Is The Universe infinite?

NowIsForever

What do current theories have to say about any resolution to this question? The visible universe might be finite, but that says nothing about the totality. Should we include the Multiverse in this discussion (if there are many worlds)? What about the nature of quantum foam? Could that have a finite basis. And on and on... Is there any end to it all?

Nabeshin

Judging from the WMAP data, we deduce that the universe is extremely close to being spatially flat. In the standard FRW cosmology, a spatially flat universe is infinite.

Passionflower

The question whether space is finite or infinite is not related to whether a spacetime is open, flat or closed. For instance a closed spacetime could be infinite and a flat spacetime could be finite. The fact that FRW models cannot handle finite spaces is obviously not a valid argument here.

filegraphy

Welcome to Physics forum. That question has not been answered yet. My view of the universe is that it will expand forever. Therefore being infinite.

NowIsForever

If the time component of space-time is infinite, then indeed the universe must be infinite even if the space component is finite every step of the way. Although, then the space component could be potentially infinite.

NowIsForever

I don't see how a flat smooth manifold would not be infinite (although perhaps that is a failure of my imagination). Maybe you refer to a non-smooth model? Could you give an example of such a model embedded in Rⁿ?

fouvy

It seems that there is no relationship between expand forever and infinite. For example, the tree is growing forever but it isn't infinite high for the reason that the time isn't infinite long, is it?

Chronos

A tantalizing question. My gut instinct is we will never entirely resolve the question of flatness. CMB anisotropy gives us conficting hints. Too close to call, IMO.

Emu

As Chronos mentioned in the thread " Why is Space Black"

George Jones

A space can be flat and compact if it is not simply connected.

robheus

The Big Bang has taken care of that issue, you can not resolve the issue of finiteness/infiniteness of space and/or time from that paradox.

I think the question is even theoretically unanswerable, and we just use the gut feeling that spacetime is unbounded (there is no *edge* to spacetime) but that also leaves the issue of finiteness/infiniteness open.

My gut feeling is that spacetime is infinite.

TrickyDicky

How could it be? maybe spatially, but, doesn't the Big Bang make it time-finite, at least in one direction?

robheus

Nope. There is nothing in the Big Bang Theory that implies that. The Big Bang Theory is not an explenation of the origins of the universe, but of it's development.

robheus

On the subject of infinites in the realm of physics, you have to be careful about some subtle aspects of infinites. First of all, real infinites (like the mathematical infinite) do not belong to physics. Any measure in physics always is of finite proportions. However note that this does not preclude that space and/or time cannot be infinite.
Look at it like this: consider the natural numbers and select a number. Now, whichever number you came up with, it definitately is a finite number, and also, you can always select a number bigger then the number you just selected.
So the seemingly contradictionary conclusion is that you can select an infinite amount of numbers, yet no number you ever get is itself infinite. Infinite itself is not considered a number.
The infinite exists only in the forms of finite elements.

So, if we consider space-time like the set of all possible spatial and temporal measures that can be made, the set itself is clearly infinite, although any element in the set is of finite measure.
There does not exist a point in time or space infinitely far away.

Andrey

This is not entirely true but i can see where you are coming from. Yes the infinate can be measured using finate elements, i.e. your number example. But lets say there are no numbers left to pick. Lets say that there is no finate elements which we can use to measure the infinate, the vastness of space. What do we get then? Well, absolute zero because there can be no set of elements that can define the measurement. Furthermore, you can't even call it Zero for Zero is a measurement of some sort. In fact it is utterly impossible to measure something without giving it a set of measurment, i.e. distance, time, dimention, space etc. Thus don't think of Universe being a set of simple measurements that one can simply use for one's own convenience because the moment you do that, you've restricted yourself from the bigger picture.

heusdens

(heusdens=robheus)

Perhaps anyone familiar with the Kalam Cosmological Argument. It's a famous but false argument against the infinity of time. The argument goes something like this: if we suppose that time did not have a begin, we could have never arrived at the moment of "now" because it is impossible to have traversed an infinite amount of time.

For people that do not immediately grasp the incorrectness of the argument, just ask yourself, at what point on the time axis is it supposed that we have started the traversal of time.

The point is of course, that wherever you have chosen to start traversing the time axis, you already smuggled in as a premise that time had a beginning, since else, you could not have started traversing the time axis at all.

The only validity of the argument is that there is no point on the time axis in the distant past that is infinitely far before the present point in time, since we cannot traverse an infinity of time. [ and pls. note, that is just what infinity is by definition, that it can never be exhausted or completed, no matter how hard or how long we try. A "completed" or "exhausted" infinity is nothing more as a contradiction in definition. ]

Yet, at the same 'time' this is not to be held against the infinity of time itself, since we can always design a point farther back in time on the time-axis, and thus show that there is no upper limit to a past point in time.

POWERSHIFT

If the universe was infinite, wouldn't it, according to the 2nd Law of Thermodynamics, have no heat? And if it isn't infinite, that means it's finite, and thus had a beginning. Which also means time had a "beginning." So to speak