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Utility of observational Hubble parameter data on DE

  1. Jul 12, 2015 #1

    Garth

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    Another paper in Friday's physics arXiv on using the H(z) v z plot to investigate any possible evolution of DE: Utility of observational Hubble parameter data on dark energy evolution.

    From that eprint:
    upload_2015-7-12_15-45-38.png

    As discussed in the Marginal evidence for cosmic acceleration from Type Ia SNe, this paper includes two further high z plots, all three are binned in the fourth (purple) bin in their Figure 1 below.

    As Chalnoth said in #41 of that thread:
    Here are two more measurements at z ~ 2.3 and they agree!

    Also of course that fourth bin is consistent with linear expansion (ho = 0. 67) but not with the [itex]\Lambda CDM[/itex] model. Just a thought.....

    Garth
     
  2. jcsd
  3. Jul 12, 2015 #2

    Chalnoth

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  4. Jul 12, 2015 #3

    Garth

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    Okay, three observations from the same survey.

    Then as they conclude:
    We wait for the next generation of space and ground telescopes....

    Garth
     
  5. Jul 12, 2015 #4

    Chalnoth

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    Better would be a measurement using different observable parameters.
     
  6. Jul 13, 2015 #5

    Garth

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    Such as the ages of passively evolving galaxies as in The Age-Redshift Relationship of Old Passive Galaxies?

    upload_2015-7-13_10-8-47.png

    (emphasis mine)

    At least with less flexibility the linearly expanding model is easier to falsify - strange then (in the matter dominated era) that it hasn't been......

    Just a thought.

    Garth
     
  7. Jul 13, 2015 #6

    Chalnoth

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    That seems like an extremely tricky measurement that is going to be difficult to do precisely.

    And the parameter estimates from that paper are weird. Their best-fit ##\Lambda##CDM model has ##H_0 = 94##, ##\Omega_m = 0.12##. Both are pretty far away from other measurements of the same parameters. The ##H_0## measurement isn't too ridiculous, because the error bars are very large and encompass the much more precise values measured by other experiments. The estimate for ##\Omega_m##, however, seems to indicate some unaccounted systematic errors.

    This measure clearly still has errors that are too big for use in determining cosmological parameters.

    It's very easy to falsify. Just include CMB data. Or notice that there is matter in the universe.
     
  8. Jul 13, 2015 #7

    Garth

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    Well as I said the model would be linearly expanding in the matter epoch, as CMB is transmitted at about the transition stage it gives information about the radiation dominated epoch.

    The linearly expanding model's EoS would have to be [itex]\omega = - \frac{1}{3}[/itex] in that matter epoch.

    Garth
     
    Last edited: Jul 13, 2015
  9. Jul 13, 2015 #8

    Chalnoth

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    The matter dominated epoch started quite a while before the emission of the CMB.
     
  10. Jul 13, 2015 #9

    Garth

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    True, but as the EoS would have to change from [itex]\omega = +\frac{1}{3}[/itex] in the radiation dominated epoch to [itex]\omega = - \frac{1}{3}[/itex], in a linearly expanding matter dominated epoch then I would envisage this being a gradual transition process and some time passing before true linear expansion sets in.

    If, for example, the linearly expanding EoS is delivered by some sort of scalar field coupled to matter, its influence would gradually kick in as [itex]\rho_m[/itex] begins to dominate [itex]\rho_r[/itex].

    So much for speculation, but here the question is, "What is the data (BAO, galaxy ages etc.) actually telling us?

    If [itex]\Lambda[/itex]CDM then well and good, but for the sake of having models to test it against I suggest the linear model to be one good candidate - and others seem to have taken the same view.

    Garth
     
    Last edited: Jul 13, 2015
  11. Jul 13, 2015 #10

    Chalnoth

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    Except you have no theory at all for why the we should have ##w = -1/3## in the first place. "There's probably some gradual transition" is just a cop-out. Why would there be a gradual transition? What causes the transition? Precisely how gradual is that transition?

    Furthermore, as I pointed out, the universe was matter-dominated for quite some time before the CMB was emitted. The main reason why I say the CMB will completely destroy these theories is not because of the dynamics prior to the CMB, but because of the redshift/distance relation that the CMB itself represents (comoving distance of 14.2 Gpc at z = 1088). What would your R=ct model predict for that?
     
  12. Jul 14, 2015 #11

    Garth

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    That's a work in progress. Actually there is a theory that delivers [itex]\omega = -\frac{1}{3}[/itex] (A New Self Creation Cosmology) but I am in the process of rewriting that paper. Here in this thread I am looking at observational evidence that might support a linear expansion, whether there is a valid theory at present or not.

    If linear expansion keeps cropping up from observational evidence and such a valid theory does not at present exist then the community should look for one.

