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Accelerating universe?

  1. Mar 23, 2006 #1
    I've heard that in the late '90s two independent groups discovered experimentally that the expansion of the universe is in fact accelerating, which might imply a non-vanishing cosmological constant in Einstein's relativity.

    So first of all, can anyone provide some more information about these experiements? Which groups made the discovery?

    And second, is this discovery commonly accepted within the scientific community? Do theoretical physicists assume a priori that the universe is accelerating outwards, and that their theory needs to explain this?

    Thanks,
    Chen
     
  2. jcsd
  3. Mar 23, 2006 #2

    Garth

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    In 1998 Saul Perlmutter, team leader of the Supernova Cosmology Project at Lawrence Berkeley National Laboratory, found distant super-novae type Ia at around red shift z ~ 1 to be fainter than expected.

    Therefore, these SN may well be further away than previously expected in the Friedmann cosmological model. If this is the case then that would mean the universe has not decelerated as in that model and may well have accelerated in its expansion since that light was first emitted.

    Such behaviour can be explained by the presence of negative pressure. (Positive pressure, has a positive mass equivalent that causes the universe, counter-intuitively, to increase its deceleration). This is delivered by something called Dark Energy that has an equation of state [itex] p = \omega \rho[/itex] where [itex]\omega[/itex] was negative. Fitting this parameter to the SN Ia's apparent magnitudes indicates that [itex]\omega \sim -1[/itex] and indeed the universe was shown to accelerate rather than decelerate.

    The universe appears to be flat, (see the recent threads on WMAP), which in the Friedmann model requires a cosmological density equal to the critical density [itex]\Omega = 1[/itex] (or very nearly so), yet the observed density of all forms of matter is only about a third of this, so the density of this Dark Energy makes up the difference. This can be considered a verification of its existence.

    The latest WMAP3 release indicates that the universe consists of, (by mass)
    So this DE is said to make up over 3/4 the density of the universe!

    As we have not identified either DE or DM in the laboratory this is somewhat an unsatisfactory state of affairs, nevertheless it does explain why the universe could be accelerating in its expansion.

    The presence of DE would also open out the short time scale of the early, high z, universe in the Friedmann models and explain how large scale structure had time to develop, together with the earliest stars and galaxies. (BTW the DM is also necessary to assist in the collapse of those structures.)

    Garth
     
    Last edited: Mar 23, 2006
  4. Mar 23, 2006 #3

    mathman

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    In previous reports, the ordinary matter fraction was about 4.5%, with an error estimate less than 0.5%. The change to 1.5% seems pretty drastic! An independent check is supposed to include light element (H1, H2, He4, etc.) ratios. Is 1.5% consistent with these data?
     
  5. Mar 23, 2006 #4

    SpaceTiger

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    It was a mis-type. The number is still around 4%. See the WMAP thread for details.
     
  6. Jun 15, 2006 #5

    Garth

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    Juan as selfAdjoint said in the dark matter, dark energy & gravity thread you have posted this text several times, however its rightful place is here, so that is where I shall answer it!

    You are correct, hyperbolic space will cause initially parallel light rays to diverge and objects will appear smaller, further away and fainter than in flat space, similar to a concave lens effect.

    This effect is convoluted with the expansion rate of the universe, because an accelerating universe will also result in objects that will actually be further away than at the same red shift z in a non-accelerating or decelerating universe.

    There is always a degeneracy in the interpretation of cosmological effects, however in this case both the curvature and expansion effects have already been taken into account in the models that then are compared to the observations of these distant Type Ia supernovae.

    The standard model fits several parameters to the data, not only that of these standard candles but also the WMAP data, quasar lensing data and other cosmological constraints.

    That mainstream fit does require DE.

    However as an example of your suggestion see Figure 2 in the primary paper on the subject of high red-shift Type Ia supernovae: Perlmuter et al's paper:Measurements of Omega and Lambda from 42 High-Redshift Supernovae (page 23)
    The ([itex]\Omega_M[/itex],[itex]\Omega_{\Lambda}[/itex]) = (0,0) plot being the empty, “Freely Coasting” model universe which has hyperbolic space and no DE. (It also doesn't require undiscovered non-baryonic DM either, but that is another story!)

    As a caveat I must also add that this ([itex]\Omega_M[/itex],[itex]\Omega_{\Lambda}[/itex]) = (0,0) plot is not such a good fit at higher red-shifts where the supernovae begin to become brighter than expected.

