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Arifz
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Does the universe has boundaries?, is it finite?
mathman said:Universe does not have a boundary. Finite or infinite is an open question.
Arifz said:But there is no such thing as infinity in physics !
Arifz said:But there is no such thing as infinity in physics !
phinds said:Oh? Can you prove that? Do you have an accredited references to back up such a categorical statement?
jackmell said:I believe there is a simple solution to this problem and it is based on behavior all through the Universe so I do not believe it is an inappropriate stretch to apply it to the entire Universe: we observe phenomena in the Universe having critical points in their dynamics. Breaching such a point often causes the dynamics of the phenomenon to change qualitatively and by doing so, the rules change. For example asking what does swimming mean beyond the critical point of freezing? What happens to a hydrogen atom beyond the critical point of fussion? In a small section of the ground it looks flat, even my whole yard. But it's not always flat, beyond the horizon the rules change and asking whether the Earth is infinitely flat or we just fall off is simply not following the new rules of a spherical Earth in a gravity field.
Therefore, in regards to a "size" of the universe, I do not think it is unreasonable to suggest at some large "size", a critical point is reached, the rules change, the concept "volume" loses meaning, and asking for a "size" of the Universe beyond that point is simply not following the new rules.
Arifz said:I think the big bang "Theory" can proves it
, however I still don't know what do we mean by "universe", is it the space that we know, or it also inclues beyond space "Vacume !".
What do you call the "No Space/No Time" before the big bang?!
Arifz said:But there is no such thing as infinity in physics !
I can't find a post claiming it does. OTOH, does anyone think it does have a boundary?andrewmh said:the big bang does not prove that there is no boundary to the universe.
But those are not infinities in the physical world, just in mathematical theory.and there can be an infinity. if you follow these two patterns you will notice that they will go on forever
1 2 3 4 5 6 7...
2 4 6 8 10 12...
divide them and you still get a number yet they are both infinitely large.
phinds said:I did not ask what you think, I asked what you can support by science.So you are making a definitive statement about something you can't even define. Now that's REALLY scientific.I don't call it anything since there is no evidence that there was such a thing and it is not part of the big bang theory
andrewmh said:to add to the idea of no space or time before the big bang, well you pretty much said it. there was no time. kind of a hard thought for human logic but time started at t=0 so yes, time does have a beginning.
Ken G said:The answer to that question is, "we have no scientific evidence that the universe is finite." That's it, that's all we can use science to say. /QUOTE]
I think we can go a little further than that. Many/most people would assume that if space has no boundaries then it must be infinite in volume. Science can be used to produce models, consistent with physics as far as is known, in which space can be finite yet unbounded. There may even be implications of such models which could be tested.
It's not terribly relevant, but that doesn't follow. Space could curve back on itself, have finite volume, and still have no boundaries. But the real issue here is, despite looking very hard (and quite possibly as hard as we can ever look), we have no evidence that it does have boundaries, nor do we have any evidence that it does not have boundaries, nor do we have any evidence that it curls back on itself, nor do we have any evidence that it does not curl back on itself. All we know is, what we see looks flat, and we have no idea how long it stays looking flat. That's it, that's the scientific evidence in its entirety.haruspex said:I think we can go a little further than that. Many/most people would assume that if space has no boundaries then it must be infinite in volume.
But making and testing models has nothing to do with answering the OP question. The models we make are intended as idealizations, and the standard idealization is that of a flat infinite universe. That model works quite well. Is that evidence that the universe really is flat and infinite? Of course not. If I am digging a foundation for my house, I'm certainly going to use a model that the surface of the Earth is flat and infinite (in that I will certainly not model any curvature of the Earth), and it will work great for digging my foundation, but I'm never going to conclude that any of this is evidence that the Earth really is flat and infinite.Science can be used to produce models, consistent with physics as far as is known, in which space can be finite yet unbounded. There may even be implications of such models which could be tested.
There is zero evidence that this is the case, and there is also zero evidence that this is not the case. And humanity should be prepared for the possibility that this situation will never change for us, as it seems quite likely at present. Even if efforts to detect a tiny spatial curvature do eventually succeed, it won't require that the universe maintains that same curvature everywhere, that will simply be an idealization of the model, like any other idealization of any other model. It will never be testable as fact, we pretty much already know this.shreyakmath said:The universe is finite but with no boundary. It is similar to a bubble or sphere
Yes, that's the point I was making.Ken G said:... that doesn't follow. Space could curve back on itself, have finite volume, and still have no boundaries.
