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Dark Matter — "Alternative Gravity" Team Responds 215

Posted by kdawson
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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Dark Matter — "Alternative Gravity" Team Responds

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  • So We Must Wait. (Score:4, Insightful)

    by Mikkeles (698461) on Thursday September 07, 2006 @09:41AM (#16058771)
    Basically, then, until the mass of the neutrino has been tested, dark matter or alternate gravity are just speculations with the arguments being:

    is too!
    is not!
    is too!

    ...

  • It's the Ether (Score:4, Insightful)

    by Fysiks Wurks (949375) on Thursday September 07, 2006 @09:44AM (#16058785)
    Dark Matter is the 21st century's ether.
    • Does that mean I can take dark matter to recover my MP?
      • Does that mean I can take dark matter to recover my MP?

        Only if your black magic comes from the dark side of the Force.

        (Got a female illusionist of African descent?)

    • by Fordiman (689627)
      ^_^

      Very good analogy. Much better than BadAnalogyGuy's.
    • Re: (Score:2, Informative)

      by SixByNineUK (949320)
      No, Gravity waves are the 21st century ether, Dark matter is the 21st century phlogiston
    • I like, know what you mean. Last night I was huffing Dark Matter, it was like I was on FIRE man. I went tango dancing with Stephen Hawking and then we watched Charles Darwin and Al Sharpton settle the whole Intelligent Design vs. Evolution thing once and for all by having a hot dog eating contest.

      That dark matter stuff is better than ether, man.

    • by Stalyn (662)
      You mean aether. The big difference between the luminiferous aether and dark matter is that there was never any evidence of the aether however there is lots of evidence for dark matter, albeit indirect.
      • Re:It's the Ether (Score:5, Insightful)

        by dmatos (232892) on Thursday September 07, 2006 @10:25AM (#16059076)
        To the scientific understanding at the time, there was evidence of the aether. It had been observed that light exhibited wave-like characteristics, and could, in fact, be understood as a wave. At that time, all waves were known to travel through a medium. There were no waves that could travel without one. There was no other medium in the vacuum of space, so it was decided that there must be an aether.

        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?
        • by Stalyn (662)
          The categorical difference is that the aether was created to fit into a model, 'all waves were known to travel through a medium'. Dark matter was created out of a need to explain empirical evidence. There is no real model for Dark matter and we don't know why it exists. Some say that the Standard Model could explain dark matter but we really don't know.

          WIMPs are just one possible candidate for cold non-baryonic dark matter. WIMPs could be wrong. Like you said it could be something where we have to change th
          • by lubricated (49106)
            Dark matter was created out of a need to explain empirical evidence because the motion of the galaxy did not fit our model of gravity. Same thing really.
  • by Anonymous Coward on Thursday September 07, 2006 @10:01AM (#16058918)
    They're just proposing that there is no "exotic", new kind of dark matter.

    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.
    • Re: (Score:2, Insightful)

      by phirzcol (447454)
      but,,, when looking at electron even with their small mass i ask what is the mass of every electron ever traveling in space since the start of the universe also what about the expansion of the universe is their a crest of mass at the "edge" of the universe, perhapse the extra mass is only reflected to us as a result of the distortion that exists as a result of the expantion of the universe, ie light that we see is old so a possible explanation is that there is a fraction diffrence between the speed of light
    • by spun (1352)
      From reading the summary, this is my understanding of what these guys are saying, too. Call it modified MOND.
  • by Rob T Firefly (844560) on Thursday September 07, 2006 @10:09AM (#16058975) Homepage Journal
    This article presents an alternative to dark matter
    Just as dark as your regular matter, but with only 1/3 the calories!
    • by Chacham (981) *
      >>This article presents an alternative to dark matter
      >Just as dark as your regular matter, but with only 1/3 the calories!

      Heh.

      Of course, there's alma [m-w.com] matter, which is just plain wet, eh. But, being full of DiHydrogen Monoxide (DHMO) [dhmo.org] which some want to ban [netreach.net] but other defend [armory.com].
  • by sweetser (148397) <sweetser@alum.mit.edu> on Thursday September 07, 2006 @10:15AM (#16059004) Homepage
    Here's Newton's law of gravity:

            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
    • 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

      What the hell?! Where did that d/dR come from? "Relativistic swap-out"? And why are those total derivatives? I fear your methods are .... unsound.
      • by sweetser (148397) <sweetser@alum.mit.edu> on Thursday September 07, 2006 @11:44AM (#16059673) Homepage
        The relativistic 4-force law is d mV^u/dtau, where V^u has four places, and tau is the spacetime interval, sqrt(t^2 - R^2). Nothing is going fast, so we get classical laws. In the case of gravity, the road from d/dtau goes to d/dt. Simple, and standard enough.

        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.
  • by jpflip (670957) on Thursday September 07, 2006 @10:16AM (#16059012)
    People should NOT take the impression from this article that there is doubt that dark matter exists. The only doubt being raised is over what form the dark matter takes. Let me clarify:

    (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...".
    • by radtea (464814)
      Many people (particularly those who do not understand the evidence) dislike the idea of dark matter, thinking it sounds too much like epicycles

      Some of us were skeptical because we did understand the evidence, and understood that extraordinary claims require extraordinary evidence.

      Galactic dark matter is something (almost) everyone believes in, because the evidence is compelling and it does not require additional, and thus far quite undetectable, particles.

