Is the universe infinite?

Chalnoth

This does not follow.

Deuterium2H

Huh?? I don't even understand what you are trying to say.

But as a possible correction, do not confuse a doppler red-shift (which IS a measure of relative velocity between two objects in space), and Cosmological Red-Shift, which is NOT a measure of relative velocity, but is a result of the expansion of space itself. They are goverened by two very different formulas/equations.

ilsley

I have read that, if the universe is flat or hyperbolically curved, then it must be infinite. I've also read that all observations to date suggest that the universe is flat. Would that make it infinite? If it is indeed flat, then to be finite, it would have to have an edge and this would seem to contradict quite a few well accepted ideas in cosmology. If the universe is infinite now, then, looking back towards the big bang, it must always have been infinite - you can never halve the size of an infinite object and make it finite. It seems to a simple mind like mine, therefore, that the big bang didn't start with an infinitely small infinitely dense single point, but with an infinitely large infinitely dense affair. Is there any reason why that can't be the case? It would mean of course that the universe isn't actually getting bigger - it's just spreading out.

Chalnoth

No, not quite. There are two problems with this:
1. Even if the universe is flat, it can still be finite as it is entirely possible for a flat universe to wrap back on itself. An example of this type of universe is displayed in the old arcade game Asteroids, which is entirely flat, but move the ship off one side of the screen and it appears on the other. This is known as a toroidal topology, and it is entirely possible for our universe to be flat and finite in this way. I'm reasonably sure that you can do similar things with a negatively-curved space-time as well.
2. Unfortunately, our vision is limited both in time and space. We cannot observe the whole of the universe. And in practice, our local, observable region can easily have a curvature that deviates somewhat from the average curvature of the universe. So measuring some curvature or no curvature actually doesn't say much of anything about the curvature of the universe as a whole: the curvature we measure could just be a local feature.

However, let me just end with a little statement. As far as we know, our universe will expand forever into the future. This means that at least in one dimension, our universe is infinite: the time dimension. So if our universe is infinite in one dimension, why can't it be infinite in the other dimensions as well?

phinds

That is unsupported speculation on your part and should be stated as an opinion, not a fact.

Deveno

i understand very little of cosmology. i am given to understand that the conclusion that "the fabric of the universe is stetching apart" is based on the cosmic microwave background, which is (as far as we know so far), "too uniform" to support the notion that space existed first, and that just the stuff expanded later into it. better data collection efforts may substantiate, or revise this idea.

but as a mathematician, i feel i must point out that there is a difference between "infinite" and "unbounded". a circle is bounded, but i don't think anyone would claim it consists of only a finite number of points.

if space is a continuum, then it is infinite, even if it is embedded in a bounded manifold in some n-dimensional ambient space. it is not possible for us to tell, at the moment, if this bound is just very large (compared to us), or non-existent. it is my understanding that the basic assumption in cosmology is that the universe is (relatively) uniform, so "local" measurements of curvature should tell us about the universe in general, but of course, this assumption may be wrong (the energy content of "our corner of the park" may somehow influence its geometry).

on the other hand, if space itself is quantized in some manner, then it's conceivable our universe is "absolutely" finite (it is a discrete structure). i think this unlikely, but some have suggested that a finite-dimensional lattice could propagate instructions in such a way as to create the illusion of states evolving over time (the universe itself could be some form of complex-behavior automaton).

it is difficult to tell how many dimensions we "need" for our (perceived) space to exist inside "a larger one". if certain algebraic relationships hold, the choices are not entirely free, as some numbers work better than other ones (4, for example, is a better choice than 3, and 8 is better than 7...there are good reasons for believing it should be an even number).

of course, the very idea of our universe existing in some larger structure, sounds very much like saying: "the universe isn't the (whole) universe", but it's still possible that the "enveloping universe" somehow leaves evidence in our discoverable universe that tells us it's there (or rather; if we hypothesize such a universe, we may be able to "explain" things that have predictable value, that might be borne out by experiment. this isn't really "proof" per se, but if it works in practice, we are likely to adopt this view).

there isn't any pure logical reason, that i know of, for thinking the universe is finite, or non-finite. my guess is, is that since the universe exhibits similar levels of complexity across all the scales of resolution we have; it is infinite in depth, as well as breadth. it's possible this question may never be answered, due to our limitations.

josewrivera

if the universe is expanding, it must be expanding into something, isn't that something also part of the universe, if it is expanding how come we keep the same distance from the sun, the big bang was just one more explosion in the universe, one of billions and billions of explosions happening as we speak, if we are moving or expanding we are just looking to occupy a different location in the infinite universe, there is no beggining or end because there is no beggining to time, and if there is no beggining to time, there is no beggining to the universe, and if there was no beggining. there was no creation, no matter how you diced or sliced, by the way im no scientist and forgive my spelling

Chalnoth

Not at all. The expansion simply means stuff within the universe is getting further apart. It is, in a very real sense, just a change of shape of the universe. There is no reason whatsoever for there to be any "outside" at all. In fact, in General Relativity, the very concept of an outside doesn't work very well.

