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I How is the timing of the big bang known

  1. Oct 19, 2016 #1
    If it is not know what happened in the first fraction of a second after the big bang, how is it know that it was really only a fraction of a second?
     
  2. jcsd
  3. Oct 19, 2016 #2
    you can't talk easily about time without setting it relative to something. What is a second? Usually it's define by electron flips in a hydrogen atom, but there was none of that back then. It could be defined by the speed of light (causality,) but there are theoretical models where that changes too. The equations all work as long as the units are the same. If it took two seconds to do what normally takes one second, how would you know unless you have some way to compare?

    Near a black hole, a second is still a second, regardless of the fact that that from our perspective it's going slower due to GR
     
  4. Oct 19, 2016 #3

    PeterDonis

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    Please give a more specific reference for where you are getting this from and what you mean by it. What do you think is not known? There are lots of different ways of explaining our best current cosmological model.

    The times usually quoted when referring to our best current cosmological model are FRW coordinate times, which are the same as proper time along "comoving" worldlines (worldlines of observers who always see the universe as homogeneous and isotropic). These times are easily calculated in the model, and those calculated times are what are usually quoted. (The input to the calculation is usually the average temperature of the universe when some particular event of interest happened; our best current model basically gives you a correlation between FRW coordinate time and temperature.)
     
  5. Oct 20, 2016 #4
    This is from Wiki
    In Big Bang cosmology, the Planck epoch or Planck era refers to the earliest stage of the Big Bang, before the time passed was equal to the Planck time, tP, or approximately 10−43 seconds.[15] There is no currently available physical theory to describe such short times, and it is not clear in what sense the concept of time is meaningful for values smaller than the Planck time.

    So, I can see why it would be the "earliest stage of the big bang" but I don't understand why it would necessarily be of such a short duration.
     
    Last edited by a moderator: Oct 20, 2016
  6. Oct 20, 2016 #5

    PeterDonis

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    Which is not a good reference. Have you looked at actual textbooks or peer-reviewed papers on cosmology?

    Briefly, there are several misleading statements in what you quote:

    (1) The "Big Bang" properly speaking does not refer to an "initial singularity"--we don't even know for sure if there was one. It refers to the end of inflation, when the universe was in a hot, dense, rapidly expanding state, but certainly not of "zero size" or even Planck size. This "Big Bang" state is the earliest state of which we have reliable knowledge.

    (2) The "time passed" referred to in the quote is not the actual proper time along a comoving worldline; it is a notional (i.e., wrong) time based on taking the temperature of the universe at some event of interest (in this case the universe being of Planck size) and converting it to a time using an idealized model in which the universe starts with an initial singularity and there is no inflation. But as above, we don't know if there even was an initial singularity, and we do know, at least with pretty high confidence, that there was inflation, so the idealized model used to obtain this "time passed" is, as above, known to be wrong. It's a shame that cosmologists continue to quote times from this model that is known to be wrong, but unfortunately they do.

    (3) Not only do we not know if there was an initial singularity, we don't even know if the universe ever was of Planck size; there are some models in which it never gets that small. And we also don't know for sure that the concept of time (or spacetime) ceases to be meaningful at the Planck scale; that is a plausible speculation based on what we think we know of quantum gravity, but we don't know very much about quantum gravity and we don't have a good theory of it yet, so any such speculations are tentative at this point.

    (4) As above, we know with pretty high confidence that there was inflation, but we don't know how long it lasted (though we have some lower bounds, based on how much inflation had to take place to make the "Big Bang" state at the end match the properties we observe) or what came before it; all we have, as with quantum gravity, are various speculations that are all tentative at this point.
     
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