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Dark Matter — "Alternative Gravity" Team Responds
Posted by
kdawson
on Thu Sep 07, 2006 08:33 AM
from the not-so-fast dept.
from the not-so-fast dept.
An anonymous reader writes, "Following previous results, an international team of astronomers answers, defending the case for a modification of the theory of gravity. This article presents an alternative to dark matter and states constraints on the neutrino mass. In short, dark matter is still not a necessity, provided that neutrinos weigh 2eV. This is allowed by what we currently know and should be tested in the KATRIN experiment in 2009."
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So We Must Wait. (Score:4, Insightful)
is too!
is not!
is too!
...
It's the Ether (Score:4, Insightful)
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Very good analogy. Much better than BadAnalogyGuy's.
Re:It's the Ether (Score:5, Insightful)
A perfectly valid scientific theory, as it was also falsifiable - as demonstrated by Michelson and Morley. When it was falsified, it required a major change in how the scientific communtiy thought about light. It is entirely possible that we'll see something similar with dark matter. Sure, an unobserved WIMP could explain things like the rotation of galaxies at their current rates. But, what happens when we get out there and don't find any? What then? Well, maybe it will require a major change in how we think about gravity. Maybe there's an entirely new force out there, that's weak enough that we can't see it on terrestrial or even solar scales. Who knows?
Technically, neutrinos are dark matter (Score:3, Informative)
Incidentally, I'd watch the Cosmic Variance [cosmicvariance.com] blog in the coming days for a discussion of this point; Sean Carroll's post there on dark matter was linked to in the last Slashdot story.
Responding to other posters: the amount of photons in the universe can be estimated based on how many of them reach us, as well as from theoretical predictions on the emission of light from stars, the Big Bang, etc., and is woefully inadequate to produce the needed gravitational effects — not to mention it is too "hot" to be the kind of dark matter needed to explain early universe structure formation.
An eV, or electron volt, is a measure of energy: the amount of energy acquired when an electron is accelerated through a 1-volt electric potential difference. It is about 1.6 * 10^-19 joules. By E=mc^2, it also corresponds to a mass, about 1.8*10^-36 kilograms. An electron, by comparison, masses about 511,000 electron volts.
Dark Matter Lite! (Score:5, Funny)
Use the chain rule Luke (Score:4, Interesting)
d mV/dt = - G M m/R^2 R_hat
It doesn't work for galaxies, it doesn't work for the big bang, it is broken for almost anything BIG. It also has a tiny bit of error that GR corrects, but that is minor. The problems with this law are HUGE. So we have two schools of thought. One wants to stuff the big M box with dark matter:
d mV/dt = - G (M + Dark_M) m/R^2 R_hat
These folks get to put Dark_M wherever it needs to go to get the answer right. Then there the MOND folks who want to mess with the R:
d mV/dt = - G (M + Dark_M) m/R^2 or if dV/dt is small, d m V/dt = - a_0 sqrt(G M/R^2) m R_hat
where a_0 is a new constant in nature that changes the form of gravity's law if tiny. I got my own proposal. Remember the chain rule from calculus?
d mV/dt = m dV/dt + V dm/dt
That V dm/dt is the stuff of rocket science. We know it is not relevant for stars cause those big star things and galaxies don't change. But we could, just for the fun of it, do a relativistic swap-out, and consider:
d mV/dt = m dV/dt + V dm/dt + V c dm/dR
Force is a change in momentum, which can be seen either as the usual acceleration, the rocket-ship effect, or as where stuff is distributed in space. That sounds like what is going on. So my proposed modification is this one:
d mV/dt = m dV/dt + V dm/dt + V c dm/dR = - G M m/R^2 (R_hat + V_hat)
Too bad I suck at numerical integration or I'd try and see if it could match real data sets. I like it because it uses stuff we know is true (the chain rule) with a fun twist to make an old law point in a new direction.
doug
Re:Use the chain rule Luke (Score:4, Interesting)
d/dtau is asking about changes with respect to spacetime intervals. We know the changes with respect to time work for our little solar system. What I am suggesting is that a change in spacetime may in the classical limit also be seen as a change in space. That would require c*d/dR to have the same units.
There are limits to what can be done in ASCII, so they appear as derivatives.
There is nothing radical about the V dm/dt, which people sometimes mention does not amount to squat. There is nothing radical about saying the "truer" force law must be a 4-force law. There is nothing wrong with the units in the switch from d/dt to c*d/dR. Don't worry, I do think it is a darn strange thing to do, but the data is forcing us in an odd direction, and at least the math here is far more constrained, as there are NO new factors or mass distributions, just relativistic rocket science.
Clarification: dark matter is STILL real! (Score:5, Informative)
(Note: Baryons are protons and neutrons. "Non-baryonic" means not made up of the building blocks of ordinary atoms.)
