Vast Web of Dark Matter Mapped 86
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."
Re:What are the odds... (Score:5, Informative)
Looking longer and longer by the day. Aether was invented because people felt it SHOULD exist, but expected consequences of it completely failed to show up. Dark matter was invented because there were observations that are very hard to explain any other way and fit increasingly precisely with one another if dark matter is the cause -- there are several different ways of measuring the distribution of dark matter among various clusters of galaxies, and they are giving remarkably consistent answers.
A better example would be phlogiston, which was invented to explain observations, but eventually failed to explain all observations, so it was replaced by a better theory. The same could happen to dark matter, but there are no signs at the moment,
Re:Did I miss... (Score:5, Informative)
...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.
Re:What are the odds... (Score:5, Informative)
The part you're ignoring is that unlike the aether, there is actual evidence for dark matter, quite a lot of it actually. It's true that at the time it was conceived it was little more than a fudge factor, but that time has long since passed. The Bullet Cluster [wikipedia.org], for example, is probably the stongest single piece of evidence, though by no means the only one. It has a core of regular matter surrounded by a large halo of dark matter which can be observed by measuring the gravitational lensing of light passing through the region.
Fair enough, the same effect could be produced by bending spacetime in some other way, but the only way we know about today is with gravitational mass. Scientists find the assumption that there is a kind of matter we can't see much more readily than they will take the assumption that there is some force other than gravity (or some source of gravity other than mass) that warps spacetime to such a degree over such large volumes of space.
Re:Did I miss... (Score:5, Informative)
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.
Re:Stop reading magic and SF into it (Score:5, Informative)
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.