I'm far from expert in this area, but my understanding is that General Relativity posits a "flat" spacetime, but a finite, curved space, with a compensating curvature in time.nor do we have any evidence that it curls back on itself,
I disagree. The question was "Does it have boundaries? Is it finite?" Establishing that 'finite without boundaries' cannot be ruled out is a partial answer.But making and testing models has nothing to do with answering the OP question.
phinds said:Oh? Can you prove that? Do you have an accredited references to back up such a categorical statement?
No, GR says that in comoving-frame coordinates (which is what is generally used in cosmology to talk about what the universe as a whole is doing), all the observed curvature due to gravity is in the time dimension (associated with cosmological redshifts), none is in the spatial dimension. We say the universe is "spatially flat" in this sense. The observations cannot rule out some small spatial curvature, but they can rule out the idea that the universe curves back on itself over the range that we can observe or in some way test our inferences about-- and of course we have no idea what it does beyond that range. Even if we do detect some small spatial curvature, it would not require that this curvature is maintained beyond what we can observe-- the cosmological principle applies to explanations of what we actually observe, it is not a philosophical claim about what we cannot observe.haruspex said:I'm far from expert in this area, but my understanding is that General Relativity posits a "flat" spacetime, but a finite, curved space, with a compensating curvature in time.
No, the Big Bang model includes neither finiteness nor spatial curvature at present. So the model is one of an infinite universe. However, the model need not make any claims that this is actually true, it just means we have no reason to model finiteness of the universe.I would also have thought that an infinite universe was inconsistent with the Big Bang model, and there is much evidence for that.
I said it could not be ruled out. I also said the alternative could not be ruled out. In fact there is no evidence at all either way. When there is no evidence in favor of a proposition, nor evidence against it, it doesn't leave you with a whole lot more to say, which is the point.I disagree. The question was "Does it have boundaries? Is it finite?" Establishing that 'finite without boundaries' cannot be ruled out is a partial answer.
Ken G said:Even if we do detect some small spatial curvature, it would not require that this curvature is maintained beyond what we can observe-- the cosmological principle applies to explanations of what we actually observe, it is not a philosophical claim about what we cannot observe.
No, the Big Bang model includes neither finiteness nor spatial curvature at present. So the model is one of an infinite universe.
When there is no evidence in favor of a proposition, nor evidence against it, it doesn't leave you with a whole lot more to say, which is the point.
haruspex said:I'm far from expert in this area, but my understanding is that General Relativity posits a "flat" spacetime, but a finite, curved space, with a compensating curvature in time.
Algren said:There is a logic behind the beginning of time. Any events which occurred before the big bang does not affect us or the universe today. There is no need to assign these useless and unknown events with a time. Thats why we have BIG BANG occurred at t=0;
Ken G said:Even if efforts to detect a tiny spatial curvature do eventually succeed, it won't require that the universe maintains that same curvature everywhere, that will simply be an idealization of the model, like any other idealization of any other model. It will never be testable as fact, we pretty much already know this.
Right, and what we find, when we do that, is zero evidence of any spatial curvature, which is consistent with inflation. If inflation is correct, this will always be true, no matter how good our observations get.twofish-quant said:But CMB power spectrums can and do constrain large scale anisotropy. We can directly measure what's in our bubble, we can infer things for some distance outward.
If we detect that, you can throw away inflation completely!Also, if we do detect small scale curvature, this is going to very strongly constrain the details of inflation and we can use that to infer a lot of stuff.
You don't see what I'm saying. LCDM is not a statement about what the universe is really like, it is a good model of the universe. That's a rather important distinction, and cuts right to the heart of what physics and astronomy does! What's more, LCDM is flat, and invokes the cosmological principle, and so it is a model of an infinite universe. Of course these are idealizations, physics deals exclusively in idealizations, it makes models. As I said, that does not mean it asserts the universe is infinite, it means it is an infinite model of the universe. Which is just precisely what it is. Newtonian physics was never an assertion that the universe is deterministic, it was always a deterministic model of the universe, which is quite different.This is incorrect. LCDM doesn't require finiteness, but it doesn't exclude it.
