Good bye Dark Matter, Hello General Relativity 688
dr. loser writes "The CERN newsletter reports that a new paper by scientists at the University of Victoria has demonstrated that one of the prime observational justifications for the existence of dark matter can be explained without any dark matter at all, by a proper use of general relativity! What does this imply for cosmology and particle physics, both of which have been worrying about other aspects of dark matter?"
Recent Sci Am article treats waves in galaxies (Score:5, Informative)
Re:Neat (Score:5, Informative)
Re:From the Abstract (Score:2, Informative)
Yes there are analogues and easily visible manifestations are spiral arms.
Treating galaxies as fluids has been done for many years and the models have been quite successful. I think it was found back in the 70's that spiral arms could be modelled rather well as density waves in a rotating disc of fluid.
Paul
Dark matter still needed in cosmology (Score:5, Informative)
Re:From the Abstract (Score:3, Informative)
CC Lin has been using this approach to model the evolution of the spiral structure of galaxies for some time (mid 70's or earlier).
http://www.worldscibooks.com/mathematics/0412.htm
Re:No magic pixie dust after all (Score:5, Informative)
The neutrino, when originally discovered, was discovered because something was missing. Particle collisions were seemingly violating the conservation of energy and momentum. Postulating the existence of an unknown, massless or nearly massless particle that interacted only weakly solved that problem.
Only later was the neutrino discovered.
Unanswered questions, very specific unanswered questions (we need *something* to do *this*) often do lead to new discoveries in science.
I'm not saying that dark matter necessarily has to exist, but the galaxy and cluster gravitational dispersion evidence were strong indicators that there had to be more gravity there. Postuatling that we weren't seeing all the mass was a very reasonable postulate. Now there are lots of other reasons (e.g. CMB, large scale structure evolution) to suspect it's there. And, possibly, in the next decade, we will finally identify the dark matter particle in the lab. We'll see.
-Rob
Re:Have they been using Newtonian physics?! (Score:3, Informative)
I'm told there is a mistake in the general relativistic metric used in the paper. Basically, a small error left them modeling the wrong situation. The situation they actually studied was one with an axially symmetric cloud of self-gravitating gas (the galaxy) AND a thin, heavy disk. The thin, heavy disk screws things up and produces the effect they observe.
something lost in the translation (Score:3, Informative)
There are two problems in astrophysics in which dark matter is invoked as a possible explanation. One is the "galaxy problem": galactic rotation curves imply a distribution of matter different that you would infer from looking at the luminous matter. The other is the "cosmological problem": observation of redshift vs. distance, and of the cosmic microwave background, and other similar measurements imply a total mass density in the Universe different from what you would infer from looking at the luminous matter. Each of these problems can be explained with dark matter (e.g. some kind of extremely massive particles that only participate in the weak interaction, not yet observed). Sadly, the properties of the dark matter needed to solve these two problems are not necessarily the same.
This paper claims to eliminate the need for dark matter to solve the galaxy problem, but does not address the cosmology problem.
Re:Alternative sets of laws of physics (Score:3, Informative)
It can and does, all the time.
Consider the chair as a fundamental particle. It can be described in terms of its mass, as a particle, or of its wavelength, as a wave. How far you can expect tunneling in a chair can be observed is a function of its wavelength, and for an object as massive as a chair its wavelength is terribly terribly small...
Re:Neat (Score:3, Informative)
They do, however, need additional mass in the galaxy to explain its motion, just not nearly as much as the Dark Matter theories and the mass is roughly distributed in the same manner as the luminous matter, rather than being further away from the core, as DArk Matter mysteriously would need to be.
Re:So does that mean... (Score:1, Informative)
Re:So does that mean... (Score:3, Informative)
Some people are so dim witted.
Comment removed (Score:5, Informative)
Re:It's Copernicus all over again! (Score:5, Informative)
Wrong. Copernicus still had the planets moving in circles. The big difference (and the reason of the rejection by the church) was that in his model he put the sun instead of the earth in the center.
It was Kepler who found that the planets don't really move in circles, but in ellipses.
Re:As usual... (Score:3, Informative)
Sensationalist headline and non-TFA-reading posters aside, the paper merely shows that there is an interpretation of general relativity that someone's only just discovered that eliminates the need for dark matter to explain one type of observation where the theory didn't quite fit reality. There are several other scenarios where Dark Matter is still thought to be a possible explanation, and this paper doesn't appear to touch those subjects.
Dark Matter required the relatively-trivial addition of a new particle (or class of particles) to the Standard Modelm and a small amount of retrofitting of a few other theories.
Fairies require a complete reworking of the fundamental basis of physics, some kind of scientific explanation of magic, and either the location of a large amount of hidden real estate for the Faerie realm, or the postulation of prallel dimensions and a method of traversing between them that doens't require wormholes or gravitational singularities.
So year, Dark Matter is simpler,
Re:My question: (Score:3, Informative)
Which is precisely why such proposals are deeply problematic.
Consider the traditional SR simultaneity paradox -
You have a train, which is carrying a photon torpedo. At time t, klingon saboteurs detonate the torpedo, sending out a pulse of light in all directions. By conventional SR, a viewer on the train and a viewer off it would observe that the speed of the pulse's propagation is as expected - at c, but while the train guy observes that the carriage is completely vaporised at the same time, that relative motion guy would not that the back of the train got vaporised first. In fact, not merely observe - the key result of SR is that both conclusions are equally valid.
But if gravity perturbances moved instantaneously, then we could fit a gravitionic transmitter on each end of the train, so the viewers would know when each end of the train got vaporised. This inevitably leads to some sort of discrepancy - for example, one observer would end up calculating a different speed of light, or we could violate causality, which would entirely invalidate one of the key postulates of SR, and the entirity of modern physics. It would contradict a century of evidence.
Proposing instant communication of information is a crazily serious claim, and needs crazily strong evidence.
More (coherent) info available via wikipedia: http://en.wikipedia.org/wiki/Faster-than-light [wikipedia.org]
Re:NOT Informative (Score:4, Informative)
Re: NOT Informative (Score:3, Informative)
Funny, I call it "the best hypothesis so far". It may lose that status if the new one stands up to scrutiny, but that's no justification for dismissing it as a hack.
Re:Well it clearly matters to some people... (Score:3, Informative)
So the only possible counterpoint is: "There is a Creator."
And since using "God" as a counter argument doesn't fit into the Scientific Method you have the convenient fall back of dismissing the only possible counter argument as "not science."
So, if I may, I'd like to point out that the question of "where it all started" doesn't belong in a Science classroom. It belongs in a Philosophy or World religion classroom. If you are going to teach it in the Science class though, then don't use that as a convenient excuse to exclude the only possible counter argument. That's just Intellectual dishonesty. Evpolution as a non origin's study, if you can keep it that way, is perfectly acceptable in the classroom. It's not presented that way currently though, nor is there any discernable desire to do so.
You dont' know anything about evolution don't you?
Evolution is based on the principal that "traits that can be passed onto progeny(genes) that are also hetrogenious(not all members have the same genes) and mutable(mutations) in some way will result in groups of living things changing over time to response to selective factors(observed often)". The assumptions about god are immaterial. God may have started it, there may be no god. Neither possibility changes evolution.
Your idea about evolution is deeply deeply flawed. It doesn't reflect it's current form or any of it's previous iterations. You are operating under a deep logical fallacy. Evolution has nothing to do with denying a god, only explaining a mechanism for biological change. It is often used as evidence to refute the exsistance of a god how ever God is not an idea that can be directly refuted. I am in fact a christian but I don't beleive in this IS/creationist propaganda.
ID/creationism aren't scientific at all. They are a politically/religiously motivated PR campiagn to assert a certain religions dominance in the American society. They have nothing to do with science and outisde of the US they are not given any credience.
Like I pointed out, it doesn't matter if a alien species/God/FSM/cthulu/me came to earth, tampered with the genetic material and created man, because the basic mechanisms of evolution stand.
Re:Dr. Cooperstock (Score:5, Informative)
As an aside, what's different about his lectures is that he uses a transparency roll and an overhead projector instead of the blackboard, and writes/derives everything with us in class, unlike many other professors who merely present slide shows or just talk a lot and write very little (very common among astronomy professors). I really dislike slideshows, and prefer Dr. Cooperstock's method because as he does so, we learn about how he thinks, why he makes the decisions he makes in the derivations, and the usual pitfalls in dealing with all the notation used in GR. That for me is far more valuable than just seeing an amalgamation of details presented on slide shows with a short verbal summary from a professor. Any textbook could provide me with that. The other advantage with the transparency roll is that if we ever need to go back to a previous lecture to revisit something that was discussed there, he just has to put up the roll corresponding to that lecture, and we have it right there in front of us. If we missed any lecture notes, we can just borrow the transparency rolls from him and copy the notes from them.
Back to the topic, I believe that what's important is that we must realize that dark matter is still just a hypothesis. There may be overwhelming signs pointing to something that we call dark matter, but this paper reminds us that dark matter is still only just a hypothesis. It is one of the easiest hypotheses to make, because simply adding a spherical distribution of dark matter to a galactic halo produces the observed rotation curve, but is not the simplest, because it postulates the existence of particles that we cannot yet prove to exist, at least not in such large quantities. If simplicity is a valid reason to accept or refute theories, then Dr. Cooperstock's model appears to me to be simpler because it requires fewer postulates to make things work.
However there are other observations such as satellite galaxies and gravitational lensing and galaxy clusters, all of which appear to require a huge amount of dark matter that we cannot observe. While Dr. Cooperstock's model may not explain all of these yet, this is work that has yet to be done, and so his model cannot be ruled out. One must realize that dark matter is really just a fudge factor to make the theory work out the same way as observations. Until there is good evidence from astronomers and from particle physicists, the arena should be open and impartial to other candidate hypotheses. It is good to see that despite most of the world jumping on to the dark matter band wagon, there are people who stand back and persist with their own ideas. We've seen this happen so many times in history.
Besides, it is still possible that despite GR explaining the galactic rotational curves, dark matter may still exist, but then its role and distribution would change. Oh, the fun of physics...
Re:New discoveries lead to new theories (Score:3, Informative)
Singular disk (Score:3, Informative)
Actually, about a month after this article was submitted another paper came out [arxiv.org] saying that the proposed model is not physical as it requires the disk generating the gravitational field to be singular. Although I did skim the Cooperstock article (he was my prof for GR, so I have a bit of a bias), I didn't read the other article.
I would be surprised if the Cooperstock & Tieu model completely dispells dark matter. For starters, we know that only about 5% of the mass (actually, density) in the universe is baryonic matter (normal matter) from Big Bang baryogenesis models and the match to cosmological observations (WMAP). We also have some confidence (also from WMAP, but also from BOOMERANG) that the Universe is very nearly gravitationally flat (this result is independent, IIRC, of assumptions about dark matter). This means that 95% of the mass/energy density in the Universe is something else. Current models and observation suggest that dark matter makes up about 35%; the remaining 60% is 'dark energy'.
However, if a simple re-application of GR can make at least some of that dark matter disappear from the models, that makes life interesting.
Subtle error (Score:3, Informative)
Re:NOT Informative (Score:3, Informative)
Re:Well it clearly matters to some people... (Score:5, Informative)
I'm not quite sure what you mean by "Dark Matter is far from an accepted Hypothesis". It is certainly not far-fetched to imagine that there is some quantity of matter, perhaps substantial, that does not "glow" like stars do. This is why it is "dark". The original problem was one of galactic rotation curves --- matter in the outskirts of galaxies rotated around the center in a fashion exactly mimicing what it would do if there was a spherical distribution of matter extending beyond the glow of the visible disk. The hypothesis that there was just such a distribution that we cannot see is not so far-fetched. It has been admittedly difficult to identify the "conventional" bodies that could be responsible for the lion's share of such a halo. Upper limits on the numbers of brown dwarfs, Jupiter-sized objects, and small black holes have shown that no one of these are primarily responsible. Still the search continues, as it would in any good scientific theory. Any of these possibilities are seen as a simpler approach than modifying our most basic models of gravitational behavior, especially when there is no similar pattern of deviation from known laws on different scales. And, as shown by the follow-up paper in the archives, there is a real possibility that the authors have made an honest mistake.
Re:So does that mean... (Score:3, Informative)
I think the reason that scientists 'made up' dark matter, was to describe what they were seeing (or not seeing), based on their current understandings of phyiscs. If the math says that there is missing matter, scientists figure out how it would behave and try to find it, so the theory can be proven or disproven, and achieve greater understanding of existance. Exotic Dark Matter wasn't created to just make the math work out, and wish away data that doesn't make sense.
