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We see that the Universe is also called (sky) when Aristotle talks about it.

But I thought about it and I came to the conclusion when someone asked the question about my belief in what Aristotle said that I understood what they were saying and it was this: If the sky was the universe then the earth is inside of the sky. I asked this question because I did not believe his claim.

What is the boundary of the universe? Since the universe is always getting bigger because matter is expanding. This means that the universe would need to be inside of something for it to expand. My question is: What is the universe inside of if it is always able to get bigger because matter expands?

If the universe is not inside of anything then that means that it has a boundary and matter would not be able to expand because there is not enough room to expand.

Do you think that the Universe has a boundary?

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    This sounds like a question in astronomy, not in philosophy, even if it mentions Aristotle. Modern science has taken quite a chunk out of philosophical speculation.
    – Frank
    Apr 14, 2023 at 17:14
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    This seems like a fundamentally philosophical topic in the philosophy of science. My copy of Durbin's: Philosophy of Science has three chapters devoted to measurement, extension, and space-time with passages from Aristotle to Newton to Einstein. The average philosopher's ignorance on the topic is not the same same as the topic lacking philosophical import.
    – J D
    Apr 14, 2023 at 18:01
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    It is a question of fact, but the observations are not available.
    – Boba Fit
    Apr 14, 2023 at 20:26
  • "This means that the universe would need to be inside of something for it to expand." This is wrong, as explained in this Physics.se post: If the universe is expanding, what is it expanding into?
    – Sandejo
    Apr 15, 2023 at 3:38
  • I’m voting to close this question because it is based on misconceptions about physics.
    – Sandejo
    Apr 15, 2023 at 3:40

2 Answers 2

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I would consider this a philosophy of physics question since 'What is space? What is time? and What are space-time?' have undergone a conceptual evolution from the time of the Ancient Greeks through Einstein's special and general relativistic theories which is a primary topic of discussion in the philosophy of physics; aren't fully answered; and raise a host of questions like, if general relativity and quantum mechanics aren't unified, is our understanding of the universe incomplete? (Obviously yes.) The notion that physicists have a definition or explanation of the universe that has universal support is silly. One hundred years after the quantum revolution, not everyone even supports the Copenhagen Interpretation let alone is any work on unifying GR and QM decisive.

So what is the physical universe? Is it the same as the mathematical models we use to describe it? Is it possible to think of QM mechanics as describing the substance of the universe and the universe being a container for substance? Do we have a consensus on the fundamental nature of the universe? Is it part of a multiverse?

Lot's of question and we'll start with 'What does it mean for the physical universe to have a boundary and is it even meaningful?'

In traditional macroscopic experience, we can say an apple is a sack, and that there is space (a collection of points in the sack, outside the sack, and the sack) that is filled in one way or another, including empty space which apparently can be understood as being filled with quantum foam. But to say, what sort of space is all space contained in is a meaningless question, because it's a contradiction to have one space of which purports to be all space be a subspace of a yet larger space. There are mereological rules (SEP) at play here that might be violated. These "rules of containment" are dealt with by physicists by making two claims. That the universe is fundamentally understood with the mathematical abstraction of the curvature of space-time is done with sophisticated models like the tensor calculus and Minkowski space, and that such mathematical models do not require other models to contain them: that is, there is no embedding space which can be understood as that there is no sack or space that contains the sack. Such explanations often invoke the finite but unbounded surface of a balloon as an analogy. Two simple explanations about this are:

If you watch these arguments, the essence of them is that the universe by definition can't be contained by something else, so to understand the notion of expanding space time, you cannot use what your everyday sensory experience tells you about containment. To understand space expanding without filling something else means you have to your mathematical intuitions instead. Imagine the density property of the reals. Let's consider an analogy.

