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BOSS questions cosmological constant (undecided)

  1. Apr 8, 2014 #1

    marcus

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    The PF home page has some news items in the righthand margin and one is about this:
    http://arxiv.org/abs/1404.1801
    The popular news account is this BBC piece:
    http://www.bbc.com/news/science-environment-26329320
    "They find that some three billion years after the Big Bang, the cosmos was pushing itself apart by another 1% every 44 million years.

    It is the latest result to come from the Baryon Oscillation Spectroscopic Survey (BOSS).
    ...
    ...
    What is interesting about the new result is that the BOSS-measured expansion rate 10 billion years ago is quite a bit slower than that expected from the standard model of cosmology.

    "This is the most precise measurement that's ever been done, and all I'll say at the moment is that there is a tension there," explained Dr Matthew Pieri, a BOSS team-member from Portsmouth University, UK."
     
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  3. Apr 8, 2014 #2

    marcus

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    If I use Jorrie's calculator to find the expansion rate back at redshift 2.34 (the figure given in the technical article) I get Hubble time= 4.13 Gy
    If you want to check: http://www.einsteins-theory-of-relativity-4engineers.com/LightCone7/LightCone.html
    put in stretch factor 3.34

    But THEY say at z=2.34 the Hubble rate was 222 km/s per Mpc which translates to Hubble time 4.40 Gy. so the difference is that at a certain moment in history THEY say distance growth rate was 1% per 44 million years and the standard model implemented in the calculator says it was 1% per 41.3 million years.

    the calculator assumes CONSTANT LAMBDA (cosmological constant) and if their measurement is good it would seem that it is not quite constant. So that is described by one of the researchers (Matthew Pieri) as a "tension".
     
  4. Apr 8, 2014 #3
    If this plays out the way that they're suggesting it might, that's big news.

    Do we know if these results are compatible with Quintessence yet?

    Could it turn out to be evidence of Linde's chaotic inflationary universe?

    Are there any other hypotheses that could explain these results?
     
    Last edited: Apr 8, 2014
  5. Apr 9, 2014 #4

    Chalnoth

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    BAO analyses are very difficult, as they rely upon galaxy observations for their data, and galaxies are complex. You can run into all sorts of subtleties with regard to lensing and mass bias and other issues. And their use of the very far-away galaxies of the Lyman-alpha forest makes this all the more tricky.

    So I think the initial expectation when this sort of thing crops up should pretty much always be that there is some unaccounted-for systematic error. Also, the tension appears to be about two standard deviations, which is right around the expected amount of tension for this sort of issue.

    I'd generally want to be cautious and wait until we have other experiments with very different systematic effects weigh in.
     
  6. Apr 9, 2014 #5
    ...I like how this thing pans out. Assuming of course. Thanks for the link Marcus! lol
     
  7. May 21, 2014 #6
    Other Models for varying Lambda

    There is a prediction of (stochastically) varying Lambda from Causal Sets. See http://arxiv.org/abs/0710.1675 and http://arxiv.org/abs/astro-ph/0209274.

    (In fact I should add that the model also predicted a non-zero cosmological constant of the right order of magnitude before observation, e.g. see p. 22 in http://www.physics.syr.edu/~sorkin/some.papers/66.cocoyoc.ps [Broken]).

    Abstracts:

    "The evidence for an accelerating Hubble expansion appears to have confirmed the heuristic prediction, from causal set theory, of a fluctuating and ``ever-present'' cosmological term in the Einstein equations. A more concrete phenomenological model incorporating this prediction has been devised and tested, but it remains incomplete. I will review these developments and also mention a possible consequence for the dimensionality of spacetime."

    "A variety of observations indicate that the universe is dominated by dark energy with negative pressure, one possibility for which is a cosmological constant. If the dark energy is a cosmological constant, a fundamental question is: Why has it become relevant at so late an epoch, making today the only time in the history of the universe at which the cosmological constant is of order the ambient density. We explore an answer to this question drawing on ideas from unimodular gravity, which predicts fluctuations in the cosmological constant, and causal set theory, which predicts the magnitude of these fluctuations. The resulting ansatz yields a fluctuating cosmological ``constant'' which is always of order the ambient density."
     
    Last edited by a moderator: May 6, 2017
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