Hubble Telescope Maps Dark Matter in 3D

dido writes "The BBC reports that the Hubble Space Telescope has been used to make a map of the dark matter distribution of the universe, providing the best evidence of the role dark matter plays in the structure and evolution of the universe. From the article: 'According to one researcher, the findings provide "beautiful confirmation" of standard theories to explain how structures in the Universe evolved over billions of years.'"

Re:Enlighten me

[bloobloo]

Anything with a temperature above absolute zero glows. You've just got to be about the temperature of a star to emit most of your light at visible (to human) wavelengths.

Re:Enlighten me

[ceoyoyo]

Nobody knows.

Some portion of it could be ordinary matter that's simply non-luminous, but I think there are observations that limit that to a small proportion.

The rest seems to be something that interacts only gravitationally... it might be a particle we haven't discovered yet. That's not as far fetched as it sounds -- neutrinos are just such a particle. They have mass so they interact gravitationally but they interact with ordinary matter extremely weakly in all other ways. Massive neutrinos were also candidates to explain some of the dark matter for a while, but I believe once their actual mass was measured it was too little to explain more than a bit of the dark matter.

Re:Does Dark Matter exist?

[HarveyTheWonderBug]

Yes, there is, but we do not have a clue yet of what it is made of.
Astronomers have ways to measure the mass of objects, like galaxies, and cluster of galaxies, using a theory of the gravitation. For galaxies, the classical newtonian theory is enough: they just measure how fast the stars and the gas orbit around the galaxy, and derive directly their mass from kepler laws. For clusters of galaxies, or large structure, they use the bending of light by mass from general relativity. These measure are getting reasonably accurate. When they compare these masses to the mass they actually can see (stars, gas, etc..), they find that they can only account for 1/6 of the total mass they measure, well above all the uncertainties of the measurments. Therefore, there must be some matter (that is, something with a mass), that we cannot see (that does not interact via electromagnetism). This is the dark matter.
For more info, there is a [wikipedia] [wikipedia.org] entry.

Re:dark matter does not exist

[HarveyTheWonderBug]

"beautiful confirmation" of standard theories?????

Yes indeed. The standard paradigm for the evolution of the Universe is the widely accepted lambda-CDM model, or Cold Dark Matter with a cosmological constant (or dark energy). The recent results of WMAP, of the high-z supernovae, all point toward a set of cosmological parameters where the energy density of the universe is made of:

• 70% of dark energy
• 30% of matter
  • out which, stars, gas, neutrino are making at most 5%
  • so we are left with 25% of dark matter

So yes, dark matter is widely accepted. It's not satisfying because we have no clue about what it is (it clearly does not interact electromagnetically), but we can feel its gravitational pull. Coming up with a good theory on its nature is one of the hardest challenges in modern astrophysics.

Re:Enlighten me

[Anonymous Coward]

The general belief is that it is an actual particle. There are other competing theories such as MOdified Newtonian Dynamics which have slight corrections to our current laws of gravity, but more and more evidence is ruling out the simplest of these models and it's pretty clear that at least some of the dark matter is actually particles. Another thing people thought dark matter could be was normal matter which doesn't produce light, things like planets and failed stars. However, extensive astronomical searches for these objects (called MACHO's) using both the fact that they should occassionally block our view of stars and their potential to cause gravitational lensing have turned up nothing. This basically leaves some sort of new particle as the dark matter canidate. The current theory is that this particle only interacts via the weak nuclear and gravitational forces. It Is is called a WIMP (weakly interacting massive particle). There are a bunch of different models of what this particle is. Basically nearly every theory of physics beyond our current Standard Model has some sort of particle that it's proponents hold up as a dark matter canidate.

There are a wide variety of dark matter searches being conducted which directly search for the particle. The general idea is to see their interaction through the recoil of an atom when one strikes the atom's nucleus. This is very difficult. The most common current technique is searching for the "sound" a dark matter particle interacting with cryogenically cooled germanium crystals produces.

