Radius of observable universe (correction)
About this video
A short correction to the previous video. The narrator had said a moment about 300,000 years after the Big Bang was 13.4 billion years ago, but 300,000 years after the Big Bang is essentially 13.7 billion years ago, because the difference is 300,000 years, not 300 million. He notes the error does not change the answer at the level of significant digits.
You'll be able to
- convert between 300,000 years and 300 million years when reading a cosmology timescale.
- explain why a moment early in the universe is almost the same age as the universe itself.
- judge whether a small numerical slip changes a conclusion.
Key words
- Big Bang
- The starting point of the expanding universe, about 13.7 billion years ago.
- Observable universe
- The portion of the universe whose light has had time to reach us since the Big Bang.
- Significant digit
- A digit that meaningfully affects the precision of a measured value.
- Photon
- A particle of light, which the narrator uses to trace light that left the early universe.
- Order of magnitude
- A power-of-ten scale, the sort of difference between 300,000 years and 300 million years.
Transcript
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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