The beauty of the Clowes work (the "proof that dark matter exists" from a couple of weeks ago) is that the colliding clusters they worked on give simple, clean evidence that galaxy clusters are really dominated by invisible, non-baryonic dark matter. At it's core, it's a very simple argument. Two clusters collided, and the baryonic clouds (hot gas, seen with X-rays) experienced drag and got a bit hung-up passing through one another. Most of the mass, however (seen with gravitational lensing), passed straight through with no drag. We see the X-rays and lensing in two different places on the sky - they really are two different kinds of stuff. This is VERY direct proof that most of the mass in galaxy clusters is not the ordinary matter we see on earth - it's something non-baryonic that does not interact with light and does not interact much with ordinary matter. In other words, dark matter is real, physical stuff!
This article argues only about what that dark matter might actually be. It's generally believed that it can't be neutrinos, because neutrinos are so light that they would mess up galaxy formation, and so must be some new, exotic kind of particle. The logic here is that very light particles move so fast that they don't clump together well under their own gravity, which would disrupt the formation of galaxies and smaller clusters of galaxies. All this paper argues is that the dark matter might not be a truly new particle - the combination of modified gravity and neutrinos can be made to work. They still conclude that the invisible neutrinos must outmass the baryons in the clusters by a factor of at least 2.5.
Many people (particularly those who do not understand the evidence) dislike the idea of dark matter, thinking it sounds too much like epicycles. That's understandable, and it's good to be very skeptical of such a weird idea (I know I was). The truth is that there is now enough evidence to say that it really does exist, no matter how strange it may seem to us. The future lies is figuring out what the dark matter is actually made of, not bland assertions that "that just can't be right...".
Re:Clarification: dark matter is STILL real! (Score:4, Informative)
The thing is, people HAVE been questioning those assumptions for decades. Even with a lot of fancy theoretical footwork, no one has yet managed to explain our observations without assuming that the bulk of the universe's mass is invisible (including the work described in this article!). It's not like everyone has gotten brainwashed by the dark matter gospel and it never occurred to any of them to question gravity. EVERYONE has thought of questioning gravity! EVERYONE has a revulsion for the idea of invisible matter dominating our galaxies. They just haven't had much success in the ultimate test of science - explaining observations.
- Gravitational lensing and rotation curves agree that a galaxy (or cluster) has much more matter than can be accounted for by visible baryons (or even less-visible hot gas), and that the distribution of that matter is much larger than the visible structure indicates.
- Studies of big bang nucleosynthesis and the cosmic microwave background agree that the vast majority of the universe's gravitating matter is non-baryonic.
- The Bullet cluster shows a situation where the dark matter and baryonic matter are segregated from one another, in a way that makes perfect sense with dark matter and stymies MOND-only theories.
Any one of these observations can be explained by modifications to gravity, but it turns out to be very hard to make them ALL work out. I obviously can't say it's impossible, and maybe someday someone will come along and show how it all works. But right now the SIMPLEST theory which fits the facts extraordinarily well says that the bulk of the universe's matter is not visible and interacts weakly (if at all) with ordinary matter.
At a certain point you get so beaten over the head with evidence that you have to (at least tentatively) accept something that sounds crazy at first. Common sense isn't always right...
Re:Clarification: dark matter is STILL real! (Score:5, Informative)
This theory was extremely popular for many years, but has fallen out of favor for two main reasons:
(1) Using studies of the cosmic background radiation and light element abundances, you can conclude that the bulk of the matter in the hot early universe was not made up of baryons ("ordinary matter"). If it were, you would expect very different abundances of deuterium in the universe today and a very different spectrum of fluctuations in the microwave background. So we need most of the universe's matter to be non-baryonic anyway (e.g. WIMPs), and baryonic MACHOs cannot make up all of the missing mass
(2) Every now and then a MACHO should pass in front of a distant star (say, in the Large Magellanic Cloud), producing a "micro-lensing" event. Many collaborations around the world studied the skies for years looking for such events, and did find a few. The number and kind of lensing events they observed, however, was insufficient to account for all of the missing mass.
For these and other reasons, the cosmological community has rejected the MACHO hypothesis. There are objects like that out there, but the bulk of the dark matter must be something else.
simple definitions (Score:5, Informative)
"Dark energy": an invisible repulsive force operating on universe-size distances (at billion light years). Size: about 73% of the energy-mass of the observed universe. Evidence: Hubble expansion is accelerating over time when gravity would suggest eventual deceleration or collapse. Suspects: energy in fabric of space-time, unknown force, observational error
"Observed matter": stars, galaxies, gas clouds, neutrinos; Size: about 4% of the energy-mass of the universe.
From someone who just finished *reading* the paper (Score:5, Interesting)
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Re:why would matter be dark (Score:5, Interesting)
Photons lack mass but (since they move at 1,0 c) they do have momentum. This is wat makes solar sails work.
