If the center of the universe is everywhere does this mean that the edges of the universe are likewise everywhere? And if the Earth is on the edge of the universe shouldn’t we see nothing but blackness in one direction? (Apologies if it’s a naive question, it is indeed hard to get a mental grip on these things).
The universe doesn’t have an edge any more than the Earth does, and likely for the same reason. When we talk about the “edge of the visible universe” what we mean is the boundary from our particular location where light that was emitted in the past can no longer reach us because of the speed at which the intervening space is expanding.
Let’s try an analogy in fewer dimensions (2+1 versus the 3+1 dimensions of our universe): say you are an ant on the outside surface of a balloon that is being constantly inflated. (The surface is topologically two dimensional even though a balloon obviously exists in three dimensions). As the inflation continues, objects some distance from you will become further because the skin of the balloon stretches, even though in their refernce frame they are motionless and you are moving away. At some point, they’ll fall over the horizon and disappear, and if the rate of inflation increases without limit, they’ll move away at speeds faster than you can possibly reach them no matter how long you travel or any arbitrary speed you move at.
Now the surface of a balloon is obviously a finite area, whereas the volume of the universe may or may not be infinite, but both are (as far as we know) without boundary in their respective spatial dimensions, and ever increasing in extent in their temporal dimension. Either way, although there are no boundaries and no unique center or privliged reference frame, there are limits to the areas on the balloon you could ever reach, and the same is true on any other arbitrary starting location on the balloon.
Stranger
All,
I will just note that authors, singular and plural, writing in Scientific American often use a V like representation. This is undoubtly due to the constraints of a print medium and ease of visualization.
Prior to this thread, I don’t think that I realized this was merely a representation.
I am wondering is perhaps the OP has seen something similar and like I, was mislead and hence the inqury.
Zuer-coli
It comes down to the cosmological principle, which in its weaker form says that (broadly speaking) everywhere should be mostly the same as everywhere else. This is just the idea that physical laws are Universal and so work in the same way everywhere, meaning that on a large enough scale the differences due to random variation average out making the Universe distinctly samey throughout its entirety. We could also called the principle of homogeneity.
We know that general relativity introduces the possibility of curvature, but for the Universe to be the same everywhere (again remember we’re talking on very big scales, bigger even than galactic superclusters), the curvature must be the same everywhere. The three model spaces with curvature that is the same everywhere in 3 dimensions are:
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Flat Euclidean space. This really needs no introduction, we’re all familiar with it as that is how we are naturally inclined to think of space. Parallel lines stay parallel and the space has an infinite volume
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Positively curved 3-spheres. These are just an extra dimensional versions of the spheres we’re all familiar with. Lines starting as parallel move towards each other and will eventually meet, just like lines of longitude on a globe. And just like a sphere has a finite surface area, a 3-sphere has a finite volume. Similarly, you can head off in a straight line on a 3-sphere in any direction and eventually come back to where you started.
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Negatively curved hyperbolic space. This is the least familiar of the three, probably as the 2D equivalent (the hyperbolic plane) can’t be embedded in 3D Euclidean space. Hyperbolic space is is perhaps best thought of as ‘anti-spherical’: lines starting as parallel will move away from each other. Hyperbolic space has an infinite volume.
Now these in fact are not the only spaces of constant curvature, a classic example is the 3-torus (which is just an extra dimensional doughnut shape). Parallel lines on a 3-torus stay parallel, but the 3-torus has a finite volume (just like a doughnut has finite surface area). You can head off in a straight line on a 3-torus and eventually come back to where you start, but how far you have to travel to get back to your starting point depends on the direction (think of how the radius of a doughnut’s rim is always larger than the radius of its thickness).
This brings us to the stronger version of the cosmological principle which states that, not only should space should be the same everywhere, but it should be the same in all directions (i.e. no getting to your starting point quicker by heading in a different direction!). This is just the idea that it would be bizarre if some directions were different from others, because all our known physical laws do not have a directional bias. We could also call this the principle of homogeneity and isotropy.
It can be proved when this principle applies the only allowable spaces are Euclidean, spherical and hyperbolic as the other spaces of constant curvature are homogeneous, but fail to be isotropic. We can’t ever absolutely prove the cosmological principle as that would mean a complete survey of the (possibly infinite) Universe, but it hasn’t let us down yet.
For the Universe to have any kind of non-arbitrary edge would be a clear violation of the strong cosmological principle and possibly the weak one too. We can’t say 100% that there is no edge, but it would be very difficult to explain how such an edge could come to be when physical laws have no bias as to location or direction.
