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B How is the Age of the Universe Determined?

  1. Jun 27, 2016 #1
    How can the age of the universe be determined when there is no absolute measure of time or distance in the universe. Every other place in the universe has another rate of the passing of time. Since time expands with the expansion of space, trying to use expanded time to measure time is circular and self refuting.
     
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  3. Jun 27, 2016 #2

    PeterDonis

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    Because "the age of the universe" is not an absolute measure of time. It is the elapsed time on the clocks of idealized "comoving" observers (observers who always see the universe as homogeneous and isotropic) since the Big Bang. Observers in different states of motion would measure a different age.

    What does this even mean?

    I think you might want to spend some time working through Ned Wright's cosmology FAQ and tutorial:

    http://www.astro.ucla.edu/~wright/cosmolog.htm
     
  4. Jun 27, 2016 #3
    So if the other observer calculates a different age of the universe, which is the correct age?
     
  5. Jun 27, 2016 #4

    PeterDonis

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    What do you mean by "the correct age"?
     
  6. Jun 27, 2016 #5
    Which age is the actual age of the universe?
     
  7. Jun 27, 2016 #6

    Bandersnatch

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    Depends on who's measuring.
     
  8. Jun 27, 2016 #7
    : )
     
  9. Jun 27, 2016 #8

    PeterDonis

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    What do you mean by "the actual age"?

    In case you haven't caught on yet, there is no such thing as "the correct age" or "the actual age". All there is is the age as measured by a particular observer. Different observers can measure different ages. That's all there is to it.
     
  10. Jun 27, 2016 #9

    phinds

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    Peter, correct me if I'm wrong but I believe that it is also true that everywhere in the universe it is possible to determine an age that is associated with a co-moving (to the CMB) observer and that all such measurements/calculations no matter where taken/done will give the same answer since they compensate for the movement of actual observers. Further, that age is about 13.8 billion years by our latest calculations from the European Space Agency's Plank Mission.
     
  11. Jun 27, 2016 #10

    PeterDonis

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    Yes, this is correct. You don't have to be a comoving observer to calculate the age a comoving observer would measure, as long as you know your own motion relative to a comoving observer at your spatial location. We estimate that by looking at the dipole anisotropy that we observe in the CMB and calculating our motion relative to a comoving observer (who would see the CMB as having no dipole anisotropy) from that. (Note that virtually all published data for the CMB temperature distribution already subtracts out the dipole.)
     
  12. Jun 27, 2016 #11

    Chronos

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    A variety of other measurements have also been conducted and are consistent with CMB measures - e.g., relative abundance of long lived isotopes, the age of ancient stars. Gravitational simulations of large scale structures and galaxy formation, and temperature of remote dust and gas clouds in the universe.
     
  13. Jun 27, 2016 #12

    Drakkith

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    As other have said, both are correct. The basic idea is that most structures in the universe (planets, stars, galaxies, etc) occupy frames of reference that move relatively slowly with respect to a convenient reference frame where the CMB is extremely uniform. This makes it convenient to measure the universe's age with respect to a co-moving observer, which is just an observer anywhere in the universe who is in our "convenient" frame of reference where the CMB is uniform. We'll call this convenient frame the CMB frame. (Note that this convenient frame is not an "absolute" frame nor a "preferred" frame. It's just a convenient one)

    Observers moving with respect to our CMB frame experience time dilation with respect to that frame. This means that the age of the universe as measured by these frames would be different from what the observer in the CMB frame measures. An observer moving very, very quickly with respect to the CMB frame would measure the universe as being much older than 13 billion years. You could say that the age of the universe measured from the CMB frame is the "minimum age" any observer would measure. (I think so at least)

    As always, someone correct me if I'm wrong.
     
  14. Jun 27, 2016 #13

    Jorrie

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    Should it not be the other way round, i.e. that such observers, using their own clocks, would measure the universe to be younger than13 billion years?
    Knowing their velocity relative to the comoving frame, they can transform their measurement to the comoving frame and get the standard 13.8 billion years for the universe.
     
  15. Jun 27, 2016 #14

    Drakkith

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    Now that you mention it, I'm not sure. I can remove that part of my post if necessary.
     
  16. Jun 27, 2016 #15

    PeterDonis

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    AFAIK Jorrie is correct; the proper time elapsed for comoving observers is maximal, in the sense that any other timelike worldline connecting the Big Bang and a given event on a comoving worldline will have less elapsed proper time between the two events than the comoving worldline does.
     
  17. Jun 27, 2016 #16

    Chronos

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    Isotope abundance is not affected by observer reference frames, AFAIK.
     
  18. Jun 28, 2016 #17

    Drakkith

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    Roger.
     
  19. Jun 28, 2016 #18

    mfb

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    Observers moving quickly relative to the CMB do not get a single well-defined age of the universe, because different directions will appear to have a different age. They can extrapolate their motion back to figure out how long they could have been moving since the big bang, then they'll get a shorter time.
     
  20. Jun 28, 2016 #19

    phinds

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    I don't get how different directions make any difference. Different speeds, yes. Since the CMB is everywhere, what does it even mean to have a DIRECTION relative to the CMB ?
     
  21. Jun 28, 2016 #20

    PeterDonis

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    It means you don't see the CMB as isotropic; you see a higher temperature in the direction you are moving, and a lower temperature in the opposite direction. We actually observe this here on Earth; the usual term is "dipole anisotropy" in the CMB. But practically all published data on the CMB corrects for this by subtracting out the dipole in order to display what the CMB would look like to a "comoving" observer at our location.
     
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