Observable Universe Explanation

[Yoseph]

Hey guys,

 

I have read and heard a lot about astronomy but I recently realised there's something about the basics I don't quite understand.

For us to be seeing galaxies that formed in a comparatively short time after the big bang, how did we get so far away? Would we not have had

to travel at near light speeds so that the light from these galaxies now has to catch us up as it were? In fact, would we not have had to travel faster than the speed of light, otherwise the light would have gone past us as the universe expanded? As you can see I'm confused, any help?

[Strange]

This raises lots of interesting questions.

 

First, it is important to understand that neither we nor the other galaxies are moving away from each other through space (1). It is (slightly) more accurate to say that the amount of space between them and us is increasing. In other words distances are being multiplied by a scale factor over time.

 

So everything was once much closer together. If we see light from galaxies that was emitted about 13 billion years ago, then those galaxies were about 4 billion light years (2) away when the light was emitted. But the light took 13 billion years to reach us because the space it was travelling through was constantly expanding (so it is like the light was trying to swim against the tide).

 

Now, those galaxies are about 45 billion light years away (3).

 

Note that the "faster than the speed of light" thing isn't a problem, because nothing is moving through space that fast. That limit comes from special relativity which just deals with local motion in non-curved space-time. The expanding universe is described by general relativity.

 

(1) Or at least, not significantly: there is some local motion. For example, our galaxy might collide with Andromeda in a few billion years

(2) I think ... from memory

(3) Ditto