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Measuring red shift

  1. Dec 30, 2007 #1

    wolram

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    Over a period of several years could the red shift of some ideal body be used to confirm the Hubble flow?
     
  2. jcsd
  3. Dec 30, 2007 #2
    The resolution of current spectographs does not allow this. However, future extremely high resolution spectrographs may do the job.
     
  4. Dec 30, 2007 #3

    wolram

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    Can you explain spectographs? i think they may be a measure elongated waves, but i am not sure.
     
  5. Dec 30, 2007 #4

    marcus

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    as stop-gap, absent matt.o, a spectrograph is an instrument used to measure wavelengths. (and record the rainbow spectrum of the light from a star)
    the light from the star is put thru (or bounced off) a diffraction grating which makes rainbows somewhat like a prism
    you can buy 25 cent diffraction gratings from science stuff mail order (Edmonds Scientific for example)

    a diffraction grating bends light by an angle proportional to the wavelength
    (or whose sine is proportional to the wavelength which for small angles is the same)

    so you spread the light out in rainbow band. You can capture it onto photographic film and take a picture of the rainbow, as stripes or lines of different color. Or you can SCAN the band with photosensitive electronics---the more modern way.

    the amount each bit of light got bent tells you its wavelength

    ===HERE'S WIKIPEDIA===
    A spectrograph is an instrument that transforms an incoming time-domain waveform into a frequency spectrum, or generally a sequence of such spectra. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves.The first spectrographs used photographic paper as the detector. The star spectral classification and discovery of the main sequence, Hubble's law and the Hubble sequence were all made with spectrographs that used photographic paper. The plant pigment phytochrome was discovered using a spectrograph that used living plants as the detector.More recent spectrographs use electronic detectors, such as CCDs which can be used for both visible and UV light. The exact choice of detector depends on the wavelengths of light to be recorded.

    The forthcoming James Webb Space Telescope will contain both a near-infrared spectrograph (NIRSpec) and a mid-infrared spectrometer (MIRI).
    ===END QUOTE===

    so you look where some recognizable lines are, in the band,
    how far they have been displaced from their normal un-redshifted position.
    and you tell the redshift from that.

    So a spectrograph is just an instrument that can spread out the light of a star and measure the wavelength of each portion very accurately. And redshift is one of the things it can measure
    ==============
    the wikipedia article is called SPECTROSCOPE
    a spectrograph is one particular type of spectroscope
     
    Last edited: Dec 30, 2007
  6. Dec 31, 2007 #5

    wolram

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    Thanks Marcus, but now i can not under stand why this method can not measure the Hubble flow, surly some change is seen over several years.
     
  7. Dec 31, 2007 #6

    marcus

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    you see it very clearly! indeed some change is to be expected!
    the change is extremely gradual (I will explain why) and therefore, to detect it, one needs very precise instruments and one needs to measure redshifts over an extended period of time

    matt.o would be the person to discuss this. I have seen just one article about it----proposing a programme of measurements over the course of several decades----perhaps half a century (I don't remember the exact numbers)

    matt.o was talking about using improved spectrograph instruments to carry this out, so perhaps it could be done on a shorter time schedule
    ======================

    the basic idea, I think, in case anyone else is reading this, is that the Hubble law is
    v = H D

    the recession speed is proportional to the distance.
    so as time goes on, the distance increases, so the recession speed increases, so the REDSHIFT should increase
    and this should be detectable.

    Now measurements of redshifts have been made for many years, so there are records. One should be able to see an increasing trend----if only they were accurate enough out to enough decimal places.

    To understand the slowness with which redshifts change, recall that the current rate of expansion is ONE PERCENT EVERY 140 MILLION YEARS.

    So every 140 million years the distance D increases one percent---and the recession speed v is proportional so it increases by one percent----

    or if you think of it as stretch during travel, the travel time increases by one percent, and the rate of stretch is fairly constant over periods of time like 100 million years, so the stretchout is greater

    either way you picture it, there is a one percent increase in the quantity (1 + z)

    =====================

    but if you can only watch over a period of 140 years, then you get a change of a MILLIONTH OF A PERCENT

    this sounds borderline unmeasurable.

    I may have made some errors so this may be numerically wrong but the idea that comes thru is that the change is very SLOW. it is expected to be there, and to eventually be measurable, but only with improved instruments

    maybe Wallace will have something more definite to say, or matt.o will get back to us on this
     
    Last edited: Dec 31, 2007
  8. Dec 31, 2007 #7

    wolram

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    But yet it is not used, or is it, i would think 3 or 4 widely separate targets could be selected
    and observed over several years to give a very accurate measure.
     
  9. Dec 31, 2007 #8

    marcus

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    distances are not known very accurately
    and indeed they are mostly inferred from redshift

    the kind test I was talking about is you watch the redshift for ONE OBJECT change

    I don't think an instantaneous comparison of several objects can get more information than they already have----they already do a lot of that

    the new test, with improved instruments, would be to watch how the redshift of a given thing changes over the course of several years as the thing gets farther away
     
  10. Jan 3, 2008 #9

    Garth

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    This paper in today's physics ArXiv might be interesting Time drift of cosmological redshifts as a test of the Copernican principle.
    Garth
     
  11. Jan 3, 2008 #10

    marcus

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    Garth thanks for catching that George Ellis et al paper about checking Copernicus.
    Just this past week there was another paper about checking the Copernican principle, incidentally also by South Africans (Bruce Bassett was one of the co-authors.)
     