Hey, maybe that's the answer: substitute momentum for mass in all gravity calculations and see if that makes it all work.
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Nope.
Quoth wikipedia:
A solar wind is a stream of charged particles (i.e., a plasma) which are ejected from the upper atmosphere of a star. When origi
Re:why would matter be dark (Score:4, Insightful)
http://en.wikipedia.org/wiki/Light_sail [wikipedia.org]
There, see? He was right about solar sails.
You were thinking of a magnetic sail:
http://en.wikipedia.org/wiki/Magnetic_sail [wikipedia.org]
Which is something completely different. Magnetic sails do use solar wind; solar sails use sunlight. Big big difference between the two.
Re:why would matter be dark (Score:4, Insightful)
Except that solar sails do not depend on "solar wind", ie. particles. The main thrust (when above a certain distance form the sun at least) is delivered by massless photons, ie. light. Hence the more correct term "light sail".
No mass for photons (Score:5, Informative)
THEORY: In our current understanding, photons are forbidden from having mass because of the way quantum electrodynamics (the most precisely tested theory in the history of science) works. It's an exercise in field theory to show it, but the gist is that electromagnetism (light, charge conservation, electric and magnetic forces...) are a consequence of a symmetry of nature, and that symmetry only works if the associated carrier particle (the photon) has exactly zero mass.
EXPERIMENT: If the photon had even a very tiny mass, it would also mean that the electromagnetic interactions would become short range (just like the weak interactions, which are mediated by a massive carrier). The usual inverse square law would become an exponential falloff. This has been tested for in laboratories (and in astronomy!) very precisely, so there are ridiculously strict upper limits on the photon mass.
This doesn't mean photons don't feel gravity!! Gravity interacts with all energy, not just mass, and so the energy of a photon is enough to cause it to bend around massive objects.
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Well that's not always true.
In a star, it loses mass which is converted to the energy of the photon. In a lightbulb, the photon's energy comes from the covering of a potential difference in v
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When you shut the light off, the light bouncing back and forth would keep pushing.
After each "push" the photons will not be reflectet with the same frequency (the sails are moving away from the light. hint: doppler effect)
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The distinction comes from the full version of Einstein's famous equation: E^2 = (p*c)^2 + (m*c^2)^2, where p is the momentum of the particle an
Re:why would matter be dark (Score:4, Informative)
Newtonian mechanics implies that for gravity to affect an object it must have a mass, however General Relitivity does not impose this restriction.
I am pretty sure that the gravity effect is caused by the distortion of space time such that the 'shortest' path (That which the light must follow) is curved.
I am not an expert in GR, but perhaps someone here can verfy my claims.
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Re:why would matter be dark (Score:5, Informative)
(ok, ok, simply *massive* oversimplification here - to the point of error, but I hope you understand my motives.)
Re:why would matter be dark (Score:4, Informative)
Somewhere in the great online book Reflections on Relativity [mathpages.com], there is a discussion of "kugelblitz"s, which is a theoretical black hole that consists entirely of energy, which could be just a lot of photons. The term isn't in much use in science (though I did find at least one arxiv.org reference) because it's not very useful; in practice, a photonic kugelblitz is impossible, and once such a black hole forms, it would be indistinguishable from any other black hole. But it is theoretically possible, because all mass-energy contributes to the gravitational field.
Re:why would matter be dark (Score:5, Informative)
The term "c" is the speed of light in a vacuum. Experiments like these don't happen in a vacuum. Light isn't really being slowed down per se; it's still moving at c when it's away from the atoms. These experiments are a very clever way to keep the light pulses intact while keeping the light itself from actually going very far. The net effect is to slow down the light pulse without actually slowing the light itself, so the mass of the light is unchanged, as is its net momentum. Inside the system the momentum of the light is bounced around and interacting with electrons, but at that level the light is just behaving as light does with nothing to affect its speed beyond plain old quantum juju, so there's no change in mass or momentum there, either.
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Re:Anti-dark-matter scientists are like ID scienti (Score:5, Informative)
Re:Anti-dark-matter scientists are like ID scienti (Score:2)
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Wait, there might be a flaw in the logic here somewhere.
Re:Anti-dark-matter scientists are like ID scienti (Score:2)
Re:Anti-dark-matter scientists are like ID scienti (Score:2)
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They know their mass is >= 0 and 2.2 eV, hence why the proposed 2eV is possible.
Re:Anti-dark-matter scientists are like ID scienti (Score:3, Informative)
Um.
