> ""90 thousand millions ly in diameter"
This is the diameter of the observable universe. We don't know if the universe is infinitely big, but all signs (e.g. the flatness of space as far as we can measure it) point to it being infinite, or very very much larger than our observable sphere."
Our observable sphere is nowhere near 90 thousand million ly in diameter since that would violate relativity. Our observable sphere in ly is always smaller than the age of the universe in years (so ~14 thousand million years).
Also, what you said is no fact, basic or not, it's just some misconception you have about cosmology so you should stop spreading that word since you are misinforming people you talk to.
In a nutshell: A Universe that begins in a singularity cannot have an infinite diameter and while it's true that we do calculate the size of the Universe by assuming the expansion rate right after the Big Bang and that rate might be wrong, it still cannot be infinite without violating the major principles of physics and the all big bang theory.
My "fact" doesn't refer to the universe being flat. I would never call this a fact. The "fact" I referred to is that the universe appears to us to be flat as far as we can tell.
Our observable sphere in ly is always smaller than the age of the universe in years (so ~14 thousand million years)
This ignores the basic fact that the universe is expanding at an accelerating rate. I'm comfortable calling this one a fact. Many 9s behind that confidence percentage (even if the number of nines was recently lowered).
A Universe that begins in a singularity cannot have an infinite diameter
Based on what logic? You are probably imagining an infinitesimally small sphere expanding out and becoming the universe. This is a good description of only our observable universe. There's no reason not to think that point wasn't part of an infinitely large manifold. This would point to an infinitely large universe.
I'm not an expert, but in my curiosity I've spoken to Dr. Don Lincoln from Fermilab about this very topic, so that's the source of my confidence. Along with my own subsequent (armchair) research. I've since followed up and done my best to understand why this misconception is so prevalent. I'm convinced it's because many sources say "universe" when they mean "observable universe", leading to many misconceptions.
> "This ignores the basic fact that the universe is expanding at an accelerating rate."
And that ignores the basic fact that information about those parts of the Universe didn't have time to arrive yet (that is where relativity is violated). You are mixing the concept of "observable universe" with "observed universe". At a given time, given the expansion rate of the universe you have an "observable universe" (those ~90E9 ly), but it will only be fully observable somewhere in the future, in the present you only get to observe ~14E9 ly). In a nutshell what you are proposing is that we are observing particles with infinite redshift. Or perhaps in a simpler way: You can be observing today parts of the universe that are (for instance) 25E9 ly away today, but you are observing particles of light that left that part of the universe a long time ago so that the c x t condition was never violated.
> "This is a good description of only our observable universe. There's no reason not to think that point wasn't part of an infinitely large manifold. This would point to an infinitely large universe."
Based on the theory of the Big Bang which is the de facto theory accepted by Cosmology. It is true that you could have an infinitely large manifold and that is a very interesting theory. But I was answering inside the actual theory that is the accepted one now-a-days the Big Bang (and I actually referred that in the answer). Also, even inside the manifold theory theory, our universe that is what is being discussed here, would still not have an infinite diameter.
There is a difference between the size of the observable Universe now and the size when the light was emitted. It does not violate relativity to state that objects that were 14 billion light years away when the light we see from them left are now much farther away due to expansion.
Furthermore, your reference to relativity is also incorrect. The expansion of the Universe is not caused by objects moving away from each other, which would be limited by the speed of light. The expansion of the Universe is actually an expansion of space itself, which is not limited by the speed of light. You should look up the "metric expansion of the universe".
In general, relativity is known to be incomplete on a cosmological scale. The situation is much more complicated due to the existence of dark energy and dark matter. You should read about the Lambda-CDM model if you want to learn more about the state of modern cosmology.
I didn't quote you or claim you said anything. I made some assumptions about why you made certain statements like the fact that you think the Universe being ~90 billion light years in diameter violates relativity (it doesn't and this is the currently accepted estimate for its size).
You could just clarify your statements/reasoning to try and progress the discussion instead of getting overly defensive.
> "you made certain statements like the fact that you think the Universe being ~90 billion light years in diameter violates relativity"
I was the one explaining to the OP of the thread that the universe is 90E9 ly in diameter (and also, just to clarify for the future so that it doesn't get mixed up yet again, that this is just an approach based on what we know about the history of the universe), it is therefore ridiculous that I also said that it violates relativity. So, sorry if I'm "overly defensive" about you mixing everything up and me having to use my time to answer you so that you don't twist everything I said on the thread.
Our observable sphere is nowhere near 90 thousand million ly in diameter since that would violate relativity. Our observable sphere in ly is always smaller than the age of the universe in years (so ~14 thousand million years).
Also, what you said is no fact, basic or not, it's just some misconception you have about cosmology so you should stop spreading that word since you are misinforming people you talk to.
In a nutshell: A Universe that begins in a singularity cannot have an infinite diameter and while it's true that we do calculate the size of the Universe by assuming the expansion rate right after the Big Bang and that rate might be wrong, it still cannot be infinite without violating the major principles of physics and the all big bang theory.