# Is the universe an open or a closed system?

I recently was having a discussion about entropy and the concept of the universe being an open or closed system came up. Assuming that the universe is infinite, would it be considered to be an open or a closed system?

A closed system being a system in the state of being isolated from the environment.

If the universe is infinite, it is not isolated from anything and therefore an open system. But then you could also make the argument that there is no affect on it from an outside environment therefore it must be closed. Which is it?

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How do you come to the assumption that the universe is infinite?
If there exists no space nor time outside of the universe, how can you assert that there is anything for the universe to be 'isolated from'?

If I'm not mistaken (which I wholly could be, :P), the 'open or closed' system discussion in regards to the universe usually refers to whether or not the universe has enough mass s.t. the rate of expansion decreases and ultimately the universe begins coming back upon itself by gravity or not.

He's asking whether it's an open or closed system in the thermodynamics sense of the terms. The universe is open if it is affected by something outside of it. If there is no outside of it then obviously it is a closed system. I think most people define "universe" in terms that require it to be a closed system, though a few people (i.e. brane-world string theorists) use the term somewhat differently.

For the purposes of your question, the answer is probably "closed". You could have come to that answer just by asking yourself why it is that you care whether it's open or closed. The reason you care, I assume, is because you want to know whether the law of conservation of energy and similar laws apply. So then, why does whether it is open or closed affect that? Because an open system might have energy added to it or subtracted from it by an external source/sink of energy. So then, shouldn't it be obvious that if you think the universe isn't connected to anything else, or you define the word "universe" so that it includes everything that could possibly affect anything, then there's no reason (at this level of discourse) the laws of thermodynamics wouldn't apply?

Thanks for the replies.

Xezlec, the reason why I would consider(considering, not stating) it open is because of the second law of thermodynamics. For the first law, I would make the assumption that it is closed because the quantity (although infinite) of energy remains constant. But with the second law however things get more complex.

The second law states entropy can not be destroyed and is forever increasing. Based on this you could also state that the universe has existed for an infinite amount of time which would imply that some point along the way the universe reached a saturation of entropy since it is always increasing, obviously this is not true. The only way for this that statement to hold true in the thermodynamics sense is if the universe is open and exergy can be added. But this would violate the first law.

Taking this a little further, consider a large enough amount of galaxies with enough mass and energy to create a "big bang". Consider this big bang system closed, as in nothing gets in or out. If the big bang holds true, then that ball of energy and mass will explode outward for a very long time, galaxies and stars will form, maybe even life. But eventually it will all come together and create another ball of mass and energy for another big bang, in other words an infinite cycle. At the beginning of each cycle though, wouldn't you consider the amount of total entropy in this closed verse to be zero or relatively close to it no matter how many cycles occurred? If this is true, entropy has to be destroyed at one point or another right? Is there a fundamental flaw in my thinking?

Xezlec, the reason why I would consider(considering, not stating) it open is because of the second law of thermodynamics. For the first law, I would make the assumption that it is closed because the quantity (although infinite) of energy remains constant. But with the second law however things get more complex.

The second law states entropy can not be destroyed and is forever increasing. Based on this you could also state that the universe has existed for an infinite amount of time which would imply that some point along the way the universe reached a saturation of entropy since it is always increasing, obviously this is not true.
No, the universe has existed for 13.7 billion years, not eternity, and its entropy has always increased during that time. In fact, the heat death of the universe is one of the many predictions for how it will end. In this scenario, the entropy increases to such an amount that there is no structure like life left at all.

Taking this a little further, consider a large enough amount of galaxies with enough mass and energy to create a "big bang". Consider this big bang system closed, as in nothing gets in or out. If the big bang holds true, then that ball of energy and mass will explode outward for a very long time, galaxies and stars will form, maybe even life. But eventually it will all come together and create another ball of mass and energy for another big bang, in other words an infinite cycle.
Not necessarily. That's only one theory, it depends on the exact values of a lot of physical constants, and it's not the theory that's winning right now. Current results indicate that the expansion of the universe is actually accelerating. There will be neither a Big Bounce nor a Big Crunch (which are two different things, BTW), it appears.

At the beginning of each cycle though, wouldn't you consider the amount of total entropy in this closed verse to be zero or relatively close to it no matter how many cycles occurred? If this is true, entropy has to be destroyed at one point or another right? Is there a fundamental flaw in my thinking?
Maybe. The emergence of structure in the universe (see http://en.wikipedia.org/wiki/Structure_formation ) is one of the greatest problems in Cosmology. Also it is not clear whether or not a singularity contains information -- I'm not sure whether this actually relates to the problem or even if it applies to the Big Bang, but check out http://en.wikipedia.org/wiki/No-hair_theorem for more on that.

Based on all these subjects, I think the Wikipedia pages you are looking for are here: http://en.wikipedia.org/wiki/Big_Bang and http://en.wikipedia.org/wiki/End_of_the_universe

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Taking a mathematical viewpoint. Assuming the universe is infinite. That means certain thermodynamic properties would no longer be defined by the simple fact that most mathematical operations involving infinity are undefined or have no meaning. The conclusion therefore is that, if the universe is infinite, then the thermodynamic state is undefined.

Xezlec,

Also, in cosmology, the Universe can be considered infinite in the sense that it contains all that is; but not necessarily that there is a never ending number of things.

So, in physics, it is assumed that no measurable quantity could have an infinite value. It is, for example, presumed impossible for any body to have infinite mass or infinite energy. There exists the concept of infinite entities (such as an infinite plane wave, ideas) but there are no means to generate such things (physical world.)- wiki

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I always thought of the Big Bang referred only to our 'local universe', the entire universe having no starting or ending concepts.
The universe is infinite outwards and it must be infinite inwards, from any infinite given point in both instances. And it is.

For some reason going infinitely inwards gives me the same feeling as going infinitely outwards, at the same time, like I never went anywhere.

I believe that we are at the same place regardless where we think we are, and everything is one; infinity.

This might be related;

"Time as a concept of change. Change as a concept of motion. Motion as a concept of energy. Energy as a concept of matter. Matter as a concept of existence. All being one and the same thing ? Does this make any sense, some youngster asks."

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If there are other 'universes' outside our 'universe', then this 'universe' by definition must not be a closed system, but all of reality, if finite must be closed.

Let me explain that first statment. Suppose there is another universe out there, like ours, but we can't get to it. Nothing from ours can get to that one, and nothing from that one can get to ours. Does it exist? NO! if there is no possible way to reach said place, that means that there is no defined interval of separation (space, time, other) between the two. Both of those mean it can't exist. Anyone follow?

From a QM standpoint, the wavefunctions of such a place, never to be observed by us, would probably be ambiguous to the point of nonexistence.