B What is the Universe expanding into?

AI Thread Summary
The universe is everything, including all matter and energy, and observations indicate that galaxies are moving away from Earth, suggesting an expansion. However, the universe does not expand into anything, as there is no outer edge or center; it is considered infinite. General relativity describes this expansion as a change in the distance between galaxies across different slices of spacetime rather than an actual expansion into a void. Misconceptions about the nature of the universe, such as comparing its expansion to an explosion or misunderstanding black holes, complicate public understanding. Ultimately, spacetime itself is a constant, not something that is created or expanded into.
Akshaya dhakal
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If it is everything then what is it expanding into?
As we know universe is everything having every matter and energy. From the observation using Hubble's law it was said that the galaxies are moving away from the Earth which means the universe is expanding. If it is everything then what is it expanding into. Is there some outer edge that it can expand into or any center it is expanding from? Is it infinite?
 
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Our current best model of the universe is infinite, yes. But even finite and closed models don't expand into anything. As far as we are aware there is nothing outside the universe for it to expand into.

The thing to remember is that general relativity describes 4d spacetime, and space at any given time is a 3d slice through that. So what we mean when we say that "space is expanding" is that there is a particular way to slice spacetime so that everything looks more or less the same everywhere in a slice, and when we do that, in each successive slice galaxies are slightly further apart. Nothing is expanding into anything because nothing is really expanding - things are just further apart in different slices of spacetime.
 
Akshaya dhakal said:
Summary:: If it is everything then what is it expanding into?

As we know universe is everything having every matter and energy. From the observation using Hubble's law it was said that the galaxies are moving away from the Earth which means the universe is expanding. If it is everything then what is it expanding into. Is there some outer edge that it can expand into or any center it is expanding from? Is it infinite?
Doesn't he expansion which basically started at the big bang been create the fime/space continuum in the process? Ex nihilo so to say. It's pretty difficult wrapping ones head around...

Explaining this to someone is pretty steeply up-hill. There are many misconceptions surfacing again and again. Some of the more annoying are people thinking that an explosion in vacuum looks exactly like a fireball in an atmosphere. Also that black holes basically works like a giant vacuum cleaner, gobbling up everything within it's gravitational sphere of influence. I'm sure there are tons of others.

Regards.
 
sbrothy said:
Doesn't he expansion which basically started at the big bang been create the fime/space continuum in the process?
This is currently not known.
 
DennisN said:
This is currently not known.

What would it even mean to "create time/space continuum"?
 
sbrothy said:
Doesn't he expansion which basically started at the big bang been create the fime/space continuum in the process?
No. Spacetime doesn't get "created"; it just is. It is a 4-dimensional geometry that already contains all of the information about "time".

DennisN said:
This is currently not known.
Yes, it is; the answer is no. See above.
 

Thread 'Matter density right after the decoupling'

https://en.wikipedia.org/wiki/Recombination_(cosmology) Was a matter density right after the decoupling low enough to consider the vacuum as the actual vacuum, and not the medium through which the light propagates with the speed lower than ##({\epsilon_0\mu_0})^{-1/2}##? I'm asking this in context of the calculation of the observable universe radius, where the time integral of the inverse of the scale factor is multiplied by the constant speed of light ##c##.
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Thread 'Rutgers finds a transparent Einstein Cross - apparently a dark matter "halo".'

The formal paper is here. The Rutgers University news has published a story about an image being closely examined at their New Brunswick campus. Here is an excerpt: Computer modeling of the gravitational lens by Keeton and Eid showed that the four visible foreground galaxies causing the gravitational bending couldn’t explain the details of the five-image pattern. Only with the addition of a large, invisible mass, in this case, a dark matter halo, could the model match the observations...
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Thread 'Big Bang and Infinite Universe'

Hi, I’m pretty new to cosmology and I’m trying to get my head around the Big Bang and the potential infinite extent of the universe as a whole. There’s lots of misleading info out there but this forum and a few others have helped me and I just wanted to check I have the right idea. The Big Bang was the creation of space and time. At this instant t=0 space was infinite in size but the scale factor was zero. I’m picturing it (hopefully correctly) like an excel spreadsheet with infinite...
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