Dark Matter Exists 459
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."
Dark Matters (Score:5, Informative)
The announcement [slashdot.org] of the pending announcement [nasa.gov] regarding Dark Matter [wikipedia.org]
I guess he's never heard of Zaphod Beeblebrox.
"A universe that's dominated by dark stuff seems preposterous, so we wanted to test whether there were any basic flaws in our thinking," said Doug Clowe of the University of Arizona at Tucson, and leader of the study. "These results are direct proof that dark matter exists."
Also a bit of info on physorg [physorg.com]
How does the Coalsack Nebula [seds.org] fit into this? It's dark and it's matter, right?
More info from a server that's not on fire... (Score:5, Informative)
Full Paper (Score:5, Informative)
We present new weak lensing observations of 1E0657558 (z = 0:296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated. By using both wide-field ground based images and HST/ACS images of the cluster cores, we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8 sigma significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen.
Re:Dark Matters (Score:5, Informative)
This link isn't Slashdotted yet (Score:2, Informative)
Better Links? (Score:5, Informative)
The new result, in a nutshell (Score:5, Informative)
The visible light image shows the galaxies within the cluster. It also shows, much fainter and much smaller, a very large number of BACKGROUND galaxies -- these are objects way, way farther away than the big cluster. As the light from these background galaxies passes through the big cluster, it is bent very slightly by the gravitational field of the cluster. This gravitational lensing distorts the shapes of the faint, little background galaxies just a bit, but with care, we can measure the effect. We learn from the lensing where the matter is in the cluster: that is, we can figure out where the stuff which produces gravitational effects is distributed. That's part one: a map of the matter within the cluster, based on gravitional lensing.
The X-ray image shows emission from hot gas within the cluster. 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. In most clusters, the amount of this hot gas -- its total mass -- is much larger than the amount of mass we can see in stars. That is, counting the stars in the galaxies suggests a total amount of mass-in-stars M, but computing the amount of hot gas necessary to emit all the observed X-rays yields a mass-in-hot-gas of around 10*M, ten times as much.
On the other hand, the amount of mass derived from the gravitational lensing of background galaxies is about 10 times larger still, or about 100*M. The stuff which produces the gravitational lensing does not emit visible light, nor X-ray light, nor, as far as we can tell, any electromagnetic radiation. Therefore, we call it "dark matter". It produces a gravitational force, but that's about all we know about it. (There are additional reasons for believing that this mysterious stuff is not made up of electrons, protons and neutrons, but that's another story).
This new result is interesting for this reason: the X-rays appear on one region of the cluster of galaxies, telling us that the bulk of the ordinary matter is RIGHT HERE. The map of total mass we can make from gravitational lensing appears in a different region of the cluster, telling us that the bulk of the dark matter is OVER THERE. It is very clear that the dark matter and ordinary matter are distributed in different places. This isn't too surprising, perhaps, if one small group of galaxies rammed into a big cluster -- the gas ram pressure might push on the ordinary hot gas in a different way than on the dark matter (which wouldn't feel any ram pressure at all, actually).
As Martin Hardcastle pointed out to me in a Google newsgroup a few days ago (thanks, Martin!), this is certainly not the first evidence for dark matter -- we have a number of examples in which gravitational forces are larger than the amount of visible matter would suggest -- but it is the first good case in which the distribution of the dark and ordinary matters are so clearly displaced.
Re:Stargate? (Score:2, Informative)
Re:Dark Matters (Score:4, Informative)
Re:The new result, in a nutshell thanks (Score:3, Informative)
This data provides no evidence for the makeup of the dark matter.
Other observations suggest that the dark matter is not Massive Compact (Halo) Objects, or MACHOs. The idea that dark matter might be composed of some sort of Weaking Interacting Massive Particle, or WIMP, is a bit out of fashion these days, but still a possibility, as far as I know.
Re:Silly Musings..... (Score:5, Informative)
Beyond that, though, it's estimated that about half of baryonic matter is invisible for various reasons - thus, the Universe appears to be composed of 2% luminous baryonic matter, 2% invisible baryonic matter, 23% dark matter and 73% (and increasing) dark energy.
Re:I don't see any proof... (Score:5, Informative)
Essentially it goes like this. They see a collision and make an assumption about what it was that collided.
Then, they looked over the area and determined where the mass is right now (from our point of view).
