Is Universe as large as we think(and observe)

We (modern physicist people) think the Big Bang is roughly a 4-d hypersphere with a characteristic radius of about 10^21 lightyears. That's trillions of times bigger than the "observable universe" inside our event horizon.

We also think there may be many such Big Bangs, perhaps an infinite number.

I would think that would be big enough for anyone.

I can't imagine how that could be true.

"...Is Universe as large as we think(and observe)..?"
At the instant of the Big Bang an event known as 'inflation' occurred. Inflation didn't last long, but while it did the universe expanded faster than the speed of light! This is okay with Einstein's relativity because it was *space* expanding FTL rather than any thing moving *through* space FTL. This means that there are regions of the universe orders of magnitude more extensive than we can ever observe.

Also, there's a boundary at about 13.7-billion light years beyond which we'll never be able to see due to the Hubble expansion of space, ≈72 km/sec/MPc. Today we might observe a galaxy at 13.7-billion light years, but in the intervening 13.7-billion years that galaxy has moved into that region of the universe we can never see.