Science Questions

Why didn't light from this distant object pass us when we were closer?

Sat, 4th Jun 2011

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Mike Coe asked:



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:†


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!




Dominic -   Thatís a really good question actually.  Just to recap, the question is, this light was emitted when the universe was made at an age of half billion years and so, the universe could only at that time have measured a billion light years across.  And so, the greatest distance this object couldíve been from us is a billion light years and that light would then take a billion years to reach us and yet, we know the universe is 13.8 billion light years, so why are we seeing this light?

What this boils down to is Einsteinís theory of special relativity, which I know sometimes makes a lot of peopleís head hurt and sometimes makes my head hurt too, but I'm going to do my best to explain this.  One of the rules of Einsteinís theory of special relativity is that moving clocks are said to run slow.  So what that means is that if you take a clock and you put it in a fast moving spacecraft for example, that clock will appear to run slow.  The laws of physics will appear to run slow so you'll age more slowly.  Everything appears to happen in slow motion. 

Now, weíre very used, obviously, to the fact that there's absolute time.  So if I sit here and do nothing for 10 seconds then you would all agree whether you're moving in a fast car or whether you're sat still at home that that was 10 seconds.  But Einsteinís theory says that's not actually the case and that if you're moving in a fast car, then you would appear to see me in slow motion and that 10 seconds will be longer for you.

So what's happening to this galaxy is that because itís on the other side of the universe, itís moving away from us very fast and that means weíre seeing it in slow motion.  Itís evolving in slow motion and so, though the galaxy, in its own reference has only evolved for half billion years.  In our frame of reference, that would take about half the current age of the universe to get to that point.

Chris -   What about the other half?

Dominic -   And then the other half of that time was spent with the light travelling across the universe from the object to us.

Chris -   And stretching out in the process because the universe is expanding presumably.

Dominic -   That's right and that's the cosmological redshift.

Chris -   And that's why we now detect the light that wouldíve been very, very bright high intensity short wavelength light has now got a very long wavelength out in the invisible microwave regime because itís stretched out in that way.

Dominic -   That's right and the reason that we talk in terms of the age of the object being half billion years is because obviously, weíre interested in how far this object has evolved down its evolutionary track, not when it was in the history of the universe that light happened to be emitted because that's not very interesting.  


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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! Pikaia, Wed, 1st Jun 2011

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 imatfaal, Wed, 1st Jun 2011

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