If the assumption about what collided is correct then the result should have been a mass of hot gas that is distributed like you'd expect if a ball of hot gas collided with another ball of hot gas.
Dark matter supposedly only interacts by gravity. Normal matter interacts by gravity plus nuclear and electromagnetic forces. That means that in a collision, normal matter collides with other normal matter while dark matter is merely slowed down and pulled by gravity.
The mass distribution that they observed matched up with the mass distribution implied by the dark matter theory. It can't be accounted for with just normal matter.
The parts of the theory that would need to hold up:
- the assumed initial configuration of the matter before collision.
- the current mass distribution that they observed.
- the calculation about how the collision should behave if it's all normal matter.
- the calculation about how the collision should behave if it's part dark matter.
If those parts hold up then it's a pretty striking discovery.
Re:Dark Matters (Score:5, Informative)
What they claim to have found is a very hot galaxy undergoing gravitation not explainable by the conventionally visible matter.
All of the conventional matter in the area should be hot enough to be conventionally visible.
But since they can't see enough matter to account for the gravitation we have to conclude:
1) It's dark matter. That mysterious stuff that just doesn't interact like conventional matter, but does cause gravity.
2) It's conventional matter in some seriously surprising state that we don't understand, causing it not to be visible.
And their conclusion is that #1 is the more likely explanation. #2 seems unlikely because you would expect to observe this surprising state in the local galaxy or in experiments we perform in colliders.
Re:Blog First, Then Scientific Journals. (Score:5, Informative)
They're physicists (I think Sean Carroll works in cosmology, formerly of the U. of Chicago, now at Cal Tech). It was announced, and the paper has been written. The blog, by the way, is really good.
Re:Blog First, Then Scientific Journals. (Score:5, Informative)
I can't speak for the others, but Mark Trodden and Sean Carroll are theoretical cosmologists. I majored in astrophysics as an undergrad and read some of their papers. Also, Sean Carroll is quoted in the press release [harvard.edu] on NASA's web site.
So these aren't just random guys talking, but professionals in the field. Also, as Sean states in his post, the result was embargoed, which means it was being kept under wraps before publication in a journal. This article and the one I mentioned above are just talking about the results that are published elsewhere. If you really want to read the journal article, it's available here [harvard.edu].
Re:Blog First, Then Scientific Journals. (Score:5, Informative)
is this good enough for you?
Re:Silly Musings..... (Score:5, Informative)
Zeroth, stray neutrons decay to a proton, electron, and an electron anti-neutrino. n->p+n doesn't conserve charge.
First, "ordinary" baryonic matter like protons can only be (according to the well-verified Big Bang Nucleosynthesis) a few percent of the total mass density of the universe, and perhaps 10% of the total amount of Dark Matter. We think the dark stuff is largely, if not almost completely, non-baryonic (not made of quarks, not strongly interacting).
Next, for any isolated mass of protons (essentially ionized H), you'd have to explain where all the electrons went, since the Universe appears to be electrically neutral on even small scales. Also, since the electric force is so overwhelmingly much stronger than gravity, any such cloud cannot be gravitationally bound and would explosively disperse. It wouldn't be perfectly transparent, since protons (being charged) have some cross-section to scatter photons just like free electrons do. In fact, the X-ray emission mentioned in the article comes from hot, ionized H.
Re:Age of the Universe? (Score:3, Informative)
Re:I don't see any proof... (Score:5, Informative)
Step 1: Observe galaxy cluster 1E0657-558 through the Magellan optical telescope, note positions of lensed galaxies.
Step 2: Observe galaxy cluster 1E0657-558 through the Chandra X-Ray observatory, note positions of colliding gas.
Step 3: Using statistics and vector calculus, compute where the centers of mass causing the lensing are.
Step 4: Note that the computed center of mass (green contours) doesn't match the position of the gas which composes ~90% of the cluster's visible mass (false-color smear), as shown on page 2 [arxiv.org].
Conclusion: Something that we can't see comprises ~90% of 1E0657-558's mass. This something emits no EM radiation, no particle radiation, and thus does not interact with the normal matter in the cluster via electromagnetism or the nuclear forces. It's only measurable property is it's mass, hence "Dark Matter".
Why should a divergent nuclear chain reaction reveal or be affected by the presence of something that doesn't interact by the strong, weak, or EM forces? Dark matter doesn't come up when experimenting with forces that don't affect it.