  12. Jan 6, 2008 #11

    Wallace

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    I saw a talk once where this kind of survey was explained in detail. In a nutshell the conclusion was that you would need to use a 30 metre class optical telescope (current state of the art is ~10m with 20-30m telescopes possibly coming in the next 10-15 years) continuously for at least 50 years to get even the most marginal measurement. The number are ball-park figures from my memory, but it's somewhere in that kind of range.

    Barring global cataclysm or a world-wide trend to stop supporting huge astronomy projects, I'm sure eventually this kind of survey will be done, and the results will be incredibly interesting for innumerable reasons. We will probably have to wait for around a century or so before we have this though.
     
  13. Jan 6, 2008 #12

    marcus

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    Since there seems to be some interest in checking the Copernican principle (evidenced by Garth and Wallace comments) I'll give the link to the paper I mentioned earlier:

    http://arxiv.org/abs/0712.3457
    A general test of the Copernican Principle
    Chris Clarkson, Bruce A. Bassett, Teresa Hui-Ching Lu (UCT & SAAO, Cape Town)
    4 pages
    (Submitted on 20 Dec 2007)

    "Here we present an observational test for the Copernican assumption which can be automatically implemented while we search for dark energy in the coming decade. Our test, which relies on the constant curvature of FLRW models, is entirely independent of any model for dark energy or theory of gravity and thereby represents a model-independent test of the Copernican Principle."

    It seems that the Bruce Bassett et al proposal does not involve much of a diversion of resources from other programmes of observation. I leave you to judge.
     
  14. Jan 6, 2008 #13

    Chronos

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    The old measurements are too low resolution to be useful. New instrument readings are, however, highly accurate. It will only take about 10,000 years to achieve the necessary level of resolution.
     
  15. Jan 7, 2008 #14

    marcus

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    I realize you are joking, but we may be talking about different things. Have you read the Bassett et al article I mentioned? The timeframe there is on the order of 2010-2030.

    You may not have understood the method they are proposing to test the Copernican Principle. A figure like "10,000 years" seems totally irrelevant to their method since it obviously does not depend on waiting for redshifts to change.
     
  16. Feb 12, 2008 #15
    Just out today on astro-ph. I haven't read the paper (and probably won't), but it seems relevant:

    http://arxiv.org/abs/0802.1532

    Cosmic dynamics in the era of Extremely Large Telescopes
    Authors: J. Liske, A. Grazian, E. Vanzella, M. Dessauges, M. Viel, L. Pasquini, M. Haehnelt, S. Cristiani, F. Pepe, G. Avila, P. Bonifacio, F. Bouchy, H. Dekker, B. Delabre, S. D'Odorico, V. D'Odorico, S. Levshakov, C. Lovis, M. Mayor, P. Molaro, L. Moscardini, M.T. Murphy, D. Queloz, P. Shaver, S. Udry, T. Wiklind, S. Zucker
    (Submitted on 11 Feb 2008)

    Abstract: The redshifts of all cosmologically distant sources are expected to experience a small, systematic drift as a function of time due to the evolution of the Universe's expansion rate. A measurement of this effect would represent a direct and entirely model-independent determination of the expansion history of the Universe over a redshift range that is inaccessible to other methods. Here we investigate the impact of the next generation of Extremely Large Telescopes on the feasibility of detecting and characterising the cosmological redshift drift. We consider the Lyman alpha forest in the redshift range 2 < z < 5 and other absorption lines in the spectra of high redshift QSOs as the most suitable targets for a redshift drift experiment. Assuming photon-noise limited observations and using extensive Monte Carlo simulations we determine the accuracy to which the redshift drift can be measured from the Ly alpha forest as a function of signal-to-noise and redshift. Based on this relation and using the brightness and redshift distributions of known QSOs we find that a 42-m telescope is capable of unambiguously detecting the redshift drift over a period of ~20 yr using 4000 h of observing time. Such an experiment would provide independent evidence for the existence of dark energy without assuming spatial flatness, using any other cosmological constraints or making any other astrophysical assumption.
     
  17. Feb 12, 2008 #16

    Wallace

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    Interesting. I think these numbers are comparable to what I've heard before. Interesting that they choose a 42 metre telescope, since this is a class above even the ELT's planned for the next couple of decades. So that means waiting 20 years or more for a 42m telescope to be built, then using at least 500 nights of observing over the next 20 years to do the experiment. Optimistic time line for this would then be around 2050. A long time away, but worth the wait I would think.

    One problem with this kind of project is that even if you have a 42m telescope, how do you get funding for a 20 year survey? Even the most languid doctrate student can't extend their thesis that far, and established guru's within a field would be retired (or dead!) before the thing finished! Still, I'm sure that if we ever have telescopes this big such a survey as suggested here is an obvious key project for it, so I'm sure it would happen somehow.
     
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