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Time and the relative speed of an Expanding Universe

  1. Nov 24, 2009 #1
    I would appreciate assistance in answering the following.

    Does the existence of time depend on the universe expanding at an accelerated rate?
  2. jcsd
  3. Nov 24, 2009 #2
    'existence of time' is something difficult to define

    Less fundamental thing, the 'appearance of flow of time for the observers ' is caused by the macroscopic arrow of time. The macroscopic arrow of time exists because of the boundary conditions at the Big Bang (low entropy). So yes, in some sense the TIME has cosmological explanation. Time is always pointing to the direction where entropy increases, so, it is always pointing away from the Big Bang -> BB is always in the 'past'

    So the expansion is important. Accelerated expansion is not.
  4. Nov 24, 2009 #3


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    I think Dmitry67 is correct.I would also point out that you can have time without having an arrow of time -- not sure which notion of time markjuliansmi had in mind.

    Re the original question, there is a sense in which the truth is nearly the opposite of what markjuliansmi conjectured. You can have a universe that's dominated by (1) radiation, (2) matter, or (3) the cosmological constant. In cases 1 and 2, you can observe the local conditions in the universe and infer something about "what time it is." In case 3, it turns out that you can't; a cosmology that's purely of this type has a kind of time-translation symmetry that 1 and 2 don't.
  5. Nov 25, 2009 #4
    At the point of the Big Bang there was no time outside the singularity. The edge of the universe, from that instant on, represents the moment of creation of time. This is why you can't explain what is outside the universe. There will be stuff there, as soon as time reaches there.

    I have no idea how this realtes to acceleration, though.
  6. Nov 26, 2009 #5


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    This is incorrect. If you want to understand this kind of thing better, a good nonmathematical book is Gardner's Relativity Simply Explained.
  7. Nov 28, 2009 #6
    Thanks everyone for replying. I suppose being rather a scientific novice I wondered, if to be in a position to observe change occurring between two points in time, as we do today, do we need to have the Universe accelerating at the current rather than expanding at a constant velocity or accelerating less than we are observing today.

    I suppose I also wondered if what we are observing in regards the expansion of the Universe is more spin than actual expansion – a question most likely emanating from my incapacity to actualise in my mind the enormity of what surrounds us rather than on any actual scientific fact.

    If we humans survive for another thousand years I wonder also if we will not require Babel Fish to communicate between the guilds of knowledge one would expect to develop - an absolute necessity for BBQs to save the sanity of both sides.

    bcrowell thanks for your suggestion I will order the Gardner's Relativity Simply Explained.
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