B Why the expansion after the "Big Bang" is showed in cylindrical form?

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The discussion addresses why the expansion after the Big Bang is often depicted in cylindrical form rather than spherical. It explains that such representations simplify the complex four-dimensional motion of the universe into a two-dimensional diagram, where time is illustrated alongside spatial dimensions. The observable universe initially was extremely small and expanded rapidly during the inflationary epoch, with the expansion rate changing over time. The lack of spherical representations online is questioned, with the suggestion that adding a third dimension may not enhance clarity. Overall, the conversation emphasizes the importance of understanding the physics behind cosmological models rather than relying on random online images.
crakedhead
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Everywhere on the net, there is no image that the expansion has a departing point, through an expansion in spherical form... Why is that?
 
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crakedhead said:
Everywhere on the net, there is no image that the expansion has a departing point, through an expansion in spherical form... Why is that?
We can't be expected to explain why random stuff on the Internet is the way it is.

If you want to learn the actual physics involved in cosmology, you should be looking at a textbook or peer-reviewed paper. Random pages on the Internet are not a valid basis for PF discussion in any case. You need to give a specific reference.
 
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Let's take this as two separate questions:

1. Why the expansion after the "Big Bang" is showed in cylindrical form?

Presumably you're referring to diagrams such as this:
1671055986853.png

The reason the is represented like this is because the BB's motion actually occurs in all four dimensions of space-time, but we can't represent that in a static 2-dimensional diagram. So what we do is drop one of the three spatial dimensions - reducing the 3-D spherical observable universe to a 2-D disc. Now we can use that 3rd dimension to represent time. IN the diagram above, time flows from left to right, and this shows us how the expansion rate changed over time. The observable universe started off very small - much, much smaller than an atom - then expanded very rapidly in what is called the inflationary epoch to a macroscopic size. Then the inflation slowed to a more leisurely rate, as it is doing today. But that rate is slowly increasing and we expect it to continue to do so.

2. Everywhere on the net, there is no image that the expansion has a departing point, through an expansion in spherical form.

What do you suppose that would look like, and why do you think it would be more explanatory than a diagram that ignores one of the three spatial dimensions?

Consider this physics projectile motion diagram:
1671056549355.png


It renders only two physical axes (x and y), even though any real world soccer kick is in all three. Do you think adding a third dimension to the diagram would add clarity to what the diagram is trying to portray?
 
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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##.
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...
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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