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No dark energy?

  1. Feb 7, 2006 #1

    ZapperZ

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    This should throw another spider into the web:

    http://www.physicsweb.org/articles/news/10/2/4/1

    As far as I can tell, this is not MOND, is it?

    Zz.
     
  2. jcsd
  3. Feb 7, 2006 #2

    Garth

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    This is here on the physics arXiv, but it is not MOND.

    This theory, like MOND, treats gravitation as a two regime phenomenon with a modification to GR/Newton that disappears in the 'close' regime inside a threshold.

    MOND treats that threshold as an acceleration a0 ~ 10-8 cm.sec-2.

    This theory, "generalized modified gravity", treats the threshold as a distance from the solar system of r ~ 10 pc.

    This is a three parameter theory, beginning with a complicated modified gravitational action. The authors find a parameter fit with the distant supernovae data and intend in future to study its fit with large scale cluster structure and CMB also. I wish them well!

    This may be the 'next big thing', or, on the other hand it could just be a case of "you can prove anything if you try hard enough" (or have enough variable parameters to fit).

    By adapting the gravitational action they are introducing new physics, are we allowed to ask whether such physics may be tested in a local laboratory?

    Garth
     
    Last edited: Feb 7, 2006
  4. Feb 7, 2006 #3

    SpaceTiger

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    Well, I would say it's more like a step forward in understanding one of the spiders. The paper is parameterizing and evolving alternative gravity models that attempt to explain dark energy. There has been no shortage of these theories in recent years, but they're usually proposed by field and GR theorists who don't do a thorough check of their model with astronomical data. Also, it's important that we have a simple framework within which we can work to test the models. It's unrealistic to expect observers to independently evolve every alternative gravity model out there to check with their data.


    No, in fact, the paper says that they still require dark matter.
     
  5. Feb 19, 2006 #4

    Chronos

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    I would be bold enough to assert dark matter has not been ruled out by any existing model. In some respects, however, it is even more difficult to rule out dark energy. Unless GR is very wrong, there is no viable alternative that explains expansion, much less accelerated expansion in the observable universe. WMAP and SN1a studies offer independently derived, and powerful evidence supporting this conclusion.
     
  6. Feb 19, 2006 #5

    Chronos

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    Unfortunately, no one else seems to be interested in this discussion. I kinda miss the crackpots. Lurkers? I think there are plenty of them. That is annoying. If you have something to say, just say it. I quit, I am tired of this nonsense.
     
    Last edited: Feb 19, 2006
  7. Feb 19, 2006 #6

    vanesch

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    There you have it ! First we clean everything up, crack down on crackpots, and now we miss them ! :rofl:
     
  8. Feb 19, 2006 #7

    selfAdjoint

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    Well, I've been lurking - I thought the paper itself was substantively handled by Garth - but I will comment on this:

    What you need here is a middleman. In particle physics such middlemen are called phenomenologists; they specialize in working out the hopefully empirical consequences of abstract theories. It is a valuable job description there, and the current proliferation of different explanations for dark matter and dark energy suggests that it would be a productive specialty in cosmology.
     
  9. Feb 19, 2006 #8

    Garth

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    [Second attempt, my computer crashed just as I was trying to post at the first :mad:]

    The evidence from SN1a would be powerful and more robust if:
    a) We knew definitely what SN1a actually are.
    b) We knew that there was no evolution over cosmological time scales and therefore we knew that they can be taken as 'standard candles'.
    c) That all IGM absorption has been properly accounted for.
    d) That the degeneracy between expansion (R(t)) and curvature (k) effects has been robustly resolved.

    The WMAP evidence for a spatially flat universe would be powerful and more robust if:
    a) There was no 'axis of evil'.
    b) There were no low l-mode anisotropy deficiencies, particularly with the quadrupole.
    c) That conformally flat geometries, which could explain these deficiencies, and would also affect the interpretation of the SN1a apparent luminosities (expansion/curvature?), have been ruled out.

    Furthermore, as far as:
    is concerned, consider Figure 2, pages 23/4 in Perlmutter et al's seminal paper Measurements of [itex]\Omega[/itex] and [itex]\Lambda[/itex] from 42 high-redshift supernovae


    Garth
     
    Last edited: Feb 19, 2006
  10. Feb 19, 2006 #9
    Oh! Now you want crackpot ideas to consider? Fine, here's mine:

    "Dark energy" may be nothing more than an apparent effect of relativity at a distance. View this thread: Are we in a black hole? Particularly note this reference from the current issue of NewScientist: "Life inside a black hole"

    "Dark matter" may be nothing more than a macroscopic effect of the Heisenberg uncertainty principle. Basically, VPs pop in, emit a graviton, absorb a graviton, and pop out. At the edges of large regions of space, they exchange these gravitons with ordinary matter. The matter reacts to the absorbed gravitons, but the VPs simply take the gravitons from ordinary matter as a necessary replacement in order to pop out. Conservation holds, and yet galactic orbits are affected.

