The edge of the universe

Edwin hubble discovered that the universe is expanding…IIRC 70 lightyears per megaparsec.

The farther away, the faster the recession from us.
At some point, the expansion away from us is lightspeed.

Now what I remember from special relativity is that an object approaching the speed of light flattens out in the direction of travel, mass increases to infinity, time slows down, and radiant wavelengths/frequencies redshift to zero.

Does that mean at the lightspeed event horizon we are surrounded by a spherical shell of quantum singularity?

i have another question while we’re at it: what is the universe expanding into?

70 km/s/Mpc. Mass is not affected by motion, and anyway, things out there aren’t precisely what you’d call moving, either. And it has nothing to do with quantum mechanics, which nobody knows how to apply to general relativity, anyway. There is a horizon at that distance, relative to us, but an observer out there wouldn’t see anything qualitatively different from what we see.

The question is ill-posed. The Universe has no boundary, so it would be incorrect to say that it’s expanding into anything.

But…but…the universe is 156 billion LY across. But it’s only about 15 billion years old. So isn’t it expanding at about ten times the speed of light?

No, because it’s only an effect of our perspective; from the perspective of anything far away they are normal and we are the ones redshifted and distorted.

It is theorized that early on it was expanding much faster than ten times lightspeed. And note that the figure given in that article is only a minimum size.

And before you ask how the universe can expand faster than light; the lightspeed limit only applies to matter and energy; it is space itself that is expanding, and it isn’t limited by lightspeed.

How much faster, exactly? It may shed some light (pardon the pun) on a question I posed in a recent thread that I felt I never really got an adequate answer to.

Wow, good question. I see many of our resident Dopers-who-understand-fantastically-complicated-physics are present… can one of you perhaps explain “it is space itself that is expanding” in a way that I could get my head around?

If the universe has no boundary, and space itself is expanding – what exactly does that process entail? What is happening?


Are you sure? Because if I think about this for any length of time, my brain starts to leak out of my ears.

do you really expect a bounded brain to be able to encompass an unbounded universe? Moreover, the universe could be finite but unbounded. Think about flatland creatures whose universe was a sphere.

How’s this?

Imagine some (tiny) two-dimensional creatures, living on a surface. Relativity applies to them: No objects in their 2-D universe can move faster than light, etc.

Now you come along and notice that they’re not living on a flat plane, but instead on a sphere. Their universe is bounded (it’s not infinite) but it doesn’t have any edges either: they can never reach the end of their universe.

Finally, you notice that the sphere they’re living on is actually a balloon that something is blowing up. Their universe is expanding, and to one of the creatures, everything is moving away from it, and things farther away are moving away faster (remember, any distance to them is along the sphere). But there’s no center of the expansion: everything is moving away from everything else.

That’s always been a popular illustration of the concept. But it always begs the question of ‘What the hell? How can SPACE be expanding?’

It demonstrates the troubles of analogy as an instructional tool, really.

What it demonstrates, Jonathan Chance, is that no one has a good understanding of just what spacetime is at present. Physicists and philosophers have debated for centuries the question of whether there exists an absolute (if dynamic) canvas or if space and time can only be defined in a relational sense. The overwhelmingly predominant interpretation of General Relativity ties gravity to distortions of spatiotemporal geometry; implicit in that statement is the notion that there must be something there to carry those distortions. The mathematics of GR are easiest to use in this formulation, however it would be just as valid to construct it in a completely relational way.

Bottom line: we cannot answer the question of what it means for spacetime to be expanding until we understand what spacetime is. This is one of the fundamental questions that a successful theory of quantum gravity will have to answer.

Edit: I should add, though, in the context of this example about Universe expansion, it is important to remember that we do already know that spacetime is part of the Universe. Space and time did not exist before the Big Bang, the Big Bang created them along with everything else. Once you abandon the notion of spacetime as some sacrosanct background ruler, it becomes easier to accept that it might do something like expand.