Just looking for some clarification here–I thought this deserved it’s own thread since it diverges from the post linked above. Sorry if I’m mistaken in that…
As I understood it current thinking is that the Universe is both Finite and Infinite at the same time. Think of it like this–The Earth is a great big sphere and completely smooth (just for ease of picturing this). While the Earth is finite in size you could walk essentially forever and never come to the ‘end’ making it appear infinite.
Also, if the Universe is truly Infinite and there are an Infinite number of galaxies/suns/planets out there then wouldn’t that suggest an Infinite amount of energy released at the Big Bang? If so wouldn’t the entire Universe be awash in infinite energy levels probably precluding the existence of life (at least as we experience it)? If the Universe is finite in age then doesn’t it have to be finite in size (i.e. it expanded from a singularity with the speed of light a limiting factor it can’t be infinite…just REALLY big)?
Since the Universe has a starting point, it is finite. However, for all intents and purposes the Universe is infinite to humans. The reason being is that we can’t even manage to cross the smallest portion of our own solar system in a short amount of time. The times required for interstellar or intergalactic travel makes the Universe seem as though it is infinite.
Some clarifications on what I said before: A truly infinite Universe is one of several possibilities currenttly under serious consideration. Another very real possibility is that it’s finite in size, but loops around like an Asteroids screen (if you fly off one edge of the screen, you come back on the the other side). Yet another is that it’s finite like the surface of the Earth. The first two possibilities are generally prefered today, as the evidence seems to be pointing to a flat Universe, but the case is not yet ironclad to eliminate the third. What I was getting at, is that a truly infinite Universe is still a reasonable possibility, with the conseqences I mentioned. Besides which, even if it is finite, it’s awfully big.
Yes, an infinite Universe would imply an infinite amount of energy in the Big Bang. Some theories of the Bang can accept this, some can’t. Note, too, that even the theories which require finite energy can still allow for arbitrarily large finite energies.
No, the finite age of the Universe does not limit its size. The speed of light is not a limiting factor in the Universe’s expansion, but it is a limit on the portion of the Universe that we can see. If there’s a civilization at just under 15 Gly from us (assuming an age of 1.5*10[sup]10[/sup] years), then the Universe would look pretty much the same to them as to us. In one direction, at the edge of their sight range, would be where we are now, and they could look the same distance in the other direction, too.
So, after the Big Bang was space pretty much instantly infinite? Or, is the volume of the Universe growing as we speak (kinda like a balloon inflating growing ever larger)? If it is kinda like the balloon analogy how fast is the ‘edge’ (I know we can’t find the edge since it’d be in 4-D space) moving? Light speed? If the expanding thing goes does that mean space is stretching? If not stretching then is ‘new’ space being created all around us? If not that then where does all this ‘new’ space come from (I have the feeling the answer is nothing)?
The Asteroids screen analogy I get and is how I thought the Universe was supposed to work (or it works with the surface of a sphere analogy too…travel long enough and you return to where you started). Technically I thought if you travelled long enough in a straight line through the universe you’d come back to where you started (except the Universe is growing faster than you can travel so you’d never really make it).
What I still have a problem with is infinite energy and/or infinite planets/stars (not to be cinfused with the infinite energy density at a singularity). Even if the whole of ‘space’ blinked into infinite existence didn’t all of the energy (and ultimately stars and what-not) expand from a singularity? If so could you use your HyperSpeed Starship to fly out past the point where there are galaxies (i.e. space that nothing has had time to float into yet)?
Ok…so there’s too many questions in there and some answers may never be known. I’m just curious as to what the ‘best guess’ is today among astrophysicists and their kind.
I wish I could contribute something noteworthy to this thread, but it looks like everything I had to say has been said. (I suppose I could describe some freaky Cosmologies of My own, but that’s not necessary. :)) But I can clear up one aspect of the confusing terminology:
Jeff_42 said that “the Universe is both Finite and Infinite”. When you say “Infinte” here, I think the word you’re looking for is “Unbounded”. Without boundaries, just like the surface of the Earth. The Universe can be Finite and Bounded, Finite and Unbounded, or Infinite and Unbounded.
I do sort of have a problem with people equating some small number like, say, 10[sup]80[/sup] with Infinity. Infinity doesn’t care how many zeroes you tack onto the end. So I think there’s a big difference between real Infinity, and “seemingly infinite to humans”.
Until we can be certain of the amount of mass in the universe we can’t be sure if its closed/finite or open/infinite. The last couple high-profile experiments on dark matter and background radiation suggest an open, ever expanding, universe.
Jeff_42
The singularity at the Big Bang was “everywhere”. It was not a localized explosion. You can’t travel past the edge of the explosion because there is no edge.
I don’t think any new space is created; it just gets streched.
“How fast is the edge moving? light speed?” The speed is not constant. And light speed has no meaning outside the universe. The balloon anology is not perfect. It is just one way of visualizing a finite expanding universe. The hyperspace that the universe is embedded in has no real existance. Chronos
The asteroid screen is a torus. Is this really a possibility?
I have trouble understanding how the universe went from a singularity to infinite size. If the universe is infinite, was the singularity also infinite or a single point?
I’ve also got a question. IIRC, if the total mass in the universe is less than or equal to some critical value, the universe is infinite. Wouldn’t this make the density zero? Achernar
I’m not sure how the universe could be bounded. What is past the boundry?