    Notice also that this cosmic acceleration interpretation depends on the assumption that Type Ia supernovae are standard candles. They are standard candles in our own galaxy, which is why this type of supernova is used as such, but that is no guarantee that they remain of the same intrinsic luminosity over cosmological time, especially if physical constants actually vary over such time scales.

    There may be several alternative explanations for these observations.

    Garth
     
    Last edited: Jun 15, 2006
  7. Jun 16, 2006 #6

    Chronos

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    Juan, the problem I see with the hyperbolic curvature conjecture is it predicts a smoothly accelerating universe. As Garth noted, that is not consistent with observations [e.g., Perlmutter].
     
  8. Jun 16, 2006 #7
    Garth,
    Thanks for your wise reply.
    The empty DeSitter model is not a bad fit to Perlmutter data you quoted indeed. But, obviously, it is unphysical in the framework of Friedman equations.
    Consider however this scenario:
    In an infinite and homogeneous Universe, the gravity force on each single galaxy should be practically cancelled out (I do not consider here local, but overall effects) as the vectorial sum of the mass distribution all around up to an infinite distance, so that gravity should not decelerate the expansion due to symetry reasons. If gravity is not controlling the cosmic expansion, no matter which is the density at every moment. Under such simple assumption, a exponentialy growing Universe with a really constant H, i.e. a DeSitter model, could be a feasible way of describing such expansion in a nearly flat, slightly hyperbolic Universe.
     
  9. Jun 16, 2006 #8

    Garth

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    With space-time curvature, i.e. GR, the universe does not have to be infinite, just unbounded.

    This is the situation for each of the Friedmann models, gravity does affect the evolution of the universe, the universe is symmetric in space but not in time. We observe this gravity effect as red-shift, which is interpreted under the GR paradigm as cosmic expansion.

    Garth
     
  10. Jun 21, 2006 #9
    Yes, but assume universe is infinite, as it could be in fact...even within GR. Could my earlier reasoning be ruled out?

    Furthermore, if gravity is not decelerating the expansion, the GR paradigm would be useless regarding this issue (even if GR is correct wherever aplicable).

    By the way, the Hot Big Bang model, in its modern form, interprets redshift as a measure of the change of the universe scale factor since the emission of the observed light, rather than as an effect of gravity.
     
  11. Jun 21, 2006 #10

    Garth

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    That hyperbolic space makes objects look further away than Euclidean space? Sure.
    Not, as we have said, if DE had negative pressure.
    That scale factor is an effect of gravity, or to be precise, an effect of the curvature of 4D space-time in an homogeneous and isotropic universe.

    Garth
     
  12. Jun 22, 2006 #11
    I think the problem is you can't or you don´t want to imagine the possibilities that there is no DE or that gravity could not decelerate the Universal expansion...
    But DE is not granted. For instance, up to a few years ago DE was not invented in order to explain the accelerating expansion, and Big Bang theory was confortable without DE.
    Furthermore, an empty DeSitter universe would indeed expand (i.e. increase the scale factor) even without any effect of gravity (since there is no mass on it). So, there are examples in wich you can get out of the standard paradigm...Please try it.
    Thanks for your comments.

    Juan
     
  13. Jun 22, 2006 #12

    Garth

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    You obviously haven't seen my own work...Self Creation cosmology
    The standard model does treat the possibility of hyperbolic space and the empty universe properly, it's just that it finds (WMAP) the universe is flat or nearly flat (possibly 'just' spherical).

    Garth
     
  14. Jul 4, 2006 #13
    I don't see how the universe can be infinite and expand. How can infinite expand? If the universe is finite in time it must be finite in space. Infinite cannot expand out of a singularity which was finite to begin with.
     
  15. Jul 4, 2006 #14

    Garth

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    Every point within the universe moves away from every other point. That is the expansion.

    Every observer sees themselves as the centre of the expansion.

    The universe will only be non-infinite if it is spherical, otherwise, if it is homogeneous and isotropic, it will be infinite.

    In which case the infinite expands - after all [itex]\infty \times x = \infty[/itex]

    Garth
     
  16. Jul 8, 2006 #15
    Early in the Universe's development, there was actually a period of deceleration caused by gravitational 'braking' between relatively near galaxies (relatively near because the Universe hadn't enough time to sufficiently grow to make intergalactic gravitational attraction negligible). Because the Universe was so small back then, Omega-Matter was actually greater then Omega-Lambda, but at present Omega-Lamda is greater, whence the Universe's acceleration.
     
  17. Jul 9, 2006 #16
    One question, if the Universe is accelerating, thus expanding faster and faster, does that imply time is moving faster?
     
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