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Cosmic Humility

  1. May 25, 2007 #1
    I would like to discuss this:

    "Now, physicists Lawrence Krauss from Case Western Reserve University and Robert J. Scherrer from Vanderbilt University predict that trillions of years into the future, the information that currently allows us to understand how the universe expands will have disappeared over the visible horizon. What remains will be "an island universe" made from the Milky Way and its nearby galactic Local Group neighbors in an overwhelmingly dark void.
    "We may feel smug in that we can detect a host of things future civilizations will not know about, but by the same token, this suggests we wonder about what important aspects of the universe we ourselves may be missing. Thus, our results suggest a kind of a 'cosmic humility'".

    I couldn't find the artice they wrote online. I guess I have to wait until it is published. anyone have access to it?

    Is this conclusion a surprise to anyone? I mean 3 trillion years from now, the CMB will be redshifted to the point that it is past no longer within our Hubble Sphere?
    Does this treatment assume our universe is not closed?
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  3. May 25, 2007 #2

    George Jones

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  4. May 25, 2007 #3


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    I read it, and really liked it a lot.

    The Return of a Static Universe and the End of Cosmology
    Authors: Lawrence M. Krauss (1,2), Robert J. Scherrer (2) ((1) Case Western Reserve University, (2) Vanderbilt University)
    5th prize 2007 Gravity Research Foundation Essay Competition, to appear, GRG October 2007
    (Submitted on 2 Apr 2007 (v1), last revised 23 May 2007 (this version, v2))

    "We demonstrate that as we extrapolate the current LambdaCDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our "island universe" will be fundamentally incapable of determining the true nature of the universe, including the existence of the highly dominant vacuum energy, the existence of the CMB, and the primordial origin of light elements. With these pillars of the modern Big Bang gone, this epoch will mark the end of cosmology and the return of a static universe. In this sense, the coordinate system appropriate for future observers will perhaps fittingly resemble the static coordinate system in which the de Sitter universe was first presented.

    As you say, it can give one a humble feeling. It can also make you feel grateful that we live in a time where so much of the universe is visible---we live in an information-rich era, when it is possible to find out so much about the universe by observation.

    Things won't always be this good. We are lucky animals.
    Last edited: May 25, 2007
  5. May 25, 2007 #4


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    What do you understand to be the meaning of the term "closed" in cosmology?

    The terms "closed" and "open" cause a lot of confusion because among professionals they changed meaning around 1998.

    It used to be that when an astronomer said "closed universe" he meant two thing: a universe that is SPATIALLY FINITE and a universe that does not continue to expand forever.

    then "dark energy" was discovered which could make a spatially finite universe continue to expand forever.

    So now "closed" is apt to mean spatially closed, i.e. spatially finite, but a spatially closed universe can continue to expand indefinitely.

    The conclusions of the Krauss paper do not depend on assuming "not closed" in this sense----their conclusions apply equally well to a spatially closed and finite universe. As long as it continues expanding according to the effect of cosmo constant Lambda.

    So the answer to your question is No. The treatment does not depend on assuming not closed.
    That is a double negative: so I will say it more simply.
    The treatment covers the case where the universe is closed.
  6. May 25, 2007 #5
    Hi. thank you for your response.
    I was using the definition that if the density is greater than the critical density the universe is closed and finite.

    Two things then
    1. In an open universe, the density is less than the critical.
    In this case, the universe can expand indefinitely (with or w/o dark energy)
    and you also say that in a closed universe (with dark enery),the universe can expand indefinitely as well.
    so what is the observational difference between the two if the end result for both could be the same (indefinite expansion)?
    The geometry of the universe is different. anything else?

    2. Just so i have it straight, you are saying that these results apply for any type of universe, open flat or closed. right?

  7. May 25, 2007 #6
    I know this is highly unlikely and I don't want to sound like a crackpot but I can't help it.
    If trillions of years from now, future physicists were unable to detect the CMB from earth, could they at least know that they coulnd't detect something?
    I ask this, because what if some key piece of information has been redshifted out of our visible horizon? I don't know just makes you wonder. Perhaps we should take the anthropic principle more seriously?
  8. May 25, 2007 #7


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    I don't think so. Just a difference in geometry (which might have some subtle observational consequences. But someone with more specialized knowledge might respond differently.

    You are right to say that the three cases are Omega <, exactly =, or > 1.

    the only new wrinkle is that even when Omega > 1 we can still have endless expansion.
    Yes that is what I am saying, for any realistic case of LCDM, whether it's open flat or closed. I'm pretty certain that's right---and if not someone will surely correct me.
  9. May 25, 2007 #8


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    BTW blumfeld, since you seem interested in the open flat closed issue, you might like to have a look at the March 2006 Spergel et al.
    It is an authoritative paper on the third year WMAP data which was released simultaneously with this paper explaining the implications for cosmology.
    Wilkinson Microwave Anisotropy Probe (WMAP) Three Year Results: Implications for Cosmology
    D. N. Spergel, R. Bean, O. Doré, M. R. Nolta, C. L. Bennett, J. Dunkley, G. Hinshaw, N. Jarosik, E. Komatsu, L. Page, H. V. Peiris, L. Verde, M. Halpern, R. S. Hill, A. Kogut, M. Limon, S. S. Meyer, N. Odegard, G. S. Tucker, J. L. Weiland, E. Wollack, E. L. Wright
    91 pgs, 28 figs. Accepted version of the 3-year paper as posted to this http URL in January 2007
    (Submitted on 19 Mar 2006 (v1), last revised 27 Feb 2007 (this version, v2))

    I underlined Ned Wright and Joanna Dunkley because i especially like them.
    David Spergel is a way prominent guy, but you get the idea already from his being lead author. So is Charles Bennett. this is a blue ribbon paper.

    the remarkable thing is that in one of the figures it gives a 68 percent confidence interval for Omega that is ALL TO THE RIGHT OF ONE. It is something like [1.01, 1.04].
    for several reasons this is not to take too seriously. for one thing 68% is not 95%. But it is suggestive.
    In their conclusions they said "nearly flat."

    Flat is still the dominant picture among rank-and-file cosmologists, for several reasons.

    I should find you the number of that figure, and the page, in Spergel et al.

    Yes, here it is: page 50, Figure 16.
    68 percent interval for so-called Omegak [- 0.041, - 0.01]
    which means that the 68 percent interval for Omega is [1.010, 1.041], or not to put to fine a point on it: [1.01, 1.04]

    A recent Ned Wright paper gives a "best fit" LCDM to all the available data that is relevant, which has Omega = 1.011

    so the prevailing picture among great majority of cosmologist is still flat but there are hints of closed (spatial finite).
    It could be unsettling the field for the next few years, forcing a rethink of inflation or consideration of other ways to solve the
    horizon problem etc.

    EDIT: here is the Ned Wright paper in case anyone wants
    It has the "best fit" Omega = 1.011, thing in a figure or table right near the end.
    for concreteness, what that translates into is a (3D not 2D) sphere with radius of curvature 130 billion LY.
    (for most purposes, flat 3D euclidean space, or call it "nearly" flat :smile:)
    Last edited: May 26, 2007
  10. May 25, 2007 #9
    thanks. i am going to check that out right now.
  11. May 26, 2007 #10


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    So it remains interesting that we live in interesting times. Three trillion years is boring. Something is obviously wrong with that formulation. I would vote collapse [perhaps several], but the jury is still out. The more obvious part is our universe has always been boring and obvious to all observers. But it is not my place to explain why.
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