Most Detailed View of Dark Matter Mapped By Hubble 93
astroengine writes "Building on previous studies by the Hubble Space Telescope, new analysis of gravitational lensing data has revealed the most detailed map of the distribution of dark matter yet. The distribution appears as a beautiful ghost-like or ethereal haze and could have serious ramifications on our understanding as to how galaxy clusters form and evolve."
Re:You want pictures of Dark Matter? (Score:2, Informative)
Re:Just a question (Score:3, Informative)
Dynamical studies of our own galaxy show there's a lot of invisible matter in it. This means that something has to happen with gravitational theory within a region much smaller than the observable universe, at speeds of only a few hundred kilometers per hour. The modified theory also has to conform to the known motions of solar system objects, which are known to extremely high accuracy. These conditions are very hard to meet.
Re:Look for the unicorn (Score:5, Informative)
TFA is firewalled off, but I found a better FA [nasa.gov] -- straight from NASA's JPL. here [nasa.gov] is a hi-res photo of the "dark matter" lensing.
Allan Sandage (Score:5, Informative)
Re: Just a question (Score:4, Informative)
I'm just asking the question, because I don't have a great deal of knowledge about this, but could an alternative explanation be that our theory of gravity is wrong?
Yeah, that or a lot of other things. There was a popular explanation called MOND - Modification of Newtonian Dynamics - but AIUI the evidence shot it down. However, I think there's some MOND variants still out there. But most cosmologists apparently lean toward DM as the best explanation for the available observations.
Re:Just a question (Score:5, Informative)
but could an alternative explanation be that our theory of gravity is wrong?
Yea. The discovery is that, IF our theory of gravity is correct, this is more evidence for the existence of dark matter.
It is something more along the lines of this:
We have a good number of formulas and calculations that work properly with the things we can measure - planets, the sun, cars, planes, kitchen scales.
One of these might be:
y + 3 = 5
Nice and simple for this example. Lets say that the "y" here represents gravity and the formula has been proven in every experiment we have done.
We therefore assume that this calculation is correct and true. BUT when we try to use this calculation when looking at things like galaxies, we seem to find the wrong answer:
y + 3 = 7.2
This is clearly not correct, but as we don't want to throw out all the formulas and understanding we have about how things work, we add another variable to the formula like so:
y + 3 + x = 5
The "y" still represents gravity, but now we add the "x" which represents something we don't understand and we don't know where it came from. We call it Dark Matter because we can't see it, don't seem to be able to interact with it and have no real idea of what it is - but with this new addition to the formula, the answer once again comes out at what we know (think) to be true. We just now need to find what this x variable is.
THAT is why finding/understanding Dark Matter (and on that note, Dark Energy) is so important. We know (think we know) the right answers, but our formulas just don't seem to fit so well when applied to certain really, really, really big things (like clusters, superclusters etc). When we find this "x" in the formula, it will once again work perfectly for all our calculations.
Re:Just a question (Score:1, Informative)
That's what the MOND [wikipedia.org] theory is all about
Short answer: No (Score:5, Informative)
Long answer: Dark matter wasn't invented just because someone saw some anomalous behavior that didn't agree with theory, and said to themselves: "Oh, there has to be something mysterious at work here, we'll call it dark matter.". There are several reasons for believing in dark matter, for example that when measuring gravity we notice gravity coming from directions where we can't see any matter. However, the source of this gravity behaves a lot like matter would. For example we can observe these "invisible gravity sources" being thrown around when two galaxies collide. Because these "invisible gravity sources" acts a lot like matter, except for the fact that we can't see it, it's called dark matter.
If you're not yet convinced, take a look at this recent blogpost by a professional astrophysicist: http://scienceblogs.com/startswithabang/2010/11/the_simplest_argument_for_dark.php [scienceblogs.com] In this post, he basically explains how we can derive the existence of dark matter from: A) Assuming that the theory of general relativity is valid, B) assuming that the big bang theory is valid, and C) our observations of the cosmic microwave background.
Re:Just a question (Score:3, Informative)
You have it backwards : nothing in physics is "provable correct".
A theory is only useful till it is proven incomplete or incorrect. If it holds a very long time, it is only "probably correct" or "correct enough for today".
The phenomenon that led scientists to develop the concept of dark matter could very well be hints that our theory of gravity is wrong/incomplete.
