Hubble and the VLT Uncover Evidence For Self-Interacting Dark Matter 117
astroengine writes: A new study carried out by the ESO's Very Large Telescope and the NASA/ESA Hubble Space Telescope has revealed for the first time that dark matter may well interact with itself — a discovery that, at first glance, seems to contradict what we thought we knew about the nature of this invisible mass. "In this study, the researchers observed the four colliding galaxies and found that one dark matter clump appeared to be lagging behind the galaxy it surrounds. The dark matter is currently 5000 light-years (50 000 million million kilometers) behind the galaxy — it would take NASA’s Voyager spacecraft 90 million years to travel that far. A lag between dark matter and its associated galaxy is predicted during collisions if dark matter interacts with itself, even very slightly, through forces other than gravity. Dark matter has never before been observed interacting in any way other than through the force of gravity."
Could someone (Score:2, Funny)
Shed some light on this so called Dark Matter?
Self-Interacting Dark Matter (Score:2)
Great, the Universe masturbates.
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That's because the existence of other universes is purely hypothetical, just like an AC's girlfriend.
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Fantastic, a lone genius who has seen behind the curtain and is laughing at all the ignorant graybeards. On Slashdot of all places. Who'd have thought a casually arrogant armchair blowhard would reside here of all places.
What keeps you from blowing the lid on all this? Worried they'll do a Galileo on you?
Ups (Score:2)
Is there a way to edit/delete my own posts?
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Yes, see the little "Preview" button?
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Yup. It returns a display that cleverly hides the stupid things I typed, as far as I can tell, and clicking on Submit puts those things where I can see them.
At least that's my empirical observation.
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"So where it takes light 5000 years to travel a distance, the Voyager would take only 90 years
That means Voyager speed is: 5000 / 90 * lightspeed = warp 55.5!"
In the Original Star Trek, "warp Factor" is cubed to give the equivalent in light speed. Thus the original 1701 Enterprise could travel 64 times lightspeed at Warp 4
By the time we get to Voyager (and presumably TNG as well) Warp 10 seems to be nearly infinite speed, and they could have got home from the Delta Quadrant fairly quickly if the could achie
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They said that, but 64c (or even 216c) isn't really fast enough to see some of the things that supposedly happened. 64c means three weeks to alphacent, or five months to Vega, as I recall. They were tooling around much faster than that....
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We need to cast off the shackles of what we want to be true, and look at the evidence in a cold, anyalytical light.
Yeah, we need to study things we don't like and don't want to be true... like how dark matter was described for the first couple decades. Astronomers didn't like dark matter, especially when easy solutions like MACHOs were questioned or almost eliminated. Yet despite continued analysis and work on both dark matter and several alternative theories, alternatives continue to fall short, and dark matter continues to work well.
At this point, I would think a plea for an analytic mind and considering things yo
Re:Dark matter doesn't exist. (Score:5, Interesting)
In a rush to tailor the evidence to a flawed theory, dark mentor was invented by humon minds in an attempt to save a beloved theory. We need to cast off the shackles of what we want to be true, and look at the evidence in a cold, anyalytical light. When this is done, I'm quite certain that there will be no need for the magical fairy dust matter that is there but isn't there.
The term dark matter is just the name for a discrepancy. For example, the galaxy rotation speed is 220 km/s at our position in the galaxy (8kpc), and stays the same until 30kpc. But the number of stars, which are the mass we can see, declines exponentially. So some mass (10x more than what we see) must be there to keep the rotation fast (otherwise it would be like the solar system -- Pluto rotating around the sun much slower than Mars).
Then in clusters we see that gravitational light acts as a lens and we can infer the mass that bends the light behind it. And it is much more than we see in stars and gas.
In the cosmic microwave background, which is a relic from the last time electrons and photons interacted very strongly, 380000 years after the "Big Bang", we can estimate the density of the universe there. Also, the fraction of matter interacting with photons, is only a fraction of the total matter there.
All of these *different, independent* probes, and several others, point to the same ratio of total matter to electromagnetically-interacting matter.