    See? Anyone can come up with a zany hypothesis that isn't testable locally. Why should mine be any less valid? At least mine doesn't require any unexplainable/unknown mass and/or energy. Remember Occam's razor?

    P.S. To the moderator: I can provide references to establish the validity of the concepts expressed in this post if required. However, I'd like it to be understood that the context of this post was in response to questions raised above and is not intended to present any new theory.
     
    Last edited: Feb 19, 2006
  11. Feb 20, 2006 #10

    Chronos

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    That's more like it! Thanks SA. Garth raises the issues that are truly worth discussing - the ones I find interesting. Starting from the top - SN1a are believed to be accretion events. Your basic binary star system where a white dwarf star is accreting mass from it's big, red giant buddy. After accreting a certain mass of hydrogen [?], it detonates. The resulting explosion may, or may not destroy the white dwarf. A few of them have been known to explode more than once.
     
  12. Feb 20, 2006 #11

    Garth

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    That is the standard model, however, there are more than one possible routes to such a conflagation such as A New Evolutionary Path to Type Ia Supernovae: Helium-Rich Super-Soft X-Ray Source Channel
    A different elemental relatative abundance would be expected to produce a different luminosity.

    Garth
     
    Last edited: Feb 20, 2006
  13. Feb 20, 2006 #12
    Okay, so you didn't like my crackpot hypothesis. Sheesh, no sense of humor around here!

    Anyway, here is an interesting hypothesis published in The Scotsman that doesn't require dark matter either. Unfortunately, since it throws Newton and Einstein out the window, I doubt it'll be seriously received.
     
    Last edited: Feb 20, 2006
  14. Feb 20, 2006 #13
    It says that they're "theorist".Sounds like it's short for crackpot theorist...
    It thorws both Einstien and Newton out the window:surprised That's pretty thowring out 2/3 of physics and pretty half of cosmology...These crackpots are starting get support popluar secience this isn't good:uhh: ...
    Edit:Diffently a crackpot theory
     
    Last edited: Feb 20, 2006
  15. Feb 20, 2006 #14
    scott1,

    Ah, but they're crackpots with degrees that got published! Isn't that all that matters around here? :rofl:

    In seriousness, I have a problem understanding the dark matter hypothesis. Isn't dark matter supposed to consist of weakly interacting massive particles (WIMPs)? If so, how do scientists explain the stability of such a particle. Wouldn't Hawking radiation preclude this hypothesis?
     
    Last edited: Feb 20, 2006
  16. Feb 20, 2006 #15

    Nereid

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    One thing at a time ...

    "DM" is what you need, in the form of 'mass' (per Newton, or Einstein) to account for things like the observed X-ray emission from rich clusters, the observed 'peculiar' motion of galaxies in said clusters, and the observed gravitational lensing in said clusters (the estimates of the amount of such Newtonian mass, from the three independent sets of observations, are consistent, within the error bars).

    What the 'mass' is is the subject of a very great deal of work. Certainly some of it is boring old baryonic mass (dust, gas, pebbles, rocks, rogue planets, brown dwarfs, red dwarfs, white dwarfs, ...) - though these are usually excluded in modern analyses (cf Zwicky's landmark papers). There are some interesting reasons to think that some of the 'non-baryonic' mass may be sparticles.

    So, it depends on your taste - the observations are sound, the explanations variously interesting, crazy, compelling, ... - but if you fail to understand just what sort of shorthand 'dark matter' is, you will be lead willy-nilly into making some pretty darn silly statements.
     
  17. Feb 20, 2006 #16
    I understand that "dark matter" is a term used to plug a hole in theory versus observed reality, as is "dark energy." Basically, either the standard model is wrong, or there's something out there, correct?

    "Sparticles" are an interesting concept of SUSY, but as far as I know no one has detected any. I suppose they might when the LHC gets fired up. However, I think I read somewhere that they supposedly only existed for only a fraction of a second during the big bang. Is this correct?

    I was referring to the common references that dark matter might be micro-blackholes. If they are, wouldn't this blow a hole in Hawking's hypothesis? Also, considering that sparticles might be like a thousand times more massive than ordinary matter, shouldn't they fall apart too (rapidly radiate away)?
     