Re:Enlighten me

[Dachannien]

Some string theorists believe dark matter may be gravitons, emitted by matter on adjacent branes, that intersect our own universe's brane, resulting in a gravitational distortion that becomes huge at cosmological scales. A similar concept is used to explain why the gravity exhibited by real matter in our own universe has a strength many orders of magnitude smaller than the other forces - most of the gravitons leave our universe's brane, while the mediating particles of other forces (gluons, photons, etc.) are constrained to move within the brane.

See also http://en.wikipedia.org/wiki/Brane_cosmology [wikipedia.org] .

Re:dark matter does not exist

[Ambitwistor]

What standard theories?

The standard theories of large-scale structure formation in the early universe, which is mediated by non-baryonic dark matter.

Dark matter does not exist, as least not as far as anyone (except astronomers with good imaginations) knows.

Wow, that's a compelling counterargument. However, it neglects the decades worth of observational evidence in favor of dark matter in the form of galactic rotation curves, the motions of satellite dwarf galaxies, gravitational lensing, measurements of galactic gas temperatures (which depend on the local gravitational neighborhood), anisotropies in the CMBR, the rate and structure of large-scale cosmological structure formation, etc.

There is a very nice (and complete!) standard model of physics, and dark matter holds no place.

Actually, one of the leading dark matter candidates is the axion, which was introduced into the Standard Model to resolve the strong-CP problem. However, the astronomical evidence indicates that the Standard Model of particle physics is most likely not complete, and that at least one new weakly-interacting massive particle is needed.

Regular matter, that is simply dark - i.e. cold, and not emiting light, does not bother me. But making up particles no one has ever seen just because you don't understand what you are seing is fitting facts to the data.

There is nothing wrong with "making up particles no one has ever seen" in order to explain discrepancies in either theory or observation. It's rather the point of science, to frame new hypotheses. Historically, see the prediction of the positron, on the basis of theoretical consistency between quantum mechanics and relativity, or the prediction of the neutrino, on the basis of apparent non-conservation of energy.

Scientists often discuss new theories, etc, and in that context dark matter has it's place, but to claim it exists - as this story does - without being able to actually measure anything is quite silly and premature. If you don't understand something, say so, don't invent plausable explanations that have nothing supporting them except your lack of knowledge.

Dark matter is a plausible explanation precisely because it is supported so well by numerous disparate observations. There are other ways one can attempt to explain various discrepant observations (e.g., by modifying the laws of gravity), but dark matter is far and away the most successful, as it passes all known independent tests. There's no reason why an ad-hoc patch designed to explain galactic rotation curves should also end up explaining, say, cosmological expansion, or large-scale structure. And it's silly to claim that we cannot measure anything: we can measure the gravitational effects of dark matter.

Sure, everyone would love it if we could detect dark matter particles directly — and if they interact non-gravitationally, we hopefully will someday. But what's silly is to claim that we have little reason to believe that dark matter particles exist.

Re:Does Dark Matter exist?

[Ambitwistor]

Why is it that scientists think that dark matter exists simply because the observed galaxies don't conform to Newton's Laws? Wouldn't a simpler solution be to take a step back and consider that, maybe, Newton's Laws are flawed?

You want a solution that is simple enough to explain the facts, but no simpler. Modifying the laws of gravity runs into difficulty explaining everything that dark matter can, although you can get it to explain some things (such as galactic rotation curves).

Can someone explain to me why dark matter is the prevalent theory?

In short, because it works and none of the alternatives people have proposed over the decades work as well. I can get into details if you want, but you should probably just start at Wikipedia.

Or perhaps why something like MOND is always ignored?

MOND isn't ignored. Go to the astro-ph arXiv or the Smithsonian/NASA ADS Abstracts and search for MOND papers. You'll find them, along with criticisms of MOND. Here [preposterousuniverse.com] is a nice but somewhat outdated set of slides on how well MOND fares against the evidence, and a more recent [cosmicvariance.com] blog post by the same author discussing newer evidence that tightens the screws on MOND even further.

