Hubble's law

  1. I was reading one of John Gribbin's (popular science)books a few days back when I thought if there could be alternative explanation(s) for the Hubble's law.One simple explanation could be that the value of G(universal gravitational constant) could be changing with time--if G were higher in the past(i.e. G increasing as we go into the past),clocks would be moving slower then(than today).Now since clocks are moving faster,light from distant galaxies would appear red-shifted to us---red-shift would be more for more distant galaxies because they are more into the past when G was still higher.My question is:-how is this explanation ruled out by cosmologists before jumping to the expansion model?

    Another observation:-(say) G does not change with time.As we go back into the past,things become nearer and nearer--so gravitational fields become higher and higher i.e. clocks get slower and slower leading to the kind of effect mentioned above(even without G changing).So the redshift is now due to two reasons--expansion and 'change of clock rate'.Is the latter effect taken care of in present theories?

    Jagmeet Singh
     
    Last edited: May 1, 2006
  2. jcsd
  3. Garth

    Garth 3,443
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    Time dilation due to increased gravitaitonal fields is taken care of in the standard theory.

    What might present an alternative intepretation of the observation of Hubble red shift is cosmologically increasing particle masses, the variable mass hypothesis, see Non-standard cosmology.

    You will also find this interpretation in the Jordan frame of Self creation cosmology. The latter theory is being tested at this moment by the Gravity Probe B experiment.

    Garth
     
  4. Variable Mass Hypothesis has some merit when we consider the variations in alpha detected by Australian radio astronomers. There are other indications that something is not quite right with the standard model but this is a fairly strong indicator.
     
  5. Garth:What about my first question:-how do you rule out variations in G?
     
  6. Garth

    Garth 3,443
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    G may vary - not in the standard model but in alternative gravitational theories such as the Brans Dicke theory.

    But this in itself would not cause red shift, although it would affect the rate of the universe's expansion. Are you confusing gravitational red shift with cosmological red shift perhaps?

    Garth
     
  7. Garth: No,I am not confusing gravitational red shift with cosmological red shift---read carefully the first para of my post no. 1 to understand my question.
    One can say that the answer is CMB radiation,which can not be explained by anything other BB.Well,I am not against BB/expanding universe,but the red shift could be due to the expansion plus other reasons(like the changing value of G)--that would have a bearing on the age of the universe.
     
  8. Garth

    Garth 3,443
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    Yes, I have read your OP, the confusion arises because varying G in itself would not cause atomic clocks to go slow, only gravitational ones such as a pendulum clock, which would actually go faster if G were larger in the past as your OP suggested.

    Only a change in atomic constants, such as particle masses, would cause atomic clocks to 'go slow'.

    I hope this helps,

    Garth
     
  9. 1.I am certainly not talking about Newton's law here.I think I need to repeat what I said in post 1.If clocks were slower in the past(compared to today),light emitted then(i.e. by a distant galaxy) would appear red shifted to us(Agreed?).

    2.Now,clocks run slower in higher gravitational fields.One of the reasons why gravitational fields were higher in the past could be that the value of G was higher.My question is:-how do you rule out a changing G?

    3.The other reason(for an expanding universe) would be that everything was nearer to everything else(!) in the past,so gravitational fields were higher.So now,in this case,there would be two sources of cosmological red shift---expansion plus changing clock rates.You told me that this effect(changing clock rates due to changing strength of gravitational field) is taken care of in GTR(Right?).
     
  10. Chronos

    Chronos 9,951
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    An interesting idea, gptjems. Propose observational evidence that would validate, or falsify it. Try comparing it to this database: http://cdsweb.u-strasbg.fr/viz-bin/Cat?VII/248. I've already plotted the date. It would be interesting to see how you fit the data to your scheme.
     
    Last edited: May 2, 2006
  11. Have you read the book "The Time Illusion" by Dennis A Wright (ISBN 0-646-02803-0) Published in 1989? The claim in that book was that all time occurs in a single instant, time accelerates as the universe expands a new book is coming soon "Physics God and the End of the World" (ISBN 1-58112-923-8.) due for release (I believe) on 06/06/06.

    Not really for the physics purist because of the amount of interconnectivity - science, religion, parapsychology you name it but some interesting concepts. I will look up some of the math and post it here.
     
  12. I don't have enough time or background in cosmology or GTR to take this up in a serious way.Why don't you take up the problem since you've already plotted the data and know enough GTR to do the calculations--may be we can write a joint paper if you come up with something interesting!
     
  13. Haven't read it.All time in a single instant---hope it's not crackpot stuff.

    Please do.
     
  14. Garth

    Garth 3,443
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    [All time in an instant - perhaps Dennis A Wright is travelling at the speed of light! :rolleyes:]

    When we talk about 'clocks being slower' we need to be very specific about about what we are talking about.

    A clock measures time at 'one second per second' - time for that clock does not slow down or speed up.

    Time dilation/acceleration only makes sense when we are talking about comparing one clock with another, that is, comparing the two clocks' measurements of the time interval between two separate events.

    The comparison requires a signal has to pass between the two clocks, most often a light/radio signal; hence the way light propagates also affects whether we observe time speeding up or slowing down, or not.

