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Q & A: seeing the early universe

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Most recent answer: 05/15/2013
Q:
if a gamma ray burst is witnessed on earth 13,000,000,000ly away that occurs 750,000,000 years after the big bang, does that mean the universe is expanding faster then the speed of light? for the shear fact that we can see it now? i ask because if you look at sound, if you was to ring a bell then travel faster then the speed of sound to pass it and stop, you will heir the sound a second time.
- Daniel Wynter (age 29)
NSW, Australia
A:

The key point is that the type of things we see from that source depend on its age in its own reference frame, not some other reference frame we might choose. When the source was 750,000,000 years old in its own frame, it would be much older in a conventional extension of our usual local frame out that far. You can see that easily in a special relativistic  calculation (which I can handle, unlike a proper general relativistic one). Objects moving away from us have,  according to our frame, slowed clocks- the "time dilation" effect.


see also:



Your other question was whether some of the universe is moving away from us faster than c. The answer depends on what you mean by "our universe". It sure looks as if some stuff that is part of the same mathematical object as us got inflated to moving away faster than c, departing past the horizons of our visible universe.

Mike W.


(published on 05/15/2013)

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