As an addendum, general relativity doesn’t just say space is curved, it says spacetime is curved, which allows space to be dynamic. However the only way it can be dynamic and maintain constant curvature is by expanding or contracting. Cosmic spacetime on a large scale can therefore be entirely described with just two things: whether it is flat, spherical or hyperbolic and by a function which describes its expansion/contraction in time. These can be related to the mass-energy contents of the Universe and in fact there are not any remotely realistic mass-energy contents that lead to a Universe without expansion/contraction.
It’s worth noting that eternal inflation has the Universe on a much, much larger scale divided into regions of differing rates of expansion (hence violating the cosmological principle) and the boundaries between these regions would be edges of sorts.
A V-like representation of what?
I believe he is referring to diagrams depicting the metric expansion of space-time such as this one. They depict space-time at a given time as a disk of space and material, not just because of “the constraints of the print medium” but also because we can’t actually visualize diagrams in four dimensions (well, most people can’t, anyway). Space-time is, of course, a volume, and that volume may well be infinite; we can only observe the parts of our universe that we can see, and we know that there is space and matter out beyond that.
Stranger
As far was we can tell the laws of physics are the same in all parts of the universe. But of course, we can’t see the entire universe. It’s possible in an infinite universe that some parts of it that aren’t causally connected to us have different physical laws. But that would be very surprising, because there would have to be some boundary where the two realms meet that were causally connected, and how would that work?
The most we can say is that at the farthest distances into space that we can observe things seem more or less the same as in the closer parts of the universe. So if distant parts of the universe are different they’d have to be very distant, and we’d never be able to interact with those parts of the universe. But even if the universe is infinite it could be that the laws of physics properly understood apply in the same way in all parts of it.
Or it could be that the universe is finite. Like, suppose you had a really good telescope, could you look all the way around the universe and see yourself from behind? But it looks like even if this is the case the universe is large enough and expanding fast enough that there wouldn’t be time since the big bang for any light to make a complete trip around.
And it is just a theory isnt it. Only in vogue because there isnt a better theory and when that better theory comes along all of the above will be bovine excrement?
Trillion universes started from a pinpoint. Yeah right.
This is what theory means, and everything we “know” in science is in the exact same boat. New data either validates or invalidates all knowledge. In other words, you don’t demonstrate an understanding of science.
Thank you, Stranger On A Train and Asympotically fat. Responses like these are why I love this board!
Indeed. Thanks to all you guys. I have been enjoying this thread.
All these years, I thought I had a fairly firm grasp on how the BB worked. Boy was I ever wrong! But hey, at least ignorance was fought here in this thread. Good job guys.
No its not that simplistic as i know more than you. You have accepted a theory blindly. Many others haven’t.
Does not make you better at understanding science.
:dubious: What’s “blind” about scientific acceptance of the Big Bang model? ISTM that it’s universally (ha ha, see what I did there) recognized that there’s still a lot we don’t understand about the early universe, but the basic fact of the Big Bang is very solidly supported by, e.g., the cosmic background radiation mentioned above.
Also, if you’re objecting to the Big Bang model for some reason, what scientific hypothesis are you saying should be adopted in place of it?
I haven’t been involved in this discussion, but on behalf of several posters that you’ve responded to, let me say that your use of phrases like “only a theory”, “i know more than you”, and especially “Yeah right” in response to someone patiently explaining the correct conceptual view of the Big Bang, are all red flags suggesting that you know a good deal less than you think you do about science in general or the subject matter here in particular.
But I have a clear understanding of what you wrote, and it’s rubbish.
Scientists never accept anything blindly. What we accept, we accept based on the best available evidence. Now, there are plenty of people who reject scientific theories blindly.
We go down this path many times. These are the usual misconceptions about the nature of science and the nature of a theory.
Use of the word theory in science is not the same as everyday usage. It isn’t just a guess.
The entire idea that it will be swept aside by a new theory, and is thus worthless, presupposes that it is actually wrong, and there will actually be a new, better, theory. You have no reason to know this is true. A great many scientific theories are so well established that there is no realistic chance of them being wrong, even though we do leave the door open for that possibility. (I blame Khun for some of this attitude.)
A scientific theory must be falsifiable. Means that you must be able to come up with ways it can be realistically shown to be false, and it should survive such tests without requiring significant change. This generally means that it is predictive. The theory says something that can be checked to be so. Ideas for which this isn’t possible are not scientific theories.