  18. Feb 20, 2006 #17

    Nereid

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    Well, it's old news that the Standard Model (of particle physics) is incomplete - there is a good article by Gordon Kane (sp?) a few years ago, in Scientific American, that covers the bases pretty well (neutrino oscillations will do as a good example for now).

    And isn't it wonderful? There are wonderful things still to learn about the universe! :smile: :cool:
    Who knows? If you've not captured one and sat it down on your lab bench, how do you know what it looks, tastes, smells, and feels like? Maybe DM is axions, or wimpzillas, or something truly wonderous that we haven't even got the faintest hint of so far?

    Again, it's old news that the universe is capable of generating tonnes of particles with energies that will make the LHC running flat out to make just one or two barely noticable (you do know, don't you, how many OOM - in energy - the most energetic cosmic rays detected so far are above that of the best the LHC will possibly reach? IIRC, it's something like 9). If you've got an amateur astronomer friend, ask them to point their telescope at a BL Lac object, and take a peek .... you'll be looking 'down the barrel' of a cosmic gun that emits in one second more high energy particles than the LHC is likely to produce if it ran for a billion years (I'm guessing).

    Have you read up on the history of the neutrino? How Pauli 'created' it? How it was many, many decades before any were detected? How many, many more decades passed before the 'solar neutrino problem' was solved? Is there some requirement that all unknowns in science be solved within a year or two of discovering 'something fishy' going on? (If you don't like neutrinos, think of Darwin/Wallace and Crick et al.)
    Maybe they are (though so far no Hawking radiation has been detected), maybe they aren't (how many ways can one say "We don't know yet"?). And what "Hawking hypothesis"? And why a hole?
    I guess that depends somewhat on what sparticles are, doesn't it? Perhaps the LSSP (lightest super-symmetric particle) is stable?
     
  19. Feb 20, 2006 #18

    Nereid

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    You seem to have skipped over something rather important.

    There are three, independent methods that give the same result, re 'dark matter' in rich clusters. In other words, whatever it is that's there, it behaves like mass.

    I think it's just too easy to be blase about this ... the challenge is of developing an alternative (to the sets of independent observations pointing to something which behaves just like mass) is extraordinarily difficult. What is it that leads folk to say 'plug a hole in theory' rather than to marvel at how powerful the theories we have actually are? Where do such muddle-headed attitudes come from?
     
  20. Feb 21, 2006 #19

    Chronos

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    I second Nereid's point. How long did it take to experimentally confirm the existence of the atom after its existence was postulated [a couple thousand years as I recall]? Unexplained gravitational affects that have been found using many different techniques, as Nereid pointed out, is what led scientists to conclude [reluctantly] there is a huge amount of some sort of hitherto unknown form of matter in the universe. This suggests our theories are incomplete, but, not necessarily wrong. Standard theory does not forbid the existence of cold dark matter, it merely never anticipated it. I call this a discovery, not a flaw in the theory. Isn't this how Pluto was discovered - detection of anomalous perturbations in Neptune's orbit? Did this not present scientists with the same dilemma - choosing between the existence of an hitherto unseen mass, or that Newtonian gravitational theory was wrong? History has taught us it is unwise to throw out the old model until it has been proven flawed beyond a reasonable doubt.

    Attempts to reverse engineer observations into a new, improved theory [e.g., MOND] have not been terribly successful to date. They create at least as many headaches as the mainstream model. That is not what I would call progress. Getting the right answer is not as important as knowing why it is the right answer.
     
    Last edited: Feb 21, 2006
  21. Feb 21, 2006 #20
    You bet! What would be the fun of knowing everything? Where would the adventure be?

    Right, like I so flippantly stated above, everyone's hypothesis is about as good as the next at this point, don't you think (like my Dad used to say, "it's anyone's guess")?

    Yeah, but these cosmic rays have a lot of relative momentum to earth and the solar system. If cosmic rays create micro-blackholes (MBHs) in our system, the MBHs should pass right on through, right?

    Oh sure. It's a good story (and a great demonstration of the veracity of QM). I still remember seeing pictures of neutrino trails(?) in my 4th grade science class!

    I was referring to Hawking's blackhole radiation hypothesis. If they don't evaporate, then his hypothesis is incorrect. And, if they don't evaporate, wouldn't this mean that creating them at CERN might not be such a good idea (considering that they might have no relative mometum to the earth)?

    Who knows? I certainly don't. All I really know is that I know less than I thought I knew! :)

    Muddle headed? Perhaps. I certainly apologize if I seemed blase about it. I have the greatest respect for the work that's gone into the development of our understanding of physics. If the standard model didn't exist, then no one would've known to look for anything, right?
     
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