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Thread: Time dilation

  1. #1 Time dilation 
    Forum Bachelors Degree PetTastic's Avatar
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    As far as I understand, time dilation corresponding to redshift has been confirmed for distant supernova.
    However, galaxies are being detected only 480 million years after the bigbang at redshifts of around 10 or more.
    Does this mean they took less than 45 million years to form in real time?


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    Quagma SpeedFreek's Avatar
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    No, it means they took less than 480 million years to form, but we would see events in those galaxies (say, a supernova) last at least 10 times longer than it actually lasted, due to time-dilation.


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    Quote Originally Posted by SpeedFreek View Post
    No, it means they took less than 480 million years to form, but we would see events in those galaxies (say, a supernova) last at least 10 times longer than it actually lasted, due to time-dilation.
    I don't understand that, the link between time dilation and redshift is fundamental to general relativity.

    Are you saying time runs at different speeds for different types of event?
    Galaxies build without time dilation, but supernova are effected by time dilation?
    The galaxy is redshifted but not time dilated relative to us?

    ----------------------
    Edit:
    I was forgetting about light travel distance, so the 480 million already includes the effects of time dilation.
    Thats better, I was getting worried for a while.
    Last edited by PetTastic; October 27th, 2011 at 03:54 AM.
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    Quagma SpeedFreek's Avatar
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    The age of the universe at any given time is calculated after taking account of any cosmological time-dilation. It is the time measured by a hypothetical clock that has been around since the Big-Bang*, and time on that clock always ticks away at 1 second per second. That clock would have read t=480 million years when the light from those galaxies was emitted towards us, and it reads t=13.7 billion years as that light reaches us.

    If we were able to watch the history of the early universe unfold, from the Big-Bang till after those galaxies formed, 480 million years later, that sequence of events would be seen by us today to take many many magnitudes longer than 500 million years.

    Every event we see with a redshift of z=10 (those early galaxies) would be seen to occur over a duration at least 11 times longer than it actually happened. If we could watch the "cosmological clock" in that z=10 galaxy, we would see it ticking slower than our own clock by factor of 11, even though it always ticked at 1 second per second. If we saw, for instance, a supernova at z=10, it would burn for 11 times longer than a similar supernvova local to us. If it were at z=1, it would burn for twice as long.

    Every event we see with a redshift of z=1100 (the release of the CMBR) would be seen to occur over a duration at least 1101 times longer than the actual time the event took to occur.

    The light from distant events is effectively "stretched" by the expansion of the universe, by the same amount that the universe has scaled up since that light was emitted. This means we see those distant events play out over a longer period of time than they would have been seen to play out for an observer who was local to them. z relates to the scale factor in the form 1+z.

    It seems to us that time was "running slower", relative to time today, the further back we look. But if everyone in the universe had a clock that had been around since the Big-Bang*, it would "today" show an elapsed time of 13.7 billion years.

    *The cosmological clock is actually a series of clocks that co-move with the expansion of the universe and thus each clock is at rest in relation to the expansion, and all local gravitational influence has been accounted for. A clock that had actually been around since the Big-Bang and was in the region of our Solar system when it formed and was now on Earth would read a slightly lower figure, due to the local gravitational influence around that clock when compared to the gravitational influence around a clock deep in the void between the clusters of galaxies. The clock deep in the void would read a figure closer to "cosmological time".
    Last edited by SpeedFreek; October 29th, 2011 at 07:46 AM. Reason: Added extra info
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