This really makes my head hurt.
A flat universe? Huh?
Last time I checked, we’re all pretty tridimensional.
Also, since the universe is expanding, and we’re moving, wouldn’t it be possible to extrapolate where we’re moving from (The Milky Way and the cluster of galaxies and the superclusters ASF)? I mean, the whole package is moving in a direction. Then, we should be able to look that way and see where we where a billion years ago?
Flat in the four-(spatial)-dimensional manner, I imagine. I guess my earlier hypersphere example is considered erroneous and obsolete by scientists.
Or maybe “flat” in terms of universal expansion? Is that what you mean, Chronos? A “flat” universe (as opposed to open, which continues expanding forever, or closed, which will eventually contract in the “Big Crunch”) will forever slow down exponentially, but never quite stop expanding… even if it gets to the point where it’s expanding at just one nanometer per trillion years.
No. If our relational size remained the same, there would be no way to detect the expansion. So, it is not meaningful to say that the universe and everything in it is expanding at the same rate.
The two models are not distinguishable. Why then, do we say that the universe is expanding rather than that we are shrinking? We prefer to use a definition of length where rigid rods do not change length with time.
Well, that’s one of the main objections to the Hoyle-Narlikar theory (alias the variable mass theory) that it doesn’t make any predictions and is merely a just a clever mathematical construction without experimentation to back it up. The main thing it has going for it is the fact that helps to explain the inital singularity and at the same time explains properties such as red-shift (which in this model is caused by the fact that electrons would be at lower energy levels because of the atoms size and therefore emit light of longer wavelength).
If the Universe is “infinite” now, what was it five minutes ago when it was still expanding? If its larger now than it was immeditaely after the big bang how does it qualify as infinite?
Using examples like walking farther north at the north pole and asteriods screens and going to the edge only to end up back where you started all point away from “infinite”.
I’m confused.
Is it "infinite because it’s creating the “space” in whic it’s expanding? If so, doesn’t that violate some kind of law? Isn;t that like a perpetual motion machine or something?
Well as it expands the universe cools so that might address your last question.
Would a better description than flat and infinite be Euclidian (2 parallel line will never cross) and unbounded (if you shoot off east you’ll never return to your origin)?
There are various degrees of infinity, it’s a math thing. It’s both true that the universe is infinite in size - as demonstrated by the whole “you can keep going forever” stuff - and that it’s getting larger, at an increasing rate, no less. I see that someone else posted earlier that it’s like time: time keeps increasing, but it’s not going INTO anything.
This doesn’t affect the total energy of the universe. It’s just moving it from place to place.
As I understand it, most of the current mathematical models propose that the dimensionality of the universe was much greater immediately following the big bang (10 or 26 dimensions) than the three spatial dimensions and fourth time dimension that we currently percieve. This may, of course, be an artifact of the math, since the laws of physics become simpler when expressed in higher dimensions, and that is the only way our models can be consistent. It does lead one to wonder, however, that if 6 or 22 dimensions have since collapsed, with our 3/4D world become unstable on the future and collapse further?
Does this make any sense? IANATheoreticalPhysicist.
Both senses of “flat” are equivalent, mostly. Strictly speaking, when we say “flat”, we mean that the large-scale structure is Minkowskian, which means that the spatial part of space-time is Euclidean. In other words, if space is flat, then you can use the Pythagorean Theorem and all the other geometry you learned in high school. Now, it turns out that, if you don’t have a cosmological constant, then a geometrically flat universe will expand forever, slowing exponentially, as you describe. If you do have a cosmological constant, as we appear to, then a flat universe will eventually expand enough for the cosmological constant to take hold, at which point the universe will start accelerating, ultimately expanding at an exponentially increasing rate (this appears to have happened already, for us).
As for an infinite universe expanding: Picture an infinite sheet of graph paper, ruled at four squares to an inch. Now picture that graph paper stretching, until it’s two squares to an inch. The size of the paper is still infinite, but it’s reasonable to say that it’s doubled in size.
It is possible that the Universe is flat and finite, by some nontrivial topology, but there’s no direct evidence for such a topology, so it’s simplest to assume it’s infinite.
And as for Fuji Kitakyusho’s question about dimensions collapsing away, the best I can answer is that we probably won’t lose any more dimensions. But we’re not too clear on what caused the others to shrivel away, so we can’t be sure.