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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?"
That's really interesting. It makes sense to model a galaxy as a fluid on a very large scale. After all, gravity is a relatively weak force. I haven't gone through the paper, but if their math is right, since the assumption is relatively benign, this seems like it would be experimentally verified.
Since the model assumes that a galaxy is a fluid (on a large scale), the model would predict fluid-like phenomena. What I wonder is if there is a galactic analogue to solitary waves. How would these manifest? (A friend wrote his thesis on solitons)
After all, I am strangely colored.
The poster title is misleading, the paper still leaves a place for dark matter, but on very smaller amounts and far from the halo. So, this matter could easily be barionic (paper's conclusion).
What is really interesting is that the third galaxy didn't fit the model as well as the others. It may be because of the inacuracy of the calculations (is the inacuracy measurable? The paper should have said that) or because there is something different on this one, maybe a smaller concentration of dark matter near the center.
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Black holes are, well, dark... so all the 'dark' matter is concentrated in localized places, namely the center of the galaxies.
Black holes at the center of galaxies have masses of 10^6 to 10^9 times the mass of the Sun. (Our Galaxy's black hole is towards the smaller side of that range.
Large galaxies themselves have masses of 10^11 to 10^12 times the mass of the Sun.
The black holes at the centers of galaxies, as far as just gravity is concerned, are dynamically unimportant to the outer parts of the galaxies.
Plus, the problem is more than that. It's not just that we don't have enough matter to explain the rotation curves of galaxies or the velocity dispersion of galaxy clusters, it's not in the right place. As you get farther from the center of the galaxy, you need more and more matter compared to what we see. Adding more matter right at the center wouldn't help that, even if the black holes were big enough (which they aren't).
(The black holes may be dynamically important to the evolution of galaxy structure for other more complicated reasons-- the generation of energy in their accretion disks can create jets and such that may limit the growth of galaxies-- but that's a separate issue from expalining the rotation curves we see in spiral galaxies.)
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If gravity has no "speed" then the advisories against instantaneous communication are violated as a change in the relative position of mass A to mass B would instantly be signaled even across the galaxies.
If gravity does have a speed then wouldn't this "dark matter" be explained as all of the extra grativational "signals" making their way through the universe?
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I'm not in this field anymore, but I spent 4-5 years in quantum gravity, black hole astrophysics, and inflationary cosmology. Summarizing my reactions to other comments in this thread:
First, I will say that I have not gone through this new paper in detail. I'm skeptical at a gut level that their results seem to depend on general relativity, because GR should not be relevant on the scale of galactic rotation curves: there is good reason why all the calculations ignore GR. It makes me think that there is a flaw in their calculation, and indeed another poster referred to a potential rebuttal of their GR analysis.
Second, as yet another poster mentioned, galactic rotation curves are just ONE evidence for dark matter. We have evidence from the aforementioned stellar orbits in galaxies, plus the motions of satellite dwarf galaxies, gravitational lensing, measurements of galactic gas temperatures (depends on the local gravitational neighborhood), anisotropies in the CMBR, the rate and structure of large-scale cosmological structure formation, etc.
(There are also a bunch of theoretical reasons to believe that dark matter particles could exist purely on the basis of particle physics, even if you ignore the astrophysical evidence; see axions, supersymmetry, etc.)
It's not surprising to come up with an alternative that can explain ONE of these phenomena. In fact, there is already another alternative that can also explain galactic rotation curves: MOND (MOdified Newtonian Dynamics), an alterating of Newton's laws of gravity. (There is a relativistic extension by Bekenstein, although it's currently even more ad hoc than dark matter appears to be.)
The problem is coming up with explanations for ALL of these phenomena. Dark matter is the only theory that has been able to do so, and it's not for lack of trying. Contrary to popular Slashdot groupthink, scientists are not in love with coming up with the most absurd and exotic possibilities they can. Most astronomers hated dark matter. For decades. I even know one who only came around to it a few years ago. It's simply that dark matter works, and everything else people tried to propose in its place didn't. As Carl Sagan said, "No physicist started out impatient with commensense notions, eager to replace them with some mathematical abstraction... Instead, they began, as we all do, with comfortable, standard, commonplace notions. The trouble is that Nature does not comply."
Now, this is not to say that dark matter is the end-all, unassailable dogma. It's possible there are alternatives, including modifications to gravity. I like to compare it to the discovery of Neptune and the perihelion precession of Mercury. People say that it's ad hoc to postulate unseen matter to explain gravitational anomalies. But that's precisely what led to the discovery of Neptune: its gravitational effects on Uranus. On the other hand, you can't always get away with postulating unseen matter: when Mecury's orbit wasn't behaving right, people tried inventing an unseen planet ("Vulcan"), but it turned out that general relativity was the answer, modifying the laws of gravity. Either can be right a priori.
In the dark matter case, it was once true that the evidence in its favor was strong and there were a number of competing theories, but now there is a lot more evidence, and higher standards for theories, and dark matter is pretty much all that's left. People should and do continue trying to come up with alternatives, but as of now, dark matter is still the best game in town. Far from claims of ad-hockery and epicycles, dark matter is actually a robust physical theory: most theories of dark matter have already been falsified because they make such specific predictions about what we should see. It's only a very specific type, quantity, and distribution of dark matter that can work. That's the hallmark of a good theory, not unfalsifiable wish-fulfillment.
