Hubble Telescope Maps Dark Matter in 3D 174
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 (Score:3, Informative)
Re:Enlighten me (Score:5, Informative)
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? (Score:5, Informative)
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 (Score:5, Informative)
Re:Enlighten me (Score:4, Informative)
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 (Score:5, Informative)
See also http://en.wikipedia.org/wiki/Brane_cosmology [wikipedia.org] .
Re:dark matter does not exist (Score:5, Informative)
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? (Score:4, Informative)
Re:dark matter does not exist (Score:5, Informative)
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 (Score:2, Informative)
Re:dark matter does not exist (Score:3, Informative)
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?
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?
That being said,
Re:Does Dark Matter exist? (Score:3, Informative)
Re:Enlighten me (Score:5, Informative)
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? (Score:1, Informative)
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. (Score:3, Informative)
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 (Score:2, Informative)