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RingNebula57
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How can the universe be infinite and yet have a finite age?
It could have started off infinite. We don't know whether it is finite or infinite but whichever it is, that's how it started off.RingNebula57 said:How can the universe be infinite and yet have a finite age?
The observable universe has a finite age, and it has a finite size. We know nothing of the universe outside what we can observe.RingNebula57 said:How can the universe be infinite and yet have a finite age?
Usually, yeslaymanB said:Is it not the case in physics that when we get infinities as a solution to our equations, it usually indicates a problem?
Because infinite density makes no physical sense.For instance, we say that GR cannot be the whole story because the equations give us singularities with infinite densities at the beginning of the universe and in the center of black holes. Why do we think that this indicates a problem with the theory and that we need new physics, as opposed to saying that there is indeed infinite density there?
Because infinite extent does not seem to be physically impossible.Now my question is: why don't we think this points to something wrong with the model, instead of embracing this infinity and trying to get rid of the other infinities (like infinite densities).
laymanB said:I thought I would resurrect ones of these infinite universe threads to make some points and hear feedback.
Is it not the case in physics that when we get infinities as a solution to our equations, it usually indicates a problem? For instance, we say that GR cannot be the whole story because the equations give us singularities with infinite densities at the beginning of the universe and in the center of black holes. Why do we think that this indicates a problem with the theory and that we need new physics, as opposed to saying that there is indeed infinite density there?
Let me take this idea into the question of whether the universe is spatially infinite or finite. We have confirmed that the local geometry is flat with small error bars. We don't observe anything that would lead us to believe that the local topology is anything besides Euclidean space. So, we plug these observations into a FRW model and the solution that we get out is a spatially infinite universe.
Now my question is: why don't we think this points to something wrong with the model, instead of embracing this infinity and trying to get rid of the other infinities (like infinite densities). It seems to me in the history of physics, that trying to get rid of infinities has led to meaningful progress.
This is part of what I'm trying to make sense of. The place where mathematical abstraction maps onto physical realities. Let's say that we consider a square with sides of length 1. We know that the length of the diagonals are ##\sqrt{2}##. We know that ##\sqrt{2}## is an irrational number and that the decimal places never end nor repeat. But don't we conclude that the length of the diagonal is a discrete length with end points in reality when we build this square out of wood or some other material?PeroK said:In mathematical terms, that set not only has an infinite number of elements, but the distance between any pair of elements is unbounded.
Note that when it comes to cosmology, physicists tend to use "infinite" or "of infinite extent" to mean "unbounded" in the mathematical sense.
laymanB said:This is part of what I'm trying to make sense of. The place where mathematical abstraction maps onto physical realities. Let's say that we consider a square with sides of length 1. We know that the length of the diagonals are ##\sqrt{2}##. We know that ##\sqrt{2}## is an irrational number and that the decimal places never end nor repeat. But don't we conclude that the length of the diagonal is a discrete length with end points in reality when we build this square out of wood or some other material?
We can ask a mathematician to show us different types of infinities, discuss the Hilbert Hotel, and talk about the continuum hypothesis, but do any of these mathematics give us justification for discussing infinities in physics? I'm thinking along the lines of Zeno's paradox, where only traveling half the distance between two points never gets us to the end point. While it is true if you can only travel by halving distances, real world particles are under no such constraint. They simply traverse the whole distance.
Good point.PeroK said:You have infinite sets at the heart of physics, because calculus relies on the real numbers. You can't do (normal) calculus on finite sets of points. And, it is problematic to try to model spacetime as a discrete set (of points a non-zero distance apart).
Sorry, this was non-linear rambling on my part. Let me try again.PeroK said:I don't see the relevance of Zeno's Paradox. Which is fairly feeble in any case, IMO.
As far as that sort of thing is concerned, infinity is just a mathematical fiction and does not need to exist to do the computation.laymanB said:Good point.Sorry, this was non-linear rambling on my part. Let me try again.
