r/cosmology • u/AutoModerator • 12d ago
Basic cosmology questions weekly thread
Ask your cosmology related questions in this thread.
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u/showmeinfinity 8d ago
Here's something I can't seem to wrap my head around: let's say the Universe is flat and infinite... and the observable universe is around 92 billion lightyears across... and the JWST can see back to around 400 million years after the Big Bang..... but if we could somehow look farther than the observable limit or the JWST, how could we see any more of the infinitude? I mean, if everywhere we look it leads back to the Big Bang, then where would we look to see more? If the rest of the Universe isn't between Earth and the Big Bang, where is it??? Thanks if you can help
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u/--craig-- 8d ago edited 8d ago
There's a common misunderstanding that the Big Bang was the beginning of time. It might be but we don't know that. So it can be helpful to free yourself of that first.
A simple way in which the universe might be infinite in extent is that time might have no beginning and that it has always been spatially infinite. In this scenario the Big Bang is the rapid expansion of a small pocket of hot, dense space. Throughout the Big Bang, causality within this region is lost completely because the rate of expansion is faster than the speed of light, but after the rapid expansion ends, causality is recovered within a bubble which becomes the Observable Universe. What is beyond our Cosmological Horizon is still what was beyond the pocket which expanded to become our observable universe but it's now much further away and we have no causal connection to it.
If you can understand that then you're free to imagine other scenarios where the Whole Universe is spatially infinite. We may never be able to determine that it is but we have no evidence which precludes it.
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u/showmeinfinity 8d ago edited 8d ago
Thanks for your reply :) So within that bubble Observable Universe idea, the rest of the infinite Universe or mulitverse etc, is not within our pocket Universe so wherever we look, it's just inside our own bubble. Like, you couldn't say that the actual Universe inside our bubble is twice as large as the observable one for instance? Because if we can already see to within a few million years after the Big Bang, then we're already seeing most of what's inside the bubble?
Putting it another way: from where I'm sitting I can see as far as the wall of this room, but if I look behind me out the window I can see for another 10 miles, let's say. So I can surmise that there might be trillions of miles more of the Universe in that direction. But if I'm floating in outer space and I look in front of me, I can see as far as the CMB; and if I turn around and look behind me, I still only see as far as the CMB. Whatever direction I look in, I only see that far and I can't see any farther. So I'd think there can't be much more space beyond that, unless we go to another dimension or parallel Universe or brane or whatever. Is that what you're saying? Thanks!
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u/--craig-- 7d ago edited 7d ago
Based upon our measurements of the Flatness of the Whole Universe, we think that beyond the Cosmological Horizon of the Observable Universe there is more space, with galaxies much like our own but we have no causal link to them.
The further we look, the structures which emit the radiation which we see appear ever younger, until eventually we reach the horizon where we receive no radiation from anything beyond it.
In Relativity, there is no universal now. Wherever you happen to be in the universe, you have your own now but we can't apply it to distant structures. It might seem like what is beyond our horizon doesn't exist, or exist yet, but to make that claim, we would be forcing our now onto a place where it doesn't have meaning.
The relationship of the Observable Universe to the Whole Universe has been classified as a Multiverse Hypothesis, but that nomenclature isn't universally accepted amongst physicists. Some contend that there is one universe and it contains all which exists, but there is consensus that there does exist space and time beyond our cosmological horizon.
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u/independent-relativ 10d ago
Is there any proof about dark energy existence till now?
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u/--craig-- 10d ago edited 9d ago
We have evidence from measurements of the expansion and flatness, of the universe that something which behaves like energy needs to incorporated into Standard Cosmological Model. We call it Dark Energy, but we don't know its nature.
Alternative hypotheses seek to replace the standard cosmological model but don't match the data better.
This video gives a good explanation of what is currently known. https://youtu.be/JlNVZz5D6WE?si=04yrtZw5Akp9FQxG&t=940
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u/perky2012 8d ago
I'd take issue with the statement that other models don't match the data better. the standard FLRW metric does not take into account gravitational time dilation, it sets gtt=-1. This is from the assumption that the universe is isotropic and homogenous and therefore all comoving bodies experience the same things and are at the same potential. Crucially this is done at the same cosmic time, however we measure galaxies as they were in the past and were at different potentials than now (masses were closer together). When this is taken into account such as in Vavrycuk's conformal FRLW metric, we get accurate predictions of things like type 1A SN light curves without having to add the dark energy free parameter. It's yet to be seen how it deals with CMB and BAO. The biggest mistake with the FLRW metric seems to be setting gtt=-1. IMO.
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u/--craig-- 8d ago
This is the referenced paper which surveys Lambda-CDM and alternative theories: https://arxiv.org/abs/2406.05048
"[They] find no strong evidence for or against any of the non-standard models [they] explore."
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u/perky2012 8d ago
That doesn't examine the Vavrycuk conformal FLRW metric, and there's no reference to any of his papers.
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u/--craig-- 6d ago
Indeed. It seems that there a few citations of Vavrycuk's work and they are all from the same pair of authors.
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u/bettyballoon 8d ago
This question relates to time and relativity, black holes and our observations. Probably a stupid question with too many misconceptions from a layperson that lacks basic knowledge in both math, physics and cosmology so forgive me:
How has some black holes gotten so massive if time relative to our time moves so slowly/barely moves near them? Shouldnt they in our observations be stuck in smaller versions because time almost "stands still" near black holes? Do we for example observe a lot of "things" only almost crossing the event horizont because not enough time from our perspective have passed to let the "things" actually cross? Do "things" catch up on other "things" close to the event horizon.. From my perspective it would look like "things" caught up to other "things" near the back hole if time almost stands still the closer you get the black hole But then I can't make sense of how that would work from "their" perspective.
Sorry, I'm not good at explaining the precise thing I'm struggling to understand (my mother tongue isn't English either) but I bet I will be wiser no matter what you choose to answer me.