I have never been talking about "what the current model allows." The current model allows for unicorns, space aliens, and teleportation beams. But none of those are included in the current model, because there is no need for them, and no evidence in favor of them. Again this is a rather important distinction.Also whether the current model allows for a finite universe is an observational equation that changes from moment to moment.
Right-- and what got changed is we got a flat model! Which is what I have been talking about all along.Before the discovery of dark energy, the amount of dark matter in the universe was clearly insufficient to close the universe so there was a period of a few years in which the preferred model was infinite and negatively curved.
Then we have dark energy and everything changed.
Ken G said:Right, and what we find, when we do that, is zero evidence of any spatial curvature, which is consistent with inflation. If inflation is correct, this will always be true, no matter how good our observations get.
If we detect that, you can throw away inflation completely!
What's more, LCDM is flat, and invokes the cosmological principle, and so it is a model of an infinite universe.
Ken G said:I'm talking about the evidence that exists today, and the models we build based on that evidence. And that evidence is used to build flat models of an infinite universe-- with no claim whatsoever that this is the truth of the matter, it is just our best model.
Right-- and what got changed is we got a flat model! Which is what I have been talking about all along.
Right, but that's exactly why the CDM model was uniformly rejected by just about everyone. That is in complete contrast with the models of today, with which we often hear the phrase "precision cosmology", and has been related to several Nobel prizes.twofish-quant said:One other thing to note is that before the discovery of "dark energy" in 1998, the best available model (CDM) resulted in a negative curvature model of the universe. It's only after you add dark energy that you get something like a flat universe.
No, that's not true. I was an astronomer in 1995 also, and few thought the universe was not flat, they thought the model was wrong. That's also why there were no Nobel prizes awarded for the CDM model. Indeed, it was considered a huge problem that the flatness parameter came out 0.3, which was way too close to 1 to not be 1 (a flatness less than 1 gets exponentially less flat with time, so to be 0.3 now, it would have had to have been extremely close to 1 in the past, but still strangely different from 1). Even in 1995, inflation was commonly taught, and it was widely expected that the flatness should be 1. The missing energy was just considered a paradox that no one knew how to solve, but made people worried that we were missing something really crucial. Today that is not the sentiment, hence all the Nobel prizes, though of course there are plenty of people still not completely happy with dark energy, and that's why we have some people claiming that you need multiverses to explain it. I'm not banking on that approach myself, however, I just think we are still missing some key physics, but the models of the universe will still be flat (except for local fluctuations with no global significance), and we will just never get to know anything beyond that for the simple reason that we cannot look.Which is why I dispute your statement that a flat universe is *required* for inflation. As of 1995, it was believed that we didn't live in a flat universe, because without dark energy flatness is excluded to pretty high certainty, but that didn't kill off inflation.
I'm not sure where you get that, but it is incorrect. See the WMAP website at http://map.gsfc.nasa.gov/universe/uni_shape.html , where we find quotes like:And I'm saying this is false. If you look at the parameterizations for WMAP, you'll find that the model that they use to calculate observational constraints is not flat.
It comes from modern astronomy textbooks, and websites like the WMAP website I quoted above. So, where are you getting your misinformation, given that you are "keen" to stamp it out?twofish-quant said:Also, I'm interested in where you are getting your information since it's wrong.
Ken G said:It comes from modern astronomy textbooks, and websites like the WMAP website I quoted above.
So, where are you getting your misinformation, given that you are "keen" to stamp it out?
Either one. Just not textbooks expressly designed to investigate speculative areas of astronomy. Such books always appear at the fringes of any science, they are certainly not quackery, but they are usually forgetten in a few decades-- such is the nature of controversial speculation. No doubt there are graduate textbooks on MOND, on loop quantum gravity, and on microscopic black holes.twofish-quant said:Which textbooks? Graduate or undergraduate?
Good luck with that, I'm sure they'll be thrilled to have your expertise weighing in.The WMAP website oversimplifies things. I'll e-mail the maintainers of the site to get it changed.
They told you that eternal inflation is a mainstream consensus idea? I doubt that strongly.1) from the graduate courses that I took in cosmology when I got my Ph.D. in astrophysics
I don't see any quotes from them in your argument. What are you claiming they said, and why don't you think it is making it to the WMAP website?2) from talking with cosmologists and supernova people, include one of the lead co-authors of the WMAP paper, one person that was a co-author on the supernova Ia investigation papers, and one person that has a Nobel prize in physics.