In a NPR News story from a few years back:
"A gamma-ray burst from about 13
billion light years away has become
the most distant object in the known
universe."
I'm a layman when it comes to physics, so cut me some slack if this is an ignorant question, but assuming the universe is around 14B years old, and has been expanding since the Big Bang, how is it that we can see events so far back in time?
I understand how it would work if you had a static universe and the GRB happened 13B years ago at 13B Light years away and the light just arrived. However, at the time of the burst wouldn't we (or at least the matter that we are made from) have been much closer to the source of that burst, and wouldn't the light have blown by us eons ago? How is it we are seeing it now?
If we were expanding away from it at close to light speeds it would seem to make sense for why it took so long for it to get here, except for that whole notion that light moves at the same speed relative the to the observer, which I think would blow that idea out of the water.
Perhaps gamma rays travel at sub-light speeds? But, I'd still think the math would require that they travel MUCH slower than light for this scenario to play out.
Another, possibility is that the light has wrapped around a finite universe a few times before reaching us. Of course if that were the leading theory, there wouldn't be any remaining controversy about the finite vs. infinite universe models.
What am I missing here?
Best Answer
There are at least two ideas involved.
First is that the expansion of the universe is not linear. While the Big Bang happened around 14B years ago, that does not mean that 13B years ago, the Universe is 1/14th of its present size. Current theory suggests that a large portion of the cosmological inflation (where the Universe increased by 26 or more orders of magnitude in linear dimensions) happened within much, much less than a second after the Big Bang. And as another example, the current theory estimates that at the time that the cosmic microwave background was emitted (which was about 0.5 million years after the birth of the Universe, placing it about 1/30000 the current age of the Universe), the universe is already about 1/1000 its current size (in length).
Second is that the apparent recession of far away objects from us is not so much objects flying apart from each other. Rather, it is space being added in between objects. Imagine you being the photon, and two turtles (moving slower than you) being the galaxies. Put turtle one in the first carriage of a train, and put turtle two on the 10th carriage of a train. And you start walking. Say it takes you 1 minute to traverse a carriage, and it take the turtles 10 minutes. Then in the case where the turtles walk away from each other, it will take you a bit under 12 mintues to get from the first turtle to the second (you walk 10 minutes to the tenth train, and the turtle has gotten to the 11th. You walk another minute to the 11th train. The turtle is just a few steps in front of you.)
But that's not how the universe expands. The expansion of the universe is more like the following: suppose every 6 minutes, all the carriages decouple, and between each pair of the original carriages plops one more car! So you walk for 6 minutes (having traversed 6 cars), and you look up, and see that the second turtle is 8 cars in front of you (and the first turtle is 12 cars behind). And you walke another 6 minutes. Plop comes the extra cars, and now you are 4 cars from the second turtle and 36 cars from the turtle behind. And finally after another 4 mintues you catch up to the second turtle.
From the point of view of the second turtle though, you would have travelled from a turtle that is now 40 cars away from him, while taking only 16 minutes! This ties back into the funny idea that light emitted from an object 13B lightyear away can take quite a bit less than 13B years to get here, due to the inflationary Universe.
This is why cosmologists and astronomers use red-shift to measure distance, because there is no reasonable intrinsic notion of distance that is free from ambiguity: should distance be described by how far away the turtles are when you started walking? or when you finished walking? or the number of carriages you (the photon) have traversed? Instead of that, they measure it using red-shifts, which can roughly fit into this turtle-you framework as how flushed your cheek is from all that walking when you reached turtle number two. Based on the redness of your cheeks, the turtles can calculate how much you exerted yourself, and thus for how long you've been traveling, and using known rules of the addition of new cars (the value of Hubble constant), the turtles can estimate the distances to other turtles.
:-)(I'm going to skip discussion of standard turtles, which are turtles from which you will always depart well rested and not flushed, nor how the turtle simiano-ferroequinologists found out about their rates of locomotive expansion.)