Originally posted by Xeven
If the light from the furthest oldest known galaxy has traveled 13 billion years to get to us then how far away was the galaxy from us when the light
That's easy: 13 billion light-years.
Also how far apart would that galaxy and earth be today?
That's a bit more difficult. It depends on the rate of recession of the galaxy, which can be calculated from its
. However, this assumes that space has been expanding, or
rather inflating, at the same rate for the last 13 billion years. It has not.
The inflation of the universe seems to be a relic of the Big Bang. At the very beginning of things, before matter and therefore mass began to exist,
this inflation was orders of magnitude greater than it is now. When matter began to form, however, gravity came into the picture and began slowing the
expansion. However, this expansion has not been even over the life of the universe; at some times it has sped up, at others slowed down.
It is not, however, impossible to find an answer to your question. We can correct for the changing rate of inflation and, ignoring local phenomena
like the so-called Great Attractor, come up with some kind of a ballpark figure. This is, however, pretty advanced mathematics, and there would be
other physical complexities to be accounted for. I confess these are beyond the scope of my knowledge.
Would that not mean the universe is that much larger than it was when the 13 billion year old light left that galaxy?
Oh, very much larger. It's even larger than your question implies.
The farthest objects we can see are about 13.7 billion light-years away. This doesn't mean the universe is just 13.7 billion light-years in diameter;
just that anything farther than that is moving away from us faster than the speed of light itself.
This is good physics, not some pseudoscientific woo paradox. The relativistic speed limit applies only to massive objects (photons in motion have
mass). Space has zero mass (being made of nothing) and can 'move' just as fast as it wants to, carrying the mass-energy it contains along with it.
Precisely what is meant by the inflation of nothing is a problem I'll leave you to consider for yourself.
Objects carried away from us by the inflation of space at faster than the speed of light become invisible. This is not, as you may be forgiven for
thinking, because their light never reaches us; remember, light always moves at the same speed through a medium relative to a given observer, no
matter what its source. No; it is because the light emitted from the receding source is red-shifted
so far down the electromagnetic spectrum it
becomes undetectable even as long-wave radio signals.
How far apart would the furthest galaxy in one direction be from another equally distant from us in the oposite direction? Would that not imply
the universe is larger than 13 billion light years?
Oh, even the observable universe is much bigger than that. This Scientific American
article will tell you more:
Misconceptions about the Big Bang
[edit on 2/7/10 by Astyanax]