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Cato
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Are there any indications on how large the universe might be? Not the observable universe, but the universe which came into being 13.8 billion years ago and of which our observable universe is a part?
The only indications that exist are very indirect. For example, if we measure the spatial curvature of our observable universe, and extrapolate that curvature outward, we get a universe that is at least 250 times as large as the observable universe. But that's an extrapolation: there are other effects that could lead a flat universe to be smaller. For example, if our universe were torus-shaped, then it could be only a little bigger than the observable universe but still be flat. It would still have to be bigger than the observable universe because the torus-shape would leave evidence would observe otherwise. Or it could just be that our observable universe is unusually flat, and the rest of the universe is more tightly-curved, leading to a smaller overall size.Cato said:Are there any indications on how large the universe might be? Not the observable universe, but the universe which came into being 13.8 billion years ago and of which our observable universe is a part?
Cato said:Are there any indications on how large the universe might be? Not the observable universe, but the universe which came into being 13.8 billion years ago and of which our observable universe is a part?
Then all your questions boil down to one: what, if any, evidence do we have that bears on the question of whether the universe is infinite or not? The answer to that is that we have plenty of evidence that constrains the parameter space of the standard hot big bang model of cosmology to a pretty narrow range, a range which makes it very unlikely, based on that model, that the universe is spatially finite, and very likely that it is spatially infinite. AFAIK there are no other models in serious contention with the standard hot big bang model in this area.
Sounds like this is a good candidate for the Astronomy and Cosmology FAQ (which is actually hidden in the Astronomy forum). Even when these questions are repetative, I always enjoy reading interesting and fresh perspectives like the one from @kimbyd.phinds said:And by the way [USER=469185]@Cato obvious questions like this one have been asked here hundreds if not thousands of times. A forum search is always the best place to start for such questions.[/USER]
stoomart said:Sounds like this is a good candidate for the Astronomy and Cosmology FAQ (which is actually hidden in the Astronomy forum). Even when these questions are repetative, I always enjoy reading interesting and fresh perspectives like the one from @kimbyd.
ilenesmith said:Well, I guess this is one of the unanswered question till now. How big is the universe is really very difficult to know. It may be so big that even light has not had that much time to cross it in nearly 14 billion years and it is still getting bigger all the time.
Gigaz said:It may in fact even be much smaller than what we think. If the universe is flat with a periodic boundary in each direction, most of the galaxies would be duplicates of other galaxies but seen from another direction and at another time.
Yup. Repetition would produce a pretty clear signal on the CMB. If our universe does wrap around on itself, it must do so outside of the observable universe. So it could be smaller, if that were the case, than the upper limit mentioned earlier in the thread of 250 times larger than the observable universe. But it's definitely larger than the observable.Drakkith said:Sure. But I think we've looked and so far haven't found any patterns that would be suggest this as being likely.
kimbyd said:Yup. Repetition would produce a pretty clear signal on the CMB. If our universe does wrap around on itself, it must do so outside of the observable universe. So it could be smaller, if that were the case, than the upper limit mentioned earlier in the thread of 250 times larger than the observable universe. But it's definitely larger than the observable.
But isn't it inconsistent with known physics? It assumes "boundary conditions" not in evidence.Gigaz said:Not necessarily. The universe can be an expanding box of flat space with periodic boundary conditions. Such a scenario is clearly not inconsistent. Just think of Pacman ;-).
phinds said:But isn't it inconsistent with known physics? It assumes "boundary conditions" not in evidence.
That's more or less the scenario I was thinking of. This is the toroidal universe. It still has to be larger than the observable universe or else there are large effects on the CMB.Gigaz said:Not necessarily. The universe can be an expanding box of flat space with periodic boundary conditions. Such a scenario is clearly not inconsistent. Just think of Pacman ;-).
Well, the "evidence against" is just that it would require new physics at the boundaries and there's just no reason to make such an "out of left field" assumption.Gigaz said:Not really. It's true that there's no evidence any cyclic repetition in space but there is also no evidence against it, at least if the scale is sufficiently large. The CMB only tells us that the large scale curvature is consistent with zero.
I'm not sure that's entirely true. PBC is basically just a toroidal topology. I don't think you need any new physics to describe our universe using a toroidal topology, as General Relativity allows this possibility, but it would raise questions of how such a topology could have been generated when our observable universe first formed.phinds said:Well, the "evidence against" is just that it would require new physics at the boundaries and there's just no reason to make such an "out of left field" assumption.
What is "PBC" ?kimbyd said:I'm not sure that's entirely true. PBC is basically just a toroidal topology. I don't think you need any new physics to describe our universe using a toroidal topology, as General Relativity allows this possibility, but it would raise questions of how such a topology could have been generated when our observable universe first formed.
phinds said:is this "periodic boundary" model viable?
Scientists use various methods to measure the size of the Universe, including the cosmic microwave background radiation, the expansion rate of the Universe, and the distance and brightness of celestial objects such as galaxies and supernovae. These measurements are then used to calculate the overall size and volume of the Universe.
The current understanding among scientists is that the Universe is not infinite, but rather it has a finite size. However, the exact size of the Universe is currently unknown and may be beyond our current capabilities of measurement. Some theories suggest that the Universe may be infinitely expanding, but it is still considered to have a finite size.
The observable Universe is the portion of the entire Universe that we are able to see and study. It is estimated to be around 93 billion light-years in diameter. However, this does not mean that the Universe ends at this point, as the expansion of the Universe may have allowed us to only observe a small portion of its actual size.
Due to the vastness of the Universe and our limited technological capabilities, it is unlikely that we will ever be able to know the exact size of the Universe. Our current methods of measurement are constantly improving, but it is possible that the true size of the Universe may always remain a mystery.
Some theories in physics and cosmology suggest the existence of multiple universes, known as the multiverse theory. However, this is still a topic of debate and there is no concrete evidence to support the existence of other universes. The size of these hypothetical universes, if they do exist, is also unknown.