As I said, I don't know what is right, but it just seems like a hack-job to me.

I don't know why all the hate for dark matter. Screwing around with the laws of gravity isn't any more elegant, and there are plenty of plausible candidate particles for dark matter lying around in various extensions to the Standard Model.

Re:dark matter does not exist

[Carmelbuck]

Every time an article regarding dark matter is posted on Slashdot, there are nonsense "fudge factor!!1!" postings like the above. And every time, like-thinking idiots mod them up as "Insightful" or "Interesting". And every time, I suspect, people like me get the urge to go through and respond to every single one, but have to limit ourselves.

So let's start at the beginning, shall we? Galaxy rotation curves indicate that there is more mass in galaxies than would be inferred from the luminous matter. How do we know that it's not clouds of cold gas? Because that's ruled out by 21cm observations and by studying the absorption spectra of extragalactic objects. How do know that it's not clouds of hot gas? Becasue that's ruled out by UV and X-ray observations. How do we know that it's not brown dwarfs and black holes? Because that's ruled out by microlensing surveys.

Now, studies of galaxy dispersion velocities in clusters indicates that there's more mass in galaxy clusters than than would be inferred from the galaxies themselves, plus the intracluster medium which is observed in the X-ray. This is verified to high accuracy (i.e., the estimates of the total cluster mass are in close agreement) by hydrostatic X-ray mass measurements and by weak lensing observations. How do we know that it's not clouds of cold gas? Because that couldn't coexist with the hot gas, and because the dark matter spatial distributions are clearly different from the gas distributions. How do we know that it's not clouds of hot gas? See "intracluster medium" above. How do we know that it's not brown dwarfs and black holes? Because there's no mechanism for moving large numbers of objects out of the galaxies into the ICM (there are some intracluster stars, yes, but relatively very few--and the number of those gives us hints as to the number of non-luminous objects similarly ejected). How do we know that it's not neutrinos? Because neutrinos are experimentally shown to be too light and too fast, and cosmological constraints show that too few would have been produced in the Big Bang.

Now, studies of cosmological structure formation indicate that the size and number of galaxy clusters in the universe are not consistent with what would be expected given an all-baryonic universe. How do we know that...er...well, that's that. Cold collisionless dark matter is required to make the simulations work.

How do we know that modified gravity isn't the answer? See multiple independent lines of evidence above. There are no theories of modified gravity that come even close to explaining all of the above. The MOND people cheerfully acknowledge this, even if their advocates on Slashdot don't.

Look, the history of physics is replete with things whose existence was inferred long before they could be directly observed--neutrinos, quarks, atoms themselves, and much, much more. It's simply asinine to suggest that "we haven't directly measured it" means "it doesn't exist". Heck, we only really "see" subatomic particles because of the photons given off when they interact with one thing or another--"seeing" dark matter via measurements of its gravitational effects is hardly less direct.

And we'll just ignore the nonsensical "fitting facts to the data". The bottom line is, there are multiple, independent lines of evidence that dark matter exists, and that it is non-baryonic. Uninformed posters on Slashdot can pat themselves on the back for their intelligence as much as they want, but they're only fooling themselves.

Re:dark matter does not exist

[Ambitwistor]

On one level, I consider dark matter to have the same credibility as the æther. An interesting concept, but largely created as hack to the model.

Aether theories didn't make any predictions that correctly explained any new observations. Dark matter does.

Certain unexplained acceleration in the cosmos has renwed interest in the Einstein cosmological constant, which if it exists, renews the presence of the æther, albeit in a different form.

The cosmological constant is a modification of the laws of gravitation, not anything like what was historically referred to as the aether.

Dismissing a concept simply because it is a mathematical hack is a mistake. In reality we use mathematics because it is a precise language that will often lead us to an unobserved reality.

