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Dodging dark energy

  1. Oct 3, 2004 #1


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    Two new October papers.
    the first is notable because of the reputation of some of the authors. Sean Carroll and Michael Turner are among a handful of the most prominent mainstream cosmologists.
    here they are trying to see if one can avoid the need for dark energy by a modification of the Einstein-Hilbert action.
    A slight modification of the law of gravity in General Relativity which is only noticeable over very long distances.

    Sean M. Carroll, Antonio De Felice, Vikram Duvvuri, Damien A. Easson, Mark Trodden, Michael S. Turner
    The Cosmology of Generalized Modified Gravity Models
    27 pages, 7 figures

    "We consider general curvature-invariant modifications of the Einstein-Hilbert action that become important only in regions of extremely low space-time curvature. We investigate the far future evolution of the universe in such models, examining the possibilities for cosmic acceleration and other ultimate destinies. The models generically possess de Sitter space as an unstable solution and exhibit an interesting set of attractor solutions which, in some cases, provide alternatives to dark energy models."

    The next paper, though not about obviating dark energy is similarly off-beat. It is noteworthy partly because it is by Ted Jacobson and David Mattingly, both prominent in testing quantum gravity---it was their paper on Crab Nebula synchrotron radiation that effectively disposed of "preferred-frame" approaches. Or so we thought. Here they are probing what looks like yet another possible avenue to violate or distort Lorentz symmetry.

    C. Eling, T. Jacobson, D. Mattingly
    Einstein-Aether Theory
    17 pages, to appear in the Deserfest proceedings (World Scientific)

    BTW, for those who like guessing-games with pretend money here
    you can see who is ranked highly as a prospect, in the opinion of
    the players.
    Last edited: Oct 3, 2004
  2. jcsd
  3. Oct 3, 2004 #2
    Just a moment... Perhaps I'm on the fringe, but has anyone stopped to consider that: if there is an event horizon, then it is the same for all point in the universe. And as matter disappears behind the event horizon of each point, that point has less gravitational potential energy. Since it no longer feel the force of gravity from the galaxies disappearing behind the event horizon, this is equal to an increase in a repulsive force that is added to gravity. If matter only disappear behind the event horizon, then there is alway an increase in this repulsive addition. And this means that the potential derived from this added repulsive force is and has always been positive. OK, isn't this the same as adding a cosmological constant to Einstein's field equations? It seems to me that this would be a cosmological constant that is very small at first, since little mass escapes behind the event horizon. But it would grow as mass is lost in ever greater emounts but would never get larger than the energy-momentum term and would eventually lead to a zero source solution.
  4. Oct 5, 2004 #3
    The paper were Einstein-Aether theory was first proposed is this:
    "Einstein-aether waves"
    Wow, an aether theory that adds the name of Einstein to its name. Albert must be turning in his tomb
    Last edited: Oct 5, 2004
  5. Oct 5, 2004 #4
    Or is it the Hubble sphere beyond which galaxies are disappearing from our present view?

    In any event, if the disappearance of mass behind the horizon which causes the loss of negative gravitational potential (on average), is equal to an increase of a positive potential at each point, then does this increase in potential energy (=mass?) at each point cause particle creation in otherwise empty space? Does the information lost behind the horizon return to us in particle creation throughout the rest of space?

    When we talk about black holes, is it not supposed (correct me if I'm wrong) that the information lost behind that horizon is gained by the retention of some virtual particles near the horizon? If so, then since every point in space has a cosmological horizon, then perhaps the info lost behind the cosmological horizon is returned by the retention of some virtual particles at all points in space?

  6. Oct 6, 2004 #5
    So, continuing to think along these lines, then during inflation, much more potential energy is disappearing behind the horizon as the unverse expanse much more rapidly than now. Does this account for a very much faster increase in potential energy which corresponds to most of the matter in the universe being created at that time? I suppose it does not matter which came first. Might it have been just as easily stated that expansion is caused by the great amount of matter that was being created at that time?
  7. Oct 7, 2004 #6


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    I am uncomfortable with an event horizon that allows physical structures to exit or enter. I know of no theory that proposes/allows that to happen. A simplified version is a black hole. No observer on either side of the event horizon can see what happens on the other side.
  8. Oct 7, 2004 #7


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    Interesting. It is my understanding that concerning a standard (Schwarzschild) black hole, someone within the horizon can still see what happens outside the hole, as there is nothing preventing light from outside to reach him. It is only the outside that cannot know what goes on inside. There is a good chance I am mistaken, however, seeing as I am not an expert in GR.
  9. Oct 7, 2004 #8
    If there was an "inflationary" phase of expansion, then at one time some portions of the universe were traveling faster than light with respect to other portions of the universe so that light sent from one portion was not visible to other portions of the universe. So at least during inflation there was a horizon for each point beyond which there was no communication.
  10. Oct 7, 2004 #9
    But our cosmological event horizon is at a redshift of z=1'8 (comoving radial distance of 16 Gly) and actually there are galaxies crossing it. when they'll cross it, whatever pass to them will be never observed by us
  11. Oct 9, 2004 #10
    And what horizon is it that when they cross, they are not visible to us, not to imply that they may never be seen again, but only that we do not now see them? Thanks.
  12. Oct 9, 2004 #11
    I just heard that the cosmological constant is a coupling constant is some purterbative expansion of some QFT and can be interpreted as a mass? Is this true? Where can I learn more about this? Thanks.

