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Time 'measurement' (or definition) for early universe

  1. Jun 13, 2013 #1
    We define time by the rate at which physical processes (i.e. clocks) tick. With atoms for example we can define time by their energy transition rates such as in atomic clocks.
    But, what before atoms existed?
    Current cosmology theories make statements such as that '0.5 seconds after the Big Bang the universe was like this or like that' but how does time get 'measured' (i.e. defined) for such early epochs of the universe when no atomic processes existed yet?

    And a related question about 'Universal Time'. Such cosmological statements need to be expressed in an observer-independent frame of reference, that is 'in universal time'. Since special relativity tells us that the rate of passage of time is observer-dependent, how is universal time calculated so that we can make cosmological statements such as '3 minutes after the Big Bang...'? who's 3 minutes?
  2. jcsd
  3. Jun 13, 2013 #2


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    The cosmological time is the proper time as measured on a clock carried by a member of a family of isotropic observers comoving with the Hubble flow (it does not matter which member of the family carries this clock if the universe is homogenous). So indeed it is the time as measured by this preferred family of observers.
  4. Jun 13, 2013 #3
    Thanks, I think I get that as regards the 2nd question but I don't see it answering my first question. What would a 'clock' measure before any atomic processes existed?
  5. Jun 13, 2013 #4
    the second is defined currently in terms of atomic decay rates but does not depend upon decay rates. The decay rates only allow us to set a "standard" second. However if their is no atoms such as the first moments of the universe, time will still flow we would simply have a different method of defining a standard second.
  6. Jun 13, 2013 #5


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  7. Jun 13, 2013 #6
    thats a good paper thanks for pointing it out
  8. Jun 13, 2013 #7
  9. Jun 13, 2013 #8


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    It is a very nice question Gerinski! I myself rarely question the operational issues/hurdles associated with theoretical definitions/concepts e.g. the operational measurement of time during the early epochs using physical clocks. It's a bad habit of mine as a physics student.
  10. Jun 13, 2013 #9
    And I guess the same goes for space. We read cosmology statements such as 'when the universe was 10-34 seconds old (sorry I don't know how to use exponential notation) it was the size of a golf ball'. However we define 'space' (e.g. 1 meter) by the distance light can travel in a given amount of time. I guess that when time measurement itself becomes a problem, automatically size definitions become a problem too!
    And even more when photons could not travel freely!
    Last edited: Jun 13, 2013
  11. Jun 13, 2013 #10


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    Nice. Thanks.

    Followup by Rugh et al:

    Citation by Roberto Percacci in an introductory overview of Asymptotic Safety:

    Percacci's cite of Rugh et al. is in a frankly speculative paragraph near the end of his paper. See page 13, reference [50]:

    ==quote Percacci arXiv:1110.6389 ==
    Aside from these attempts to apply asymptotic safety to inflationary cosmology, one may try to make connection also to other ideas. One important fact is that physics at a fixed point is scale invariant 4. Even though the fixed point Lagrangian contains dimensionful couplings, these scale with energy according to their canonical dimension so that all observable quantities have power law dependences. Under these circumstances, defining a clock becomes impossible even in principle and the notion of time loses its operational meaning [50]. Although one may still be able to define separate points, time intervals and distances become meaningless. In this sense, one may argue that a fixed point leads to a notion of minimal distance [51]. This is also in line with the view that the metric geometry “melts down” near the big bang, but the conformal geometry remains well defined. In fact it is worth noting that if the infrared behavior of gravity was also governed by a fixed point, as conjectured in [52], then one would have scale invariance at both ends of the cosmological evolution. This would lend support to Penrose’s Conformal Cyclic Cosmology [53].
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