If we are talking about the escape velocity of an object, what is the physical meaning of the object slowing as it approaches infinity. In other words, does infinity have to exist to launch something with escape velocity? Sorry if this is still muddled.
laymanB said:If we are talking about the escape velocity of an object, what is the physical meaning of the object slowing as it approaches infinity. In other words, does infinity have to exist to launch something with escape velocity? Sorry if this is still muddled.
PeroK said:you can set up an equation for the time the rocket returns. If that equation has no solution, then the velocity is greater than or equal to the escape velocity, which then is the lowest velocity for which the equation has no solution.
No need to mention the "i" word!
Chris Miller said:from what I gather here, Nash's theory was closer to current cosmological thinking than Einstein's at the time
laymanB said:Am I correct in saying that the energy density, the stress-energy tensor, and therefore the spacetime curvature was extreme in the first second of the Big Bang?
laymanB said:what are the implications if that energy density is extended into infinity versus being finite? Does it put constraints on the scalar field for inflation?
laymanB said:Does it make it much more likely that the spacetime curvature will result in an immediate collapse?
laymanB said:What, if any, are the implications of a spatially infinite universe at t > 0? Let me elaborate.
Am I correct in saying that the energy density, the stress-energy tensor, and therefore the spacetime curvature was extreme in the first second of the Big Bang?
Assuming this is right, then what are the implications if that energy density is extended into infinity versus being finite? Does it put constraints on the scalar field for inflation? Does it make it much more likely that the spacetime curvature will result in an immediate collapse?
Ha! I am much lazier than you think. I'm only about a quarter of the way through, and haven't touched it for the last month or two. I'm trying to get more big picture ideas while trying to learn the theories. Instead of going through the standard protocol of learning the math and foundations first, before moving on to more complex subjects. I would not advise it for a college curriculum.PeroK said:If you are already on Chapter 22 of Hartle, you must have bashed through Chapters 1-21 fairly quickly!
I think this, because I find it hard to conceive of infinite, flat space expanding. Really, as the poster here has noted very well, I think, the term "physical infinity" is an oxymoron. Certainly opens up some philosophical cans of worms.PeterDonis said:Why do you think this? Current cosmological thinking is that the universe is spatially flat and expanding, not spatially flat and non-expanding.
Chris Miller said:I think this, because I find it hard to conceive of infinite, flat space expanding. Really, as the poster here has noted very well, I think, the term "physical infinity" is an oxymoron. Certainly opens up some philosophical cans of worms.
Chris Miller said:I think this, because I find it hard to conceive of infinite, flat space expanding.
Thank you for addressing my concerns/confusion. Of course, what I think/believe/can or can't get my head around, is irrelevant. My assertion re Nash's model was based entirely on posts I've read here pertaining to an infinite, flat, expanding universe, by persons more educated than myself.PeterDonis said:To be blunt, so what? We have a perfectly consistent model of a flat, expanding universe. There is no requirement that the model be easy for you, or anyone, to conceive of, just that it be consistent and make correct predictions. Our current best fit model does.
Besides that, the claim of yours that I questioned was that Nash's proposed model was closer to current cosmological thinking than Einstein's. That has nothing to do with whether or not you personally find current cosmological thinking hard to conceive of. What is your basis for your claim about Nash's model vs. Einstein's?
Chris Miller said:My assertion re Nash's model was based entirely on posts I've read here pertaining to an infinite, flat, expanding universe, by persons more educated than myself.
One of the problems with this kind of question is that "physically infinite" and "very, very large" are indistinguishable when it comes to predictions about what we might be able to observe. That makes it the kind of question that most people working in physics aren't likely to be very interested in. Usually physicists work with models that are physically infinite because it makes the math simple, and avoid making strong claims about whether or not it's actually true. If they work with a finite universe model, it's usually to try to come up with ways of measuring the features of that model (e.g. if our universe was a 3-sphere with a sufficiently small radius, we might be able to measure its curvature).Chris Miller said:I think this, because I find it hard to conceive of infinite, flat space expanding. Really, as the poster here has noted very well, I think, the term "physical infinity" is an oxymoron. Certainly opens up some philosophical cans of worms.