Because the expansion is a large-scale, average phenomenon. Overdense regions, such as our own galaxy or solar system, are quite stable within an expanding universe.

skydivephil

I think this impression is created by poor explanation of what the expansion means. We measure disatnt galaxies as receeding away from us at a speed proprtional to their distance. The important point to note is that this only applies to distant galaxies. Why only distant galaxies?
Becuase for nearby obejct they are gravitationally bound and the expansion of the universe is not strong enough to unbind them. It might be in the future, google "big Rip" for this possibility, but it is not now.
Hence gravitationally bound strucutres do not see any expansion. The solar system is grvaitiationally bound, so is our galaxy and so are nearby galaxies. For example Andromeda is on a collision course with the Milky Way. If everything felt the expansion this could not be the case.
Bottom line: for local structures, think nearby galaxies and closer , the expansion of the universe is irrelevant.

skydivephil

We dont know whether or not the universe is infinite. We do know there was a big bang event, but whether there were others and how far back they go back is currently unknown. Perhaps you are referring to certainly interesting models such as eternal inflation, CCC, ekpyrotic and bounce cosmologies that imply our big bang was not a unique event. But we have to be honest and say none of these models have been experimentally veirified yet. So I think a "we dont know" attitude is best. We should wait for the data to tell us the answeres and until that happens we should not presume anything.

Deuterium2H

Jose, I encourage you to read the FAQ section of this forum (listed at the very top of this section). Many of your questions are answered in descriptive, non-rigourous and very accesible explanations.

As has already been explained (e.g. Chalnoth), the known physics of our Universe does not require an additional "dimension" within which to expand. The shape/curvature of our Universe is an intrinsic geometric property, and does not require a higher dimension in which it is embedded. That is to say, our Universe can be infinite, open, and expanding...but it is not expanding into any "external" pre-existing volume. This fact is precisely why thinking of the Big Bang as a single "explosion" IN Space is misleading and incorrect. The correct concept is to understand the Big Bang as occuring everywhere, and to imagine it is an explosion OF space. The Big Bang occured simultaneously in the space now occupied by the current position of your belly-button, as well as any (and all) arbitrary points in the Andromeda galaxy.

According to the Standard model of Cosmology, there was a definite beginning of "time", which was the instant of the Big Bang. The Big Bang created our Universe which contains space and time, and our Universe does not exist "in" space and time. This is part of the "Containment Principle", which is an integral aspect of modern Cosmology.

skydivephil

Everything you say may be true but I think the picture is more nuanced than that. I think most comslogigst that work on the very early universe would agree that "The Standard Model" is not to be trusted as we get v close to the Planck scale. Hell all of my text books say that too, so this is nothing new. In order to say there was a beginning of time at the big bang we need to trust the mdoel all the way to the Planck scale which i think very few people would say is wise.

josewrivera

we have trouble undestanding events that ocurred only a couple thousand years ago, but we think we have the answer to what happened 5 billion years ago, the big bang didnt create the universe nothing no matter how big can affect a infinite universe in its totallity some time in the future humanity is going to come to this conclusion no beggining no end and there was time before the big bang, and is not possible to reach the beggining because there is always a second before, and a minute and an hour.

phinds

You really need to read some basic cosmology before posting such nonsense on a forum where people take science seriously.

"big bang" has two meanings

1) the singularity / t=0 and nobody pretends to know what this was all about, it's just the place where the models break down.

2) everything since one Plank time after the singularity. This is remarkably well understood, although there are still puzzles. I recommend you read "The First Three Minutes" by Weinberg.

Chalnoth

This is really sad. Why not try learning a little bit about how we have learned these things before throwing out blanket condemnations of science you know nothing about?

jay.yoon314

I think the universe is infinite in space and time, but finite in energy and mass.

phinds

Do you have any SCIENCE to back up this statement or is it merely unsupported, and unsupportable, personal opinion?