
After taking about 30 minutes and reading no less than 6 heavily biased PR pieces... I say this stinks. It's certainly not science - (yet).
So what's new then? All along the whole case for "dark matter" was that galaxies -- _all_ galaxies -- rotate strangely like a rigid body, except right near the centre. According to newtonian mechanics the stars in a galaxy should behave basically like the planets in our solar system: the farther from the centre you get, the slower they move. But in a galaxy stuff moves like that only near the centre, and then it's like gravity changed gradually from 1/(R*R) to 1/R, and the stars rotate at an almost constant angular velocity around the centre.
;)
So from there it's that either:
1. there's a metric buttload of matter we can't observe other than through gravity, in some weird distribution all through the galaxy's disc, or 2
2. we accept that gravity isn't working like we think it does
(Or my favourite: 3. galaxies are just a rotating texture there, so _of_ _course_ they rotate like a rigid. Noone would be dumb enough to simulate the individual stars just to give us a pretty sky in this MMO we call RL
And somehow the favourite is 1, for no obvious reason than that noone wants to modify gravity theories. It's as if Galileo, upon discovering that a stone dropped from the mast doesn't lag behind the ship, would then proceed to invent some "dark wind" that pushes the stone along with the ship. Since existing wind obviously isn't strong enough to push the stone that hard, it's got to be some dark wind in there too. Just, you know, for the sake of not contradicting the existing Aristotelian system.
Anyway, all along we knew that it can't be conventional matter, because we already had plenty of galaxies in various states of illumination and they all behave the same.
So exactly how does the new one help there? It seems to me like it still can't offer conclusive proof that 1 is true and 2 is false, because it would _still_ be equally well explained by 2. What this "solves" is at most a sub-distinction inside 1, once we're dead-set on believing 1 instead of 2. It says basically that if we already decided it's 1, then, yep, it's definitely not baryon matter (rocks, gases, protons, etc), but some weird matter that interacts only with gravity.
A polar bear is a cartesian bear after a coordinate transform.
Take a gander at the published paper. A large part of the reason that this galaxy cluster in particular was chosen was because it is one of the cases known where we have a clear-cut idea of what's going on.
Their initial assumption (page 1 right column): "During a collision of two clusters, galaxies behave as collisionless particles, while the fluid-like X-ray emitting intracluster plasma experiences ram pressure. Therefore, in the course of a cluster collision, galaxies spatially decouple from the plasma." Since the area occupied by dense matter (stars) is more than 10^11 times smaller than that of the whole cluster, literally one or two stars might impact each other. Meanwhile, the intracluster gas is, however diffuse, GAS - it can't pass through itself, and is observed to contain ~80-90% of a cluster's visible mass.
I don't know the specifics of how this is done, but they used a gradient of the change in a background galaxy's size and related it to the curvature of space (and hence amount of mass). By plotting a lot of background galaxies, they were able to integrate the gradient to find the center of mass (warning: not 100% sure of this explanation) that was causing the lensing (green gradient lines on page 2).
When this is compared with an x-ray image of the gas which is known to comprise most of the visible mass of clusters, the two mismatch by about 6 arc seconds. On page 4, they discuss the probability of other clusters creating the apparent mass (1/10 million chance) or entire filaments of intergalactic mass creating it (1 in 100 million). The only remaining conclusion from this is that something which fits the description of dark matter (in that it has mass but no other measurable property) makes up the great majority of the cluster's mass and the two clouds of it passed through each other like the galaxies.
So, they expected the stars/dark matter and the gas of colliding clusters to separate in a collision, and this is exactly what was observed
Yes! Finally someone noticed. I was thrilled with the writing of that blog entry, which is why I chose to submit that rather than a more "official" write-up (also, none of the big publications had picked the story up when I submitted it yet. On the other hand, a real publication would have been able to handle the server load. Oh well). But my original summary even said something about how well written the blog entry was, but the editor cut that part.