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Hubble Builds 3D Dark Matter Map
astroengine writes "Dark matter can't be spotted directly because it doesn't interact with electromagnetic radiation (i.e. it doesn't emit any radiation and reflects no light). However, its gravitational influence on space-time can bend light from its otherwise straight path (a phenomenon known as 'lensing'). Using a sophisticated algorithm to scan a comprehensive Hubble Space Telescope survey of the cosmos, astronomers have plotted a map of 'weak lensing' events. Combining this with red shift measurements from ground-based observatories, they've produced a strikingly colorful 3D map of the structure of dark matter."
...but I fail to see the 3D that was promised by TFA.
I agree it's a nice picture but there seems to be no explanation as to what these colours actually mean, let alone any kind of conclusion drawn from what I presume to be "pockets of dark matter".
Anyone care to enlighten me?
...especially when you consider it's a picture of something that very possibly doesn't even exist.
There isn't any "scale bar" because you are not looking at something at any fixed distance! You are looking at (theoretically) blobs of stuff at various distances.
You could do it based on movement speeds. Things in the background of an image move slower than those in the midground when you change your position--If the thing in the background, a galaxy or something, moves in a strange way, then you can be sure it's being lensed. I'm not sure if the Earth moves enough for this to be useful, though, given the scale.
Conscience is the inner voice which warns us that someone may be looking.
(To me still a imaginary excuse, based on the arrogance of not being able to admit that the math is wrong, but instead calling the universe wrong! ^^ [But a good {and compact!} explanation will of course change my mind.])
That might be something similar to what they told Einstein when he used his math to explain characteristics of nature that no one had witnessed.
I find the possibility of dark matter and energy kind of fascinating. Maybe it just a problem with their math - but then again, having huge amounts of mass in the universe be something other than what we experience every day adds a little mystery to it all.
Why are you letting these clowns ruin our country?
Sorta. As the earth goes around the sun, we do in fact get enough parallax to determine the distance to nearby stars. On the galactic scale, though, this doesn't work. The way they find lensing artifacts is that lensing doesn't just skew a single flat image of what we see, it might bend the same light source so it comes at us from different angles, producing multiple images of a single event. Something like this: http://en.wikipedia.org/wiki/Einstein_Cross
If they have good enough images with spectral plots for each pixel (which if they are using redshift to determine distance, they must have), I could see an algorithm being able to pick out which images are mirages, and therefore where the lensing matter must be.
... the disturbance I felt in the Force earlier. I thought I just had gas.
It's not arrogance; frankly, a true scientist is thrilled at the prospect of being proved wrong. It means they're answering some long-standing questions and posing countless new ones. Furthermore, the concepts of "dark matter" and "dark energy" are still only theories; scientists have yet to definitively prove the existence of these entities. These theories just happen to be the best explanations for what scientists observe.
The bottom line remains what osgeek above me said: it's easy for you to call the scientists who postulate dark matter "arrogant" considering it's something that has about as much impact on our daily lives as Einstein's Theory of Relativity does (which, when it was being proved, required very specific measurements to be taken, measurements that could only be gathered in a solar eclipse...how's that for completely unnecessary to quotidian life?).
No, right now we can't definitively prove that the 3D image referenced in TFA is indeed dark matter. But within the parameters of the current hypothesized model, that is what scientists believe to be pockets of dark matter.
"I'd just like to emphasise that taking a million years isn't a metaphor here..." -Rich Bradshaw
...how they know it’s lensing, and that the stars aren’t just positioned like that?
Sounds to me like you could never prove, which one it really is, until you fly behind that “dark matter”. (To me still a imaginary excuse, based on the arrogance of not being able to admit that the math is wrong, but instead calling the universe wrong! ^^ [But a good {and compact!} explanation will of course change my mind.])
When you see multiple images of the same object, it's lensing. This is, in fact, how gravitational lensing was first discovered. Check out this great wikipedia image of the effect: http://en.wikipedia.org/wiki/File:Einstein_cross.jpg. This is actually called strong lensing. TFA discusses weak lensing, which is a much smaller effect. That's detected by looking at very distant galaxies. Lensing changes the shape of galaxies such that there is a preferred orientation. If this orientation is statistically significant, i.e., too many galaxies are stretched in the same direction to be caused by normal physics, then it tells us that the weirdness is likely caused by lensing. Thanks to Hubble's ability to paint an incredibly dense picture of background galaxies, our statistics are based on a huge number of samples and we can trust them pretty thoroughly.
Awesome, right?
"I zero-index my hamsters" - Willtor (147206)
So luminiferous and aethereal! Almost magical like!
I'll go farther than that: I can remember how before the Hubble was launched, scientists didn't think we'd ever actually be able to observe the effect because it was too small to be imaged from any ground-based telescope.
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IANAAstrophysicist, but my understanding is that a point source, such as a black hole, would create a strong lensing effect. These observations are of a weak lensing effect that indicates a diffuse source of gravity.
The dark matter doesn't bend light. The gravity from the dark matter bends light.
Actually, it does exist. Frankly, I'm fucking sick of posting the same links over and over, so why don't you just go to Wikipedia and read about the Bullet Cluster. There is simply no question, now, even among MOND proponents: there is weakly interacting matter out there, and we have no idea what it is.
"These theories just happen to be the best explanations for what scientists observe."
Exactly! Dark matter and dark energy are just tags for unexplained phenomena that appear to have similar properties to matter and energy. They are not simply mathematical entities, they are phenomena that can be observed but cannot (yet) be explained with our mathematical models. This is no different to any other physics, Newton didn't discover gravity he discovered it could be described with maths.
And did you exchange a walk on part in the war for a lead role in a cage? - Pink Floyd.