What is 'Space' expanding into?

I'd never heard of /r/truereddit. Thank you for that.

It's interesting how the obscurity is a self-selection filter for quality contributors.

Making quality contributions requires more effort. Having the front-page content as a "honeypot" of sorts weeds out those individuals who are primarily in the "drive-by" mode, anxious to blurt out emotional statements for immediate gratification.

The interesting thing, too, is that there is a necessary friction to maintaining high editorial quality. There must be conflict to be able to enforce "creme de la creme" level of content for consumption. Which is fine, and not personal. All of us have our thoughtful/contemplative mood and the rash impatient one. At least on a relative scale.

I think it's more to do with the moderators to posters ratio that contributes to a higher signal to noise ratio. They have about 30 moderators.

Actually, r/askscience is still on the frontpage, they were only off it during Christmas due to the expected influx of users.

I am no physicist, but I still feel as though this is an immensely misleading subject.

The idea of an infinite universe conjures up images an infinite number of worlds out there, infinitely many of which must be similar to our own. And somehow, all that infinite matter/energy must have been compressed infinitely densely at the beginning.

But isn't it the case that we have only measured the flatness of space to within about half of one percent. If so, it seems much more likely that the universe is locally very flat but still finite, though very large, due to initial inflation (the event, not the process). So the universe might be 10^100 light years across, but still finite, yet definitely flat to well within half of one percent.

But let's suppose that the universe is infinite (since we have not, and may never rule it out). What does this actually mean? Does it mean an infinite number of galaxies out there somewhere at this moment in time? Somehow I doubt it.

What does it even mean for a galaxy to be out there at some point at this time? Do we mean time relative to the local reference frame of that galaxy (stationary in that frame), say with respect to the big bang or time relative to our own reference frame, which we agree is moving rapidly with respect to the other?

Even distance measurements vary between the two frames of reference.

So my guess is that to even have a concept of what this all means, we need to be expert at understanding space-time manifolds, which we are famously hopeless at.

Another obvious point: it isn't possible for an observer to travel off in one direction and return to the same point, even if the universe is curved (as I suspect it is). The reason is simply that you couldn't make that journey. There are points in the universe which lie out of causal contact with us, which means that there is no means of getting there, even theoretically. We can't even observe those points, let alone go there and see them. So we can never make the full journey in one direction and return from the other. Doing so would imply travelling beyond our cosmic horizon.

Indeed it does, there's a distance from beyond which light and information can never reach us. That's the notion of "observable universe". http://en.wikipedia.org/wiki/Observable_universe

That's an interesting idea. My answer would be that even though every volume of space would be contracting if you reversed the clock, each of these volumes don't form their own singularities, instead the near-singularities coalesce at the point of the big bang.

>>Doesn't this imply that there is a reverse event horizon around every point in space? That we are on the inside of an inside-out black hole?

Wow! Thanks for the new perspective! Very original and intriguing.

Relevant: http://en.wikipedia.org/wiki/Mayor_of_High_Wycombe#Tolling_o...

No, you do not 'loop around' the universe. It is flat, not a hyper-sphere. The 4th dimension of the universe is time, and not spatial.

     What do you mean, 'infinite in dimension'? The universe has only 4 dimensions, 3 spatial, 1 in time.

I'm was not asking how many dimensions the universe has. I was asking if the universe is infinitely tall, infinitely wide and infinitely long.

     Why do you need to invoke infinite speeds and times? 

Simply for matters of scale. I was trying to avoid considerations like "it's so big, it would take so long ...".

     If 2 particles stay in the same place [...]

Actually, it's one particle that moves, and other is stationary, thus providing two points of reference for determining a displacement. Specifically the intention was to indicate a path through space such that you always are moving further from where you started and will never arrive back at your starting point (as opposed to the spherical theory of space such that you can only go half way around the sphere until you start getting closer to the point at which you started.

      If you pick a random direction and travel really fast for a really long time, the chances of you running into any other matter is extremely remote. 

I didn't mean physically collide with other matter, I simply meant to pass by it in the "vicinity". Similar to how traveling along a highway you pass by or through towns, you don't physically collide with them though. If matter in the universe was finite, then regardless of how sparse that matter is, there's an imaginary bounding box you could draw around all matter in the universe. A particle moving really fast for a long time would eventually end up outside of that bounding box. At which point its distance not just from the stationary particle at it's original, but all particles of matter would forever be increasing. However, if matter is in fact infinite, then such a scenario can not occur.

Luckily, despite my questions barely making any sense, you've managed to provide in your reference [1] a comment that answers exactly my questions so thank you for that.