Dodecahedrus t1_j6xem6y wrote on February 2, 2023 at 4:00 PM

I saw a question in the last week along the lines of “If the universe is 13 billion years old and expands at the speed of light, then how is it 92 billion lightyears wide?”. (Oversimplified.)

I don’t think I opened it for the answers. Perhaps I should have.

Then today I saw a question here about the speed of differs with light moving through different substances (oversimplified, thread in question is here ).

Could an answer to the first question be that the universe at the moment of the big bang wasn’t a vacuum? Is this related to the undetectable dark matter that is pushed outward with the growth of the universe?

Ape_Togetha_Strong t1_j72jbac wrote on February 3, 2023 at 4:44 PM

No, the answer to the first question is "there's no reason to think those numbers should match". Asking the question in the first place requires some kind of misunderstanding about the big bang and/or observable universe, but it can be hard to pin down exactly what that misunderstanding is.

But if a question actually included the words "expands at the speed of light", it would be easy to identify that misconception, because that's not at all how expansion works. It doesn't have a speed. It's like increasing the scale factor of the universe, everything getting further apart from everything else, and a uniform dilation preserves distance ratios between things, which means the recession velocity of an object must increase with distance. Two objects can be expanding away from each other at any speed you want as long as they are far enough apart.

The distance at which things expand away from us faster than C is around 14.5 billion lightyears (the hubble radius). So the objects in the observable universe that are further than that have been expanding away from us faster than C ever since they passed that distance.

The observable universe is defined as containing everything we could have ever observed, not things that we could still receive a signal from if emitted "right now" in cosmic time. That is once again the hubble volume, which is just a sphere with r = the hubble radius.