Aug 16 2007
Prev/Next Posts: « Cut! That’s a warp! || SpaceFest Report #1 »
A dark hole
Dark matter is pretty weird stuff. We’ve seen tons of evidence for it: the way galaxies behave in clusters, the way individual galaxies rotate, and just in the past year two monster pieces of evidence supporting it were found (the Bullet Cluster (#4 on that list) and the COSMOS survey).
The deal is, dark matter doesn’t interact with normal matter except through gravity. Two clouds of DM could pass right through each other and not collide like normal matter, but their mutual gravity might distort the shapes of the clouds. Dark matter also appears to hang out in giant clouds surrounding galaxies, and in theory may be the seed which helped cause galaxies and clusters to form in the first place.
The evidence for this has been really good… but then a monkey got thrown in the wrench.
The image above was just released, and shows an optical image of galaxy cluster Abell 520 with an X-ray image from the orbiting Chandra X-ray observatory overlaid on it. The optical image shows stars in our Milky Way and galaxies belonging to the cluster. The red fuzz is hot gas made of normal matter, heated to millions of degrees in the cluster. The blue fuzz is the location of the dark matter in the cluster, inferred by its effect on the light of background galaxies (I describe how this is done in detail here).
The problem is, where there appears to be the most dark matter, near the center of the cluster, there are very few galaxies. And there are areas where there are lots of galaxies, but little dark matter. This is the opposite of what the theory predicts! In general, the DM and the galaxies should stick together, so wherever you see one you should find the other.
Note that I said "in general". While astronomers are scratching their heads over this, I would caution that we are only just now starting to be able to make maps like these, and it doesn’t surprise me that all kinds of weird things will come up. While the Bullet Cluster was a brilliant confirmation of DM theory, as was the COSMOS survey, not all galaxy clusters are equal. Since we have seen a whole pile of evidence supporting DM, it looks like Abell 520 will give us insight into details of the theory (as opposed to overthrowing the idea of DM altogether).
Do DM particle interact more than we thought? Could the galaxies in Abell 520 have interacted in some way as to separate them from the dark matter? Is there some other aspect of the theory we missed, or are wrong about? The astronomers who took this data will be getting more time not only on Chandra but on Hubble as well to investigate this mystery.
The beauty of this, as usual, is that by getting contradicted over what we expect, we learn more about what the Universe is trying to tell us. Confirmation of theories is always good, of course, but it’s that occasional slap in the face where the real progress is made.
As long as you are posting- any comments on Dr Gunter Nimtz and Dr Alfons Stahlhofen?
Nice picture BTW
Okay, BA, you’ve got MY brains oozing messily from my ears! If I understand your posting correctly, this will have a LOT of bright people scratching their heads and reaching for the Exedrin! This is big news, and I eagerly await future developments!
This is what I find so achingly beautiful about the scientific method. And what causes me head-banging frustration when people construe it as science “changing it’s mind all the time”. The kids movie The Robinsons had a really cute affirmation of the scientific method when the main character destroys dinner with an invention and they clap and tell him “What a magnificent failure!” The point being that now he has a chance to make his idea better. This is exciting stuff.
I don’t think that “Dark Matter” and “Dark Energy” exist at all. I think if you take a step back and really look at the concepts, you will see that they just don’t pass the giggle test.
Most scientific theories that have withstood the test of time are really quite beautiful, simple and elegant.
Not so with Dark Matter and Dark Energy. They both strike me as clumsey and kludgy efforts to explain something we don’t understand.
And that is my prediction…that as we learn more, we will discover that what we are calling Dark Matter and Dark Energy are really just manifestations of something far more exotic and fantastic then we can grasp today.
I’m not talking about anything mystical or supernatural! I’m talking about something fundemental about the nature of things around us.
My money (for now) is on the Ekpyrotic Universe theory. It seems to have more “truthiness” than anything else I’ve read about.
Maybe there’s two different kinds of dark matter? One stick-to-normal-matter and the other stay-away-from-matter?