Between two points, one can find a midpoint, and then between the first and the midpoint, another, and another, and another, and if we were to construct a set, we can grow that set without having to move the endpoints. That is, our set expands by adding members (by summing and dividing by two using our metric) and yet we never move the endpoints. So, there are two ways we can expand a set of points. One is to expand the interval define on the reals (0,1), (0,10), (0,100), and the other is to expand the listed points in our extension of the interval {0,1}, {0, 0.5, 1}, {0, 0.25, 0.5, 0.75, 1}, ... Thus, what we have are differences in the definition of expansion.

Yes, but you might argue, the universe isn't the math we model it with. In which I would agree with you, but not all physicists would. There are some physicists who maintain that the universe is inherently information (the marginalian.org). John Archibald Wheeler is one such person:

Wheeler speculated that reality is created by observers in the universe. "How does something arise from nothing?", he asked about the existence of space and time.[82] He also coined the term "Participatory Anthropic Principle" (PAP), a version of a Strong Anthropic Principle.[83]

In 1990, Wheeler suggested that information is fundamental to the physics of the universe. According to this "it from bit" doctrine, all things physical are information-theoretic in origin:

So, if our infinite density property of reals is at the bottom of the what is fundamentally real about the universe, then it's the representations that determine the substance. These sorts of philosophical positions are considered alternate interpretations of quantum mechanics. Some of these are forms of philosophical idealism.

So, the conventional answer is that the mathematical physics we have are sufficient to suggest that, whatever the universe is, given a materialist interpretation of the universe, that it doesn't need to expand into anything because expansion is defined in terms of the increasing distance between galaxies, for instance, and one needn't have even a finite universe, let alone something outside the universe, to make that make sense mathematically.

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  • @Frank So, you're ready to pick apart Wheeler's It-From-Bit quantum informational argument about the fundamental nature of the universe? Is your problem with his placing information as foundational to matter the specifics of his quantum mathematical models or do you have deeper ontological and epistemological concerns given what you take to be his limited understanding of physics? ; )
    – J D
    Apr 14, 2023 at 18:41
  • If philosophy is allowed to decide epistemological questions in astronomy, what is the mechanism by which philosophy will guarantee the correctness of its results? As far as I can tell, philosophy has only speculation on its side, which is not a safe way to guarantee the "truth of the matter" on questions in astronomy.
    – Frank
    Apr 15, 2023 at 15:35
  • @Frank Epistemological questions are philosophy since philosophy includes epistemology, so asking and answering them in any field is simply a philosophy-of, if the topic matter is narrow enough, and general epistemology if broad enough. My point was to rebut the notion that astronomers can do astronomy without philosophical presumptions inherent in their theories, and my methods was to show that Wheeler puts bit-before-it as opposed to it-before-bit (like we both do). There's no purely scientific way to determine who is right and who is wrong, since theories are laden with speculation...
    – J D
    Apr 15, 2023 at 21:20
  • Philosophy is technically construed the attempt to draw the line between irrational and rational speculation since induction is a form of speculation that rests on a presumption of the uniformity of the behavior of the physical universe. Even the mathematical axiom forged from the application of abduction and construction distilled to deductive certainty is speculation. My question was to provoke the question that drawing a line between speculation and logic isn't crisp or easy. Einstein speculated long before he fallibly proved. What made Einstein special, in fact, was his speculation...
    – J D
    Apr 15, 2023 at 21:25
  • since it is arguable that creativity is a form of speculation. In this regard, isn't it true that there is no science free of speculation if science is an inherently generative activity? The sciences advance on the back of speculation.
    – J D
    Apr 15, 2023 at 21:29
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This is a classic misconception, based on a faulty analogy, because both space and time began with the Big Bang (or were in extremely compact form at least, by some theories). See the discussion in response to a letter to New Scientist: The universe is expanding, but what exactly is it expanding into?

If you go in any direction you could come back to where you started, eventually. But, the acceleration in the expansion, means in the future distant parts of the universe will be seperating faster than the speed of light - creating Cosmological Event Horizons, and leading to the Big Rip scenario fir the universe, now considered more likely than just Heat Death.

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