Cosmic Background Radiation Cosmic Background Radiation
Cosmic Background Radiation
⇐ Use this menu to view and help create subtitles for this video in many different languages. You'll probably want to hide YouTube's captions if using these subtitles.
- Let's think a little bit about what the Big Bang theory suggests.
- And then based on the theory, what we should be observing today.
- So the Big Bang starts with all of the mass and space in the universe,
- an infinitely and infinitely dense singularity.
- A singularity is just something that the mass doesn't even apply to.
- We don't even know how to understand that.
- But immediately after the Big Bang, so this occurred 13.7 billion years ago
- 13.7 billion years ago. Immediately after this little tiny, infinitely small
- singularity begins to expand and so for the first 100,000 years
- it's still pretty dense, so let me just show this right now.
- so this starts to expand and maybe it gets to this level right over here,
- and I do not know if this entire universe is infinite or finite, whether
- it's a 4-dimensional sphere or whether it goes infinitely in every direction,
- or if it's just slightly curved here and there, and maybe flat everywhere else,
- and all of that, but then it starts to expand
- a little bit from the singularity and it's still extremely dense.
- So dense that atoms can't even form.
- So you just have the basic fundamental building blocks of atoms
- they're just all flying around; electrons, protons, just flying around and just ultra hot- white hot, I could say.
- Maybe even white-hot plasma.
- So this is- I'll call it white-hot plasma.
- And then if we fast-foward a little bit more and now this is a point
- that we think we understand well, but this number
- I actually looked at some old physics books and this number
- has changed in really the last 15 or 20 years, so maybe it'll change more.
- But after 380,000 years from the beginning of the Big Bang,
- 380,000 years after the Big Bang, I'll call it the B.B;
- and obviously this is give or take a couple of years;
- the universe expands enough; the universe is now large enough
- (and obviously I'm not drawing things to scale)
- the universe is now large enough and sparse enough so that it can cool down a little bit.
- You don't have as much bumping around.
- It's still a hot place, but now there's kind of
- it cools down enough so that the electrons can be captured by
- protons, and you can actually have the first hydrogen atoms that can begin to form.
- They actually condense, and we estimate this temperature to be around 3,000 Kelvin.
- So we've cooled to 3,000 Kelvin but this is still a temperature
- that you would not want to hang out with.
- It's still extremely, extremely hot.
- Now why is this moment important?
- The first atoms forming.
- So let's think about what's happening here.
- You have all of this bumping and interaction
- and because of a bump or some energy release or because
- of the heat temperature, if a photon is released,
- it'll immediately be absorbed by something else.
- If something gets- if some energy gets released
- it'll immediately be absorbed by something else, beacuse
- the universe is so dense, especially with charged particles.
- Here, all of a sudden, it's not that dense.
- So over here, things that were being emitted could not travel long distances.
- They would immediately bump into something else.
- While you go over here, the universe is starting to
- look like the universe we recognize.
- All of a sudden, if one of these really hot (and it's still nowhere near as hot as
- this universe over here) but if one of these hot
- atoms emits a photon and they would, because they are at 3,000 Kelvin,
- if they emit a photon, all of a sudden there's actually space for that photon to travel.
- So for the first time in the history of the universe, 380,000 years
- after the Big Bang, you now have photons.
- You now have electromagnetic radiation.
- You now have information that can travel over long, long distances.
- So given that this happened (it's still roughly 13.7 billion years ago)
- 380,000 years is not a lot when you talk about 13.7 [billion years].
- It still wouldn't really even change the dial,
- Because we're talking of the hundreds of thousands
- or 700 million years.
- So this is actually a very small number.
- So it's still approximately 13.7 billion.
- It's really 13.7 billion minus 380,000 years;
- but given that this was the first time that information could travel,
- that photons could travel through space, without most of them
- having to bump into something, especially something that's probably charged,
- the other interesting thing is that these atoms that form are now neutral.
- What could we expect to see today?
- Well, let's think about it.
- These left- these photons were emitted 13.7 billion years ago.
- And they were emitted from every point in the universe.
- So this is every point in the universe.
- The universe was a pretty uniform place at that time.
- Very minor regularities.
- But you could see, because it was this white-hot thing
- just began to condense. It hadn't formed a lot of the structures
- that we now associate with the universe. It was just kind of a fairly uniform
- spread of; at that time, reasonably hot hydrogen atoms.
- So this is every point in the universe. So let's think about what's going on here.
- Let me draw another diagram.
- Let's say that- so we're talking about this point in the universe
- right over here- the universe, even 380,000 thousand years
- after the Big Bang, still much much much smaller than the universe today;
- But let's say that this is- let's say that this is the point in the universe
- where we happen to be now.
- At that- at this point in time, there was no earth, there was no solar system,
- there's no Milky Way it was just a bunch of hot hydrogen atoms.
- Now if we were at this point in the universe, there must have been points
- in the universe at that exact same time-
- that were emitting this radiation.
Be specific, and indicate a time in the video:
At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
Have something that's not a question about this content?
This discussion area is not meant for answering homework questions.
Share a tip
When naming a variable, it is okay to use most letters, but some are reserved, like 'e', which represents the value 2.7831...
Have something that's not a tip or feedback about this content?
This discussion area is not meant for answering homework questions.
Discuss the site
For general discussions about Khan Academy, visit our Reddit discussion page.
Flag inappropriate posts
Here are posts to avoid making. If you do encounter them, flag them for attention from our Guardians.
- disrespectful or offensive
- an advertisement
- low quality
- not about the video topic
- soliciting votes or seeking badges
- a homework question
- a duplicate answer
- repeatedly making the same post
- a tip or feedback in Questions
- a question in Tips & Feedback
- an answer that should be its own question
about the site