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.'"

Comments

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] 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: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: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: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.