Forgive the technical terms
[…] (more…) […]
The concept of Dark Matter was created just to explain the unexplainable motions of the galaxies. Wait, the galaxy isn’t spinning as predicted by Relativity? Well then… there must be something else out there affecting it only through gravity and no other forces! It’s just a constant added into the formula to let it add up to what we currently think we know. No one has any idea what it is. Most likely… we just need to tweak the theory of gravity. Look at the Pioneer Anomaly … Both Pioneer probes have traveled a lot further than they should have according to our current knowledge of gravity and motion… No one can explain it. I think it is possible that the force of gravity changes slightly from what is predicted by relativity over stellar distances.
The problem with “dark matter” is that, since it is undetectable, except through gravity… it is therefore unseeable and unprovable. This strikes me as very similar to the concept of “God”. I think they should be looking less at trying to theorize some undetectable god particle to fix their formula, and more to refining the theory that this so-called dark matter is affecting.
Nice picture. Dark matter appears to be blue here.
But no kidding. It seems that dark matter/energy actually are huge bubbles/areas of compressed spacetime ‘in the middle of nowhere’, interacting with mass as if there actually is mass. Call it a quantum effect on macroscale. How it was formed? Maybe due to the expansion of spacetime, maybe due to huge amounts of annihilated mass a very long time ago. And yes, maybe it’s just an exotic state of energy/matter.
Nice site btw.
ET
Nice post, but… “a monkey got thrown in the wrench?” I think you mean “a monkey wrench got thrown in the works.” Unless you actually are talking about hurling simians… I imagine that could cause some confusion, too.
[…] Astronomer take: BA Blog: A dark hole __________________ 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 […]
Ed: You should be aware that the Bullet Cluster results have put a serious damper on most MOND theories, and as a result, even MOND proponents have had to admit that there must be more matter in such interactions than is visibly present. See:
http://www.astro.umd.edu/~ssm/mond/bullet_comments.html
To quote: “But one is obliged to say, in MOND, that there must still be more mass to be discovered in clusters.”
Xavier Onassison:
“Most scientific theories that have withstood the test of time are really quite beautiful, simple and elegant.”
I don’t like those dark sh.. things, either. But by today it seems to be our best explanation for some strange happenings in outer space.
I agree with you that the theory of everything will be quite elegant, beautyful and simple, I only think that we just don’t have the mathematics to formulate it (and of course the understanding).
Our problem today is that we know for sure that general relativity is not all - it predicts its own failiure (Big Bang, Black Holes,…). So there must be something else which must contain this little number, named Planck’s constant or h - a theory including quantum dynamics.
We have two attempts so far as I know, string theory and loop quantum gravity. I’d like to know how far they can explain Dark Matter, Energy and this special strange behaviour Phil wrote this nice post about.
Bad Astronomer, I think you hit the simian on the head with this possibility:
“Could the galaxies in Abell 520 have interacted in some way as to separate them from the dark matter?”
Isn’t this what happened–er, is happening–in the Bullet Cluster, enabling the detection of weak lensing where there is no luminous matter, implying that it must be dark matter doing the lensing?
perhaps there sits a primordial black hole in the center of the cluster- wich consumed the galaxies and represents a dark mass concentration
Xavier:
“Truthiness” means something that sounds or feels true, without actually being true.
DM theory may feel kludgy, but that is more likely because it is new and still needs to be refined (as the post suggests) than because it isn’t basically correct.
Hmmmm…..peanut gallery comment. Maybe the dark matter in the middle of the cluster is what is drawing the cluster towards it.
Cougar,
I think that in the Bullet Cluster, the hot gas (pink) got stripped away from the dark matter (blue) and the galaxies, but that the galaxies and dark matter stayed together. In Abell 520, the dak matter is separated from the galaxies and I’m told that it appears closer to the gas. I don’t trust my eyes on this so well, though, because I can’t tell which of those points of light are galaxies from Abell 520, and which are stars from the Milky Way. I never seem to see what I’m told I’m supposed to see anyway. It started with the constellations and just got worse after that.
The Universe is a giant Jiffy-Pop. The galaxies are popcorn and dark matter is the hot air circulating betwixt them.
Aaron - I think the nice Doctor was engaging in a little play (them edumacated folk do that now and again).
And I’ve discovered evidence of dark matter here on earth… it’s generated by people! Specifically, it’s generated by wimmins! Every time there’s a wimmin nearby, I am forced by gravity to spiral closer and closer.