Physics is nothing more than a way to model the universe and it's contents. Would you have exclaimed suprise at Einstein's use of wave-particle duality to explain the photoelectric effect because we didn't understand phyisics at the atomic scale circa 1900? The photoelectric effect, the quantum theory of the atom, black holes, and now Dark Matter are the things we use to make "known physics" jibe with observed reality. The whole reason Dark Matter is proposed because the current model of gravity acting on visible mass doesn't fit observations.
Re:Blog First, Then Scientific Journals. (Score:5, Informative)
Any non-standard gravitational force that scales with baryonic mass will fail to reproduce these observations.
This is a very nice piece of work. One observation doth not a proof make (the myth of the "crucial experiment" is, well, a myth) but if confirmed by comparable observations on similar structures it could really start to constrain inter-galactic dark matter models in ways that are much more precise than hitherto has been possible.
The fundamental importance of this paper is less in the single observation than in the development of a new technique for probing the inter-galactic dark matter distribution directly and in detail.
Of course, it says nothing at all about galactic dark matter.
Re:Blog First, Then Scientific Journals. (Score:2, Informative)
Indeed - do we have "conclusive direct proof" that the Sun is made of mostly of hydrogen and helium?
Yes, we do.
It sounds like (these) astronomers have a different standard of evidence than I do.
Yes, a much, much, much tougher one.
Proof?! (Score:2, Informative)
Full-text from Browser Cache... (Score:5, Informative)
Sean at 11:52 am, August 21st, 2006
The great accomplishment of late-twentieth-century cosmology was putting together a complete inventory of the universe [uchicago.edu]. We can tell a story that fits all the known data, in which ordinary matter (every particle ever detected in any experiment) constitutes only about 5% of the energy of the universe, with 25% being dark matter and 70% being dark energy. The challenge for early-twentyfirst-century cosmology will actually be to understand the nature of these mysterious dark components. A beautiful new result [harvard.edu] illuminating (if you will) the dark matter in galaxy cluster 1E 0657-56 is an important step in this direction. (Heres the press release [harvard.edu], and an article in the Chandra Chronicles [harvard.edu].)
A prerequisite to understanding the dark sector is to make sure we are on the right track. Can we be sure that we havent been fooled into believing in dark matter and dark energy? After all, we only infer their existence from detecting their gravitational fields; stronger-than-expected gravity in galaxies and clusters leads us to posit dark matter, while the acceleration of the universe (and the overall geometry of space) leads us to posit dark energy. Could it perhaps be that gravity is modified on the enormous distance scales characteristic of these phenomena? Einsteins general theory of relativity does a great job of accounting for the behavior of gravity in the Solar System and astrophysical systems like the binary pulsar, but might it be breaking down over larger distances?
A departure from general relativity on very large scales isnt what one would expect on general principles. In most physical theories that we know and love, modifications are expected to arise on small scales (higher energies), while larger scales should behave themselves. But, we have to keep an open mind in principle, its absolutely possible that gravity could be modified [blogspot.com], and its worth taking seriously.
Furthermore, it would be really cool. Personally, I would prefer to explain cosmological dynamics using modified gravity instead of dark matter and dark energy, just because it would tell us something qualitatively different about how physics works. (And Vera Rubin agrees [newscientistspace.com].) We would all love to out-Einstein Einstein by coming up with a better theory of gravity. But our job isnt to express preferences, its to suggest hypotheses and then go out and test them.
The problem is, how do you test an idea as vague as modifying general relativity? You can imagine testing specific proposals for how gravity should be modified, like Milgroms MOND [wikipedia.org], but in more general terms we might worry that any observations could be explained by some modification of gravity.
But its not quite so bad there are reasonable features that any respectable modification of general relativity ought to have. Specifically, we expect that the gravitational force should point in the direction of its source, not off at some bizarrely skewed angle. So if we imagine doing away with dark matter, we can safely predict that gravity always be pointing in the direction of the ordinary matter. Thats interesting but not immediately helpful, since its natural to expect that the ordinary matter and dark matter cluster in the same locations; even if there is dark matter, its no surprise to find the gravitational field pointing toward the visible matter as well.
