Not sure this needed a new thread, but I didn’t want to hijack the other one.
This thread, but specifically this post:
Started me thinking…
I googled around and there is no hard consensus as to whether the universe is finite or infinite, but if there was a Big Bang (from a time when all the matter in the universe was a singularity), wouldn’t that mean it has to be finite?
There are other similar threads but most are around 20 years old.
The vast majority of physicists agree with the Big Bang theory. As you say there is no consensus on the finite or infinite size of the universe. That means that there are plenty of physics who would agree with the hypothesis of a universe of infinite size universe that started with the Big Bang combination. It could be that those physicists are wrong, but it’s certainly possible to hold to both positions and be a respectable physicist.
As to why some hold to the position of an infinitely large universe, I don’t know. But it’s clear that many mainstream physicits do hold that position. For my part, I’ll ready admit I hold to the hypothesis of a universe of finite size in part for emotional reasons rather than based entirely on scientific evidence. The only evidence I would propose in support of the finite size hypothesis is that in every other aspect where we have looked for infinities, we have never found any.
There’s no hard evidence one way or the other. Most physicists believe that it’s infinite, on the grounds that that’s the simpler assumption. Some, however, believe that it’s finite, on the grounds that that’s the simpler assumption.
The observable universe is definitely finite.
We don’t know what exists outside the observable universe (for obvious reasons) but there’s no reason to assume it’s not just more of the same, forever. (So far, but we are trying to make very fine measurements of things like the overall shape of spacetime which might shed more light on the matter. But so far signs point to infinite).
The universe may be finite, but much much larger than it seems. During cosmic inflation, the universe doubled in size about every 10-36 seconds. But it’s possible that inflation never stopped in general–only in our little corner of the universe. So the actual rest of the universe has continued to double every 10-36 seconds for the last 14 billion years. That’s quite a few doublings.
That blows my mind. I don’t get how an infinite sized universe is a simpler assumption. It seems that would create all sorts of dilemmas that a finite universe doesn’t, even a massively huge one that’s trillions or quadrillions or more light years across. The obvious problem being that if the universe is already of infinite size, how is that it has expanded and is still expanding? If the universe is growing rather than merely being stretched out, how does an infinite size make any kind of sense?
Cosmologists’ description of the expansion is entirely in terms of it being “stretched out”. There’s no notion that there’s some actual size that can be given a number and that that number is increasing, because even if there is, we can’t know what it is.
I know that there is bunches I don’t know about this stuff but I’m having trouble squaring the fact that all matter and space started as a point of singularity, started expanding and then, somehow turned into infinity.
Am I making sense. Before the Big Bang started expanding, all of the universe was finite, right?
If something were infinite, you would never be able to see its full extent by definition. In fact, the question of whether the universe is infinite or not is for all observational purposes moot as we can only see (or experience in any other causal fashion) as far in spacetime as the cosmological horizon. Whether there is further extent to the universe beyond that distance is immaterial unless you have a time machine or a superluminal propulsion system.
The Big Bang Hypothesis (which is widely, albeit not quite universally accepted) is based upon inferences about the observable universe which may or may not apply to any volume beyond that. The distribution of visible matter in our observable universe only has very slight anisotropies at cosmic scales (and ‘dark matter’ can’t have much greater clumping and still allow the structure that we see) but it is entirely possible that other volumes of the universe have very different distributions of luminous and non-luminous matter, and perhaps even different amounts of ‘dark energy’ (whatever that is) giving spacetime very different properties. However, any model that would allow for this would be very complicated, and so cosmological models utilizing General Relativity (which is pretty much any model that would be considered credible) assume that spacetime is at least approximately homogeneous at cosmic scales.
The more interesting question isn’t whether the universe is infinite or not, but whether is is bounded; that is to say, if it is a finite (or “hyperfinite”) and bounded, or if it is unbounded and wraps back around upon itself. So far, we have no evidence of a finite-but-unbounded universe but both the philosophical and thermodynamics of such a universe are intriguing and not just slightly paradoxical. It also turns out that a workable model for the Big Bang and subsequent expansion of the universe is a black hole in reverse.
Stranger
So are you hypothesizing that galaxies are receding away from us and the further they are away from us (generally speaking), the faster they are receding. But that just might be a local phenomenon and not a property of the universe as a whole?
It’s not really a hypothesis in the sense that it can be tested in any way known to science, but just an acknowledgement that it is conceptually possible that volumes of the universe beyond our cosmological horizon are not guaranteed to have the same properties as our causally ‘local’ volume of the observable universe. For all we know, the universe beyond the cosmological horizon might be filled tusk-to-arsehole with pink elephants.
For that matter, we have inferred a model of the universe (the “Lambda-Cold Dark Matter” or ΛCDM model) that says our observable universe has one homogeneous and isentropic set of properties and expansion rate proportional to distance (the Hubble constant) but there is an inconsistency in measurements that produces two distinct values (the so-called “Hubble tension”) even within our observable universe. There have been various attempts to try to rectify this by glomming on new submodels and somewhat arbitrary changes in behavior, but what is described as the “Big Bang” is actually a whole family of hypotheses that are not entirely consistent with known physics, and indeed, some people want to throw away General Relatively entirely and replace it with a much more arbitrary and frankly inelegant model that is tunable to match observations, which in my mind is bad science. I think we all know that General Relativity cannot be a complete theory and is (likely) some kind of mean field approximation to a more unified theory with the other fundamental interactions, but it is consistent and well-tested sufficiently that I don’t see any non-speious argument for throwing it out any more than I would recommend a high school physics student ignore Newton’s laws and infer a novel theory of physics from Warner Brothers cartoons.
Stranger
What do you mean here? One would think it is the bounded universe that would wrap back around on itself, like (for example) a sphere.
There’s no real reason to think that’s the case. It kinda makes sense to the human mind because our world is a sphere and we think of it as a map that loops on itself, but there’s no reason to assume that even a curvy universe loops in that way.
I understood “bounded” to refer to distance; are you saying that in some cosmological model there is a boundary or edge or horizon
My own personal pet theory!
Absolutely nobody in cosmology takes seriously the notion that the Universe might be bounded. What would the boundary be? And if there is a boundary, what’s beyond it? Even if it’s finite, it’s surely still unbounded (possibly like a sphere, possibly like a torus or some other topological configuration).
If it’s infinite, then it has always been infinite. Never would it have gone from finite to infinite, just as your intuition suggests. We had a thread on this this past fall.
But what’s the alternative for a finite universe? Every spacetime path ultimately ends in a singularity?
I have some conceptual trouble, that others may share. If the “universe” comprises “everything that exists” then ideas of expansion can refer only to increased space between objects, not to anything like a border or edge which is growing, because that would imply some “outside” into which the universe is growing. That’s where I get into trouble and what’s left of my brain shuts down.
In every example except that involving the titled topic you can envision what comes next, what lies outside, what there might be once the end comes, but when it comes to that which holds everything there is without need of limit to space, weight, height, length or power the question that has to come mystifies me: And then…?
Yes, that’s exactly correct.