I’ve tried to explain this to my wife, but not being scietifically inclined, she has doubts on the veracity of my theory (see! there’s evidence for it, so it’s not just a hypothesis)
I think this dark matter also explains the energy transfer capabilities children have. Why invent something else when one answer will do for both? (Juvenile Energy Transfer (JET) is the phenomenon observed, but rarely noted, by almost all parents. Put a child and an adult in close proximity and the child will wind up, eventually pinging off the walls. In direct - and opposite - reaction, the adult will become more and more tired, eventually dozing off. I’m attempting to prove that, should a child become so energetic that he vibrates right through the wall/floor/ceiling, the adult will expire)
Trebla - in all seriousness, black holes aren’t the answer. Interesting and mysterious as they are, they DO interact with normal matter in more direct ways than gravitational disturbance.
I dont understand why people are always scratching their noggins over all this cosmological claptrap. All you have to do is penetrate the giant Black Hole where the Heechee live and politely ask them. The Heechee know everything cause they explored the entire Universe a half billion years ago…before they found out about the Assassins and ran off to hide, that is.
Just ask Larry Niven, he knows all about this stuff. :o)
Or maybe we should ask Frederick Pohl about the Heechee?
Yikes! Make that Frederik Pohl.
Any bets that its the FSM that is holding the galaxies together ?
As you can see your science has turned up wrong again. Does it then baffle you that your current theories like evolution will turn out to be wrong someday? And only then will you accept the His word. I pray for your soul and hope you see the light.
Hey there, you little Devil, the theory isn’t wrong, it’s just incomplete, as is Relativity, Quantum mechanics, and a whole host of other theories, which is exactly the way we expect things to be, incomplete. That’s the ESSENCE of the scientific method, slowly zeroing in on the way things really are, rather than expecting an absolutist answer.
By the way, can you prove or even show ANY empirical evidence that YOU have a soul?
GAry 7
Ooooh, interesting!!! If the lensing map can be trusted, the dark matter seems to be shaped like a bar rather than a ball, which seems very odd… especially because the gas IS in the shape of a ball, like you’d expect! There’s definitely something weird going on here, either with the lensing map or with the cluster itself, and I haven’t the faintest idea what it could be.
Fortunately, it’s not my job to find out.
p.s. Ignoring the strange shape of the dark matter cloud for a moment, is it really too implausible that the dark matter should lie at the center of the cluster, while the galaxies fly around the perimiter? I don’t understand the process by which things are supposed to lose energy and settle down into the center of a cluster, but it seems reasonable to me that because dark matter particles and galaxies are on drastically different mass scales, this process could affect them differently!
Is there a dynamics expert in the house?
I wonder if Devil’s Advocate actually read–and understood–the last three paragraphs of BA’s post, which address the very thing he (she?) is criticising about science. “See the light” indeed…
From the same point of view we could ask the question why not all the planets have crashed into the sun. This is because the planets have angular momentum. It may be possible that the dark matter interacts with itself, and thus forms a tighter region, while the galaxies are bound only gravitationally, and circle the cloud of dark matter at a great distance.
Perhaps the Galaxies are in orbit around the Dark matter? Since Dark matter doesn’t interact with normal matter it doesn’t face friction forces so it would move toward the center quicker. This would speed its rotation. Irregular density would probably produce gravity variations that would confer some of that motion to the other galaxies. Heck probably even induce star formation.
It looks like a piece of blue yarn stuck through a giant pink bead.
I find that I don’t believe in Dark Matter. I mean, it’s just a big ad-hoc handwave to explain the discrepancies between observations and models, and I think a lot of people (even scientists) forget that. It somehow offends my sensibilities to posit this whole world of particles that we can’t detect or affect, which weren’t predicted by any previous theories (like the big bang). I don’t doubt that there’s something warping our expectations, but whether it’s an undiscovered particle or a slight change of some physical constant, I don’t know. I’ll believe it, I guess you’d say, when I see it.
Maybe Louise Riofrio is right, and GM=tc^3, and therefore there is no need to invoke dark matter or dark energy at all. Dark Matter could be the aether of the 21st century.
