Shouldn't it be 13 * 2 = 26 across? 13 light years that way, and 13 light years the opposite way? Is there some "relativity trick" I'm missing? Is some space aliean on the edge of the universe going to see 13 light years in all directions?

This is a misconception. If you just want a satisfying mental image of the Universe: It's really big, it doesn't have any point defined as its center, and it doesn't have any wall-like borders. Aliens living 14.6 billion LY away from us see a Universe that looks just like what we see in all directions.

If it is limited in size at all, it's because it wraps around on itself ("if you go straight long enough you end up where you were"). Nobody has firm evidence that it actually does wrap around on itself, and there's reason to believe that if it does, it's on a scale that's much larger than we could ever hope to observe. We will likely never have any sign inconsistent with it being literally infinite regardless of whether or not it is.

If you want a better answer than that, you'll need a very firm grasp of special relativity and a basic understanding of general relativity and the metric expansion of spacetime. In which case, read this: http://arxiv.org/pdf/astro-ph/0310808.pdf

The parent post was trying to say "13 billion lightyears in <--- that direction and 13 billion lightyears in ---> this direction", because we happen to be at a relatively fixed point in space and can look in both directions. That doesn't assume we're at the center of the Universe, only that the light has reached us coming from both directions for as long as the Universe has been around.

Was it instantly huge after the big bang or did it expand much faster than the speed of light?

The Schwarzschild radius of the observable universe is 10 billion light years. Me thinks that for the mass to be compacted into a smaller space at some point in the past, it would have either had to escape it's own event horizon (since under most current definitions it would be a black hole), or the definition of light-year had to have changed from what it is today. In that regard, the only thing making up the definition of light-year is distance and time; so space-time itself was radically different way back then (from our current perspective of relatively low energy densities).

Both! If the universe is flat, then it was always infinitely big. And, according to the inflationary universe theory, it expanded faster than the speed of light too.

In this case, you have to think of "expanding" as being like stretching. I.e., things within the infinitely big universe rapidly got farther apart from each other.

How could it have always been infinitely big if there was a big bang where everything was once?

Everything we can see (which is called our "Hubble sphere") started off as a tiny little (and very massive) speck of stuff at the beginning of time. There were infinitely many tiny little specks, and they all became different Hubble spheres. But all the Hubble spheres all overlap each other, forming one continuous space. The same thing was true about the little massive specks at the Big Bang.

As I said above, at the time of the Big Bang, the universe was like a very dense sheet of rubber, and then it started rapidly stretching, getting less and less dense over time, until it is now the density that we see around us.

Infinities can definitely give you a headache, though. For instance, there are just as many odd integers as there are integers.

The latter: it went through an inflationary period; the inflationary model supplanted the big-bang theory in the 1980s.

Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space. ~ Douglas Adams, The Hitchhiker's Guide to the Galaxy

>Aliens living 14.6 billion LY away from us see a Universe that looks just like what we see in all directions.

How's that? Won't they see a completely different universe due to the speed of light? If they're 14.6 billion light years away from us, and they in our direction, won't they see what was happening in this area 14.6 billion years ago?

He doesn't mean the aliens will see an exact duplicate of what we see but that the universe is uniform enough that what they see would have the same properties (density, etc.) as what we see.

> Shouldn't it be 13 2 = 26 across?*

I guess this depends on how you define "across". Our Hubble sphere is all the space around us for which light has arrived. If the universe is infinitely big, then yes, there are things that we can see that are 26 billion light years from each other. However, they can't see each other, and so they are not causally connected to each other, even though we are causally connected to both.

>Is some space aliean on the edge of the universe going to see 13 light years in all directions?

An alien at the edge of our Hubble sphere will live in a different, overlapping Hubble sphere, from which they will see 13 light years in all directions around themselves.

that would assume that there is a center to the universe. I'm not sure there is (speaking as the lay-est of laypeople). There are articles on the topic which I won't embarras myself by summarising in a HN comment...

http://www.quora.com/The-Universe/Does-the-Universe-have-a-c...

considering I knew even less than I thought I did 20 minutes ago I am going to wait for someone who actually knows to chime in.