Now you can take the state of the Universe at 380000 years age, with its total matter, electromagnetically-interacting matter and photon budget and evolve it following general relativity. And people find that the clustering of galaxies, their total number and sizes can be reproduced quite well. And this is not possible without putting that additional, non-electromagnetically-interacting matter there. And In this experiment you can learn something about how weak the electromagnetic interaction must be (for example, a large population of Neutrinos can be excluded, because they interact to strongly, smoothing out the structures).
As you say, another path is to modify the theories of GR, and every week there are papers explaining Dark Matter with alternative theories, sometimes in combination with Dark Energy. This is a path that many people are working on. If you see the term "Dark Matter" as the *name of the problem*, namely the discrepancy between observations and normal matter + GR, then there is no conflict, it does not say how to solve it. Dark matter is real, because the discrepancy exists. And the search for particles is also not concluded yet: Larger, cold objects have been proposed (e.g. brown dwarfes, Jupiter-size planets), as well as new fundamental particles (Neutrinos, as well as as-of-yet unobserved particles, like the sterile Neutrino, or totally new particles from some theories of supersymmetry). Some of them have been excluded -- for example it can not be stellar-size black holes, because of the number of binary star systems we observe in the outer parts of the Milky Way; those would be destroyed by frequent interactions with a large population of masses. The upgraded LHC will try to produce more particles, and there is a real chance it will produce (or exclude) a specific candidate dark matter particle predicted (proposed) by supersymmetry.
Believe me, Astronomers really do not like the idea of Dark Matter, and have been fighting it for decades. But the evidence from many different experiments is there. We still don't know what it is, whether the laws have to be changed or additional particles have to be put there (and which ones). But the range of possibilities is getting smaller and smaller. And putting particles there that do not interact except for gravity has been very successful in explaining various observations. I used to be cautious because in principle you could just arbitrarily put mass where you need it -- but if you start from the Big Bang and only use general relativity, then the created galaxies with dark matte
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Mod up for a great reply. Oh, wait. No mod points again. Wish there was a kitty so that I could save a handful for when I need them.
rgb
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I am with RGB on this one, I wish I could mod you up. This is the most approachable explanation for dark matter I have ever read.
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There is zero evidence that dark matter exists - it's a fudge factor used to make bad theory fit reality, nothing more.
Bullet cluster. Galactic rotation rates. Gravitational lensing.
Funny concept of zero you have. Any other theories you don't like that you want us all to stop taking seriously?
Modified Newtownian theories are all dead in the water. They've been discredited. Enjoy wallowing in ignorance.
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When this is done, I'm quite certain that there will be no need for the magical fairy dust matter that is there but isn't there.
So dark 'mentor' is the theory that best fits the available data, but you're certain it's wrong because of ... ???. Brilliant!
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Eh, probably typed that on a phone.
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Re: Dark matter doesn't exist. (Score:2, Funny)
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" through the first principals of physics"
The first ... principals of physics? Like Aristotle, Newton? Who else?
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Dude, it's embarrassing that in your "paper" you can't spell "principle" correctly.
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One only needs to define the photon as a thermodynamic reexpansion of spacetime that was compressed by nearby matter.
Unfortunately that is not a meaningful statement. I have no idea what a "thermodynamic reexpansion" is versus a "non-thermodynamic reexpansion", for example. Nor is it clear how this would be expressed mathematically as a generalization of Maxwell's equations. Nor does your paper do anything more than repeat this meaningless statement.
There may be something meaningful and interesting to say about the thermodynamics of electromagnetism and space-time, but until you give us a mathematical statement of the phy
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" dark mentor was invented by humon minds"
Are you a Ferengi? Never mind what a "dark mentor" is, like the Emperor or something?
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May the schwartz be with you!
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If the only interaction was gravity (Score:2)
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No, because there would be no easy way for the particles to shed momentum and form a clump.
Even when you have a black hole pulling on normal matter, stuff can't easily just fall in. If you had a black hole and a single piece of something falling toward it, unless that piece directly hits the event horizon or comes close to it, it will just fling back out at the really close to the same speed it went in (if it was massive enough and in a small orbit, the orbit might decay from gravity waves, but that is st
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Then wouldn't the dark matter clouds just collapse in on themselves and form singularities as there would be no counterforce to gravitational attraction?
