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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.
They're just measuring the known affects of dark matter, as opposed to measuring the dark matter itself.
Isn't enough that I ruined a pony, making a gift for you?
...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.
.... look like demo scene?
...the story where they discovered/detected Dark Matter?
Apparently. But if you googlize cluster dark matter
you can start catching up on your reading.
Sheesh, evil *and* a jerk. -- Jade
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.
All this extra proof of dark matter is going to prevent me from making the Kessel run in less than 12 parsecs. Damn extra mass.
Silence is a state of mime.
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.
What they measured was the deflection of light from galactic sources that were 6 billion light years away. Using a method not described in the article, they were able to measure the amount of gravitational deflection between the light sources and our current observation position. I believe this is a composite measure of gravitational lensing.
I assume that their measurements are accurate. To my understanding, there is also an assumption that a specific portion of the lensing is due to dark matter. This has an appearance of circular logic. One of their assumptions about measuring the effect of dark matter is that they know how much effect dark matter has.
Have I gotten something wrong here? It's always a guessing game using an article intended for the general public when trying to understand scientific findings. Is there anyone out there with meaningful credentials who can clear this up? (I promise that I will ignore all the self appointed "experts" who don't actually have any more knowledge then I do.)
Why is Snark Required?
Dark matter is just stuff we can't see directly at the moment mostly becuase it isn't glowing extremely brightly. It's interesting enough without bringing magic into the picture.
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."
He's probably posting from the USA, where everyone knows more than the experts.
Sheesh, evil *and* a jerk. -- Jade
Pure bullshit. I have actually nothing to back that up, but the whole Dark Matter/Energy thingy sounds to weird to be true. :) Forget rationality or believability.
More likely the physicists made some kind of wrong assumption early on, painted themselves in a corner and this is what they came up with.
Same with the BB theory. They reckon that in the first few nano-secs there were no physical laws? Therefore the BB could do whatever it wanted to (if I'm allowed to add consciousness to that
Don't be apathetic. Procrastinate!
But then, that has been the case since shorty after the invention of fire and the wheel. Seriously, what physical phenomenon are we directly observing? We can't see atoms, we can't even see the circuits making up computers anymore. All we ever did was meassuring the effects of stuff, and conclude that stuff (probably) exists and have certain characteristics.
You know this is falling on deaf ears don't you. The group think on /. is that dark matter is silly and they have a much better alternative... On no wait... they don't even propose an alternative.
In other news everyone knows that you should only listen to real climatologists.
The Grey Goo disaster happened 3 billion years ago. This rock is covered in self replicating machines!
N.B. The string used in the example above has nothing to do with any of the string theories.
Of course it does. If you do the math right, that string will surely pop out of the sea of String Theories!
Dark energy and dark matter are simply lazy science.
The problem is that physics is wrong, or at least incomplete, not that there's some invisible force guiding matter to do strange things that leave only highly questionable evidence behind.
I think and hope above post was aiming for "funny", not for "troll"... And I certainly hope it was not aiming for "In*" moderation...
Does that map make it possible to calculate the temperature of the dark matter?
As far as I can understand it, if we know the spatial distribution we can infer how much kinetic enery each particle have. But I'm not so sure about that, there may be something I'm overlooking, or the current map may be less precise than the infered values from computer simulations. Do anybody knows the answer?
Rethinking email
Einstein's Telescope, by Evalyn Gates, has a very good entry-level discussion of how gravitational lensing of galactic clusters and superclusters is being used to investigate dark matter. She wrote the book so that it'd be accessible to people without a science background. Friends who aren't scientists have read it and learned a lot from it, although they may have had to read some sections two or three times for some of the concepts to sink in.
But what if the Big Bang never happened?
Every time I read about webs of matter spanning across such distances, considering the time required to form these I remember the refreshing perspective put forward by this beautiful little book.
Mi domando chi à il mandante di tutte le cazzate che faccio - Altan
This bothers me too. When you take a bunch of measurements, whatever their interpretation or value, and you make a nice visualization with them, it all becomes more real.
If gravitational lensing was how we discover black holes, dark matter, etc... Then why are they not lit up like a floodlight? The lensing that should occur around something that is infinitely dense (Such as a black hole) should also be an infinite amount of lensing, therefore curving all the light in the universe around it, to point back at us. That should (According to my small brain) make black holes the brightest point in any universe.
/..
I once drew a diagram of this for some friends, but none of them really understood it. Am I crazy, or do I just not understand fully how gravitational lensing works?
I suppose I could buy the book and find out, but I would rather ask
Black holes are not infinitely dense (caveat, volume may not make too much sense for a black hole if you get pedantic). A black hole with the same mass as the sun would (roughly speaking) be a sphere about 4 miles in diameter. Any light actually hitting that sphere would be absorbed. Light just barely missing it will be bent through large angles. Light passing a few miles away will be bent through smaller angles and so on. If you're in the right place, this can make make something behind the black hole look a bit brighter, because more of the light from it gets to you.
Dark matter is not especially dense anywhere (we think). It's spread throughout galaxies and clusters of galaxies like a sort of background haze. Actually, given the relative amounts it's better to say the Universe is full of a web of sheets and filaments of dark matter. At a few places where this is especially dense it has attracted (by gravity) enough normal matter to form a large black hole and its accompanying galaxy.
Instead think of them as having high, but not infinite, gravitational fields due to high (not infinite) quantities of mass. That can be achieved with even a "low" density (i.e. mass per volume) - such as galactic superclusters. The amount of bending that occurs when the light passes one of these strong gravitational fields depends on how strong the gravity field is, and the angle of approach for the light.
A couple of examples:
Light pointing at a black hole's event horizon will be pulled in and disappear. It wouldn't make a U-turn and come back toward us.
Light going near a black hole - but never crossing the event horizon - will be bent. How much bending depends on how close to the event horizon the light comes. Someone more knowledgeable than me can advise whether it could make a U-turn, but I guess that's possible.
On the other hand,
Light pointing toward the center of mass of a galactic cluster could go straight through, assuming no one in that galaxy pulls down a curtain that blocks the way.
Light going near a galactic cluster would be bent, once again proportionally to how close it was to the center of mass of the cluster.
All this is from memory from my bachelor's, which was a long time ago. I apologize in advance if I got something wrong.
hth.
...the story where they discovered/detected Dark Matter?
If there is to be a story that resulted in the idea of missing matter, it should be attributed to Louise Volders discovery in the late 1950's and Vera Rubin's additional work in the 1970's about the rotational speed of galaxies being faster than the observed mass of the galaxies--contradicting the acurate results observed when applying the same classical mechanics to the rotational speed of our solar system. Vera Rubin argued convincingly that Fritz Zwicky's unrelated work in the 1930's (for accurately calculating missing matter in star clusters) was a viable solution.
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