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B Time, space and spacetime

  1. Dec 11, 2016 #1
    It is my understanding that most cosmologists prefer to describe our universe in terms of space-time as opposed to simply time alone or space alone (or in discrete combination). Why is it then that when we observe a red shift in distant galaxies or stars, we describe this to a physical expansion of the universe and/or an acceleration of these objects in space as opposed to describing the red shift in terms of time slowing down in the realm of either time or space-time. If you think about it carefully you can see that the observed cosmological red shift could easily be described as, not an expansion or an increase in acceleration of the object in space, but instead as evidence of a slowing in time at the fringes of our universe with little or no physical expansion of the universe. Has anyone investigated this and are there papers that either support or refute this idea?
     
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  3. Dec 11, 2016 #2

    PeroK

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    There are no fringes of the universe. The furthest we can see is just another part of the universe like the region where we are. There would be no reason for time to slow down there (whatever that might mean). Observers on those distant galaxies would see our galaxy redshifted.
     
  4. Dec 11, 2016 #3

    PeterDonis

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    It's not a matter of "preference". General Relativity requires it, and General Relativity is our current best theory.

    We don't. At least, we don't when we're actually doing the science. Pop science sources use this terminology, but it's not really correct. A better statement is that the spacetime geometry of the universe is what is "expanding", but even that takes some unpacking, to give a proper technical meaning to "expansion" as a property of spacetime geometry.

    The redshift is not the only observation that needs to be explained. We also observe distant galaxies to have different brightness and angular size, and so we have measured relationships between all three pieces of data for each galaxy: redshift, brightness, and angular size. Different theoretical models for the universe predict very different relationships between these three pieces of data, so we can use the data to rule out many models, including ones like you propose. That is a key part of how cosmologists have arrived at the current best-fit model which involves expanding spacetime geometry.

    It should also be noted that a model such as you propose would require a very different distribution of matter and energy in the universe than the one we actually observe, according to GR. The average density would have to be much larger far away from us than it is near us. That is not what we observe (and also seems unlikely in itself since it would require us to be in a special location).
     
  5. Dec 11, 2016 #4
    As a science layman, understanding the redshift came by studying the Doppler effect, which is used in astronomy to measure the speed stellar objects are moving away from us.

     
  6. Dec 11, 2016 #5

    PeterDonis

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    This interpretation has limitations in a curved spacetime, because there is no invariant way to define "relative speed" for spatially separated objects.
     
  7. Dec 11, 2016 #6
    Is that why redshift must be correlated with angular size and brightness like you mentioned above?
     
  8. Dec 11, 2016 #7

    PeterDonis

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    I'm not sure what you mean. The predicted correlation between redshift and angular size and brightness is, as I said, model dependent--that is, it depends on the particular curved spacetime you are using to model the universe. The fact that there isn't an invariant way to define relative speed for spatially separated objects in curved spacetime is not model dependent; it's true in any curved spacetime whatsoever.
     
  9. Dec 11, 2016 #8
    OK I think I understand. Is my stated interpretation then only useful for determining if the objects are moving toward/away from us, and not for determining their speed?
     
  10. Dec 11, 2016 #9

    PeterDonis

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    No, because even "relative motion" between spatially separated objects is not an invariant in a curved spacetime. Heuristically, if you observe light from some distant object to be redshifted, it could be because it is moving away from you, or it could be because it is coming out of a gravity well. (More precisely, there is no invariant way to distinguish, in a curved spacetime, between "moving away from you" and "coming out of a gravity well"; there is no sharp boundary between these two things.)
     
  11. Dec 11, 2016 #10
  12. Dec 11, 2016 #11

    PeterDonis

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    Again I'm not sure what you mean. Type Ia supernovas are used as standard candles because their intrinsic brightness doesn't vary much. It has nothing to do with issues in defining relative motion.
     
  13. Dec 11, 2016 #12
    I assume their light isn't coming from a gravity well.
     
  14. Dec 11, 2016 #13

    PeterDonis

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    Yes, and in general that's not a good assumption, because, as I said, the concept of "gravity well" doesn't have a sharp definition in a curved spacetime. Variations in the average density of matter in the universe between the distant object and us can act like a "gravity well" in their effect on the redshift of the light.
     
  15. Dec 11, 2016 #14
    Got it, thanks as always.
     
  16. Dec 11, 2016 #15
    Thanks everyone for the explanation and discussion! Interesting!
     
  17. Dec 12, 2016 #16
    "instead as evidence of a slowing in time at the fringes of our universe with little or no physical expansion of the universe"

    If you slow time down, you lose all of your dynamics. Thermodynamics, microwaves, quantum fluctuations with vacuum energy on a string in a zero point field, all requires time. You cant have space, or any kind of dynamics, without 'smooth' space-time.
    "Cosmic Microwave Background spectral distortions from cosmic string loops" http://iopscience.iop.org/article/10.1088/1475-7516/2016/02/047/meta

    Some researchers are talking about different morphings of gravity and light 'constants' over time, but "How do you have dynamics without time."-Dr.JamesGates UMD, theoretical physicist, (nobel physics discussion on the nature of nothingness)
    ,
     
  18. Dec 12, 2016 #17

    PeterDonis

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