It is assumed in most physics discussions that the universe is a closed physical system. But is it really? I am defining the universe as the entirety of everything physical, not merely the observable universe. I want to know if it really is a closed system.
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2$\begingroup$ what do you mean by most physical discussions? (any reference?) Einstein's equations doesn't talk anithing about the border or not of the universe, they are local. But of course general relativity is not a final theory. $\endgroup$– Gabriel PalauCommented Nov 3, 2023 at 23:55
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$\begingroup$ Better yet, most assume the universe is an isolated system: not even energy leakage is allowed. $\endgroup$– Matt HansonCommented Nov 4, 2023 at 0:54
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2$\begingroup$ To define the universe as "everything" and then ask "Is everything everything?". Well, that's like those foolish people who ask "Is reality real?". JZ $\endgroup$– John ZCommented Nov 5, 2023 at 23:29
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$\begingroup$ Closed physical system does not mean it's everything. It's completely fine for the universe to be an open system, receiving or losing energy through a boundary. $\endgroup$– Ján LalinskýCommented Nov 5, 2023 at 23:35
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4 Answers
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All we have is observations and models. We check the models against the observations: often they agree, but sometimes they don't (so we have more work to do). But the models are products of human imagination: they don't exist outside of human minds. What the Universe really is, we cannot say.
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Determining if the universe is open or closed or isolated for that matter can be done in two ways. One is asking whether there is an "outside" to the universe, and the other would be the energy approach. The energy approach is just that if energy is going into the system or leaving the system then it is open and if that doesn't happen it is closed.
Regarding the geometry and whether there is a "physical" outside, it is still debated and there is not a clear answer as to whether there is more to the universe or not and some scientific discussions talk about the multiverse theory but it doesn't have as much data to support it, so in terms of geometry, the universe is most likely an isolated system.
In terms of energy though, things get more complicated. Sure, there is no energy escaping the "edges" of the universe or going into the universe, primarily because there might not be one, but there is this thing called dark energy.
Dark energy is theorized to be a form of energy that expands space faster than the speed of light, so in theory this energy can be seen as new energy and so means the universe is open from an energy perspective. There is also a cosmological redshift which means that light stretches out and this energy is lost. All of these things may seem to violate conservation of energy but they follow Noether's theorem.
So basically the universe is an open system because normally we associate open or closed systems based on the energy not necessarily on the geometry of the object(generally).
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1$\begingroup$ When not considering multiverses, there is no debate about whether or not there is an outside to the universe. There is not. $\endgroup$ Commented Nov 5, 2023 at 17:38
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$\begingroup$ Some theories require there to be multiverses, which is why I included that as a case when potentially there may be a physical exterior. Alan Guth stated that there could have been many multiverses created during the Big Bang but of course there is still not enough evidence to support that just yet. That is where the research is still going so they can make the theory have more evidence supporting it. $\endgroup$ Commented Nov 5, 2023 at 18:11
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there is no "outside" to the universe from which the universe could be observed. in this sense the universe is closed.
answered Nov 4, 2023 at 0:02
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1$\begingroup$ If the universe is topologically compact / closed, then I can see a case made for considering it a closed system, but can we really consider it closed if it is infinite? I'm not so sure. We never think about infinite regions in thermodynamics, do we? $\endgroup$ Commented Nov 4, 2023 at 0:10
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$\begingroup$ @MaximalIdeal, Lee Smolin has written extensively on this topic. $\endgroup$ Commented Nov 4, 2023 at 0:12
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$\begingroup$ @MaximalIdeal You mean that an infinite universe allows "magic" a la Hilbert's Hotel? $\endgroup$ Commented Nov 4, 2023 at 13:56
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$\begingroup$ From the perspective of physics, and science more generally, surely the answer has to be "we do not know". One can say that the models adopted in physics and in science more generally are those suited to the tasks they perform, and these models do not invoke an "outside" to the universe. $\endgroup$ Commented Nov 5, 2023 at 21:49
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It is your question, in which you define "universe" as everything, which answers your own question: yes.
Regarding the definition of "universe" that is not well defined today in professional cosmology, when "Multiverse" is used. Thus your question is asked.
It seems we are onto a similar question in contemporary cosmology: is the blob of galaxies we can possibly ever observe part of a self contained system, the only one, or is it within a greater sea of other "isolated" blobs?
