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Standard candles

  1. Aug 17, 2006 #1

    wolram

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    http://arxiv.org/abs/astro-ph/0608324

    Cosmological Implications of the Second Parameter of Type Ia Supernovae
    Authors: Philipp Podsiadlowski (Oxford), Paolo A. Mazzali (MPA, Munich; Trieste), Pierre Lesaffre (Oxford, Cambridge, Paris), Christian Wolf (Oxford), Francisco Forster (Oxford)
    Comments: 10 pages, 6 figures, constructive comments welcome

    Theoretical models predict that the initial metallicity of the progenitor of a Type Ia supernova (SN Ia) affects the peak of the supernova light curve. This can cause a deviation from the standard light curve calibration employed when using SNe Ia as standardizable distance candles and, if there is a systematic evolution of the metallicity of SN Ia progenitors, could affect the determination of cosmological parameters. Here we show that this metallicity effect can be substantially larger than has been estimated previously, when the neutronisation in the immediate pre-explosion phase in the CO white dwarf is taken into account, and quantitatively assess the importance of metallicity evolution for determining cosmological parameters. We show that, in principle, a moderate and plausible amount of metallicity evolution could mimic a lambda-dominated, flat Universe in an open, lambda-free Universe. However, the effect of metallicity evolution appears not large enough to explain the high-z SN Ia data in a flat Universe, for which there is strong independent evidence, without a cosmological constant. We also estimate the systematic uncertainties introduced by metallicity evolution in a lambda-dominated, flat Universe. We find that metallicity evolution may limit the precision with which Omega_m and w can be measured and that it will be difficult to distinguish evolution of the equation of state of dark energy from metallicity evolution, at least from SN Ia data alone.

    May be not much, just another question mark.
     
  2. jcsd
  3. Aug 18, 2006 #2

    wolram

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    Double edged sword.

    http://arxiv.org/abs/astro-ph/0608351

    The impact of neutrino masses on the determination of dark energy properties
    Authors: Axel De La Macorra, Alessandro Melchiorri, Paolo Serra, Rachel Bean
    Comments: 4 pages, 2 figures

    Recently, the Heidelberg-Moscow double beta decay experiment has claimed a detection for a neutrino mass with high significance. Here we consider the impact of this measurement on the determination of the dark energy equation of state. By combining the Heidelberg-Moscow result with the WMAP 3-years data and other cosmological datasets we constrain the equation of state to -1.67< w <-1.05 at 95% c.l., ruling out a cosmological constant at more than 95% c.l.. Interestingly enough, coupled neutrino-dark energy models may be consistent with such equation of state. While future data are certainly needed for a confirmation of the controversial Heildelberg-Moscow claim, our result shows that future laboratory searches for neutrino masses may play a crucial role in the determination of the dark energy properties.
     
  4. Aug 21, 2006 #3

    wolram

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    No acceleration

    http://arxiv.org/abs/astro-ph/0608386

    On the Absence of Cosmic Acceleration
    Authors: John Middleditch
    Comments: 12 pages
    Report-no: LAUR 06-5685

    Type Ia Supernovae (SNe) have been used by many to argue for an accelerated expansion of the universe. However, high velocity and polarized features in all nearby SNe Ia, show that the paradigm for Type Ia SNe is drastically and catastrophically invalid. By now it is also clear that an extreme version of the axisymmetry seen in SN 1987A is the correct paradigm for SNe Ia and Ic. A Ia/c is produced from the merger of two degenerate cores of common envelope WR stars, or of two CO white dwarfs. Its polar blowouts produce the observed high velocity and polarized spectral features in Ia's, and its equatorial bulge is much brighter in Ia's, due to the greater fraction of 56 Ni contained within it. These become classified as Ia's when viewed from the merger equator, and Ic's when viewed from the poles. Thus cosmology determined strictly from Ia's alone is flawed at its very foundation: the local sample is selectively biased. The problem arose with the more distant supernovae, when the high velocity polar blowout features, which initially obscure part of the Ia/c equatorial bulge, expose a greater fraction of it, particularly when viewed off the equator, during the interval when Delta m_15 is measured, leading to a smaller decrease in observed luminosity. The width-luminosity correction was thus too small, and the result was a distant SN Ia which appeared to be too faint for its redshift. When the errors introduced by this process and others are taken into account, there may be no cosmic acceleration effect in distant SNe.
     
  5. Aug 21, 2006 #4

    Garth

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    These papers may indicate the confidence placed in the standard
    [itex]\Lambda[/itex]CDM model may be misplaced.

    Garth
     
  6. Aug 21, 2006 #5

    wolram

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    I am not going to say anything Garth, i will just post any thing that seems interesting in this thread, unless any one has objections.
     
  7. Aug 21, 2006 #6

    Garth

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    The point is: Are these 'SN Ia' standard candles?

    Such a lot in the standard model hangs on the assumption that they are.

    Yet the evidence that their luminosities are known depends only on those nearby, and therefore recent, ones that are able to be calibrated. As we go back into cosmological history metallicity may be expected to change, and as your links imply this may alter their expected luminosities and call into question the conclusions based on them.

    Garth
     
  8. Aug 21, 2006 #7

    wolram

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    May be i should ask you what is standard in cosmology, is there a standard galaxy ,star, planet, moon, if not it is difficult to understand how there can be a (standard) body in the U.
     
  9. Aug 21, 2006 #8

    Garth

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    The concept of a standard candle is simply a luminous object of which we think we know the Absolute Magnitude. (Such as the Cepheid Variables)

    The object's distance can then be calculated from the distance modulus.