Nothing is provably correct (Score:3, Informative)
It's provable correct.
No model in physics is "provably correct". That's not how the scientific process works. Scientific hypothesis and their resulting models can never be proven conclusively correct, they can only dis-proven. You can support a model with vast amounts of evidence and be quite confident that it is a useful and accurate model but it only takes a single piece of evidence to establish that the model is wrong. When we say something is a physical law we are basically saying we have a mathematical model for how this works and we've studied the hell out of it and every piece of evidence we've gathered so far supports the correctness of the model. That is NOT the same thing as saying we have proven this model to be correct - it is saying we have been unable to prove this model is wrong.
Of course we can also still use models that we know are less accurate (provably INcorrect) if they provide good approximations under known circumstances. We know relativity is a more accurate model of the physical world than Newtonian models for a great many problems. But the differences are negligible under many conditions and the relativistic models are much more mathematically cumbersome.
Re:You want pictures of Dark Matter? (Score:2, Informative)
Re:You want pictures of Dark Matter? (Score:3, Informative)
With regards to your sig, it's not Apple you should thank, but the fine folks at KDE.org which did all the heavy lifting. Webkit is a modified version of khtml.
Just sayin'
--
BMO
Re:Just a question (Score:2, Informative)
In a word: yes.
In a bunch of other words: yes, but... The standard model for gravity is used for a number of reasons, first among them that it fits better than the others. It's only off by about a factor of five (working with a universe of 20% normal matter, 80% dark matter). It doesn't devolve into some easily disprovable logical contradictions like most of the others. It's easy to say "yes, but..." It's much harder to propose a theory of gravity that stands up to close inspection of the world's physicists, astronomers and tenure committees. Here on slashdot we aren't (all) physicists, mathemeticians or professors seeking tenure so let's have some fun and talk about some alternatives. I'm no expert either, but I like to play with these ideas and it turns out they can be interesting.
The force of gravity is presumed to fall off as the inverse of the distance squared. If rather than this smooth integral it's a more complex function that would explain a lot. It would however complicate many other things. It should be possible to find describe the observable phenomena with a complex function of gravity over distance rather than missing mass - but then you wind up with a complicated answer that requires considerably more proof than a simpler one and is more subject to disproof. It's still one of my personal favorites. It can pay off to not get too emotionally attached to one theory though, because theories evolve as we learn.
Then there's the fact that we're evolving our understanding from the bottom of a gravity well of our planet, sun, local cluster, galaxy, local galactic cluster, local galactic string. We're looking at the Universe from the middle of dust clouds on all those levels, across vast voids between we cannot measure the properties of, and the properties of the voids may be as important as the properties of the masses in the larger scheme. We're speculating about the evolution of nearly 14 billion years from a viewpoint of mathematics born less than 3000 years ago (Euclid). We may as well be children studying wave motion by splashing in a puddle. We may discover things, but the discovery of all things seems unlikely. We are building a ship in a bottle here. We're composing a powerpoint presentation on a cellphone. Beside our own biases, we're watching the game through a knothole in the fence. It's possible that mass alters time in some way that creates the appearance of gravitic lensing effects that are not there. It's possible that gravity has some time function we're not aware of - a number of experiments to find "gravity waves" have been proposed, but afaik none have borne fruit. It's possible (and close to proven) that at least some of the "constants" in the equations are variable according to some not-understood function, and that in the foreign lands beyond the star of our birth the local rules are different.
We know that gravity affects both space and time. We know we don't fully understand the effects of either. Our Voyager probes, for example, appear to be slowing down at the edge of System Sol for no reason we can explain. Gravity slingshots have somewhat different results than predicted. Even on this minute immediate local level we don't fully understand gravity and its relation to spacetime, though we get closer with every measure. It's possible we're looking at the Universe through some edge effects that alter our view in ways we don't understand yet. When we come to understand them better we may be able to apply corrective lenses. In the mean time some explain the difference between theory and observation with "missing mass" which, if it does nothing else, quantifies the difference between the prediction and the observation in a way that can be modeled, manipulated and explained to the press without sounding too much like you're making the whole thing up. Ultimately it adds to understanding by quantifying the depth of our ignorance with a specific map to the undiscovered country to test new theories against. Like any good lie it can be usef