Recently my mother was reading the “Letters to the editor” section of the local paper and exclaimed, “Finally! Someone had the same idea I had about global warming! That it might just be natural variations!” I think of this whenever I am tempted to poo-poo dark matter just because it flies in the face of reason. I am not a scientist, and I am not privy to all the theories, data and calculations that the scientists use to arrive at their conclusions. But I have to admit that it seems the explanation is more problematic than the question. It reminds me of string theory–a beautiful, elegant, and all-encompassing theory of everything, that works up until the point at which you have to assume the existence of a great many dimensions, not because there is any outside evidence for them, but only to allow the theory to work. Imagine a chemist discovering some new complicated process and then announcing, “But it only works if you have this duck on your head.” I am heartened by the fact that many scientists feel the same way and are critical of such theories. Why are so many people willing to accept the existence of a form of matter that is postulated only on the premise that it supports the needs of the theory, especially when such matter must exhibit properties that defy well established principles of physics? It may be that to continue going further and further trying to follow a single line of reasoning is like a bug going down in one of those carnivorous plants with the backward pointing hairs that prevent retreat. Perhaps if more scientists took seriously the study of alternate explanations which have been proposed, it will shed more light on the veracity of the existing theory.
Dark Matter is the new Black.
To Mighty Favog: Did you read anything other than the headline and the first sentence?
What we need to learn how to do is communicate with the giant life form our universe currently resides in.Then ask IT what IT knows about IT’s giant life forms Biology.
Mighty:
String theory IS a mighty pretty thing. The only real objection to it I know of is that it isn’t FALSIFIABLE, which is absolutely required for a SCIENTIFIC theory. There are zillions of theories, but ONLY those that can, in principal at least, be falsified, are SCIENTIFIC theories. All others are,,,merely,,,theories,,,
DArk Matter has been “seen”, at least in outline. It may be nothing more than an exotic form of neutrino(another seemingly ad hoc invention that turned out to be accurate).
By the way, “not privy to,,,” theories, data, calculations, etc, implies there is someone impeding your acquisition of such. All those bits of information are readily available on the ‘net. You just have to search for them. UNDERSTANDING them may be another problem altogether but,,,keep on trying,,,
GAry 7
Mighty Favog: Why are so many people willing to accept the exisentence of a form of matter that is postulated only on the premise that it supports the needs of the theory….
Well, people look for explanations to things. A number of years ago, we saw that stars in galaxies and galaxies in galaxy clusters weren’t following the paths we expected them to on the basis of Newton’s Laws, Einstein’s Theory of Relativity, and the distribution of luminous mass that we could see. They moved as if some extra, unseen, matter was pulling on them. You can Google up more info for yourself.
An alternative theory, called Modified Newtonian Dynamics (MOND), proposed that the observed motions of the stars in the galaxies could be accounted for if the old f=ma that we all learned in high school was modified. MOND worked pretty well at expaining the motions of stars in galaxies, but had trouble explaining some other things. MOND was never able to explain gravitational lensing, for instance, or the observations of the Bullet Cluster, or, more recently, a galaxy cluster called CL 0024+17. Again, I suggest Google. More and more precise experimental measurements of f=ma are beginning to impact the MOND discussion also.
In 2004, someone proposed a theory called Tensor-Vector-Scalar gravity (TeVeS), which has had some success matching observations. So if you don’t like dark matter, you can root for TeVeS or come up with your own alternative theory - but again, in the long run, the theory that best matches the observations will win out.
Wow, I was just sort of thinking out loud as I typed, I didn’t expect to get so many responses.
To Gary 7: I am aware of the non-falsifiable situation and that’s essentially what I was pointing out. But I was just adding my two cents, admittedly more from gut reaction rather than knowing any of the math, that as I hear string theory explained it seems to acquire new assumptions in order to make the previous assumptions work. But the math is beautiful, I’m told.
Good analogy to the neutrino, but still, could such particles collect in such concentrations so as to create a sizable gravitational field and yet still not interact physically? In other words, how can it have mass yet not interact? I am really in the dark about that, I’d like to know if there is a plausible hypothesis.
As to “not privy to…”, sure, I admit I didn’t follow a career in science because of the math. So even though I have these questions (as the one I just stated above) I have to take it on faith that scientists know what they’re talking about.