    Wouldn't that be interesting? That would mean that the GR effect of expansion may be responsible (or may be an equivalent expression for) QFT, right?
  13. Oct 9, 2004 #12
    will the central mass of a BH be effected by the mass outside the BH
    will two BHs orbit eachother outside the event horizons of both
    it sure looks like gravity can pass thru an event horizon

    so "IF" our big bang was a local event, and other big bangs happen outside our univerce why couldnot their gravity pass thru our big bangs event horizon to
    power the observed expansions in our univerce esp if curved space is not limited to lightspeed time limits

    so maybe we are seeing effects of things beyond the limits of our event horizon
    but calling them dark energy rather then what they truely are, gravity
  14. Oct 9, 2004 #13


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    You are correct. I was careless in making my point. In the case of the observable universe, the equivalent of an event horizon exists at z ~ 1100. With the exception of relic neutrinos [courtesy of Nereid], there is nothing left to see beyond that point. And since that barrier has and will always recede faster than less distant objects, nothing will ever cross that barrier. While objects we currently see may someday become too distant and faint to be seen, they will never exit the observable universe.
  15. Oct 9, 2004 #14
    According to the Robertson-Walker metric of the universe, the Hubble sphere is the distance from us that is receding faster than the speed of light. Any light emitted at that distance will never reach us.

    I would be curious to know... given the distance to the event horizon (or is it the Hubble sphere), and the density of all matter in the universe (or at least baryonic matter), and the Hubble constant of the expansion rate, is it possible that a conservation law applicable only within the horizon require that the amount of mass and/or energy leaving the horizon must be equal to the mass/energy associated with the cosmological constant? Maybe someone intimate with the numbers involved can do the calculation. Thanks.
  16. Oct 9, 2004 #15


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    I disagree with that interpretation.
    It is not an event horizon if structures are free to depart.
  17. Oct 10, 2004 #16


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    Current LCDM models (flat, infinite U w/ accelerating expansion) have what is called a "future event horizon". If you do some Google searching, you will find some descriptions and diagrams that will explain the concept better than I can. According to these models, after some arbitrarily long time, the only galaxies visible from our viewpoint will be those of our local group. Here is a non-technical explanation of Abraham Loeb's work.


    I personally do not subscribe to the LCDM matter model, but you defend it quite vigorously, so I'm surprised that you do not embrace the concept of the future event horizon. If you do not accept the concept of the future event horizon, you might have to modify standard cosmology by modeling the universe as closed like this author does:

    Last edited: Oct 10, 2004
  18. Oct 10, 2004 #17
    I thought it was defined as an horizon only if matter did disappear behind it.

    But I understand your question. Since we can see the CMBR which is the farthest thing from us and thus the fastest moving thing away from us, then we should be able to see anything less distant, namely everything. But I also wonder if the CMBR is not instead the radiation from every point in space near and far. As I understand it, light was first able to travel across the entire universe when things cooled down enough for nuclei to capture electrons to form stable atoms. Since the temperature of the unverse was pretty much the same everywhere, this recombination happened everywhere pretty much at the same time and photons started to travel across the universe from every point in space in all directions. So the photons were evenly distributed throughout all of space at recombination. Then as the universe expanded, the photon soup remained evenly distributed throughout, but the density of photons decreased with expansion until now they represent only about 3 degrees Kelvin. So the CMBR does not represent the farthest object that we can see. They represent a "background" that exists everywhere amongst which stars and galaxies are placed. Others are welcome to correct me if I'm wrong.
    Last edited: Oct 10, 2004
  19. Oct 10, 2004 #18


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    It is a horizon if we cannot see behind it. If we cannot see objects today they are behind our particle horizon, if we will never be to see objects ever they are behind our event horizon.

    I'm not correcting you only clarifying what you have said. The CMB consists of a "background" of photons emitted at the surface of last scattering, the glowing fog of ionised hydrogen before the electrons and protons combined; in the foreground of which are placed the galaxies etc.
  20. Oct 10, 2004 #19
    The graphs I've seen (available on line) show the particle horizon presently at about 45 Billion light years away whereas the event horizon is only about 15Gly away and the Hubble sphere is about 13.5Gly away. If at the time of the BB (about 13.5Gyrs ago) there were objects that emitted photons at the particle horizon some 45Gly away we would be able to see their light. However, there were no objects that far away at that time, so we do not see them.

    Can we see any kind of lensing effect on the "background" of the CMBR caused by a "foreground" of distant galaxies? I've never heard of such a thing.
  21. Oct 10, 2004 #20


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    I think this may be as much a matter of semantics as substance. The cfa paper is saying exactly what I thought I was saying [in an apparently bumbling way]. There is no horizon that objects we presently see will some day cross and suddenly disappear from view. There is, and always has been an observer horizon, but that horizon continuosly recedes and is forever beyond the reach of any object inside of it.

    Bear in mind that not only are distant galaxies increasingly red shifted, they are increasingly time dilated. We are watching them in slow motion. Cosmologists routinely factor this in when plotting the light curves of distant type Ia supernova [they do not fade as quickly as relatively nearby ones]. Enormously red shifted galaxies, whether now or in the future, are / will be so severely time dilated, they will appear to be virtually frozen in time. Ultimately, they will simply fade from view as they become to faint to be seen, not suddenly vanish as if they crossed some arbitrary horizon.

    What I am saying is we can already see everything that will ever be possible for us to see [but only as it looks in our current particle horizon]. Since the CMBR photons have already made it here, and it is probably safe to say there was no large scale structure prior to recombination, it seems reasonable to conclude that photons emitted from structures that subsequently formed have also had time to reach us. Will new galaxies come into view in the future? Of course. But they will not suddenly pop into view fully formed as if they crossed some arbitrary horizon. We will watch them coalesce out of primordial gas clouds already visible to us and light up star by star.
    Density is not the issue, CMBR photons are redshifted to 3k due to expansion. Read this and see what you think.
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