This point is well taken. However,

Recall that special relativity is based on Einstein's assertion that equations should be symmetric.

That wasn't Einstein's motivation for introducing special relativity. (On the other hand, the complete form of Maxwell's equations of electromagnetism were based on Maxwell's assertion that the equations should be symmetric.)

And, for the record, we must also admit that there is no evidence for a particle of magnetic or gravitation force.

The electromagnetic force is mediated by the photon, for which there is abundance evidence. Gravitons, as you say, currently have no experimental evidence in their favor.

Re:dark matter does not exist

[Ambitwistor]

Because galaxies don't rotate the way our current theory of gravity says they should, because gravitational lensing isn't working the way our current theory of gravity says it should, because of a bynch of other thing I guess,

You act as if having a bunch of observational evidence for dark matter is unimportant.

the accepted solution it to declare that 95% of the universe is made of stuff we can't directly detect,

Would dark matter be more palatable to you if it only made up 5% of the universe? Why does it suddenly become more implausible if it makes up most of the universe. It's because it makes up most of the universe that we can even tell it's there.

can't do experiments on,

That remains to be seen; we may be able to create such particles in accelerators, and we may also be able to detect them in the Sun, in cosmic ray experiments, etc.

That being said, even if we can't do experiments on dark matter, why does that suddenly make dark matter implausible, in the face of all the other astrophysical phenomena it explains? Is there some law of the universe that says that all matter must be easily producible and manipulable by humans?

doesn't exist locally,

That's false. It surely does exist locally.

That being said, even if it didn't exist locally, why does that suddenly make dark matter implausible, in the face of all the other astrophysical phenomena it explains? Is there some law of the universe that says that everything interesting or important in the universe has to exist nearby?

and is completely different from the universe we do observe and interact with

It's not that different. Ordinary neutrinos have most of the properties needed to be dark matter; they're just not massive enough.

That being said, ... well, you get the idea.

Re:Does Dark Matter exist?

[Ambitwistor]

No. Neutrinos aren't massive enough to be (most of the) dark matter. See here [princeton.edu] for a brief but more detailed discussion.

Re:Enlighten me

[radtea]

Some portion of it could be ordinary matter that's simply non-luminous, but I think there are observations that limit that to a small proportion.

Big Bang nucleosynthesis limits the amount of baryonic (that is, "normal") matter to a relatively small fraction of the total observed mass of the universe. The basic idea is that we know how big the universe was when protons and neutrons (collectively known as nucleons) were being formed--at some point the cosmic fireball cooled off to the point where quarks were no longer free, so they condensed into nucleons. We also know that the lifetime of a free neutron is about 15 minutes, so there was only about an hour for nuclei more complex than hydrogen to form.

So, if the universe was VERY dense in the hour or so after nucleon formation then every single proton would have run into a neutron or two and there would be almost no plain old hydrogen in the universe--everything would be helium and deuterium. On the other hand, if the the universe were extremely diffuse during that single hour there would be hardly any helium--only the few percent made by stellar fusion and supernova in the past ten billion years. As it is, we are pretty sure based on observations and theory that about 20% of the helium in the universe was formed in the Big Bang. That, plus some more problematic numbers from deuterium and lithium and helium-3, give us a very good estimate of the total baryonic mass in the universe.

The visible mass is quite a bit smaller than the total baryonic mass, and there is some reason to believe that the flat rotation curves of spiral galaxies are due to baryonic dark matter, although it would have to be in the form of small clumps of matter like comets or dead stars or something to not do any significant scattering of light.

Dark matter on larger scales is completely unrelated to galactic dark matter--the use of the single term "dark matter" for these totally unrelated problems is unfortunate and confusing, as I point out every time this topic comes up on /.

The observation reported here, like the colliding galactic clusters observation reported a month or so ago, is amongst our first clear view of extra-galactic dark matter, which is too copious to be explained as normal baryonic matter.