Brian: Perhaps MOND explains one set of observations, and something else–Dark Matter, for instance–explains others?
Thomas Siefert: Sure, I read the whole thing, which prompted my comments, based on the fact that Phil’s posting was about problems with the theory. Note that I didn’t say outright that I didn’t believe in Dark Matter, as others before me did. I was just examining my reactions to what I read. Although it’s just possible I was making loaded statements in order to provoke a reaction.
I take no pride in being a holdout, but I always have that little voice in the back of my head that reminds me that scientists are prone to much of the same limitations as us regular folk…such as following the flock, linear thinking, and having biases. Also, that there could be much we don’t yet know. Ever read 19th century explanations for volcanism? I love hearing alternate theories, even if they are somewhat suspect, because it gives rise to the occasion to re-examine the prevailing view, and to find reasons to defend or question it.
Dark matter theory does seem to be somewhat robust these days, I’m just not one to jump on the bandwagon right away…I’m probably just hoping that if I wait long enough, something stranger will come along.
Birds from dinosaurs, anyone?
A comparison occurred to me. Politicians might say, “Let’s argue about it”, “let’s take out a lot of full page newspaper ads to try to get people to agree with us,” or “we’ll drop bombs on you until the survivors acquiesce to our poiint of view.” The researchers in the article applied for and were granted additional viewing time on the Hubble and on Chandra to try to discern as much as they can about what actually is going on..
Mighty Favog: the issue with dark matter is not so much that it could exist– as Gary Ansorge and Brian point out, we already have one form of matter that is dark and very weakly interacting: neutrinos. The issue with it is, how can there be *so much* of it? It does not appear that there are enough neutrinos, but there *could have been*, there’s no fundamental reason why the dynamics of the universe couldn’t have been ruled by neutrinos, just because they don’t make light.
But the main problem with neutrinos is that they have very low rest mass, so act like “hot” dark matter, and don’t clump at all. We need “cold” dark matter, that does clump gravitationally, but doesn’t interact and doesn’t make light, yet rules the dynamics of the universe (with dark energy, but that’s another thread). Is this so far-fetched of an idea? Not if you look at the way we probe our universe: we only use light! So is it any surprise that if our universe is ruled by something that doesn’t make light, we wouldn’t know about it until we started looking closely at its gravitational influence?
As for the idea that it is bad science to try to postulate new types of matter to help allow our present theories to be correct, I have two words for you: positrons and neutrinos. Both of those particles were postulated to make an elegant theory correct, before they were ever detected. That doesn’t mean every new particle that would be nice to exist really does (say, the Higgs), but it doesn’t make it bad science to look for it, even to expect it to exist, in the absence of any preferred model. That’s precisely where we find ourselves today, and we are indeed looking for the dark matter in laboratories, just as we did with positrons and neutrinos.
As for Abell 520, I have no idea what the dynamical explanation there is. But I also had no idea what the explanation was for the weird dynamics seen in recent supernova explosions– yet no one said “the theory of gas dynamics must be wrong, we need to look for new forces”. We figured that the dynamical puzzle would be solved, and that’s where my money is here. I could be wrong, of course! That’s the fun of science, and horse racing. But I do know the best theory of the alternatives when I see it.
Ack! I never said it was bad science! I was just remarking on how certain things are so fervently accepted. Again, I also didn’t say I didn’t believe in dark matter, it may very well be real.
When I went to Carlsbad Caverns when I was 12, at the end of the tour I felt kind of let down. As amazing as it was, it was no longer legendary. It was now finite. It just made me want to discover more, stranger caverns.
I suppose that if I’m honest with myself, my gut reaction to even a good, sound theory is “Hmm. Is that all? I wonder what ELSE might work?” Although I am not a scientist, I understand that that’s how many scientists think.
Ken G talked about how much fun this. I can sit in my office, go through my activities here, and just check a few web sites to keep abreast of a lot of fascinating stuff.
I understand what you are saying, Mighty Favog, but to my eye, most of the “fervently held” ideas come from those who reject dark matter on the grounds that is seems like a “cluge” to the theory. As if a totally new theory is not! The issue is not do we “believe in dark matter”– this isn’t religion, we don’t “believe in” anything here. The issue is simply, what direction should we take to find the best explanations. So we should pursue all avenues, but calling dark matter an artifical “fix” is missing the point, in my view.