What we really want is to ta
Re:"To be published..." (Score:5, Informative)
Re:Full Paper (Score:5, Informative)
We know musical talent must come from either training or predisposition. We assume there are no other factors, because those two cover the reasonable possibilities. Consider this logical statement then: John Doe has no musical training, yet he is very skilled. John's musical skill cannot be explained by training, and thus proves that there exists some sort of predisposition to musical talent. It doesn't tell us (e.g.) whether it's genetic or not, but knowing for sure that it's there helps us refine our further studies.
Now the real version. There is "more" gravity than we can account for with the combination of Baryonic (regular) matter and Einstein's theories of gravity. A LOT more. There are only two possibilities: Gravity gets stronger under certain conditions (regular matter pulls harder), or something "unseen" is pulling. Of course, both could be true, but at least one of them MUST be true to match observations. We assume there are no other explanations, because those two are broad enough to cover the entire range of reasonable possibilities.
This experiment showed that the center of gravity of certain galaxies doesn't correspond to the center of the regular matter. In other words, the galaxy's gravity is pulling in a different direction than the normal matter would indicate. "[This] cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen."
Re:What is "dark matter"? (Score:3, Informative)
No. We can also see things by the light they BLOCK from stars. If it was just matter, we would have seen it, and conclusively proved it's existance, long ago.
Re:What is "dark matter"? (Score:3, Informative)
As I understand it, the way they did it in this case is by knowing the temperature of the galaxies. If there were regular matter causing the observed gravitation, it would be hot enough to give off some sort of radiation that could be picked up via telescope. They may also have measured how much light from behind is being absorbed. In addition, if it were regular matter that interacts with other regular matter, it probably would have followed a similar pattern to the gas clouds, which means the center of gravity would have stayed with the observable clouds rather than separating like it did. (They probably have more reasons why it can't be regular matter, but that's what I could come up with off the top of my head late at night.)
Re:"To be published..." (Score:4, Informative)
Re:Remember Vulcan? ( no, not startrek vulcan ) (Score:3, Informative)
He calculated that Vulcan must have a revolution period of 33 days, an orbit 18 1/2 million miles from the Sun, inclined 12 degrees to the ecliptic. And by Newtonian cosmology, he was right.
And people did see it. As much as, or more so, than they have seen dark matter. The first sighting was made by Dr. Lescarbault, a doctor - and amateur astronomer - from the town of Orgenes on March 26th, 1859. It was right where Le Verrier said it would be. More than 30 people claimed to have seen it over the next decade or two. Many of them were serious scientists, such as James Watson, director of the Ann Arbor Observatory.
So much for the facts. I was going to elaborate on the logical structure of my previous post, but donaggie03 already did it better than I could.
Re:Why does dark matter only hang around solid mat (Score:5, Informative)
Re:Why does dark matter only hang around solid mat (Score:3, Informative)
Good question! (Score:3, Informative)
Re:Flat or closed universe, open ruled out ? (Score:3, Informative)
Re:As I understand the process ... (Score:3, Informative)
... this would constitute a confirmation if a prior prediction was made, and the observed results match the prediction.
The prediction was that mass distribution wouldn't match baryonic mass distribution because the non-baryonic part only interacts with itself gravitationally. Hence, there would be far more diffusion of non-baryonic component than the baryon component in the collision described in the article. The article claims that they indeed observed a mass distribution derived from the study of gravitational lensing that doesn't match (to a substantial degree of deviation) the distribution of visible, baryonic matter (which is illuminated by X-rays).Re:Stargate? (Score:1, Informative)
http://slashdot.org/article.pl?sid=06/08/22/13022
Re:this stinks (Score:4, Informative)
But, MOND and the related theories DO NOT remove the need for dark matter (or dark energy). MOND does away with dark matter on galazy scales, but clusters still require dark matter to match observations (for the record, I do simulations of galazy clusters).
There really is no big conspirency. Lots of astronomers are not comfortable with dark matter or dark energy. But they aren't trying to fake their way into making other believe it. At the moment, dark matter fits the data very well (without breaking relativity and other well tested physics). I've been to lots of talks and seen lots of papers where people take the idea of modified gravity seriously. Unfortunately, it is hard to come up with a modified theory of gravity that explains the data without getting something else well tested wrong. It doesn't mean it can't be done.
Re:Full Paper (Score:3, Informative)
I think your observations fit well with what the parent poster was saying.
Understand that both options are meant to cover the entire range of comprehensible possibilities. If you can think of any possibility that does not fall under the umbrella of one of these, then you should publish a paper, because you are thinking outside the box and you might be the next Einstein.