Google "electron volt in amu":
That's five whole orders of magnitude lighter than an electron. That sounds like a good reason they don't interact; it'd be like saying a dust cloud should inte1 electron volt = 1.07354412 × 10-9 atomic mass units
Re:Anti-dark-matter scientists are like ID scienti (Score:5, Insightful)
Neutrinos *do* have mass, and this fact is accepted by pretty much all physicists. The argument for this comes from discovery that they change states over the course of their lives, which means that they experience time, which means that they cannot travel at the speed of light, which means they must have a small mass. (This explains the apparently deficiency of solar neutrinos which was a problem in the 70s) Pinning down the exact value of this mass is more troublesome, though - for now, we know only that it's small, but positive.
What more puzzles me about this statement is that neutrinos have generally been counted as *part* of dark matter - in particular, they are proposed to constitute some of those so-called Weakly Interacting Massive Particles (WIMPs) which is one of two possible models for dark matter. I don't see how changing the details of these particles would change how neccessary they are, unless these guys are trying a bait and switch by redefining dark matter to be unneccessary. (Which would be a very dirty trick.)
No, YOU'RE like an ID scientist (Score:3, Interesting)
And more to the point, fails to reflect radiation, fails to block radiation, in general, um, fails to interact except in terms of gravity.
What was that you were saying about explaining some
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This is incorrect. Theory exists regardless of the existance of any one theorist who believes that the theory must be true or is the o
Re:Anti-dark-matter scientists are like ID scienti (Score:5, Insightful)
I think I'd regret responding to the complete misunderstanding of forces and neutrinos in the body of your post. That would take pages.
Let me just respond to your title. That is completely wrong as well. Now, I think the alternative gravity guys are probably wrong and at this point I think they are stretching their theories to their limits. Dark matter is the "easiest" explanation. But, what they are doing is science. They are coming up with an alternate theory that makes predictions and testing them. The are countering circumstantial evidence for DM with another theory. They are not picking just one small thing, saying "Well that can't be true because of [insert some non-science babble like you just posted] so clearly God created everything." in contradiction to vast bodies of scientific evidence. And the alternative gravity people are publishing in peer-reviewed journals.
ID can't say any of those things. While the motivations may be similar (not wanting to give up on old ways of thinking about things) the methodology is completely different.
Re:Anti-dark-matter scientists are like ID scienti (Score:4, Funny)
Perhaps neutinos are similar.
Occam's Razor != Science (Score:4, Interesting)
At one time, all the scientists thought there was this stuff called ether, and it was the best explanation for the observations we had. Then people did more tests, and discovered incongruities. In the end it was proven an incorrect idea, and was supplanted with a better model.
Einstein spent many years trying to find a deterministic alternative to quantum mechanics. There were many respectable scientists that felt that QM was merely a useful approximation, but after years of testing, a the consensus finally turned, and the community accepted that the non-deterministic aspects of QM were real.
Should we have blindly accepted Ether or QM, just because preliminary results showed promise with the ideas? No - we continued to question them and test them until they were disproved or time had shown them to be solid ideas. Dark Matter is in the same place as these theories once were. I don't know whether it will turn out to be correct or not, but I do know we should continue to challenge it, to think of new ways to test it, and to think of alternative explanations, because that is what science is about and that is how we take good ideas and turn them into a rigorous and well-established understanding of the universe.
You would call these people pseudo-scientists, and yet your only argument an application of Occam's Razor (and as others pointed out, faulty understanding of principles). But that's the funny thing about Occam's Razor - it is dependant on one's personal opinion of what is the most likely, or most simple explanation. Some would consider making up new particles that we have never observed a real stretch, others consider tweaking the existing rules a hack. That someone has a different view of what is elegant than you, does not make their ideas pseudo-science. What matters is if they are predictive and falsifiable, which these are.
Honestly, if you can't tell the difference between people that present testable alternative hypothesis, and people whose best "theory" that they could present amounts to "does this not appear irreducible", then you are the one that needs a refresher on what is and is not science.
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Interesting observation, if a bit off. The difference being, of course, that we will eventually have a factual basis for dark matter ( whether it exists or not ),
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Re:weighs 2eV? (Score:4, Informative)
After relativity and E=Mc^2, physicists have preferred to measure mass in terms of energy rather than silly units like grams and ounces. In short, we give the energy equivalent for the particle if it was somehow completely annihilated. 1 electron volt refers to the energy of one electron under an electric field at a point of 1 volt of potential difference. It doesn't have anything to do with the actual mass of the electron, but rather the electrical potential energy such an electron would hold.
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Re:Useful research (Score:5, Insightful)
For those who say, "that's technology, not science!" I will note that the examples I gave were based largely on previously abstract, largely government-funded scientific research whose applications were not immediately obvious, but which have since transformed the way we live. If you don't understand the research, that's fine; you don't have to in order to take advantage of it. But just because you don't give a shit about the way things work doesn't mean that you get to stand in the way of people who do, and whose work will benefit you and your children's lives, no matter how little you deserve it.