The idea that falsifiability of theories is a weakness is a misunderstanding of science. It is not a feature that allows anyone to come along and dismiss the theory because it might eventually be wrong. It is there to distinguish science from religion and psuedo-science. Where facts are presented as revealed truth, with no possibility of refutation, or are presented as a constantly moving set of assertions that is modified with get-out exceptions every time it is disproved.
Big Bang is predictive, and has made some very specific and tested predictions. It also fits the known, observed, facts. It is an uneasy theory from some philosophical points of view, but by the same token, any alternative theories yield equally difficult philosophical results. Everything from requiring various gods to unjustifiable new physics.
(Note clearly - trillion universes from a pinpoint has almost nothing to do with the Big Bang. Either it is a straw man argument, or a misunderstanding of the theory.)
Anything that might supplant the Big Bang must do everything it has done, and more. Dismissing it because you don’t like it, or you have a big floppy book somewhere that unequivocally states that it is turtles all the way down isn’t science.
Definitions: Universe … the infinitely large 3d space we are in.
“Our Big Bang Region”… whether its collections of collections of collections of galaxies,
or collections of collections of collections of collections of collections of collections of galaxies, its just our region, a finite volume that the singularity can fill in the finite time since its Big Bang.
Ah but I understand you are saying an infinitely large “THE Universe came out of one singularity” … No… thats a stupid white lie the overly philosophical astrophysicists (Hawking etc) keep touting, to sell their imaginative ideas of possible scenarios…
In more objective, elementary, 'stick to the known facts" , OUR REGION is assumed to be created by something like a Big Bang, measurements of our region are admitted to be “only so far as we measured”, yes a finite size, this gives the time back to big bang like event … ( An implication of that is that if a single big bang did produce an infinitely large universe, we couldn’t know how long ago that big bang was ! )
Here’s were the confusion starts…
The basic evidence is that every distant galaxy one looks at seems to be red shfited about the same amount and therefore that suggests everything is moving away, and no pattern is found to give any HINT at a centre.
The strong contradiction in the “the big bang created the entire , infinite universe” line is that would suggest that a single black hole created the entire universe … But hang on, we can see many black holes… So can’t they create their own regions ?
There’s two possibilities…
- The entire universe is like that… (evidenced)
- OUR REGION is like that, and we can’t know anything about outside our region since we haven’t yet observed anything OUTSIDE of our region… (the evidence only says so much… only says out that far… )
Conclusion: it sells books to go on and on about possible universes where “1” is true, but its just totally evidenced.
We can only detect galaxys which are sending enough photons coming our way to be above background noise, so the lack of ability to detect the “edge of our region” does not mean “our region” is infinite… we just can’t find the edge of “our region” …yet.
Why add any qualifiers? The “universe” is simply everything there is. The question of infinite size is an answer to a question, not part of the definition. Similarly so the number of dimensions.
Some truth to the point that some well known physicists enjoy speculating.
This need not be true.
Nitpick. It more precise than this - the redshift of galaxies is proportional to their distance from us. That provides a very specific piece of evidence about the nature of the expansion of space. It also provides a time when the expansion began.
Who said anything about a black hole being the seed of the universe? Black holes may contain a singularity, and the universe may have started with a singularity. There is nothing saying they are the same sort of singularity. A singularity is a mathematical observation about the nature of the laws at that point. The breakdown of understood laws for a black-hole need not have much in common with the break-down of known laws that we have at the genesis of the universe. A singularity is not a thing. It is an observation about our understanding. We don’t understand the laws at the center of a black hole in a related, but different, manner to how we don’t understand the laws at time zero.
The very strong evidence for what happens outside what we can see is the isotropy of the observable universe. Especially the isotropy in the cosmic background radiation. The ridiculous level of precision we have observed such isotropy to is very suggestive of the nature of what exists further out, and of the history of the early universe.
That the CBR exists and fits the predictions of the big bang is very hard to ignore.
This presupposes that the notion of edge is meaningful. Thinking in simple Euclidian geometries makes it hard to fathom. But there no good reason to expect that the universe is so limited.
You’re behind the curve. The “expansionary phase” modification of the Bing Bang has been around since 1982, and says that, yes, we are only in an observable “region” of the overall cosmos. There is a much larger cosmos which we can’t observe, because of cosmic expansion.
ETR: too rude, apologies.