Science a GoGo's Home Page Forums General Discussion General Science Discussion Forum How big was the Universe 13 billion years ago?

If we look from the Earth, in opposite directions, at objects that are 13 billion LY away, they are 26 billion LY apart.

We are seeing them as/where they were 13 billion years ago, so they must have been 26 billion LY apart then.

Does this mean that, 13 billion years ago, the Universe was, at the very least, 26 billion LY in diameter?

It's not evidence of that, Bill; during the 13b years, space has been expanding so photons have been traversing an ever increasing distance. If we ran the movie in reverse we would see objects moving toward each other into a much smaller volume of space...

...but if Inflation occurred as theorized then that early universe could have been far larger than the currently observable universe - even though the observable universe at that time would have been only about a billion lt yrs across.

This is why I said "at the very least.......".

Just as I thought I was getting somewhere with the question of why we see things in the places we do, another thought came along and messed it all up. This is just an example, so don't attach any significance to the figures.

If the light from a certain galaxy first reached the Earth 3 billion years ago, that galaxy would be 10 billion LY from Earth, and that light would have left the galaxy 10 billion years earlier.
Light reaching Earth from that galaxy 3 billion years later would have left it 13 billion years ago. That is the same point in time as the time of departure of the light that arrived 3 billion years earlier; so why would that light not have reached the Earth until 3 billion years after the first lot?
Logic tells me there is something wrong with this, and I have a nasty feeling it is going to involve a complicated answer.

Fascinating question, Bill, but you say...


...if the light left it 13b yrs ago, then it would have to be 13b lt yrs away - which would require that it had receded at light velocity, in which case no photons would ever reach Earth.

I realised after I had posted that the figures didn’t make sense, but I thought I would leave it as it was, while I had another look at it. In doing so, I think I have solved the problem.

I shall run it past those who may be thinking more clearly than I, in case I still have it wrong.
PBB = post Big Bang (just to state the obvious).

Let’s assume, to keep calculations simple, that the BB occurred 15 billion years ago.

Let’s also assume that the galaxy that is now 13 billion L Y from Earth was 11 billion L Y away 3 billion years ago. (12 billion years PBB).

Light that reached Earth at 12 billion years PBB left the galaxy at 1 billion years PBB.

Light reaching Earth now left the galaxy at 2 billion years PBB.

Problem gone….right?

Looks good to me, Bill.

To me, your assumptions in this OP are logical, “airtight” and are consistent with the laws of physics.

If the universe was significantly smaller (<1billion LY across) 13 billion years ago then something else will have to come into play to account for the inconsistency. Perhaps, expansion (although this would require space as a carrier)…perhaps, gravitational lensing, etc.

The bottom line is that the inconsistency between observation and the state of the universe 13 billion years ago cannot be explained away so easily.

This isn’t the only aspect that is difficult to account for. For instance; when we think of expansion; one description that comes about is that all points (particles) in our current universe were once at the center and that therefore all points (particles) in the universe continue to be “at the center”. Not hardly…of course there’s a physical “edge” and it’s no longer in the center. Perhaps the (particle distribution) edge was once in the center but it isn’t right now. If the Milky Way happened to be near the edge of the universe one wouldn’t expect the universe to look the same omni-directionally…not without extreme gravitational lensing.

No, there’s much more to understand about our universe on a macroscopic scale and expansion alone isn’t going to get it done.

I agree with redewenur.
I guess the expansion of space itself allows for two galaxies to drift apart with higher velocities then the speed of light.

WP: "The estimated age of the Universe is 13.75 billion years. However, because the Universe has continued expanding since that time, the comoving distance from the Earth to the edge of the observable universe is now at least 46.5 billion light years."

Also the background radiation reaching us now from different sides of the observable universe has been emitted 13 billion years ago. But the points in space where this radiation was emitted were much, much closer at that time.

I came across this figure (46.5 billion L Y) a while ago, and have been meaning to look for an explanation. (sub-consciously I've been avoiding it, for fear of complex maths ) Perhaps you could explain it.