(This may be an unrelated question, but i wanna know the answer anyway)
Doesn’t the dark matter theory also predict that there should exist galaxies devoid of dark matter? I mean, if u look at the famous picture of the bullet cluster, u see the heated gas separated from dark matter. Won’t that gas eventually cool down and form its own stars and galaxies, without dark matter? What kind of galaxies will be the result of that? Are people looking for such galaxies?
It’s an interesting idea, I really don’t know how possible it is. For situations like the Bullet Cluster, the hot gas may still be gravitationally bound to the dark matter– so even as it cools and sets up to form new galaxies, it may “fall back” into the dark matter. But I’m not saying that to rule out your scenario, no doubt all kinds of interesting dynamical possibilities have to be looked at to figure out Abell 520.
I bet some other astronomers are looking for other colliding clusters of galaxies to study - assuming we know of or can find some. Seems like a promising window into the dynamics of dark matter. This result really hits the monkey on the head with a wrench.
According to Google Adsense, you can buy dark matter on ebay. This seems to be a good way to test the theory…
John A: …you can buy dark matter on ebay.
I’m selling some. It comes in a Klein Bottle.
Ah, the Klein bottle, one entry, no exit, rather like a black hole,,,
Reminds me of a memory device proposed in the 1970s, in which the storage capacity was trillions of bytes, with a tera hertz write speed. Unfortunately, the read back took,,,,forever. It was a beautiful spoof on a perfect top secret memory storage device and was called, appropriately enough, a Write Only Memory. Perfect fit for Dick Chaney,,,
Mighty: You’re in good company in your skeptical viewpoint. Richard Feynman was famous for his attitude of always questioning perceived authority. Even old, tried, true and trusted experiments weren’t beyond his pervue. That attitude led him to a Nobel prize,,,
,,,but dark matter fits our observations quite well. I much prefer it to tossing out theories that have been shown to work so reliably. We stand upon the shoulders of giants and hope,,,they don’t have a bad itch,,,
Gary 7
The answer is very simple, as to why dark matter might be distributed in an unexpected manner:
GOD DID IT!
God did it? One would think God might get a little bit pissed from being blamed for all the things that go wrong in this world. Or maybe we’re just a computer simulation and when OUR ability to create really powerful computer sims (on the order of this world and maybe even universe)(in about 2050 at current estimates of growth in computing power), in order to avoid stacking the sims, the program will be ended,,,danged programmers,,,always playing gods,,,Gee, I wonder if they’ll have any use for a really clear thinking program, like me???
Doh!!!
Gary 7
I see two simple explanations here.
1. The dark matter in the center of the cluster is opaque and is blocking the light from the central galaxies.
2. The galaxies have congregated in orbit around a central mass of dark matter.
Dark matter cannot be opaque because it does not interact with light.
Kyle: The galaxies have congregated in orbit around a central mass of dark matter.
That could be right, Kyle - I don’t know any more details than were given in the picture and the accompanying remarks. What keeps that explanation from being “simple”, from many peoples’ perspective, is that it seems to imply a mechanism by which the dark matter and the galaxies, the two of which were presumably together (in basically the same place) in each cluster before the collision, could get separated from each other after the collision. The only known force that would act on either the galaxies or the dark matter is gravity.
Some people are suggesting some sort of gravitational slingshot mechanism. We use something like this to direct some of our spacecraft. But these people can’t get the details (abode of the devil)to work out.
Others are suggesting that maybe the dark matter interacts with itself in some way other than gravitationally. That would lead us into new physics.
There has been some speculation about this on bautforum.com, some in the Universe Today Stories forum and some in the Astronomy forum.
Owlbear said: “Since Dark matter doesn’t interact with normal matter it doesn’t face friction forces so it would move toward the center quicker. This would speed its rotation.”
Maybe I’m mis-understanding what you meant, but friction forces are what usually drop things into increasingly-lower orbits. For instance, the ISS will eventually hit the Earth because of air resistance; if you put dark matter into that orbit it would sit there for a very long time (assuming dark matter really does interact only through gravity). I know in this case, it will take longer for the friction to slow anything down since the orbiting stuff is slowing itself down, but I think the same principal applies.