Gravity is the attraction of masses. The reason that things don't pass through each other is something else. It involves the electric repulsion of electrons and protons, but a more detailed answer is here [stackexchange.com]
Uh, thanks for the useless Voyager comparison (Score:4, Insightful)
The dark matter is currently 5000 light-years (50 000 million million kilometers) behind the galaxy —
it would take NASA’s Voyager spacecraft 90 million years to travel that far.
Right. Would it? Okay. How is that supposed to help me imagine 5000 light years? I already know it's a bloody long way. You might as well have told me it was the length of x football pitches or y times the length of the Amazon river.
A comparison with the diameter of the galaxy in question would have been more useful.
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So... can we have your liver, then?
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Yeah, alright, you've talked me into it.
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Think about the thickness of a dollar bill. Imagine stacking dollar bills on top of each other... $100 is a bit less than half an inch, and $1 million is about 30 stories tall. $1 trillion reaches a little over 1/4 of the way to the moon.
Take that $1 trillion stack and imagine shrinking it again, back down to the height of a single dollar bill. Take 1 trillion of those, again creating a tower 1/4 of the way to the moon.
Go back and imagine the size of that original dollar bill now, shrunken so much. If t
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Right. Would it? Okay. How is that supposed to help me imagine 5000 light years? I already know it's a bloody long way. You might as well have told me it was the length of x football pitches or y times the length of the Amazon river.
A comparison with the diameter of the galaxy in question would have been more useful.
Our galaxy is on the order of 100,000 lightyears in diameter. So 5,000 lightyears is about 1/20 the distance of our galaxy. That's a pretty large distance to lag behind our matter, considering that it also interacts with itself.
How have we ruled out measurement or model error? (Score:4)
I realize the term Dark Matter is something of a placeholder for the cause of some as-yet unexplained observations but many people (including physicists) are taking the term quite literally There are three possibilities for what it could possibly be.
1) There is some form of exotic matter (or other phenomena) whose properties have yet to be discovered but which has a gravitational effect
2) There is an error in the measurements of the matter we can see
3) There is an error in the models we are using to describe the matter we can see
What I don't understand is why so many scientists are favoring 1 when 2 and/or 3 seem to be just as likely. 1 is the most exciting possibility but we have nothing more than indirect evidence for it. I'm waiting for someone to explain why so many seem so sure that it actually is some form of exotic matter. We've been down this road before. We couldn't explain phenomena like the orbit of Mercury until Einstein showed that Newtonian mechanics was merely an approximation of the more accurate relativistic models. People were trying to use the observations as evidence that there might be some undiscovered matter when really it was an inaccurate model. We also make measurement errors all the time. I just don't get how we've ruled out a measurement error or a modeling error.
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ad 3: Plenty of people are working on modified models, such as alternatives to general relativity. There are papers coming out every week. https://en.wikipedia.org/wiki/... [wikipedia.org]
ad 2: Errors in measurements can be somewhat excluded as a possibility because many different measurements looking at different aspects and scales find the same result. Wikipedia lists 3.1 Galaxy rotation curves, 3.2 Velocity dispersions of galaxies, 3.3 Galaxy clusters and gravitational lensing, 3.4 Cosmic microwave background, 3.5 Sky
Errors versus public debate (Score:2)
Plenty of people are working on modified models, such as alternatives to general relativity. There are papers coming out every week.
And yet they are basically never discussed when the topic comes up in the public discourse. I cannot remember the last time I saw a physicist talking about dark matter as a possible modeling error. I'm certain they do but the notion of dark matter actually being matter dominates the public discourse. Not shocking since it is by far the most interesting of the possible results but still...
Errors in measurements can be somewhat excluded as a possibility because many different measurements looking at different aspects and scales find the same result.
In many cases yes but certainly not off the table. Particularly if some key pieces of the puzzle turn out to be wrong
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You should look into MOND [umd.edu].
"Dark matter" as an effect is very well established. It is a sign of a failure in our models of physics. That failure could be in the microphysics (thus, various particle models, such as WIMPs), or in the macrophysics (i.e., in general relativity, the model for gravity, which is modified by theories such as MOND).
