Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Addressing Impossibilities in the Standard Cosmological Model

  1. Dec 30, 2005 #1

    turbo

    User Avatar
    Gold Member

    some examples:
    1) inflation that turns on, then magically switches off simultaneously in causally-disconnected parts of the universe.
    2) DM
    3) DE
    4) cosmological expansion that slowed nicely for billions of years, then accelerated (again, simultaneously in causally-disconnected parts of the universe)
    5) Higgs boson
    and you can add any other number of entities depending if your model depends on supersymmetry,etc.

    This is taking the thread way OT, but I can offer you observational proof in one paragraph that the Higgs Boson cannot exist. As such, there is no need for me to learn all the arcana that led to the Higgs hypothesis.
     
  2. jcsd
  3. Dec 30, 2005 #2

    Evo

    User Avatar

    Staff: Mentor

    This post was split off from the "I think popular science is ruining science" thread in GD.
     
  4. Dec 30, 2005 #3

    turbo

    User Avatar
    Gold Member

    Thank you Evo. Things were getting OT, but I could not let them go unanswered.
     
  5. Dec 31, 2005 #4

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Far from being "magical", there are many workable (consistent with known physics) theories of inflation. The procedure of both entering and exiting inflation are described in great detail here:

    Inflation

    In particular, I recommend the link in post #19.

    So what are the caveats? Well, the fields responsible for inflation obviously haven't been observed in the laboratory and it seems pretty unlikely that they will be in the near future. In order to produce the universe we currently observe, it had to decay into those things that are now the primary constituents of our universe (an event called "reheating"). Some theories of inflation do have predictions that are testable in the laboratory, but I'm afraid they're the minority. In order to test inflation, we'll likely have to resort to astronomical observations. If anyone is interested in the details of these observations, I'll be happy to elaborate. Perhaps hellfire will be too, since he seems to know quite a lot about the subject. :smile:


    Again, there are a lot of theories for this. The idea that it might consist of weakly-interacting particles is the most popular. We already know of particles that are weakly interacting and make up a non-negligible fraction of the energy density of the universe -- neutrinos. Considering the trouble we had to go through to detect these directly, it shouldn't be all that surprising if the dark matter particle has so far escaped our notice.

    Many supersymmetry models predict a weakly-interacting particle. Not only is this not impossible, but I would even say that it's likely. We know that dark matter exists. The question now is, "is it enough?"


    The most popular explanation for the acceleration of the universe (which some include in "dark energy") is the cosmological constant. Far from being exotic, this is actually a very simple component of GR.

    That said, mainly because of the fine-tuning problem, I have my doubts that it's the culprit for the current acceleration. It doesn't make the cosmological constant "impossible", as you seem to be suggesting, but it does leave it suspect. I would be more prone to suspect some kind of quintessence, which only suffers from the less severe "cosmic coincidence problem".


    This is basically equivalent to the dark energy point...and the causally-disconnected part is addressed by inflation.


    The Higgs boson is a prediction of the standard model of particle physics, not the standard cosmological model. I'm pretty sure that LCDM could do without it, though some (not all) of the particle dark matter and inflationary theories would be ruled out by the lack of a Higgs boson

    This thread is extremely rich with topics for discussion, so you should probably be more specific about what you'd like to understand. So far, I've only given a quick summary of why these things are not impossible. If you want to discuss observational constraints or other theoretical concerns, let us know.
     
    Last edited: Dec 31, 2005
  6. Dec 31, 2005 #5

    Garth

    User Avatar
    Science Advisor
    Gold Member

    We must not let the success of a posteriori adjustments to make the standard model fit the data blind us to possible alternative theories.

    Inflation arises as a prediction of GUT and resolves the horizon, smoothness, density and magnetic monopole problems of the 1970's. The fact that the Higgs boson or an alternative inflaton predicted by those theories has not yet been found, that the subsequently necessary non-baryonic DM has not been identified and cosmic acceleration was only 'post-dicted' by the invocation of DE, is not to be forgotten in assessing the status of the mainstream model.

    Testable, viable and concordant alternatives are therefore to be evaluated alongside the standard model.

    Garth
     
  7. Dec 31, 2005 #6

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Who's saying we should ignore alternatives? This thread is about whether or not the standard model is "impossible".
     