Chronos, I’ve never quite understood how to get from a finite universe (which must have existed if the Big Bang came from a point, or at least from some small volume) to a universe which is today infinite. Even with an inflationary theory, going from finite to infinite would seem to require a discontinuous jump, not a continuous evolution.
Or was the universe already infinite when it was only a point (whatever the hell that means :rolleyes: ).
That is only a problem with a finite bounded universe, which I don’t think is a possibility. The universe could be finite and unbounded as the surface of a hypersphere.
Mr X
But all the ones next door are contracting right now. God took care of all these details.
My view as a non-scientist has long been “finite but expanding.” Expanding into what? Same answer as what existed before the big bang - I have no idea. Gotta limit the headache inducing queries somehow.
Thanks Achernar…that is what I meant. Finite and Unbounded. I tend to go with Dr Matrix and his possible assertion, ‘The universe could be finite and unbounded as the surface of a hypersphere.’
Is there some reason there can’t be many universes? I mean, it might be weird and there’d really never be any way to know (I don’t think they can interact) but is there any reason why it absolutely can’t be this way?
Here’s how I thought the Beginning was supposed to work.
There is nothing. True nothing…no time, no meaningful measure of any kind. The nothing is infinitely big or infinitely small or the size of a basketball…again, measurement is meaningless.
Big Bang happens. Like most explosions we are familiar with today it starts to expand like a sphere except this is a hypersphere (4-D spacelike). It expands ‘into’ the nothing mentioned in #1.
As the expansion continues energy densities drop eventually allowing the formation of particles then atoms then molecules and eventually stars and planets.
Some 15 billion years later I write this.
– Conclusion, the Universe should be finite in size. I believe it should be unbounded (hypersphere) for no reason beyond the fact it appeals to me (good science huh?). Basically I can’t picture a bounded universe…hard enough picturing this other stuff. Still, I’m more than open to having my mind changed.
Possibilities I’m unsure of (at least a few of them):
In #2 above I say the Big Bang happens and assume the ‘edge’ (for lack of a better term…I know it’s vague) coincided with the ‘blast front’ (again weak terminology…bear with me) basically defining the size of the Universe. The other possibility I see that could allow for an infinite universe is that ‘space’ essentially blinked into infinite being (call it a side effect of multiple dimensions unraveling and leaving dimension 1,2,3 and 4 behind) and the Big Bang singularity exploded into the space that was already there.
The Big Bang wasn’t a singularity. You get your instant infinite space (as mentioned above)and energy blinks into to being all over the place due to the stress of the whole space creation deal (not as weird as it sounds…energy spontaneously creates itself still today). This energy eventually coalesces and voila! Galaxies and what-not in an infinite universe.
I know I’m all over the place here but I’m trying to narrow in on what the answer (as best we can figure) is.
Yup, it sure is, as well as various other tesselated topologies. There was a Scientific American article about it about a year ago… Do you still subscribe? [gloat]As it happens, too, one of my professors is the main guy investigating that possibility.[/gloat] For those in the audience with less mathematical background than you or I, BTW, a torus (Asteroids screen) is distinct from a sphere in several ways: A torus is flat, meaning that you can lay a piece of paper smoothly on an Asteroids screen, whereas you have to crincle it around the edges to lay it on a globe. Also, on an Asteroids screen, how long it takes you to reach your starting point depends on your direction: if you pick an angle whose tangent is irrational, you’ll actually never quite get back to your starting point, although you’ll pass really close by it.
Quoth Jeff_42:
Jeff, do you have a cite for that? That sounds like Hoyle’s steady state theory, but nobody but Hoyle (well, no active cosmologists, anyway) accept that anymore.
Using words like infinite is ,if I understand correctly, shorthard for an infinitly expanding universe. Spacetime that eventually contracts is the finite universe as it has a limit to maximum size.
You’ll have to forgive me for not commenting on the topic of this thread–I don’t know enough about it to contribute intelligently, so I’ll refrain from contributing unintelligently. All I know is it’s damn big.
In any topology (open, flat, closed) the universe is infinite in the sense that you cannot reach an edge…either it goes on forever or space curves back on itself. As was pointed out, current evidence indicates that the universe is “flat” (Euclidean, infinite expansion). Even during the early inflationary period, when the universe was “smaller” in extent than it is today, it was still without an edge due to its curvature. The time=0 state (when the universe appeared to be a point…or really really small…or a pea instanton or whatever) is not understood.
Whether the universe expands forever or collapses is tied to the spatial curvature of the universe. If the universe has positive curvature (analogous to a sphere), it will eventually collapse. If it has negative curvature (analogous to a saddle) or is flat, it will expand forever. Is it possible for the universe to have negative curvature everywhere, and still be finite?
Not reaching an edge means unbounded, not infinite (see Achemar’s post).
I remember something about it in Scientific American some time ago. (I didn’t buy that issue, just skimmed it at the bookstore.) It’s prolly what Chronos refered to above. I’ll have to go to the library this weekend and see if I can find it. It was some weird shape that “tiled” negatively curved space.
Jeff_42
Hawking radiation is not energy spontaneously being created. The mass of the black hole is reduced by the radiation, so it comes from the black hole. It is just mass-energy being recycled.