Finally: this is a
My question about dark matter has always been "Why is it more acceptable to make up a new type of matter, rather than deal with the idea that the fundamental forces may work differently than is believed?"
Well, because there was no theoretical framework to explain the data without the use of darkmatter. Let's face it, the whole darkmatter hypothesis is extremely ad-hoc, a fudge factor added into galactic rotation calulations to make them fit to what was expected. The outcome was a predicition that darkmatter must exist.Now, there is nothing particularly unscientific about this. Go take a look at particle physics where all kinds of particles were predicted to exist, and as a result many particle physicists went out looking for these particles. When they were found, this confirmed the theory, when the particles were not found, they continued to look, or they revised the theory.
The same kind of thing happened here. People have been looking for darkmatter for quite some time, however, it appears that a revision to the models used to predict galactic rotation curves *and* galactic clustering is what's needed.
Why was the existance of darkmatter more "acceptable?" 1) Basically, because it was a prediction that fit the models. That's something that scientists like a lot, it gives the experimentalists something to really sink their teeth into. And 2) there was no way to predict that a change in the theory was needed without having already developed a theoretical framework that could explain galactic rotation curves without the need for darkmatter.
As an astronomer, I would say that you're not wrong to ask your question, however, without having any idea of how our theory might need to be changed, it's kind of a pointless question. And in this case, it sounds like we really don't need to change our theory at all, it turns out that the range of validity of Newtonian gravity is a lot smaller than we thought.
I think the bigger question in my mind is why hadn't someone tried to do this before now? In some sense, it's one of those things that just kind of surprises you, because all of a sudden you realize that *everyone* has been operating under the same false assumption about Newtonian gravity, and then you wonder why nobody thought to check that out.
Of course, this all assumes that this new model using relativity is correct... It probably is, but I think it does need to under go the usual scrutiny just to be sure.
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I hate responding to an AC who's unlikely to read the response, but for others who might still be reading, I can summarize what I know. The CMB radiation is a snapshot of the moment in history called "recombination", when the universe changed from a relativistic plasma and became transparant. This happned at about the same instant across the universe because the temperature of the universe was nearly the same at all points.
In a relativistic plasma, a photon doesn't go very far before "hitting" an electron, so the plasma is effectively opaque, but glowing with so much heat that electrons are almost never in a low-energy state, so photons are constantly being re-emitted. The "light pressure" is therefore the dominate force, and the "electron photon soup" acts like a compressible liquid that tries to expand. Over a large enough scale, this is balanced by gravity.
Given we know that the universe was at an extremely uniform temperature, we can predict that it consisted of large cells of gas alternately expanding and contracting. By observing the parrern of temperature differences revealed by the CMB radiation, we can get direct observational evidence about the size and motion of these cells. From our knowledge of plasma physics we can figure the ratio of mass to energy. From the CMB data we can figure the ratio of baryonic mass, which is affected by both light pressure and gravity, and non-baryonic darm matter, which is affected only be gravity.
We actualy have those numbers to about 2 significant digits, which is better than cosmology has ever done in the past with anything. However, the one simplifying assumption in all of this is that the non-baryonic dark matter doesn't interact with light in some strange and complicated way, and while that's the proper assumption to start with, we don't actually know what dark matter is, so who knows.
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I'm so happy this main reason for Dark Matter finally got explained with standard physics.
I didn't quite buy the whole idea of Dark Matter, it wasn't scientific enough. We took a stab and said that there were phantom particles that we couldn't see and they were causing our observations to be different from what they should be. It just seems like we assembled a mythos. DarkMatter, the God of the Slow Galactic Turn, floats unseen at the edge of all galaxies. 90% of all matter is dark matter, and no you cannot see any of it (short of one lensing effect from an unknown object). So verifying this theory is next to impossible. And after a while we took the leap to say that we were correct. Even though we just invented stuff to 'fix' the flawed equations. Not that we can't guess right the first time, but just inventing a solution with no basis shouldn't hit the nail on the head.
I think the comparison between Luminiferous Aether and Dark Matter is one of the most prudent ones I've heard in a long while. Making something up to force your data to fit is a pretty bad idea. We can't be wrong. There's something that we cannot see that exists (does some calculations)... here; that makes the data roughly fit. It might as well have been the law of invisible elves of slow rotation.
And yes, if by some odd happening this gets peer reviewed dead... I still believe everything I said.
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I think this demonstrates a fundamental misunderstanding of scientific research. Any real scientist will tell you they're wrong more than they're right.
You start with an observation, come up with a reasonable hypothesis to explain it, then test it.
Eventually your hypothesis fails at some level. So based on your observation, you create another reasonable hypothesis....
That's scientific progress. Each step along the way we learn more. And often, we get led down the wrong path, for any one of many reasons -most are not evil.
As a scientist, I can tell how I think many things work. Of course that leads to the question, "Don't you know for sure?" from a non-scientific public who wants to know that the levy will hold or the vaccine will protect them from disease and not cause it.
No, I don't know for sure. But that's not what anybody wants to hear. And that's not what anybody will report in the press. That's not what politicians base decisions on. The overwhelming majority of times you see science misused as you stated above its by companies/politicians/people taking scientific data and theory and restating it as scientific fact. Its rarely the scientist doing the study who says such things.