I think that would fall under possibility #1: gravity gets stronger under certain conditions. The condition you cited is resonance.
Actually, that sums it up quite well. I might put it this way:
1) Gravity is caused by matter
2) There exists a gravity that is not caused by observable matter
3) Therefore, that matter is not observable.
The other possibility is that (1) is wrong, but we have so much evidence confirming that, and no alternatives, so we have to continue with that as our basis.
I think what they just saw cannot be explained by the proposed changes to the mathematical model. So either we need a new proposal (none exist yet), or we need dark matter. I think at this point, dark matter just got a whole lot more likely.
My turn to make an analogy using Virtual Particles [wikipedia.org]
We could say that the concept of a "virtual particle" as a force carrier is silly. How can there be a neutral particle that just happens to show-up whenever we need it, to carry force from one thing to another. That's silly, and surely an updated mathematical model is better. And every time we see something that conflicts with the mathematical model, we can revise it so that it works again. But eventually that model fails many many times, and we revise it so often that it gets confusing and complicated. So eventually, despite our instincts saying it is crazy, we decide that it is just easier to say there is a virtual particle and move on. If it works mathematically, and it explains the pheonemon, and it is simple, then Occam's razor says use it.
Maybe there is no dark matter. Maybe it is a virtual conceptual thing that will lead us to a better solution. But for now, the alternate models have failed and this is where we stand.
Re:So what's new, then? (Score:5, Informative)
Relative to your point (2) Some scientists have proposed that indeed gravity doesn't work as simply as General Relativity explains it, in particular, the simplest one called MOND assumes that gravity weakens with distance. It is sufficient to explain the rotating galaxy artifacts that you mention, however MOND is purely phenomenological, in other words it does not provide a mechanism by which gravity should weaken. It can be adjusted to rotating galaxies observations by modifying a couple of parameters, but it explains nothing.
With regards to (1), scientists are loath to abandon GRT because it is funded on very simple principles (essentially everything is local and the effects of acceleration and gravity cannot be distinguished) and explains so much with so little. There are myriads of ways to extend GRT in such a fashion as to explain observations by playing with the equations but AFAIK none can be derived from simple first principles unlike plain Jane GRT.
Indeed the simplest explanation to the observations is to admit that there is a great deal of matter in the universe that doesn't interact with normal matter as usual (it doesn't heat up in the same way for example) and is therefore dark, but does possess mass and affects observations. Of course it looks as if an enormous list of free parameters has just been added to GRT, but this is not innocuous. Dark matters, if it exists, should show up in observations other than with rotating galaxy data.
Now the new data is not derived from rotating stars but from large clouds of galaxies attracting each other. This is precisely why this is interesting, because it does look as if the new data confirms the existence of some kind of matter that doesn't heat up in the normal way and attracts normal matter, but this time not in a rotating framework, more in a translating framework. This is something that MOND does not explain.
Also perhaps we can design experiments that would prove the existence of dark matter in the lab. We already know about neutrinos, which fits the description of a kind of dark matter. Neutrinos do not interact through the electro-magnetic force or the strong force, they don't interact with normal matter, they don't heat up. They are very hard to observe due to this fact, and to characterize. However we have been able to prove their existence in the last few decades via indirect effects, and to prove they have mass. Neutrinos are very light though, we would need absolute humongous amounts of them to explain the vast quantities of dark matter that would explain the observation, and thus a mechanism that would generate such huge quantities of neutrinos.
Or perhaps there exists other kinds of weakly interactive particles that are much more massive. This is not predicted by the standard model of quantum mechanics though.
So right now physics is at an impasse : either GRT is wrong or QM is wrong. Probably both in fact, but what we do like is a smallish set of first principles that would guide us towards a better, more comprehensive theory. It was hoped that superstrings would be it, but it's too complicated and right now untestable.
The key points in conclusion : yes you can propose changes in the way gravity works to explain older observations, and some scientists have done so. However these changes are not popular because they are essentially ad hoc and explain nothing. Furthermore the latest observations seem to imply they are not sufficient anyway. Dark matter explains both old and new observations, but we don't know what dark matter is, how it is produced and how to characterize it. Right now this is not satisfactory, but this means new awesome discoveries are awaiting us in the (hopefully near) future. Stay tuned !
I hope this helps.