Two recent studies are attempting to shine light on dark matter.
The Dark Energy Survey (DES) has published its preliminary findings. The project is attempting to map dark matter across 1/8 of the sky using 570-megapixel imagery from the Victor Blanco telescope at the Cerro Tololo Inter-American Observatory, in the Chilean Andes. The early results are a test of the project's first images and cover just 0.4% of the sky, yet it is "the largest contiguous dark matter map" ever made, revealing exactly where dark matter is concentrated amid 2 million galaxies.
A separate finding published this week in the Monthly Notices of the Royal Astronomical Society may reveal how dark matter can interact with itself:
For the first time, the enigmatic quantity may have been caught interacting with other dark matter in a cluster 1.4 billion light-years away.
[...] A team of astronomers led by Dr Richard Massey of Durham University studied a simultaneous collision of four galaxies in the cluster Abell 3827.
Although dark matter cannot be seen, the team was able to deduce its location using a technique called gravitational lensing. While dark matter does not absorb or emit light, it does have gravity.
So it bends the path of light passing nearby, warping our view of anything on the other side of it. The dark matter in Abell 3827 bent the path of light rays coming from a distant background galaxy, which happened to be aligned just right for the team's purpose.
The researchers found that one dark matter clump appeared to be lagging behind the galaxy it surrounds. Such a lag between the dark matter and its associated galaxy would be expected if the mysterious stuff was interacting with itself - through forces other than gravity.
(Score: 5, Funny) by dyingtolive on Wednesday April 15 2015, @07:10PM
"Two recent studies are attempting to shine light on dark matter."
Well. Good luck with that then!
Don't blame me, I voted for moose wang!
(Score: 2) by kaszz on Thursday April 16 2015, @01:04AM
Perhaps there's dark photons that one can catch?
(Score: 2) by FatPhil on Wednesday April 15 2015, @09:51PM
But it's also lagging behind the galaxies, because of the deep-woo-woo force. Rather than participating in gravitional interactions.
I'm glad they've cleared that up.
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 2) by Jeremiah Cornelius on Wednesday April 15 2015, @10:36PM
Gravity - which does not exist - and Dark Matter - which does not exist.
Now this valuable work has been produced, would the authors also inscribe the arcs of Ptolemaic epicycles onto the crystal sphere of the heavens?
You're betting on the pantomime horse...
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @01:53AM
I've had a feeling that gravity is an illusory force, like centrifugal force and the Coriolis effect, but I was unaware that there was any proof of this.
(Score: 0) by Anonymous Coward on Thursday April 16 2015, @04:08AM
(Score: 3, Interesting) by PiMuNu on Thursday April 16 2015, @05:02AM
> So first it sits in an annulus around galaxies in order to keep the galaxy rotating at the right angular velocities.
No. Under the dark matter model, dark matter is assumed to be more-or-less uniform in the majority of galaxies. This is because dark matter is pulled into the gravitational well of ordinary matter, just like anything else. The times when this is not true is when there is a collision between galaxies or something like that (I guess some non-equilibrium condition).
> But it's also lagging behind the galaxies, because of the deep-woo-woo force.
In this instance, there has been a collision between galaxies, so this is not "the majority of galaxies".
FTFA:
> So it bends the path of light passing nearby, warping our view of anything on the other side of it.
Perhaps you have some explanation of why light bends around this place where there is no visible matter? Perhaps some mysterious woo-woo force?
Or perhaps there is some matter there, let's call it "dark" matter because it emits not light.
(Score: 2) by PiMuNu on Thursday April 16 2015, @05:39AM
Just to add, the self-force mentioned in the article arises due to drag forces which have made some measurable, at 68% confidence, effect on the dynamics of the collision. Also, the paper authors write in the abstract
> With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments
So they are hinting that there is a dependence on the model of mass distribution in the galaxies involved. This is all done using some software tools to handle the modelling of gravitational interactions. I apologise, I didn't have time to read the paper.
(Score: 2) by FatPhil on Thursday April 16 2015, @07:13AM
Please update wikipedia, then. Because http://en.wikipedia.org/wiki/Navarro-Frenk-White_profile disagrees with you in equations (with the wonderfully non-convergent ~r^-1.(1+r)^-2), and http://en.wikipedia.org/wiki/Dark_matter disagrees with you in words: "Galaxies show signs of being composed largely of a roughly spherically symmetric, centrally concentrated halo of dark matter with the visible matter concentrated in a disc at the center."
You'd better update the universities too, as they're under the same misapprehension as I am:
"Since the luminosity density typically decreases exponentially with radius, the mass-to-light ratio becomes large in the outer parts, implying that spiral galaxies are embedded in halos of dark matter." -- http://ned.ipac.caltech.edu/level5/ESSAYS/Bertschinger/bertschinger.html
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 2) by PiMuNu on Thursday April 16 2015, @08:01AM
> Please update wikipedia, then. Because http://en.wikipedia.org/wiki/Navarro-Frenk-White_profile [wikipedia.org] disagrees with you in equations (with the wonderfully non-convergent ~r^-1.(1+r)^-2)
Oops, my mistake. Not uniform then, but centrally distributed and more dense in the middle.
r^-1.(1+r)^-2 is large in the centre and small on the outside. E.g. http://www.wolframalpha.com/input/?i=r^-1*%281%2Br%29^-2 [wolframalpha.com]
Do you know of a calculation for the expected matter profile of weakly interacting matter under gravity which shows this is not a reasonable density profile?
I buy the argument that the density profile is singular at r = 0, but I don't worry about it - they are after all just doing a fit to data. The errors are probably large at r = 0.
> Since the luminosity density typically decreases exponentially with radius, the mass-to-light ratio becomes large in the outer parts, implying that spiral galaxies are embedded in halos of dark matter.
Note that the mass-to-light ratio becomes large, because the dark matter extends further out than the matter. This is what one expects for frictionless gas embedded in a frictionful gas. It fits the model, right? Unless you have a model which shows that this is not true (I never did the calculation, I just go on intuition here)?
Do you have a better model for explaining the galactic rotation curve, which I guess is the source of this density profile?
(Score: 2) by FatPhil on Thursday April 16 2015, @08:59AM
> Oops, my mistake. Not uniform then, but centrally distributed and more dense in the middle.
That's density, yup, but the middle is very small - effectively it's just a blip. Multiply by r^2 to get actual mass at each distance, and you'll see there's more mass at r=1 than anywhere else. (Which is why I mentioned it non-converging - it scales like 1/r in the shell at distance r for r >> 1, which is a divergent integral as r->inf.)
And no I do not have a better model. For the hypothetical mass distribution, based on galactic rotation speed, that model doesn't look too bad. More measurements from more galaxies of various types should tell us more about its shape. However, the proponents of that, or any other, model don't have a reason why the distribution should have that shape any stronger than that of it making the curves fit. There's no gem of insight in that, no understanding, just plotting.
I object more to the certainty with which some (N.K-H.) put forward their explanations for what the mass *is* and how it behaves (yes, it sounds like a predictive theory - which is good - but he always goes quiet whenever an experiment proves his theory unlikely, until he's got funding for the next bigger experiment which will prove him right this time).
But I don't think that modified theories of gravity can be completely dismissed either.
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves