Slashdot Mirror
Good bye Dark Matter, Hello General Relativity
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?"
A recent Scientific American article does mention the formation of waves in galaxies. It's worth reading!
Ben Hocking
Need a professional organizer?
The paper only concerns itself with the observed rotation speeds of galaxies, for which "maybe there's something we don't understad about gravity" has always been just as convincing an explanation as dark matter. However, the recent cosmic microwave background radiation data *also* implies dark matter, and doesn't have such an easy alternative explanation. The data tells us that (at least, at the moment the univers first became transparant) baryons only account for 20% or so of mass.
Socialism: a lie told by totalitarians and believed by fools.
Even if the article was correct, and dark matter was not needed to explain rotation curves in galaxies, dark matter is still needed to explain the acceleration of the universe, its large scale structure and the primordial anisotropies in the cosmic microwave background.
So was the neutrino.
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
Comment removed based on user account deletion
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.
The Tao of math: The numbers you can count are not the real numbers.
Dark matter is divided into "ordinary" and "exotic" dark matter. Ordinary dark matter includes baryonic matter, plus black holes, neutron stars and the like (astophysicists don't seem to call free neutrons baryons, go figure; they call carbon a metal). TFA is only really talking about "exotic" dark matter, the rest is detectable through its effects in reasonably certain ways.
Socialism: a lie told by totalitarians and believed by fools.
I'm in Dr. Cooperstock's General Relativity (GR) class this semester. I must agree, he's a great teacher, and definitely a little quirky. Also he's quite old and some of that quirkiness may come from age.
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...
Actually, about a month after this article was submitted another paper came out 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.
This comment is geared towards other professional physicsts, even though few might see it. The Cooperstock paper is clearly wrong, although the reason turns out to be subtle. See astro-ph/0507619.
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.
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.
Forget magic. Any technology distinguishable from divine power is insufficiently advanced.
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.
"We returned the General to El Salvador, or maybe Guatemala, it's difficult to tell from 10,000 feet"
Name one prediction that has come to pass.
Here's the one I was talking about: link. 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.
"We returned the General to El Salvador, or maybe Guatemala, it's difficult to tell from 10,000 feet"