The problem that cold dark matter theorists have to deal with is that the extra-galactic dark matter can't just interact gravitationally, because gravity is too weak a force to produce structures in the short time the universe has been around. To clump in the manner observed, extra-galactic dark matter has to have some mechanism for losing energy. Otherwise two pieces of dark matter (or a piece of dark matter and a peice of ordinary matter) would just pass through each other. The dark matter would never be slowed down by anything, and so would never form clumps on any scale.

So it is probable that extra-galactic dark matter is pretty exotic, or that something was sufficiently different in the early universe to make gravity sufficiently dissipative to form the observed clumps. Either way, the flood of observations using these new microlensing techniques is going to start killing off theories in droves--at least those theories that make actual predictions.

Re:Does Dark Matter exist?

[pudro]

Dark matter is a crutch of a theory with so many problems they had to invent an imaginary substance to explain them.

The term "dark matter" originally referred to normal matter that we couldn't see because it wasn't lit up. Once this idea was proven inadequate, dark matter became something new and its definition was shaped solely by what the theorists needed it to be. Later on, they found even more problems with their theories and had to invent dark energy, which was once again defined solely by what the theorists needed it to be. But even with these inventions, they are routinely surprised by what they find in the universe. The only explanations they can come up with range from "it's a mystery" to claiming that previously accepted "universal" laws work differently out there in the universe.

Since you seem to be open minded, let me introduce you to a different (though never claiming to be definite) explanation. A growing number of people are starting to realize that these surprises are no longer surprising once you start to imagine that electric currents can flow throughout space (contrary to popular opinion). It is the Electric/Plasma Universe theory.I have only known about it for a relatively short time, but the bulk of what I have read is from this site:
http://www.thunderbolts.info/home.htm

I have spent a lot of time just going through the Picture of the Day archive section and reading the explanations below the pictures (which is actually the point of that section - not showing pretty pictures or something like that). (Be warned - some entries are repeated several times.) Many anomalies can be explained simply as electric discharges (or the results thereof). And all of their theories are directly relatable and scalable to physical tests carried out in labs on Earth.

I'll point out a couple of my favorite examples so far: Io and the "Greatest Surprise" [thunderbolts.info] and V838 Mon [thunderbolts.info] (which also happens to be one of my favorite pictures as well).

It never did sit right with me how things like the Cat's Eye Nebula [thunderbolts.info] were supposed to have formed through explosions and gravitational forces. Now I know why: it's electric!


P.S.
I keep having the thought of how funny (and sad) it would be if the conventional model just keeps making up more rules and invisible entities until those evolve into the Electric/Plasma model.

Re:This is pretty cool.

[andersa]

Black holes are too small to have any influence on the distribution of dark matter. It is a common misconception that black holes are these huge gravity monsters that suck up everything that get closer than a parsec to them.

In reality you have to get within a few thousand kilometers of the event horizon for you to notice anything peculiar. Further away and the gravity well looks and behaves almost identical to an ordinary star.

Black holes doesn't play any role in the distribution of cosmological dark matter, which is what this experiment focusses on.

Re:Enlighten me

[Ambitwistor]

Is there any chance that there's two classes of graviton? Like the electromagnetic force has attract opposite, repel like. Could the gravitational force which has some similarities to the electromagnetic force have attract like, repel opposite.

That would require a vector theory of gravity. General relativity is a tensor theory. There are combined tensor-vector theories that have features of both, as you suggest. But there is no observational evidence in their favor right now.

And while I'm at it, is it possible stuff escapes black holes because passing matter causes small changes in the gravitational field hence shifting the event horizon and causing the photons that are orbiting near the event horizon to escape orbit.

No. An event horizon, by definition, is a region from which nothing can ever escape. The horizon of a black hole can distort, and something near a black hole can be perturbed away from it, but anything that escapes a black hole was never within the event horizon to begin with — by definition.