[Tom Nawalinski]

Tom Nawalinski  asked the Naked Scientists:
   
On a recent podcast, your guest was explaining how he was able to detect the most distant object ever seen.

My question is: if the light from the gamma ray burst left the star over 13 billion years ago, wouldn't the Earth have been much closer at the time since the universe was only nine percent of it's current size?

And if the Earth was closer, wouldn't the light have arrived long ago? How is it that we can see it now?

Tom Nawalinski

What do you think?

[LeeE]

It certainly seems that everything, on a galactic scale, and apart from our 'local group', are all moving away from each other, so yes, it does make sense that everything was closer together in the past.

The reason that it has taken so long for the light from something that was closer to us when the light was emitted is that during the journey of the light, the distance it has had to travel has increased.

Imagine that someone sets off walking towards you, from ten metres away, at a speed of 1m/s.  If you stand still then after one second they'll be 9m from you and it will take them a total of ten seconds to reach you.  If you're walking away from them though, at say 0.5m/s, instead of standing still, then after one second the person still has a further 9.5m to walk instead of just 9m.  In the end, it will take them 15 seconds to catch up with you instead of just ten, and they will have traveled 15m instead of ten.

This is very simplified, of course, and instead of you walking across the ground and away from the person walking towards you it is more as though you are standing still on the ground but the ground itself, where you are standing, is moving, being forced away from where it was because new ground is appearing between you both.

[Fozzie]

It certainly seems that everything, on a galactic scale, and apart from our 'local group', are all moving away from each other, so yes, it does make sense that everything was closer together in the past.

The reason that it has taken so long for the light from something that was closer to us when the light was emitted is that during the journey of the light, the distance it has had to travel has increased.

Imagine that someone sets off walking towards you, from ten metres away, at a speed of 1m/s.  If you stand still then after one second they'll be 9m from you and it will take them a total of ten seconds to reach you.  If you're walking away from them though, at say 0.5m/s, instead of standing still, then after one second the person still has a further 9.5m to walk instead of just 9m.  In the end, it will take them 15 seconds to catch up with you instead of just ten, and they will have traveled 15m instead of ten.

This is very simplified, of course, and instead of you walking across the ground and away from the person walking towards you it is more as though you are standing still on the ground but the ground itself, where you are standing, is moving, being forced away from where it was because new ground is appearing between you both.

Hang on though, this analogy is OK for something which continues to exist like a star shining, Tom is talking about a one-off event which happened for a brief period, and then was gone.

Taking the original example, the gamma-ray burst actually occurred 13 billion years ago when the universe was one ninth of its present size. Lets say 10% for clarity. In that case, it would have happened 13 billion years ago minus 90% meaning that is was 1.73 billion light years away from us when it went off. But surely, however fast it moved away from us, the light from that single transient event, would have reach us 1.73 billion years later - ie 11.27 billion years ago?  []

[Mr. Scientist]

Tom Nawalinski  asked the Naked Scientists:
   
On a recent podcast, your guest was explaining how he was able to detect the most distant object ever seen.

My question is: if the light from the gamma ray burst left the star over 13 billion years ago, wouldn't the Earth have been much closer at the time since the universe was only nine percent of it's current size?

And if the Earth was closer, wouldn't the light have arrived long ago? How is it that we can see it now?

Tom Nawalinski

What do you think?

Relatively-speaking, the universe would have been smaller and more compact in the distant past, so yes - the earth would have been physically closer to these other star systems... which uniformly expanded, meaning there is no center to this big manifold!

[PhysBang]

Taking the original example, the gamma-ray burst actually occurred 13 billion years ago when the universe was one ninth of its present size. Lets say 10% for clarity. In that case, it would have happened 13 billion years ago minus 90% meaning that is was 1.73 billion light years away from us when it went off. But surely, however fast it moved away from us, the light from that single transient event, would have reach us 1.73 billion years later - ie 11.27 billion years ago?  []

Where the burst actually went off was closer, but it had to cross a distance that was expanding. When it went off, it was probably about 2 to 4 billion years away, now the event is probably 20 to 40 billion years away. The light from the past event took about 13 billion years to get to us.

Think of a bucket brigade passing water. Now imagine that bucket brigade passing water from the front of a train, along the length of the train, to put out a fire on the side of the track. As the train pulls away, the water will have to be passed farther and farther down the train to be thrown out onto the fire.

[Fozzie]

Thanks for a great explanation. I hadn't allowed for the fact that the event is now probably 20 to 40 billion light years away.

[Mr. Scientist]

To have an event horizon the universe would need a boundary. The univers seems to not have a boundary. String Theory differs those rules though.