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Perpetual acceleration of the universe expansion?

  1. Jan 11, 2013 #1
    There is a consensus on a slight acceleration of the expansion in our epoch, mainly from supernovae Ia measurements, but is there any evidence (apart from results from more distant supernovae) allowing to rule out the possibility that the universe had been expanding eternally with a tiny acceleration?
  2. jcsd
  3. Jan 11, 2013 #2
    By characterizing the detailed structure of the cosmic microwave background fluctuations, WMAP has accurately determined the basic cosmological parameters, including the Hubble constant. The current best direct measurement of the Hubble constant is 73.8 km/sec/Mpc (give or take 2.4 km/sec/Mpc including both random and systematic errors), corresponding to a 3% uncertainty. Using only WMAP data, the Hubble constant is estimated to be 70.0 km/sec/Mpc (give or take 2.2 km/sec/Mpc), also a 3% measurement. This assumes that the universe is spatially flat, which is consistent with all available data. This measurement is completely independent of traditional measurements using Cepheid variables and other techniques. However, if we do not make an assumption of flatness, we can combine WMAP data with other cosmological data to get 69.3 km/sec/Mpc (give or take 0.8 km/sec/Mpc), a 1% solution that combines different kinds of measurements. After noting that independent

    This is cut and paste from the following


    Not sure I would call that expansion slight lol. Not sure on early universe measurements though they have confirmed that expansion was slower after the initial rapid expansion of the early universe
  4. Jan 14, 2013 #3
    Thank you, but the different and consistent measurements of the Hubble parameter only determine the present rate of expansion, without saying nothing about the past or the future rates (in fact similar figures were obtained before SNe Ia observations ruling out deceleration at present)...Concerning the above quote, this is the commonly accepted model but ¿How did they observationally confirm the past deceleration of expansíon?
  5. Jan 14, 2013 #4


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    The predictions of big bang nucleosynthesis are sensitively dependent on the expansion rate of the universe, which is determined in Friedmann cosmology (the modern concordance model) by the dominant source of energy (relativistic matter, nonrelativistic matter, vacuum) in the universe at the time. Following the big bang (and/or inflation), the universe was dominated by radiation (which is considered relativistic matter) and expanded as a power law. As the universe cooled, nonrelativistic matter began to dominate. At about this time, the CMB decoupled from the relativistic plasma -- so the very existence and subsequent decoupling of the CMB is evidence that the universe underwent a transition from being dominated by relativistic to nonrelativistic energy densities.

    Additionally, the acoustic peaks in the temperature spectrum of the CMB (as well as the Sachs-Wolfe plateau) provide evidence for a universe that passed from a period of nonrelativistic matter domination to the recent phase of accelerated expansion.
  6. Jan 14, 2013 #5


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    Juan, have a look at this curve. It is the expansion history as generated by the Friedmann equation model. I want to use this to illustrate a point:

    You can see it relates redshift to lookback time (now = zero, start≈-14 Gy, i.e. -14 billion years)

    You can see in rough outline a period of deceleration followed by a period of acceleration, but that is not the point I want to make.

    The point is that in a mathematical science you aren't individually checking a lot of disconnected details, you FIT A MODEL to all the available data and look for the simplest model with the best fit.

    The Friedmann equation is a simplified form of the Einstein GR equation which comes equipped with 3 physical constants, G, Λ, c. (newton, cosmo const. Lambda, and speed of light). The Einstein GR equation is our LAW OF GRAVITY which has been tested hundreds of different ways. The Friedman equation, a version simplified by assuming approximate uniformity, inherits those 3 basic constants.

    The rest of the story is determining best estimates of things like densities of matter and radiation and current percentage rate of expansion---basically adjusting boundary conditions to get the best fit. There are many many different types of data. There is a huge amount of data.

    So you have to take this simple little equation and , by adjusting 3 or 4 numbers, make it fit tons and tons of data of all different kinds of observations.

    Here, in what I'm saying, I'm not trying to convince you of this or that proposition. I want to give an idea of the overall process. The approach is, in a sense, *holistic*. the model is a surprisingly simple equation and it generates the curve I showed you.

    but it also generates curves of TEMPERATURE and a curve of DENSITY, e.g. of matter, or of the ancient light called the "CMB". so all these things have to be cross-checked to see that they are physically consistent.
    the past rates of star formation have to check with what the model says was the past matter density

    the current temperature of the ancient light has to check with the matter density and temperature at the time it originated and the amount of expansion since then. expansion cools light by a known law.

    it is like the *cross examination* at a trial, this very simple equation is the "witness" and everything possible should be examined for consistency.

    ==quote Juan==
    ¿How did they observationally confirm the past deceleration of expansíon?
    I think the answer is that you don't directly observe an expansion rate at some moment in the past, or a rate of change of an expansion rate. You have a remarkably simple equation that gives an amazingly good fit to a huge amount of data. You continually interrogate this equation to make sure the story is physically consistent in every way you can think of. The equation is derived from the accepted law of gravity (=geometry). Alternative laws of gravity are constantly being invented and tried out, so far not demonstrating any advantage. The current consensus model will doubtless some day be successfully challenged, but so far it is passing all the tests people know how to devise. And it is what generates the curves like the one I showed. See also the "Figure 1" link in my signature.
    Last edited: Jan 14, 2013
  7. Jan 15, 2013 #6
    Could you provide any reference directly linking the spectrum of the CMB and the deceleration phase?
    As fas as I know the very existence of CMB only demonstrates that at some moment in the past the temperature was high enough to keep the universe matter in a state of plasma...
    Marcus, thanks for your clear explanation. I was aware of the way the Concordance model fits with most observational data, but to my knowledge only the farthest away (and therefore less accurate) SNe Ia results are direct evidence of the deceleration rate, right?
  8. Jan 15, 2013 #7


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    The CMB constrains the content of the universe at various epochs. Given the content of the universe at a certain epoch, the Friedmann model tells us what the expansion rate was. So the CMB data do not directly constrain the rate of expansion. You are correct that the presence of the CMB indicates high temperatures, but take that a step further. Matter in equilibrium at high T is relativistic -- the constituent particles behave like radiation. This means that the universe must evolve in a particular way, specifically, as a power law: [itex]a(t) \sim t^{1/2}[/itex]. Before the universe cools below the binding energy of hydrogen and the CMB decouples, it passes a point at which radiation and matter are in equal abundance. From this point on, non-relativistic matter dominates the expansion and the universe evolves as [itex]a(t) \sim t^{2/3}[/itex]. The matter content of the universe (both dark matter and baryonic) can be determined by examining the relative heights of the 2nd and 3rd peaks in the CMB spectrum (see Wayne Hu's excellent CMB tutorials for more details: http://background.uchicago.edu/~whu/).

    Lastly, the broad anisotropy seen at large scales in the CMB spectrum (the Sachs-Wolfe plateau) is sensitive to the recent accelerated expansion. The fact that this effect is seen only at large spatial scales is related to its being a recent phenomenon. If the expansion had been accelerating all along, we'd have a very different CMB spectrum. For one, the plateau would extend across all scales, and there would be no acoustic peaks at all.

    For references, I'd recommend you spend some time looking through Wayne's tutorials to see how the CMB gives us insights into the composition and evolution of the universe. Also, any good introductory cosmology text that covers the thermal history of the universe should be helpful.
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