XX said: “The problem with ‘dark matter’ is that, since it is undetectable, except through gravity… it is therefore unseeable and unprovable. This strikes me as very similar to the concept of ‘God’.”
Your exception negates that entire “problem”. Can you see air? Of course not, but you still know it exists because you see and feel its effects. Likewise, we can’t see dark matter, but it appears that we can see its effects, so we think it exists. Maybe it’ll turn out to be something different than what we think right now, but it’s not some hypothetical, whimsical “dark matter will allow your non-existent soul to enter an imaginary playground if you believe in it when you die” type of thing.
Okay, I’m confused. Everyone keeps saying the dark matter is clumped in the middle and the galaxies are around it. I don’t see that.
The most clumped thing is the red, hot gas. The blue dark matter spans more space and more protrusions. Wouldn’t that make the dark matter orbiting the hot gas? At least by the reasoning used in this thread.
Also, I am having trouble discerning what are stars vs galaxies, and which are galaxies that are clustered vs. ones that may be at different distances. Frankly, I don’t know what makes this a “galaxy cluster”.
I do agree with MichaelS. Friction sucks energy out of orbits, and thus causes them to get smaller. I don’t see how the lack of friction would be aiding dark matter to clump more than regular matter. It would be the other way around.
Irishman,
I’m not on completely firm ground here, but I think that the distribution of mass in the galaxy is inferred from the studies of the gravitational lensing. Wherever we infer more mass than is accounted for by the galaxies and the gas, the artist with the blue paintbrush has painted in some dark matter. The lensing also allows us to infer the location of the center of mass of the whole system, so I guess this is what they mean by the “center.”
Irishman,
I think I can answer some, but not all, of your questions.
1. A cluster of galaxies is a group of galaxies that are gravitationally bound to each other.
2. Abell 520 apparently formed from the collision of two or more clusters of galaxies. If the the various elements of the conglomerate remain gravitationally bound together, they will have formed a larger cluster of galaxies (Abell 520).
3. Each galaxy, or particle of dark matter or gas within a cluster before the collision would have its own motion relative to the other parts of the cluster added to the motion of the whole cluster relative to the rest of the universe. After the collision, there would also be a tendency to orbit the center of mass of Abell 520. This tendency to orbit the center of mass of Abell 520 would apply to all three elements, the galaxies, the dark matter, and the gas. But each object’s motion before the collision would also affect that object’s trajectory after the collision.
4. I can’t tell the Milky Way stars from the galaxies either, the astronomers may have to use the spectral shifts to determine which is which.
5. The conundrum is why the dark matter and the galaxies aren’t in the same place as each other the way they are in the Bullet Cluster.
Space.com has a really good article on Abell 520.
The Tully Fisher law could mean what it says it means.
The spreading infrared luminosity from the central cloud of hot gas in between the two clusters of Abell 520 could actually be causing the gravitational light bending. In the 1919 solar eclipse study, it could have been the hot spreading luminosity from sun that causing the observed light bending an not the yet-to-be-specified property of the mass of the sun that is causing the warping of the nearby space that the background starlight would appear as bent.
I have been trying for years to get my experiments replicated so someone besides me would start to believe that spreading infrared radiation is gravitationally attractive. And now maybe Mahdavi et al have found evidence that collaborate my experiments ,which in a simple, inexpensive way demonstrate that spreading heat is attractive.
Just a thought - I find the DM discussion fascinating, and it brought to mind a book I read a few years ago, and I’m not sure if it’s been brought up here. Anyone who enjoys this discussion (whether you think DM exists or is just a nice theory to make Relativity “work”) will like this book. The title is Nightfall, and the authors are Asimov and Silverberg. The whole plot of the novel begins when a young scientist calls attention to the fact that according to new data (from newer, better technology) the old “theory of everything,” some sort of gravitational theory, is slightly off. This leads to the theorizing of previously unknown bodies acting on the known ones, just as we are starting to look at DM as acting on normal matter. The young scientist encounters much the same resistance as the proponents of DM, that if we can’t see it it must not be real. Anyhow, I think anyone who reads this would enjoy this read.
~Maestra Myers