I get emails from Quora, some of which are questions people have asked and want an answer to. Recently, someone asked, “How much water is there in the universe”?
It’s hard to say. I’m sure it’s possible to estimate, in a very broad fashion. We could start by estimating how much hydrogen exists in the observable universe, based on how much would have formed during the Big Bang, and subtracting out losses due to subsequent fusion in the hearts of stars. We could estimate how much oxygen has formed through stellar activity, and been released into the universe. That gives us an upper limit on how much water could exist in the observable universe. We then estimate how much of the oxygen would have combined with hydrogen to form water in the observable universe.
Then there’s the issue of what exists outside the observable universe. Cosmologists estimate that what we can see is a tiny fraction of what’s out there. So we’d multiply our estimate of the water in the observable universe by the number of observable universe volumes in the total universe, which is a guess.
My answer was a lot shorter.
I answered, “All of it.”