Science Questions

What distance separates two gamma ray bursts that are 13 billion light years from Earth each but in opposite directions?

Recently, a gamma burst was detected in the sky from a star about 13 billion light years away. Would it be possible for a detection of a gamma burst from a star 13 billion light years away in the exact opposite direction with respect to our galaxy? If yes, how far would the two stars be away from each other? I don't think it could be 26 billion light years? Sean Thompson

Dave -   The concept of distance gets quite difficult when you start thinking on the scale of the universe above anything else, because Einstein's relativity work says that depending on how fast you’re moving, distances can get compressed.

The first thing is, because the (13 billion light year) distant gamma ray burst is moving at a very different speed to us, the space in between is going to get distorted due to relativity. And if we’re both moving at the same speed, then the burst would be about 30 light years away. 

The universe is expanding all the time in every direction, and as far as we can tell this expansion looks the same wherever you are. If you can see an object which is 13 billion light years away you are looking at it 13 billion years ago, in that time it is going to have moved away a lot, it should now be about 46 billion light years away.  So in some senses, the two gamma ray bursts are now over 90 billion light years apart. 

However if the two gamma ray bursters could see one another then they wouldn't appear more than 13.7 billion light years away, as the only way one could see the other is right at the beginning of the universe when they were much much closer together.

Chris -   So why would you end up with the potential light wave that had or looked like it was 90 billion light years away?

Dave -   Well, the light hasn’t travelled 90 billion light years, the light has travelled 13 billion light years, but the object that emitted it has moved another 70-80 billion light years off in the other direction, so where it is now isn’t where it was when it emitted the light.

November 2009