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Is The Universe infinite? 
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#1
Jul1410, 10:01 PM

P: 5

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?



#2
Jul1510, 12:45 AM

Sci Advisor
P: 2,193

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.



#3
Jul1510, 02:58 AM

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#4
Jul1510, 04:41 PM

P: 93

Is The Universe infinite?
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.



#5
Jul1510, 09:18 PM

P: 5




#6
Jul1510, 09:34 PM

P: 5




#7
Jul2210, 10:52 PM

P: 1




#8
Jul2310, 01:19 AM

Sci Advisor
PF Gold
P: 9,428

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.



#9
Jul2310, 10:06 AM

P: 18

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



#10
Jul2310, 10:50 AM

Mentor
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#11
Jul2310, 10:51 AM

P: 143

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. 


#12
Jul2410, 08:21 AM

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#13
Jul2410, 08:38 AM

P: 143




#14
Jul2510, 10:02 AM

P: 143

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 spacetime 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. 


#15
Jul3010, 06:29 AM

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#16
Jul3110, 01:21 PM

P: 1,620

(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 timeaxis, and thus show that there is no upper limit to a past point in time. 


#17
Aug110, 09:58 AM

P: 2

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



#18
Aug110, 10:50 AM

P: 1,620

The issue onhand is however much more complicated. Based on E=mc^2 the 1st law of Thermodynamics we need first to consider physical matter too, so the total quantity of both physical matter (mass) and energy is a constant. Second, both laws of Thermodynamics were originally constrained to laboratory scale thermodynamic systems which were closed and had a thermodynamic boundary. The 1st law (in it's contemporary form, based on GR and QM) still holds for all systems, including the universe. Physical matter creation, as what happened on a massscale in the early universe, does not contradict that, because it was a conversion of energy in another form. The problem with the 2nd law of Thermodynamics however is that it is still constrained to thermodynamic systems wich have a thermodynamic boundary. This does not apply directly to the universe, because apart from the cosmological issue of open or closed universe, there is in the strict sense no thermodynamic boundary to the universe. There's no border or boundary to the universe (cosmological principle), and that is true even when the universe turns out to be a multiversum. In the Thermodynamic sense the observable universe is an open system, since it does not have a boundary and is in thermal contact with the rest of the universe, beyond our horizon. For the universe in total the terms open or close with respect to it's Thermodynamic behaviour makes no sense since there is no boundary to the "rest of the universe" (since the total universe already encompasses that) so that it is neither open nor closed. But there is also something else peculiar about the universe. You might have seen these pictures of a sequence of moments of time in which a gas in a container spread outs through the container (due to entropy or the 2nd law of TD) and becomes uniformly spread through the container. At the microscopic level however, all physical laws work both ways, so how do we know the progress of time? When we have to order the pictures (let's say there are 3 pictures, one with local concentration(s) of molecules, the next with a medium spread of those molecules in the container but not yet uniform, and the third a uniform distribution) we would clearly say that the progress of time is from picture 1 to 2 to 3. Yet, when this picture was not of a small gass container, but was of cosmological size, we would need to arrange the pictures in the opposite order! Due to gravity, local matter clumbs together forming stars, galaxies and clusters of galaxies. So if we were not told what scale the picture represent or that the scale is a varying quantity, we could not say what the right direction of time was! In the example of the cosmological progress of distribution of matter, there are two important differences with the example of the small gas container. First, the progress of time is from a uniform distribution towards local clutterling of matter, forming galaxies, stars, etc., and the scale of the 'container' grows, due to cosmological expansion. In terms of entropy, this in fact means that the growing metrics of space allow for more possible states, so this in fact means a lowering of entropy at a cosmological scale! If you search online, you might find a lecture of Roger Penrose on this issue of cosmological expansion and entropy, which is very interesting. This is just some basic information, the issue itself is far more complicated as I can explain, but at least I think you get the basic idea that you can not simply scale up our laboratory scale experiment and conclusions based on the 2nd law of Thermodynamics to cosmological scales. I don't give a proof of it, but one could suspect that on truly cosmological scales (the universe as a whole) the issue of entropy is different as expected, and might lead to the conclusion that entropy is a conserved quantity throughout the cosmos, even if at local scales the 2nd law still applies. But perhaps someone else more educated on this subject can explain more details. 


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