    Garth
     
  10. Aug 21, 2006 #9
    I think Wolram was asking a deeper question here, Garth. He is pointing out the differences in other observed phenomena such as galaxies and then questioning how there can be standard candles if things are so different.

    I would just like to say that as scientists, we generally like to have a few more cards up our sleeves, hence there are other tests which point to a dark energy density of ~.7 independant of the SN1a results. Such studies include the CMB, baryon acoustic fluctuations, galaxy cluster mass function etc.
     
  11. Aug 21, 2006 #10

    SpaceTiger

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    That hasn't been true for some time. Pretty much everything we infer from Type Ia SNe (so far) has been checked with other methods (e.g. CMB, LSS). In fact, it would be very strange if the results were found to be invalid after having already been found consistent with these more reliable measurements.
     
  12. Aug 22, 2006 #11

    wolram

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    Thankyou, Matt.o,

    i do not know about (baryon acoustic fluctuations) i will have to look up the
    subject.
     
  13. Aug 22, 2006 #12

    Garth

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    ST - what do you think about the OP link paper?

    If SN Ia are not standard candles and the universe is not accelerating would that not alter the interpretation of the CMB and LSS data?

    Garth
     
  14. Aug 22, 2006 #13
    Actually, I don't think baryon acoustic fluctuations have been used by themselves (yet) to constrain dark energy. The current galaxy redshift surveys don't cover enough volume. They have certainly been used in conjunction with WMAP data to constrain cosmological parameters here. For a good overveiw on what can be done with lage redshift surveys, see this paper by Chris Blake and Karl Glazebrook.

    I do know of galaxy redshift surveys that are going to attempt to estimate the equation of state of the dark energy, but they are a little while off yet. There are also planned missions to survey for clusters and measure the dark energy equation of state using the cluster mass function.
     
  15. Aug 22, 2006 #14

    SpaceTiger

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    The paper in the OP says the following:

    This doesn't suggest to me that the SN Ia results have been invalidated.

    As for the Middleditch paper, I took a quick look at it and it sounds very crankish. I doubt it will make any waves.


    If SNe Ia were not standard candles, why would you infer that the universe was not accelerating? The former statement would simply mean that they couldn't be used as distance indicators.
     
  16. Aug 22, 2006 #15
    There are still some big holes in our understanding, maybe if we collect enough details of anomolies we will be able to figure out just what is going on out there.
    Here are a couple for the collection'

    APM 8279+5255 the red shift of the quasar, a vibrant galaxy with a bright central region and massive central black hole, revealed that it contained much more iron than it should for its age. The amount of iron present is greater than the amount found in our own solar system. At a fraction of the age of the solar system this quasar should contain the lower percentage of iron.

    Fossil cluster RX J1416.4+2315 Current projections state that the fossil group should not have had enough time to form given the age of the universe.
     
  17. Aug 22, 2006 #16

    Garth

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    I was simply reverting back to the pre-'cosmic acceleration' understanding.

    The acceleration or non-acceleration of the universe determined from SNe Ia at varing high z is an important handle on the behaviour and therefore nature of DE.

    The consequences of them not being standard candles would therefore be rather significant. Tzemach has highlighted again the Age problem question, I think there is a degeneracy in the interpretation of the data that may be resolved by attention to these anomalies.

    Is there an age problem at high z?
    Are SNe Ia standard candles?
    Is there a genuine deficiency at low-l mode in the WMAP anisotropy power spectrum?
    Just a few questions to chew over....

    Garth
     
    Last edited: Aug 22, 2006
  18. Aug 22, 2006 #17

    SpaceTiger

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    It's not clear to me that your above statements really make that case. It would have made a big difference historically, but with the high-precision CMB and LSS measurements we have now, I don't think you'd see many people jumping ship from standard cosmology.
     
  19. Aug 22, 2006 #18

    Garth

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    Well ST, surely a project such as Destiny
    depends on the assumption that SNe Ia are standard candles?

    Are you saying that if, for the sake of argument, they prove to be intrinsically fainter (say) at high z, then the interpretation of the high precision WMAP and LSS data would not be affected?

    Or do you think that WMAP, LSS, etc. have so proved cosmic acceleration independently of SNe Ia that the there is no question about it, and consequently SNe Ia have been verified to be standard candles?

    Garth
     
    Last edited: Aug 22, 2006
  20. Aug 22, 2006 #19

    SpaceTiger

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    Again, this depends crucially on the distinction between them not being standard candles and them being renormalized. If they were renormalized and subsequently showed no acceleration, then that would be a serious problem for mainstream cosmology in general. That alone likely wouldn't be enough to convince folks (after all, it would also demonstrate the unreliability of supernovae). However, if subsequent measurements with other distance indicators gave the same result, the mainstream model would have to be changed.

    If the supernovae were just shown to be unreliable as standard candles, then yes, I'm saying the interpretation would likely remain the same. Your choice of words in the above doesn't make it clear which case you're referring to.


    Those data sets do not directly prove acceleration, but they do suggest it. Believe it or not, [itex]\Lambda CDM[/itex] was under serious consideration even prior to the supernova results because of COBE CMB measurements.
     
  21. Aug 22, 2006 #20

    Exactly. From COBE, it was known we live in an [tex]\Omega[/tex] = 1 Universe, but the observed matter density only made up ~.3 of the total density. People were considering a cosmological constant to make up the rest of the energy density and along came the SN 1a results!
     
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