    On a space-time diagram draw the world-lines of the two clocks at different epochs and the null-geodesics of light signals between them. These null-geodesics diverge on the space-time diagram and time dilation is observed as red shift.

    If particle masses remain constant then cosmological red shift is caused by this divergence of null-geodesics that itself is caused by space-time curvature - the gravitational field of the whole universe. This gravitational curvature is observed as red shift and interpreted as doppler shift of the expanding universe - but note you could interpret it as simply "clocks run slower in higher gravitational fields", actually they don't - they are observed to run slower and that is significantly different to your statement.

    "Hubble red shift is caused by the universe expanding" and "clocks are observed to run slower in the higher gravitational fields of the earlier universe" are two equivalent descriptions of the same phenomenon.

    You can formulate a theory in which the clocks are observed to run slower for different reasons, such as a change in the atomic constants that determine the frequency of a light emitted or absorbed, for example, but changes in G are affecting the curvature of space-time alone, not the internal workings of the atom.

    If G is to vary in a particular theory then that theory requires a field equation and metric from which specific predictions of observables can be made for verification or falsification. The Brans Dicke theory is one such theory but in it both G varies and the also universe expands.

    Garth
     
    Last edited: May 2, 2006
  15. I had made it clear that clocks were moving slower then(compared to today)--so I don't see why you are objecting.

    Are you sure?Even if there were no gravitation between the bodies moving away from each other,there would still be red shift observed--so I don't see how the two are equivalent.

    If clock rates(one second per 'my (present day) second') have to depend on what clock is used,then time itself loses meaning.Atomic clock rates would also be identical to other clock rates--figure out how.
     
  16. Garth

    Garth 3,443
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    I'm not objecting - just explaining carefully in stages.
    They are equivalent in the cosmological case as they are observables deduced from the same R-W metric.

    The galaxies/quasars are not moving through space - apart from small proper motions - it is space itself that is expanding with time taking the objects along with it.

    If there were no gravitation as in your example any red shift observed would be a Doppler shift caused by their motion through space.
    Actually time is defined by the method used to measure it, we have atomic time (with Terrestial time & Geocentric Coordinate time subsets), ephemeris time, solar time and sidereal time. They are not all at the same 'rate', but so long as we know how one relates to another we have no problems.

    One question is though: "Is there a clock drift between ephemeris and atomic time?" The standard answer is no; however, such a clock drift would be an explanation for the Pioneer Anomaly.

    Garth
     
    Last edited: May 2, 2006
  17. I think it is an excellent idea gptejms - and certainly more plausible than the common view. Didn't Arp propose something along these lines but with physical constants changing with time? After all we have only been measuring G, h, etc for a short time and have no way of telling if they are time dependent or not.
    Just a thought; Hubble's law is a linear law, when you say:

    I haven't really thought it through but wouldn't the density increase as the cube of the radius whilst redshift only varies as the first power?
    Ratfink.
     
  18. Garth

    Garth 3,443
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    Nearly correct - actually inversely proportional to the radius cubed etc.

    As we go back in time we reach the radiation dominated age when the density of the radiation is greater than that of, and dominates, the matter in the universe and then all the way back to the intense gravitational fields of the Inflation and Planck epochs.

    Garth
     
    Last edited: May 2, 2006
  19. Probably true, but aren't we mixng models here?
    If our original poster (sorry but if i go back to look, i will lose this) if time dependent gravity etc is responsible for redshift then
    all this is a fairy tale and not relevant.
    Lets keep on thread and look at time dependent G, h etc being responsible for Hubble's law.
    Ratfink
     
  20. sorry, got this new keyboard and the 'i' key doesn't work too well - bit like misery in Stevens Kings novel!
     
  21. Garth

    Garth 3,443
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    Well, I was using the standard model to base my argument on.

    The relative abundances of the elements and the CMB itself are strong evidences that the universe went through a hot high density phase when everything was tightly compressed in a small volume, it is not a fairy tale. Any alternative theory would have to take these observations into account, I don't think Arp's theory is able to.

    If you do propose varying 'constants' in a theory you have to think through the consequences of their variation and often end up with the same total effect.

    For example: Fred Hoyle proposed a mass variation theory to explain the CMB in a steady state type universe. ('On the origin of the microwave background', F. Hoyle 1975 ApJ.)

    As you went back in time in this theory particle masses decreased until you approached an interface where masses were zero, beyond which they were negative. There was no expansion or contraction, space was static, and this boundary, or interface, was supposed to be that which we incorrectly identify with the BB singularity. The CMB was simply the light from galaxies beyond this interface.

    This proposal wanted to avoid the singularity of the BB, however if masses decreased then atomic sizes would increase until at the actual boundary itself they would become infinite.

    A BB singularity of finite sized atoms squashed into zero volume was replaced by a universe of infinite sized atoms squashed into a finite size volume.

    Either way the material in the universe would be equally squashed! It depended on how you looked at it and what ruler you used.

    Certainly if physical constants varied over cosmological time then that would completely turn our understanding upside down. Any such variation would have a knock on effect that would make predictable and falsifiable differences to the unverse we observe. To find such a theory that is concordant with observation is the trick - my own offering is Self creation cosmology.

    Garth
     
    Last edited: May 2, 2006
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