How much smaller was the universe 12.8 billion years ago?

This is trickier question than you may realize.

First, we need to denote the difference between the "Universe" and "Observable Universe". Usually when people are talking about the universe they really mean the observable universe.

The "Universe" is everything that exists, even if we can't see it. This includes matter that is so far away that light hasn't even had time to get from there to here. We don't know how big it is; because we can't see it all, we can't measure it. However the most popular opinion is that it is probably infinite (again this is speculation, it is by definition impossible to measure it).

The "Observable Universe" is the subset of the universe that we can see. More specifically it is a chunk of the universe centered on the Earth from which light has had time to travel to us. This is currently roughly 93 billion light years wide, right now. The further away we look, the further back in time we are looking, with the light from the edge of the observable universe being emitted very close to the time of the big bang, when photons first formed.

So, your question what size would the universe be 900 million years after the big bang? Well the "Universe" was probably infinite in size back then, just like it is now (again, only speculation, we can't know this).

However the Observable Universe would have been a different size from what it is now.

900 million years after the big bang, the observable universe would be a lot smaller in terms of diameter. However it would have a lot more stuff in it, because the universe was a lot less spread out back then. You might expect the observable universe would have been 900 million light years in diameter, but it actually wouldn't be, because of cosmic inflation.

Due to cosmic inflation spacetime itself has been expanding, which has the weird effect of pushing distant objects away from us at faster than the speed of light. This is not a violation of the speed of light, because it is space itself expanding, and the light speed limit only applies to objects moving relative to space. Because these objects are moving away at faster than speed of light, they end up getting pushed out of our observable universe. Thus, over time, our observable universe always gets bigger in terms of size, but smaller in terms of how much matter is inside it. If you rewind the clock, you get the opposite effects; smaller in size, but a lot more matter inside it.

Unfortunately I don't know the exact size it would have been at that time, but it would have been a lot smaller and a lot denser.

/r/askscience Thread