  8. Dec 31, 2005 #7

    Garth

    User Avatar
    Science Advisor
    Gold Member

    True, but I just thought the point ought to be made. A polemic can easily become polarised, I agree that there is nothing impossible about the standard model; just improbable IMHO! :wink:

    Garth
     
    Last edited: Dec 31, 2005
  9. Dec 31, 2005 #8

    turbo

    User Avatar
    Gold Member

    This might be a good time to ask how everyone treats these improbabilities mathematically. If the correctness of one improbable hypothesis is also dependent on the correctness of another improbable hypothesis shouldn't their probablilities be multiplied? In other words, if there is a 1% chance of A being true and a 1% chance of B being true, and if both A and B must be true to make the model work, the model's viability has been reduced to no more than .01%

    The improbable things in my list above are not independent - they are inextricably linked and are model-dependent. The more such entities a model requires, the less trustworthy it becomes.
     
  10. Dec 31, 2005 #9

    turbo

    User Avatar
    Gold Member

    OK, let's take the "impossible" things one at a time. Let's start with Dark Matter.

    GR gravitation is not predictive on galactic and cluster scales. Gravity in these massive domains seems to be far stronger than GR predicts. If we assume that GR is infallible, we come to the conclusion that there must be about 10 times more matter in each of these systems than we can detect observationally. This is a serious problem, because we are already pretty adept at detecting stuff and if DM was made out of any stuff that we are currently aware of, we would already have detected it. The way out of this problem is to say that DM is non-baryonic and weakly interactive, which leads to yet another problem. If the stuff is so weakly interactive, how does it manage to distribute itself "just so" in every relevant system? How can it form spherical halos with central voids around galaxies and then arrange itself in blobs and tendrils in clusters? This stuff is so elusive that we have never detected it, yet it is remarkably obedient.

    After decades of diligent searching for DM, is it not time to admit that our understanding of gravitation is inadequate to explain the behavior of galaxies and clusters?
     
    Last edited: Dec 31, 2005
  11. Dec 31, 2005 #10

    Garth

    User Avatar
    Science Advisor
    Gold Member

    Nobody is taking GR as infallible, although perhaps it should be questioned more, however, galactic rotation curves and cluster dynamics are calculated under Newtonian gravity. (Hence the interest in Cooperstock & Lieu). The extra mass required by Newtonian dynamics also lenses (under GR gravity) distant quasars and so is doubly detected.

    Once you get into explaining the detailed galactic structure you also have to include galactic magnetic and possibly electric fields as well as gravitational ones. Galactic MHD anyone? :yuck:

    Garth
     
  12. Dec 31, 2005 #11

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It's "weakly interactive" only for non-gravitational interactions. It still has mass and it will still clump gravitationally.


    The spherical halos are actually a direct consequence of a low interaction cross section. If the dark matter were arranging itself into a disk or some other shape, then we would have a real inconsistency. In the absence of dissipation (which is a form of non-gravitational interaction), the halo of particles should end up with approximately spherical symmetry (for lack of a preferred direction). For another example of dissipationless "particles" that arrange themselves into a spherical configuration, take a look at a globular cluster.

    I'm not sure what you mean by "central voids". There is no hole in the dark matter distribution in the cores of galaxies.


    Actually, this could be viewed as a favorable thing for the standard model. The weak interaction explains many things at once, including the lack of observability, the lack of detectability in the laboratory, and the approximately spherical halos around galaxies and clusters.


    This has been under consideration for a long time (read up on MOND, for example). It doesn't, however, fit the data as well as the dark matter model.
     
  13. Dec 31, 2005 #12
    Excuse me, but I would think that the non-spherical nature of spiral galaxies would effect the distribution of dark matter halos so that the halo would not be spherical either, right?
     
  14. Jan 1, 2006 #13

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    Perhaps it would be more effective to address the evidence in favor of the alternatives.... which is mostly nonexistent. The standard [cosmological] model is not based on a few conjectures, like DM, DE or the Higgs boson, it is based on all of physics. The real pillar of the standard model [cosmological] is GR. Fell that tree and the forest is yours for the taking.
     
  15. Jan 1, 2006 #14

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If I may, can I ask that we distinguish between what's 'solid' in terms of a great deal of good observational or experimental results and what's contained in various mainstream (and not so mainstream) theories?

    For example: for each of turbo-1's five OP points (actually only four, as ST has already noted), what's the status of the observational/experiemental basis?