Now, as it happens, these sorts of galaxy cluster collision observations are probably the strongest test [arxiv.org] of MOND type theories - it is hard to see how a failure of gravit
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Corrections to gravity were discussed at great length when they were still a reasonable alternative. You CAN explain some galactic rotation, and the movement of some clusters. The problem is, in order to explain all galaxies, all clusters, or all galaxies and clusters, modified gravity theories need lots of dark matter anyway.
Dark matter isn't really all that revolutionary of an idea. Neutrinos are "dark" in that they don't interact electromagnetically, and they were mysterious mathematical figments (ver
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Because to physicists who spend their time working this problem seem to agree that 1 is most likely. 3 is being attacked all the time with new theories but it is sort of an unlimited well of human imagination.
In my opinion, it is like the WTC "theories". They get started because some yokel cannot understand the official explanations when others seem as likely. However, if you pick up Popular Mechanics book "Debunking 9/11 Myths" (especially the newer addition), they pretty much destroy the reigning alternat
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Physicists in the public discourse (Score:1)
Wondering why they have not been ruled out demonstrates lack of domain knowledge and awareness of what is being worked on, not a flaw in how it is being approached in research.
Has nothing to do with lack of domain knowledge. You almost NEVER hear physicists talking about dark matter in the public discourse as a possible modeling error. They ALWAYS refer to it as matter we cannot see. Heck the term dark matter itself strongly implies that they think it actually is matter. Otherwise why call it "matter"? I could be cynical and point out that hunting for model errors isn't a great way to get funding for the next particle accelerator but I don't think that is what is going on he
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Having worked in physics, I can tell you that hunting for modeling and measurement errors is a great way to get funding. Finding a specific modeling error is a HUGE career boost, and papers are written all the time exploring possible errors and how they can be (or were) tested. Same with measurement errors, there was even a story on this very site a few months back about a paper being publishe
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Dark matter IS a modeling error.
Of course it is a modeling error but which kind? Is it an error that there is some new particle or other physical phenomena that we haven't accounted for (like a new particle) or is there some error in the mathematical treatment (the "model") of the phenomena but no new physical phenomena exists? The discussion among physicists themselves certainly talks about both possibilities but that is NOT the case when the topic is brought up to those who do something other than physics for a living.
But to claim they are never talk about modeling errors represents a fundamental misunderstanding of research into dark matter or even the very concept of it.
You are misunde
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I'm waiting for someone to explain why so many seem so sure that it actually is some form of exotic matter.
You'll forgive me for believing that that is a lie, because this has been explained many, many times. On the balance of probabilities, you are an irrational nutjob who is resistant to any actual explanation or evidence.
That said, I'll waste few minutes of my precious time pretending your question is sincere and you have a non-zero chance of changing your mind.
The reason why we focus on exotic matter is because observational evidence for a source of anomalous gravitational attraction is robust and diverse an
I don't favor any particular hypothesis (Score:2)
You'll forgive me for believing that that is a lie, because this has been explained many, many times. On the balance of probabilities, you are an irrational nutjob who is resistant to any actual explanation or evidence.
And based on your statement you'll forgive me for believing that you are a condescending asshole who looks down on people who ask a pretty straightforward question.
That said, I'll waste few minutes of my precious time pretending your question is sincere and you have a non-zero chance of changing your mind.
And the condescending asshole theory is confirmed. "Your precious time"? You're posting on slashdot. If you actually had better things to do you wouldn't be here. So drop the attitude. I didn't come here to insult you so I'd appreciate the same consideration.
Do you see why I think your question is dishonest?
I see that you are a condescending person who thinks I'm asking this question for re
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You agree that some form of matter that doesn't interact much electromagnetically is an answer. There are others, but they all have more problems than figuring that there's a whole lot of something like slow massive neutrinos (one possible version of dark matter). Therefore, if we line the answers up in order of plausibil
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Property of Dark Matter (Score:5, Informative)
Contrary to the summary, this is one of the expected properties of Dark Matter. The leading candidate that answers the dark matter observation problem (which is already well-described by buchner.johannes above) is a new kind of particle, known as a WIMP, for Weakly Interacting Massive Particle. "Weakly" doesn't just mean "not strongly", it means "through the weak force". It's postulated that this new kind of particle, predicted by various extensions to the Standard Model of particle physics, interacts with itself through the weak nuclear force.
