Cosmic Background Radiation
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- 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.
- Actually, every point in the universe was emitting this radiation.