    Personally, I'd like to begin by getting some consensus on what this is, before we dive into the theories.
    A very good, and very deep question, turbo-1! :approve:

    I suspect that discussing it will take us way, way outside modern cosmology, into the heart of what the nature of modern science is, for example. Personally, I'd love to participate in such a discussion (partly because turbo-1 and I have already danced around the edges of just such a discussion, in another forum).

    How do folk feel about starting another thread, in PF's Philosophy of Science section?
     
  16. Jan 1, 2006 #15

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    I'm game, Nereid.
     
  17. Jan 1, 2006 #16

    turbo

    User Avatar
    Gold Member

    Dynamical simulations have pointed to a cuspy distribution of DM - low density near the galactic core and increasing with radius.
    http://arxiv.org/abs/astro-ph/9710039
    http://www.citebase.org/cgi-bin/citations?id=oai:arXiv.org:astro-ph/0108505

    Regarding a modification of gravity:
    No, MOND doesn't cut it - it is an ad-hoc modification that is predictive on limited scales, but it does suggest that all is not well when we try to apply our laws of gravity to very massive structures. I think that it is instructive to remember that our understanding of gravitation arose from the study of fairly idealized systems, balls rolling down ramps, planets orbiting the sun, suns orbiting one another, etc. Newton's attractive gravitational force and Einstein's curved space-time both work well in these "ideal" domains, but they do not predict the behavior of galaxies accurately, nor can they explain the distribution of tendril-like overdense regions of space discovered by lensing surveys (again neatly attributed to DM).
    http://www.cfht.hawaii.edu/News/Lensing/

    To date, the DM model has accomplished one thing - quantifying in detail the extent to which Newtonian and GR gravitation fail to predict the behavior of large systems of luminous matter. The process has been to 1) measure the matter shortfall (assuming our understanding of gravitation is correct), 2) attribute the missing mass to an experimentally undetectable entity, and then 3) look for the entity.
     
  18. Jan 1, 2006 #17

    turbo

    User Avatar
    Gold Member

    It is done here:

    https://www.physicsforums.com/showthread.php?p=870900#post870900

    I have included a couple of simple examples to illustrate how a model with many parameters, even if each is individually at a high confidence level, may prove to be untenable.
     
  19. Jan 1, 2006 #18

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That's one of several reasons I said "approximately" spherical. You have to keep in mind that we're seeing something like 10% of the total mass of the galaxy when we look at images of spiral galaxies. Although the visible baryonic component has arranged itself into a disk due to non-gravitational forces, the stuff that we can't see (~90%) would only be slightly perturbed by it in the outer parts of the galaxy, where we're most interested in the properties of the dark matter halo. There's still a lot of uncertainty about the baryon-dominated inner parts (partially because of this "cusp" problem turbo mentions), but even there, it's possible to maintain an approximately spherical halo of stars and dark matter.
     
  20. Jan 1, 2006 #19

    Garth

    User Avatar
    Science Advisor
    Gold Member

    [itex]\Omega[/itex]observed matter= 0.003, [itex]\Omega[/itex]baryon = 0.04

    ST what are the latest hypotheses about the dark baryonic matter? What form does it take, what equation of state might it have - dust or gas??

    Garth
     
    Last edited: Jan 1, 2006
  21. Jan 1, 2006 #20

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    There seems to be a slight misunderstanding of "cusp problem" here. It's not that we're seeing a "void" or "hole" of dark matter at the center of galaxies, it's that it's less dense than predicted by some simulations. It's still more dense than in the outer parts of the galaxy. We already discussed this in a recent thread:

    Cuspy Halo Problem


    It certainly was originally, but recently, it has been put into a more complete relativistic framework by Bekenstein:

    Relativistic gravitation theory for the MOND paradigm

    Word is that it's failing to fit the third peak of the CMB:

    Does BOOMERANG hit MOND?


    Actually, this was the whole idea behind MOND, to provide an emperical fit to the galactic/cosmological regime (i.e. small accelerations). Now that it has been extended to a full relativistic theory of gravity, I'm not sure why you think it doesn't fit what you're describing.


    That's incorrect. The dark matter paradigm has contributed to successful predictions about gravitational lensing, the CMB, and large scale structure. It also successfully explains many things we previously knew, such as roughly flat rotation curves and the large velocity dispersions of galaxy clusters.
     
    Last edited: Jan 1, 2006
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Addressing Impossibilities in the Standard Cosmological Model
  1. Cosmological models (Replies: 4)

Loading...