And what is space made of, again?
Yes, actually, a topologically nontrivial universe like an Asteroids screen does have a preferred frame of reference, but only on a global scale. That is, any experiment to determine that preferred frame would have to span the entire universe. On any smaller scale, it would be indistinguishable from a universe without any preferred frame.
I don’t understand the question.
I’m still struggling here. So space can expand faster than c, and this carries the matter with it, and hence there are objects in the universe that we cannot see? Is this right?
Space.
In some ways this is a deep question, but for most ordinary purposes, including those of this thread, space is not made up of anything. It’s the container of things like matter and energy, but is not matter or energy in and of itself. Therefore it is not subject to the limitations of physics the way matter and energy are.
And that means you have the gist of it. Most cosmologists believe that while we can see about 13.7 billion light years worth of space, our universe is at least 50 billion light years in extent.
What would happen if you were magically transported out 13.7 billion light years? You would see a universe about 13.7 billion light years old, with the rest of it hidden. If you used our physics and our observations you would derive a universe with the same properties as ours and consider yourself at the center just as we do.
Doesn’t this mean that the balloon analogy does in fact hold up fairly well? Just as the expansion of the 2D surface of the balloon has a centre in 3D space, so the the expansion of the universe has a centre in 4D space-time. The centre cannot be found on the surface or in the universe, one has to travel at ‘right angles’ to the familiar dimensions to reach it.
This is a genuine question/comment, I’m not trying to pick holes in any theory or argument.
Except as far as we can tell space isn’t curved. So either the “balloon” is very, very big (so the curve is very gentle) or space is more like an infinite flat rubber sheet than a balloon. And where’s the “center” of an infinite flat rubber sheet?
Regarding the metaphor of the universe as an expanding balloon . . .
In our daily lives whenever we encounter a two-dimensional surface, it’s embedded in a three-dimensional space. So for instance when we see the two-dimensional surface of a balloon, we see also that it is embedded in a three dimensional space that includes the volume outside the balloon, the volume inside the balloon, and in fact the thickness of the balloon itself. So of course we say “The balloon is expanding from a central point; the point just isn’t part of the balloon’s surface.”
However, it is possible with math to describe a two-dimensional surface like that of the balloon, but without needing to embed it in a three-dimensional space. And in fact, it is also possible to describe a four-dimensional universe without embedding it in a higher dimensional space.
Thus it is a mistake to carry the metaphor of the balloon too far and assume that the universe is expanding from some central point which lies outside the universe. The balloon happens to be embedded in a space that contains a “center” for the balloon’s expansion, but the universe need not be embedded in such a space. Just because there’s such a thing as “outside the balloon” doesn’t mean there need be an analogous thing that is “outside the universe”. The balloon is merely an imperfect metaphor.
Yes, that’s right.
Space expanding faster than the speed of light and carrying matter with it is OK with relativity, because the matter isn’t moving relative to its surrounding space at a speed faster than the speed of light.
Okaaay… I think…
But, wouldn’t the matter, from my point of view, be moving faster that the speed of light? So at time=0 (the big bang), I’d be able to “see” every particle of matter in the universe (as it’s all in the same place), but at time=now I can’t. So, from my frame of reference, I must have been able to see (or measure) that these particles have apparently moved away from me faster than c.
Depending on the initial state of the Big Bang you might or might not have been able to see EVERYTHING. (Whether the universe was actually a singular point at the moment of creation is still open to debate.) However we can be sure that things were much, much, much closer together than they are now.
As the universe expanded, some things that were originally in causal connection with each other (light was able to pass between them) moved apart so rapidly that they were no longer in causal connection with each other (they were so far apart that light couldn’t close the gap anymore). Does that answer your question?
As a practical matter, you couldn’t have seen anything that close to the Big Bang (even if you somehow managed to get a time machine and get there). Until protons and electrons combined to form hydrogen, about 380,000 years after the Big Bang, the universe was opaque, because photons were scattered by the free electrons. Inflation took place in the second following the Big Bang.
Sorry, but I still don’t understand! What you’re saying in the second paragraph seems to me to be that things moved apart faster than the speed of light. And that if there were someone around they would have be able to see this happening. But I thought this was supposedly impossible under any circumstance (Einstein and all that)?
What they would see then is what we see today. Objects farther away move at a larger fraction of the speed of light away from us. Beyond a certain point, however (that 13.7 billion light years again) we don’t see anything at all. We can logically deduce that something must be there but we can never see it or experience its effects in any way.
There’s no contradiction in saying that at some point every particle in the universe must have been within speed of light effect of every other particle and that some of those particles have now vanished. It’s the logical consequence of faster than light expansion of space. This doesn’t contradict Einstein. It’s what he says. The speed of light only inhibits matter and energy, not space itself.
Because they weren’t actually MOVING. Space was stretching.
And the same process is going on right now. Only now the rate of expansion is much less than it was right after the Big Bang. As best as we can tell, the current rate of expansion is about 70 km/sec per megaparsec. (One megaparsec is about 3 million light years.) So if you look at an object 3 million light years away, cosmic expansion will be carrying it away from us at about 70 km/sec.
But the effect is linear. An object 2 Mpc away will be receding at 140 km/sec. And object 100 Mpc away will be receding at 7000 km/sec. And an object 5000 Mpc away will be receding at 350,000 km/sec, which is faster than the speed of light.