What we don't know very well is how efficiently this interaction takes place. Ways to measure this (and hence detect WIMP dark matter) include:
1) Direct detection: Wait for a stray WIMP to hit a block of stuff and detect a flash/vibration/decay product/whatever. Many experiments. Status: ongoing.
2) Production: Make some WIMPs in a particle collider. Status: check with LHC in a few months.
3) Indirect: The weak nuclear interaction produces some by-products, like neutrinos and gamma rays. Thus if you look at a spot where there ought to be lots of dark matter (like the center of the galaxy), you might see some extra gamma rays. The Fermi-LAT satellite is doing exactly this. Status: ongoing.
4) Behavior: The interaction will "slow down" the movement of WIMPs by introducing a little bit of drag. This would be a much much weaker version of what happens to normal matter when clouds of gas run into each other. Using gravitational lensing we can probe the mass distribution and look for such drag effects. That's what this article is addressing.
Whoever is the first to confirm the existence of dark matter (whether WIMP or otherwise) is pretty much guaranteed a Nobel, so the race is on.
If we still don't find anything in ~10 years, then we probably need to go back to the whiteboard and figure out something else.
Shameless self-plug: I'm going to discuss this more in an upcoming episode of my podcast [apple.com].
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WIMPs are actually an old explanation of dark matter, probably on the way out unless LHC can pull out a supersymmetric particle in their new run.
In any case, WIMPs only interact through the weak interaction, so it is generally assumed that these "self-interacting [arxiv.org]" particles are not WIMPs, but some new form of SIDM.
a ghostly gas inside us all (Score:3)
Immolate! (Score:1)
It's 2.5 sigma (Score:2)
It's probably not wise to put a lot of weight on a discovery that is only 2.5 sigma.
Double-rubber Hypothesis (Score:1)
Any chance the topology of space-time can vary independently of gravity?
Despite its flaws, let's use the rubber sheet analogy. Except in this case, suppose there are two rubber sheets: one atop another. Say ordinary matter (planets, stars, etc.) indent the top sheet as expected, but only black holes can over-extend the first sheet and influence the shape of the second underlying sheet.
Now suppose this second sheet deforms over greater distances than the first, which in turn influences the shape of the top
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If I understand part of GR correctly, the geometry of spacetime is relativity. (Topology, at least in the math I've studied, refers to more general properties. If you have coffee and a doughnut, the doughnut and cup are topologically identical. There may be a more specialized meaning in physics that I'm unaware of.) Therefore, you're asking if spacetime can vary independently of mass. Given space expansion, I'd suspect the answer is "yes", although you should ask a real physicist for confirmation. Ho
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Re:OK (Score:5, Interesting)
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They do as long as they don't propose anything too radical.. like that dark matter might fit very well as tachyonic material. They cant even consider that because it threatens fundamental scripture. ...
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It's a good thing nobody is suggesting magic (except you).
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Yeah, because both those operate at the same time scales...
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Bullet Cluster.
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Umm No. General Relativity's whole "mass distorts space time" thing kind of does explain what makes things move. As opposed to Newtonian physics that only mathematically (and inaccurately at stellar mass scales) predicted how things should move. That's not to say that through the Scientific Method we will not continue to learn new and interesting things. But GR does offer an explanation of what makes things move.
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They never stop to think that GR may actually be incomplete as a theory.
Wrong. The jaggy discrete quantum world and its forces are completely alien to GR. No-one denies this. Your statement is profoundly ignorant.
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No actually the real problem is that general relativity is completely wrong, at least at FTL speeds. If you actually build an FTL version of physics it becomes quite clear that an absolute frame and FTL Simultaneity are required for the universe to exist, and general relativity forbids both. The solution that reconciles the two models is to restrict the (physical) dimension of time to quantum scales. This marginally modifies the theory of space time to make dimensional time a notional abstract quantity.
Th
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Invisible mass of magic particles? Doesn't that describe a neutrino burst? What's magic about something like slow massive neutrinos?
Heck, do you believe in trans-Saturnian planets? It turned out that Saturn had some discrepancies with its predicted orbit. Rather than trying to hack Newtonian gravitation, astronomers figured that they'd be accounted for by a more distant planet with certain characteristics, and found Uranus. Neptune was found much the same way.