    All I was saying above was that if we start with the BBN and CMB evidence and accept the standard model is correct up to radiation-mattter equality but also find by observation that there has been a linear expansion later on, then there has been a DE evolution with a (gradual) change from [itex]\omega = +\frac{1}{3}[/itex] to [itex]\omega = -\frac{1}{3}[/itex] at some stage.
    In a flat, R = ct, [itex]\omega = -\frac{1}{3}[/itex] [itex]\rho_m = 0.33, \rho_\Lambda = 0.77[/itex] universe the co-moving distance at z = 1088 is 13.6 Gpc.

    I do accept there is work to be done on interpreting the CMB data - as I said it is a 'work in progress'.

    Garth
     
    Last edited: Jul 14, 2015
  13. Jul 14, 2015 #12

    Garth

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    Just to reiterate about the "observational evidence that might support a linear expansion," I have re-posted Figure 1 from the OP link Utility of observational Hubble parameter data on dark energy evolution eprint but I have added the R = ct plot for comparison with the observed data points and the yellow [itex]\Lambda[/itex]CDM plot.

    upload_2015-7-14_14-37-37.png

    There is a problem for R =ct around z = 0.5 but otherwise a good fit, especially at high z ~ 2.3.

    Bin 2 fits [itex]\Lambda[/itex]CDM but not R=ct, (Edit: actually it doesn't, it lies below both plots but nearer the [itex]\Lambda[/itex]CDM one,) whereas Bin 4 fits R=ct but not [itex]\Lambda[/itex]CDM.

    However it is more difficult to determine [itex]\omega[/itex] at low redshift; from the OP eprint:
    Garth
     
    Last edited: Jul 14, 2015
  14. Jul 14, 2015 #13

    Chalnoth

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    ...which is about 5 standard deviations away from the WMAP 9-year result, and getting close to 30 standard deviations away from the Planck 2015 results.

    Should get cracking on that, then. Because the CMB is the most precise data set we have, especially as it has the lowest chance for systematic errors.
     
  15. Jul 14, 2015 #14

    Garth

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    And in a flat, R = ct, [itex]\omega = -\frac{1}{3}, \Omega_m = 0.3166, \Omega_\Lambda = 0.6834[/itex] universe the co-moving distance at z = 1088 is 13.8 Gpc.

    Garth
     
  16. Jul 15, 2015 #15

    Chalnoth

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    I don't quite see what your point is here. That's still very far away from the observations.
     
  17. Jul 15, 2015 #16

    Garth

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    I was simply answering your question with the best values of [itex]\Omega_m[/itex] and [itex] \Omega_\Lambda[/itex] available.
    What you mean is: 'from interpretations of the observations' - they depend on the priors you assume.....

    Garth
     
    Last edited: Jul 15, 2015
  18. Jul 15, 2015 #17

    Chalnoth

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    While there is some model dependence on the parameter estimates, I sincerely doubt it will be enough to close a 20+ sigma gap.
     
  19. Jul 16, 2015 #18

    Garth

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    One prior I would question that would significantly alter observation interpretation is the assumption that SNe 1a are standard candles out to cosmological distances.

    As I said in jim mcnamara's thread Standard candle - in question - affects distance estimates, there are at least three types of SN 1a now known, which was not realised in 1998:
    (With added luminosity information provided by |Glitch| in the post following that one - thank you |Glitch|.)

    What has not been resolved is the evolution of the ratio of these three (possibly more) types over cosmological time scales.

    Garth
     
    Last edited: Jul 17, 2015
  20. Jul 16, 2015 #19

    Chalnoth

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    Uhh, what? That has nothing to do with the CMB observations.
     
  21. Jul 17, 2015 #20

    Garth

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    Really? I would have thought [itex]\Omega_\Lambda[/itex] would have quite a lot to do with the interpretation of CMB observations.

    The whole [itex]\Lambda[/itex]CDM model interpreting the CMB data is a compilation of the questionable theory of Inflation, with its requirement that [itex]\Omega = 0[/itex] or very nearly so, and the independent SNe 1a observation that apparently indicates that [itex]\Omega_\Lambda[/itex] is positive such that [itex]\Omega_m[/itex] + [itex]\Omega_\Lambda[/itex] = 0.

    Even under the assumption that they are standard candles SNe 1a only give marginal evidence (<3[itex]\sigma[/itex]) that the universe is accelerating.

    If an improved analysis of SNe 1a observations, taking into account the evolution of the ratio of the three or more types, should reveal [itex]\Omega_\Lambda \neq 0.68[/itex] or nowhere near that value, then the standard schema of interpretation would begin to crumble.

    There are a number of degeneracies between the cosmological parameters that can be extracted from the CMB power spectrum which can only be lifted by combining CMB data with other data sets such as the SNe 1a observations or large scale structure formation surveys. (Which BTW is why the presence of over-massive evolved objects at high z might also be pertinent.)

    So I would say the assumption of SNe 1A being standard candles out to cosmological distances (z = 1 and beyond) has everything to do with the interpretation of CMB observations.

    Garth
     
    Last edited: Jul 17, 2015
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