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."

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Dark Matters

[ackthpt]

The announcement of the pending announcement regarding Dark Matter

"This is the most energetic cosmic event, besides the Big Bang, which we know about," said team member Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

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

How does the Coalsack Nebula fit into this? It's dark and it's matter, right?

Re:Dark Matters

[Surt]

I'm not an astrophysicist, so feel free to consider this only mildly informed:

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.

More info from a server that's not on fire...

[mpathetiq]

Here's some info from NASA.

Full Paper

[Anonymous Coward]

The full paper can be found here. From the abstract:
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:Full Paper

[NereusRen]

I know your post was somewhat tongue-in-cheek, but it was modded interesting, so others are probably wondering the same thing. Lets play the analogy game first...

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."

Better Links?

[clang_jangle]

For those who prefer here are the salient links which TF"A" (it's a blog entry) is referencing: http://chandra.harvard.edu/chronicle/0306/devil/ http://chandra.harvard.edu/press/06_releases/press _082106.html

The new result, in a nutshell

[StupendousMan]

Astronomers observed a distant cluster of galaxies in optical light, with ordinary telescopes, and in X-ray light, with a telescope in space. This is an unusual cluster of galaxies, since there is clear evidence that one small group of galaxies are "interlopers:" members of a smaller cluster which fell into a larger one some time ago. Members of this interloping group are all bunched together at one side of the main cluster.

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:Silly Musings.....

[Xeriar]

There are very few free protons or free electrons, and no free neutrons (half-life of about 15 minutes before it turns into hydrogen) - nearly all interstellar matter is composed of hydrogen and helium. Beyond which, by your theory they would be generating an absolutely massive electromagnetic charge.

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:Blog First, Then Scientific Journals.

[Soldrinero]

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 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.

Re:Blog First, Then Scientific Journals.

[garvon]

http://www.sciencedaily.com/releases/2006/08/06082 1133930.htm
is this good enough for you?

Re:I don't see any proof...

[The Master Control P]

Show me why dark matter has to exist.

Executive summary of TFPP (The Fucking Physics Paper):
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.

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".

We've set off fifty megaton nukes for crying out loud without a single sign of anything amiss that would suggest we have a giant hole in physics requiring dark matter. We've done experiments on electromagnetic fundamentals, nuclear forces, and so on and along the way, we didn't hear of a need to invent 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.

But some people look at the cosmos and decide that despite not truly understanding the whole picture of physics at every scale yet, we can claim that dark matter exists and here's proof. Where in the Nine Hells does this stuff fit with the physics theories they already promulgate as accepted science to be taught in universities?

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:"To be published..."

[Michael Woodhams]

ApJ Lett. is as good as it comes in astronomy. ApJ is the most significant journal in astronomy, followed by Astronomy and Astrophysics. Partly this is on volume (ApJ is huge) - I don't know how the impact factors compare. ApJ Lett. presumably has higher impact factor than ApJ as a whole.

Re:So what's new, then?

[HuguesT]

You make good points, but the devil is in the details. Long post follows :

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.