Slashdot Mirror
Vast Web of Dark Matter Mapped
astroengine writes "Astronomers from the University of British Columbia and University of Edinburgh have created a vast cosmic map revealing an intricate web of dark matter and galaxies spanning a distance of one billion light-years. This is the largest map of its kind and demonstrates that this large-scale web stretches across the universe in all directions. The results of this groundbreaking discovery were presented at the American Astronomical Society conference in Austin, Texas on Monday."
...dark matter eventually turns out to be like luminiferous aether from the 19th century? I don't believe anyone has directly observed dark matter.
...the story where they discovered/detected Dark Matter?
No, there was no discovery story to miss. We have yet to directly observe dark matter. I'll try an analogy with one caveat: like models, all analogies are wrong. Still, some can be useful...
Picture a ball hanging from a ceiling by an invisible thread. Through various methods you are fairly certain how much the ball weighs, and your knowledge of how gravity works gives you an idea of what it should be doing (i.e. falling), yet it does not. You are faced with two ways to explain this discrepancy: your understanding of gravity is faulty or there is something preventing the ball from falling.
Dark matter is the latter sort of explanation. We think there is a string, and we can infer some of its properties from what we see the ball do but we cannot see it. At the risk of incurring the wrath of cosmologists everywhere I'll give another analogy, even more wrong than the first: one cannot see the air or the winds, but one can deduce their existence from their effects on things one can see.
N.B. The string used in the example above has nothing to do with any of the string theories.
If God forks the Universe every time you roll a die, he'd better have a damned good memory.
There's a bit more too it than that though, dark matter is no longer just a guess, there is more direct evidence to back it up. To extend your analogy, lets say instead of a single ball hanging you have hundreds of them. A follower of the string theory (pun definitely intended) might make a prediction: some of the balls should have a detectable periodic motion from past disturbances. A thorough survey of the floating balls shows that yes, some of them are swinging like pendulums. It doesn't prove that the balls are hanging from strings, but it means that there's yet another effect that a modified theory of gravity has to take into account, which can be explained very easily by positing the strings. Similarly, there have been a host of indirect observations which show that either there is large amounts of matter that we can't detect or there are dozens and dozens of gravitation effects that are not only not included in current theory but in some cases appear to be mutually exclusive.
This or something similar seems to come up every time there is discussion of dark matter: "Dark matter was a lazy fix, instead physicists should realise theories were lacking and come up with a new one instead."
Physicists did see physics was incomplete, they did come up with a new theory: dark matter. Dark matter is a new theory and is a way of saying previously physics was incomplete. Physicists also came up with dozens of other theories, but in the end they haven't been doing as well at matching evidence as dark matter.
Way too often people seem to yell that they needed a new theory. But what is really going on, is dark matter is that new theory, and those people just don't like it. That is ok, a lot of people studying or promoting dark matter don't like it either, but still see it as the best of current theories. At least they are trying to look at actual evidence unlike most random armchair scientists on the internet complaining how lazy physicists are, by what amounts to a very lazy and unfounded argument of their own.
In a sense, yes. Dark matter was just one hypothesis among many for galaxy rotation speeds before the CMBR studies.
But the CMBR studies were really the "discovery" of dark matter. At the point where the universe took a snapshot of itself, the distribution of matter was still fairly uniform: alternating areas of slightly-denser and slightly-less-dense matter as sound waves rolled through the universe. By measuring the size and magnitude of these compression waves, one thing that we know - by direct observation - is that only 20% or so of matter was interacting with photons, directly or indirectly.
The universe at that time was dominted by 2 forces, gravity and light pressure. Gravity would compress slightly denser patches until light pressure would cause them to "bounce". We know the force of gravity and light pressure quite precisely, and the mechanics of compression waves, and so we can measure the ratio of mass that interacts with each force. And there's abot 5 times as much mass that reacts to gravity as mass that reacts to light pressure.
So, yeah, direct measurement of dark matter, and the exact measurement (which was 2 or 3 significant digits) was just what the dark matter hypothesis had predicted based on completely unrelated measurements of galaxy rotation speeds. Of course, that gives few clues about the nature of dark matter, but we know most matter in the universe is dark.
Socialism: a lie told by totalitarians and believed by fools.
Dark matter does not interact electromagnetically. In other words is *can't* glow, it *can't* absorb light or microwaves or anything. We can see dark stuff, it blocks the CMB and other things. The fact that it does not interact except via gravity also means that is its very diffuse.
Consider a non interacting particle falling from 1 light year out towards the sun. It falls right thought and out the other side and comes to a stand still 1 light year away. It will do this forever since its does not interact with anything that can slow it down. Now consider a say consider about 9x the mass of the sun of these particles, they will always occupy a massive volume and hence be very very diffuse... but in the large scale (galactic) are a large effect with their combined gravity. The are Dark in the sense that they *only* interact via gravity.
The Grey Goo disaster happened 3 billion years ago. This rock is covered in self replicating machines!