Just because people are studying some supposed invisible intangible matter, doesn't mean they are trying to pull a fast one on you, or even that they believe it exists themselves. I suspect that a fair number of people studying the problem are skeptical, and are looking to eliminate exotic dark matter as a possiblity.
Re:So does that mean... (Score:4, Informative)
From what I can recall, the whole idea of "dark matter" started with the observation that the galaxies are spinning too fast that the gravity of their visible matter would be able to keep them together. This led to the conclusion that one of the followign must be wrong, in descending order of propability:
The scientists started with the most likely alternative, namely by assuming that their observations were incorrect. There's two variables being observed here: the speed of galaxy's rotation and its mass. The speed of rotation is pretty easy to figure out: you simply need to compare the red and blue shifts in the spectrum of the stars of the galaxy.
On the other hand, mass is impossible to detect directly; you can only calculate its presence by the gravitational effects it causes. Assuming that the galaxy is a stable structure, you can calculate the mass once you know the rotation speed. After this calculation was done, the result was much larger than one would expect from the amount of visible stars. The obvious conclusion was that the majority of the mass was in a form that didn't show up in any way but through its gravity. The scientists accidentally added some overdramatization to the concoction, and thus the term "Dark Matter" was born.
No, we had a bunch of unexplained (without quotation marks) things that then got explained (without quotation marks) using dark matter/energy. The explanation might very well turn out to be incorrect, but putting quotation marks on words does neither make any point nor show the hypothesis incorrect.
Good thing that no one asked you, then, since this seems like a prime example of how science is made: you notice an unexpected phenomenon and try to find an explanation.
It is rather difficult to think up a more substantial base than the apparent conflict between observed reality and theoretical predictions that lead to the hypothesis of dark matter. And sitting around saying "I don't know" isn't a scientists job, trying to find out is. Being humans, they sometimes get things wrong; after all, certainty is not the domain of mere mortals. However, they are men enough to risk being wrong, even if someone will mock them on Slashdot when there is a suspicion of them being so.
Re:Well it clearly matters to some people... (Score:1, Informative)
Re:Well it clearly matters to some people... (Score:3, Informative)
Re:A hunch (Score:4, Informative)
Perhaps, the more gravity the less the speed of light and the more gravity the slower the speed of light is.
Sort of. The more gravity, the more space is curved, which makes light travel a longer path and thus appear slower. Once gravity exceeds a certain limit, light is curved in on itself.
Re:Well it clearly matters to some people... (Score:2, Informative)
Even though this is disappointing from a philosophical point of view, the result that the model is unphysical is good because it saves astronomy a lot of trouble. I think that it is really important to stress here that the evidence for dark matter does not come only from flat rotation curves alone, but that there are many independent methods to determine the presence of gravitational mass, many of which do not depend on any Newtonian assumptions. Had the original result been true, the non-existence of dark matter halos of galaxies would have implied that most of these other independent experiments showing the presence of dark matter, e.g., in galaxy clusters, are be wrong as well. And that would have shaken the foundations of most of modern cosmology.
Re:Well it clearly matters to some people... (Score:4, Informative)
Name one prediction that has come to pass.
Here's the one I was talking about: link [washingtonpost.com]. Just because you don't know about it doesn't mean it doesn't exist. Also, I'm not aware of any competing theories.
Couldn't it merely be that instead of a common ancestor, we have a common creator?
Who cares? You can't test that, falsify it, or make predictions based on it.
Re:My question: (Score:2, Informative)
Note that gravity having a speed absolutely would not simply account for dark matter. Astrophysicists have taken such facts into consideration for years, and that alone is certainly not enough. Galaxies are still only rotating at a tiny fraction of the speed of light (no doplar shift) and are cylindrically symmetric.
Re:Well it clearly matters to some people... (Score:1, Informative)
Nice rant, and I'm not even sure if this correction supports or opposes your position, but...
You wrote:
They teach "facts", like "water vapor absorbs light, but absorbes blue light the least, and thus makes the sky blue".
This isn't even a fact-in-inverted-commas. All together now: "Air scatters light, but scatters blue light the most, thus making the sky blue."
If you were taught the above "fact" in school then your complaints are the least of your worries about your science education! :)