      Dark matter on larger scales, which is completely un
    • by sholden (12227)
      Please show me some.

      And no just because there's a gravitational lensing picture that implies there is mass where we can't see it isn't good enough. Maybe we don't understand gravitational lensing? Maybe we are completely misinterpreting the images and there's no lensing at all? Maybe those galaxies didn't collide at all and hence there is no "drag"?

      Anytime your theories result in the need for magical invisible stuff in larger quantities than the stuff we can observe/interact with it might be time to questio
      • by jpflip (670957) on Thursday September 07, 2006 @11:52AM (#16059744)
        I completely agree - as a previous poster said, the extraordinary claim of invisible stuff requires extraordinary evidence. We definitely need to question our assumptions about gravity, since they are the foundation of our reasoning about dark matter.

        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...
    • by RKBA (622932)
      Naive question: Have the astronomers taken into account the mass equivalent of the background radiation left over from the "Big Bang"?

      P.S.
      You can send my Nobel prize to ... ;-)
      • by jpflip (670957)
        Yes, they have. We know the temperature of the background radiation filling the universe, and so we know the energy density this represents. It turns out to be completely negligible at the present day, a few orders of magnitude less than the matter density.
      • Nice try :P
        Of course, since background radiation are photons which move at the speed of light, they have no rest mass, and their mass equivalent doesn't interact gravitationally with other masses. Also, measurements show that there is very little variation in cosmic background radiation, meaning that these photons don't clump together, and thus don't interact gravitationally.
    • by thePig (964303)
      IANAP, so just wondering;
      Why should we consider that all the gas should be hot?
      My question is such - Suppose the galaxy contains the remnants of lot of white dwarfs etc, wherein most of the elemental hydrogen has been converted to iron. Now, there is no more fusion possible, and because of which the gas, even though grouped together - stays as that - just a group of iron particles grouped by gravity, which is not hot enough to emit radiations.
      Since there is no fusion, it wouldnt emit any rays, and if it is
      • by jpflip (670957) on Thursday September 07, 2006 @12:14PM (#16059948)
        That's an excellent question! You've just described the MACHO model of dark matter (Massive Compact Halo Objects). The idea is that there could be cold, compact objects made of ordinary matter filling our dark matter halo and giving us all the extra mass. In this theory, the invisible mass exists but is still "ordinary" - no modification to gravity and no fancy new particles. It contrasts with the more exotic WIMP model of dark matter - Weakly Interacting Massive Particles.

        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.
    • by amliebsch (724858)

      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.

      Oh my, yes. You have your ordinary gravitons, which are mostly harmless, and then you have your graviolis, a neutrino on the outside with a rich, meaty graviton embedded inside of it.

  • KATRIN experiment homepage URL
    http://www-ik.fzk.de/~katrin/ [www-ik.fzk.de]
  • Prudence (Score:2, Funny)

    by Digitus1337 (671442)
    I try to be conservative, but not closed minded, in my accepting of new ideas. Concerning a concept as heavy as gravity, I think that scientists are just throwing us around, and i'm not falling for it.
  • simple definitions (Score:5, Informative)

    by peter303 (12292) on Thursday September 07, 2006 @10:52AM (#16059273)
    "Dark matter": an invisible attractive force operating on galaxy-level distances (at million light years). Size: about 23% of the energy-mass of the observed universe. Evidence: Galaxies spinning faster than the number of visible stars justify. Gravitational lenses stronger than visible stars justify. Suspects: known low mass particles like neutrinos; unknown low or high mass particles like strings, wimps; a new phsyical force; non-r-squared term in Newton's equation of gravitation, observational error ...

    "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.
  • by Anonymous Coward on Thursday September 07, 2006 @10:59AM (#16059337)
    The real result of Clowe et al's fascinating work was to show that the missing mass in the bullet cluster must be COLLISIONLESS, whatever gravity looks like (a purely baryonic bullet cluster has been *falsified*). However, a big misconception about it was to think it was a direct *confirmation* of the Lambda-CDM concordance model that everybody is supposed to believe (may I recall that real science is about *falsifying* things, not "proving" them right), or that it was falsifying MOND. Actually, it is known for years that MOND is UNABLE to fit the temperature profiles of X-ray emitting clusters from their pure baryonic content. The fix, for MOND to stay in the game, was to propose that neutrinos have a 2eV mass and can then make up for the missing mass, in clusters ONLY, because they are too light to cluster on the galaxy scales (incidentally they are also too light to form structure in GR, but this is not a problem for structure formation in MOND). However, if dark matter is indeed cold as the lambda-CDM guys tend to take for granted, and even more since Clowe's work, why does the 2eV neutrino combined with MOND seem to work in ALL clusters??? The bullet cluster being a totally new kind of constraint for MOND on the galaxy cluster scale (constraint coming from gravitational lensing instead of temperature profiles), it was mandatory to check if 2eV neutrinos were excluded even in MOND, which would have *falsified* MOND indeed! This is what those guys wanted to do, to *falsify* MOND once and for all, but the surprising result is that they didn't manage to do so, because the SAME neutrino mass as the one needed to fit temperature profiles of other clusters ACTUALLY WORKS in the bullet cluster too. Their conclusion is thus just that MOND is *not excluded* by Clowe's data. One will thus have to wait for particle physics experiments to rule out massive neutrinos to rule out MOND. Until then, place your bets...

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