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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?
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.
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)
...especially when you consider it's a picture of something that very possibly doesn't even exist.
It exists. Educate yourself.
Thanks. It's worth noting that the Bullet Cluster results you linked to are only the most recent development in dark matter's nearly 80 year history:
1933 - Zwicky studies the Coma cluster of galaxies and is surprised to find that these galaxies are orbiting each other much faster than he predicted based on their visible mass. He proposes that each galaxy actually contains much more mass than is visible.
1959 - Measurements of galactic rotational velocities conflict with expected velocities based on the amount of matter observed to be present. The dark matter concept proposed by Zwicky is found to solve this problem too.
1970s - Big Bang nucleosynthesis has trouble reconciling observations of high deuterium density with the expansion rate of the universe. Non-baryonic dark matter solves this problem as well.
At this point, dark matter was simply an hypothesis. MOdified Newtonian Dynamics (MOND) was another hypothesis with equal weight. But then in 2006 measurements of the Bullet Cluster supported the dark matter hypothesis over the MOND hypothesis.
Simultaneously, WMAP (2001-present) measured the microwave background radiation and independently confirmed the existence of dark matter. It also revealed an even larger amount of "dark energy" which confirmed the 1998 discovery that the expansion of the universe is accelerating.
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.
"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.