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Radius of observable universe (correction) | Cosmology & Astronomy | Khan Academy


I want to make a quick
correction to the last video. It doesn’t really affect the
learning of the last video. But I just want to make
sure that you understand that I got the math a little
bit wrong in the last video. I said that you had this
state, 300,000 years. So we talk about the
Big Bang happening 13.7 billion years ago. And then I talk about
this state of affairs where we’re maybe 30
million light years away from the edge of the
observable universe, the current observable universe. And I said that this
was about 300,000 years after the Big Bang. That’s what I talked
about in the last video. That was our starting
point when the photon started leaving that point. And obviously, the
universe kept expanding. The photon, it kind of
traversed more and more. But still had more
and more to travel as the universe expanded,
as all of space expanded. But this is 300,000
years after the Big Bang. Now my brain, because I was
kind of not thinking hard enough about it, I said, hey, this
was 13.4 billion years ago. That’s what I incorrectly
said in the last video. I said that this is
13.4 billion years ago. That’s what I said
in the last video. And that is wrong. Because if this was
13.4 billion years ago, this would have been 300 million
years after the Big Bang. We were talking about
only 300,000 years after the Big Bang. So it wouldn’t have taken
that many decimal places off of something in the billions. The correct answer is this
would have been only a little less than 13.7 billion years. It actually wouldn’t have
even made a significant digit. So this is still approximately
13.7 billion years ago. So I wanted to just
make that correction. It was a slight error. I shouldn’t have viewed
this as 0.3 billion years. This is only 0.3 million years. It doesn’t even basically
change the precision on this number right over here. So I just wanted
to clear that up. But hopefully, it doesn’t affect
your understanding too much.

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