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Author Topic: Why didn't light from this distant object pass us when we we  (Read 2477 times)

Offline thedoc

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Hi

I have a question about the recent star explosion which it is felt might be the most distance object ever detected.

Here's a link to the BBC article: http://www.bbc.co.uk/news/science-environment-13539914

It states in there that the star exploded some 520 million years after the Big Bang and that the light has taken 13.14 billion years to get to us.

However, if it happened 520m years after the big bang and given that we assume that every point in space and time was in the same place at the Big Bang, then it seems to me that this means that it was at most 520m light years from the piece of space that we occupy now when it exploded. By that rationale, it should only take a maximum of 520m years for the light to arrive. 

Now, while I appreciate that the universe is expanding, does this not contradict the laws of Relativity? If light takes one year to travel one light year, then in order for light to take 26 times as long to get here as the initial distance the explosion was from us at the beginning would suggest, does this not mean that either light is travelling at 1/26 the rate it should be or that the star that exploded is moving away from us faster than the speed of light? It might be 13bn light years away now, but it wasn't when it exploded.

In essence, I suppose I want to know why the light from this star didn't pass the Earth 12.5bn years ago.

I hope that makes sense!

Mike
Asked by Mike Coe


                                        Visit the webpage for the podcast in which this question is answered.

 ...or Listen to the Answer or [download as MP3]

« Last Edit: 21/06/2011 15:14:53 by _system »


 

Offline Pikaia

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Why is light from the early Universe still around today?
« Reply #1 on: 01/06/2011 19:40:00 »

However, if it happened 520m years after the big bang and given that we assume that every point in space and time was in the same place at the Big Bang,....
Your error is in thinking that the universe began as a point, which is a common mistake - it was actually infinite at the BB. The light originated 13 billion light-years away, which is why it took 13 billion years to get here!
 

Offline imatfaal

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Why is light from the early Universe still around today?
« Reply #2 on: 01/06/2011 20:17:05 »
Pikaia is dead right; but I think you may also be confused over the way galaxies separate. 

Expansion of space is not the same as travelling through space.  Nothing travels through space faster than light.  But, space itself can expand so that two distant points seem to be separating at a speed greater than light (note again nothing is moving thru space quicker than light). 

Analogies are difficult here, but here goes; imagine you are standing on a large pavement of 1m square paving stones, your significant-other is standing ten paving-stones away (ie ten metres away.  If each stone grew by ten centimetres - you would suddenly be a metre further apart; but neither of you moved with relation to the paving stones.  Now imagine the pavement is very big - it's 3,000,000,000 stones between you and your best girl/boy (delete as applicable), if in under a second the paving stones grow by ten percent you have separated in less than a second by 300,000,000 metres; and thats faster than the speed of light, without even moving! 

Now the rate of expansion is space is less that 10pct per second but the distances between objects is vast so objects can seem to be rushing away from us at enormous speeds without moving much through space.  For a bit of guidance to how unintuitive this can be look at the CMBR (cos I know the figures) - the period which caused that happened was 13.5-6ish billion years ago, ie the light has been travelling for 13.5-6ish billion years, but if we were to identify where in the universe the light we are seeing now came from it would be over 40 billion light years away.

1.  The big bang was everywhere not a point in space
2.  Travelling cosmological distance you must look at speed thru space and expansion of space
 

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Why is light from the early Universe still around today?
« Reply #2 on: 01/06/2011 20:17:05 »

 

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