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Acceleration/deceleration of the expanding universe?

  1. Jun 29, 2013 #1
    Apparently the generaly accepted interpretation is that the expansion is speeding up, mainly by observing the redgarbageing of supernovae of distant galaxies and establishing their distance and redshifting indicating the speed at which they are moving away from us.

    The further away the faster they are moving according to observations.

    This as I understand it was seen as proof of an acceleration.

    Which in turn was explained by the invention of dark energy to counter the gravitational force and accelerate the expansion which would otherwise have been decelerated and eventualy possibly reversed if the mass of the universe was great enough to counter the force of the initial "explosion" which would lead to a big crunch.

    For example a galaxy 13 billion Ly away would be observed moving away from us 3 times faster than a galaxy 1 billion light years from us. (no exact numbers just the basic idea of how it was explained)

    While the explanation of darkenergy propelling the universe towards a Rip or fading seems like it could be possible, I'm thinking it sounds like the explanation has become a theory of too much.


    Couldn't it be that the galaxy's speed observed 13 billion years back could have been faster simply because all matter in the universe was moving faster 13 billion years back before gravity had slowed it all down and decelerated the expansion.

    Which by looking at closer galaxies would suggest that the universe is still expanding but not at the same speed it was 13 Billion years ago.
    But then it would indicate a deceleration of the expansion.
    Then you wouldn't need dark energy or at the very least far less of it.

    After all the galaxies 13 billion lightyears away were 13 billion lightyears away and moving at the now observed speed 13 billion years in the past, their current state is unknown but calculations and estimates could be atempted I guess. But basicly it seems we're seeing their speed 13 billion years back while closer galaxys at 1 billion years are recorded at their speeds 1 billion years ago.

    The observed speed/distance spread seems fairly homogenous aswell. The age of the data collected seems to be what determines of their expected speed in the expansion of the universe.
    It's also possible that small deviations of the mass of the observed supernovae could interfere with the data a little even if's generaly a reliable "standard light".

    The idea that the universe is currently expanding seems reasonable enough, but the idea that it's accelerating seems less intuitive.

    Maybe I missed some of the "proof" or maybe it was overly simplified. I've tried to keep it short by not going too much into the expansion of space driven by dark energy and all that. Since my concern isn't the the idea itself but rather that I think it might be, like I said, a theory of too much. a little bit like Einstein tried to create a constant to keep the universe in a static state. Which he later considered one of his biggest misstakes.

    Maybe it's in the interpretation of the observations. Maybe someone could explain the proof provided by the supernova standard lights which tells us the universe is expanding at an accelerated pace.
    As I understand it one can theoreticaly get the distance by measuring the luminosity of the standard light type 1a supernova, and it's speed relative to us from it's redshifting, the acceleration part seems unclear though.
     
  2. jcsd
  3. Jun 29, 2013 #2

    mathman

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  4. Jun 29, 2013 #3
    Hi Peter,
    What is your source of information? I ask because that will help posters here get a better perspective before responding. Your questions may not be so easily/briefly answered. It could take a book or two!!

    COSMOLOGICAL EXPANSION...a one paragraph synopsis

    A short answer is that using a particular cosmological model, the FLRW model and the conventions and assumptions that go into it, we calculate ‘superluminal’ (faster than light] expansion both at really early cosmological times and continuing to today at really great distances. We cannot directly observe such rapid expansion [not because the light will never reach us, although that happens in some circumstances, but because such distances are measured [calculated] along a curve of constant [fixed] cosmological time. This is analogous to instantaneously reading a measure on a fixed ruler.

    Another insight which helped me is this :

    It is currently believed the scale factor [a(t)} is 'accelerating'. But it is slower currently than it has been over cosmological history.

    One of my very favorite discussions in these forums is here:
    [Up through the first 100 posts or so]

    Expanding Universe: Does Space Expand
    [Wallace]
    https://www.physicsforums.com/showthread.php?t=162727&highlight=current+flow&page=2


    Try this "frequently asked questions" which interest you here:

    [It is a reasonably good place to start]

    http://www.astro.ucla.edu/~wright/cosmolog.htm

    Another is this article:

    Misconceptions about the Big Bang: Scientific American 6/14/08 6:50 PM
    http://www.sciam.com/article.cfm?id=misconceptions-about-the-2005-03 [Broken]


    A more advanced discussion was referenced in these forums in another discussion....
    Don't read this one first.

    http://arxiv.org/pdf/1304.4446v1.pdf
     
    Last edited by a moderator: May 6, 2017
  5. Jun 29, 2013 #4
    An illustration of how the scale factor, the relative expansion of space, has varied over time via different
    cosmological parameters is here:

    http://www.astro.ucla.edu/~wright/cosmo_03.htm

    [This is in the is ned wright tutorial I posted previously]

    We are at time ZERO in the plot. Our universe is approximated via the MAGENTA [pink]
    curve...but those factors are now slightly out of date.....

    Here is a nice synopsis of the scale factor, from Marcus of these forums:

     
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