No more weird than a finite universe and in some ways a lot more rational. I mean if it's finite, what's its shape? What's its extent?kimbyd said:A physically-infinite universe is certainly very weird,
This comment highlights the problem: without observation to guide us, what seems 'weird' often comes down to personal interpretation.phinds said:No more weird than a finite universe and in some ways a lot more rational. I mean if it's finite, what's its shape? What's its extent?
So essentially we are left to conclude that the universe is much larger than the volume that we can observe, but the observational evidence gives greater credence to the hypothesis that the global geometry is flat and infinite in extent, assuming isotropy and homogeneity for the universe as a whole?kimbyd said:For now, I think the best answer is, "We don't know. Maybe it's infinite. Maybe it's finite. We don't yet have solid models validated by observation which give us a strong indication one way or another."
My understanding is that current measurements give a specific indication to within the tolerance of our measuring ability. It's likely that our measuring ability will increase at least somewhat which will likely just make the current beliefs more solid but COULD begin to lean the other way. "The other way" would be a bit of a surprise since out of all the values that "flatness" could have, it would just be downright weird that the actual value would be "really close to flat but not quite flat" instead of just flat.laymanB said:Do you foresee any future experiments or modifications to theory that would give greater confidence to this hypothesis?
As far as I remember, but I have no reference, most cosmologists don’t think about weird, rather they tend to believe that the universe is infinite simply because a trivial topology seems more obvious than e.g. a 3-torus.kimbyd said:A physically-infinite universe is certainly very weird, but that's not necessarily a guide to truth. Quite a lot about the universe that has been verified experimentally is exceedingly weird.
This conclusion is not correct. If the universe is spatially flat it isn’t neccessarily spatially infinite, its topology can be compact also, e.g. a 3-torus is spatially flat and if large enough we will never be able to confirm it by observation.laymanB said:Let me take this idea into the question of whether the universe is spatially infinite or finite. We have confirmed that the local geometry is flat with small error bars. We don't observe anything that would lead us to believe that the local topology is anything besides Euclidean space. So, we plug these observations into a FRW model and the solution that we get out is a spatially infinite universe.
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timmdeeg said:they tend to believe that the universe is infinite simply because a trivial topology seems more obvious than e.g. a 3-torus.
I have mentioned in post #33 that “a 3-torus is spatially flat and if large enough we will never be able to confirm it by observation” and was anticipating in my previous post that comologists would “prefer” a trivial topology as there are no oberservational indications for a compact topology. I’ve read this somewhere, but can’t find it. If I understand it correctly such reasoning makes sense because “trivial” in this case means less assumptions however would be glad to know your thoughts in this matter.PeterDonis said:That's not quite true. If the universe has flat 3-torus topology, there is a lower limit to the "size" of the 3-torus, because if it were small enough, we would see multiple images of distant galaxies coming from different directions. We don't. So the "size" of the 3-torus would have to be much larger than the size of the observable universe.
One possible explanation is the concept of "inflation," which suggests that the Universe expanded rapidly in the first moments after the Big Bang, making it appear infinite, but it actually has a finite age. Additionally, the Universe is constantly expanding, which means it may continue to grow infinitely in the future.
No, we cannot observe the edge of the Universe because it is constantly expanding and there is no fixed edge or boundary. This makes it impossible to determine its exact size or whether it is truly infinite.
Scientists have used various methods to estimate the age of the Universe, such as measuring the cosmic microwave background radiation and the expansion rate of the Universe. These measurements suggest that the Universe is approximately 13.8 billion years old.
Not necessarily. The concept of infinity does not necessarily mean something has existed forever. It simply means that there is no limit or boundary. The Universe may have had a beginning, but it could still be infinite in size.
The existence of infinite parallel universes is a theoretical concept and has not been proven. If it were to exist, it would likely exist within the infinite Universe, but it would not necessarily affect the overall size or age of the Universe itself.