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Let's think a little bit about what the Big Bang theory suggests.

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And then based on the theory, what we should be observing today.

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So the Big Bang starts with all of the mass and space in the universe,

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an infinitely and infinitely dense singularity.

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A singularity is just something that the mass doesn't even apply to.

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We don't even know how to understand that.

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But immediately after the Big Bang, so this occurred 13.7 billion years ago

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13.7 billion years ago. Immediately after this little tiny, infinitely small

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singularity begins to expand and so for the first 100,000 years

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it's still pretty dense, so let me just show this right now.

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so this starts to expand and maybe it gets to this level right over here,

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and I do not know if this entire universe is infinite or finite, whether

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it's a 4-dimensional sphere or whether it goes infinitely in every direction,

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or if it's just slightly curved here and there, and maybe flat everywhere else,

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and all of that, but then it starts to expand

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a little bit from the singularity and it's still extremely dense.

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So dense that atoms can't even form.

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So you just have the basic fundamental building blocks of atoms

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they're just all flying around; electrons, protons, just flying around and just ultra hot- white hot, I could say.

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Maybe even white-hot plasma.

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So this is- I'll call it white-hot plasma.

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And then if we fast-foward a little bit more and now this is a point

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that we think we understand well, but this number

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I actually looked at some old physics books and this number

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has changed in really the last 15 or 20 years, so maybe it'll change more.

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But after 380,000 years from the beginning of the Big Bang,

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380,000 years after the Big Bang, I'll call it the B.B;

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and obviously this is give or take a couple of years;

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the universe expands enough; the universe is now large enough

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(and obviously I'm not drawing things to scale)

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the universe is now large enough and sparse enough so that it can cool down a little bit.

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You don't have as much bumping around.

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It's still a hot place, but now there's kind of

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it cools down enough so that the electrons can be captured by

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protons, and you can actually have the first hydrogen atoms that can begin to form.

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They actually condense, and we estimate this temperature to be around 3,000 Kelvin.

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So we've cooled to 3,000 Kelvin but this is still a temperature

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that you would not want to hang out with.

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It's still extremely, extremely hot.

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Now why is this moment important?

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The first atoms forming.

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So let's think about what's happening here.

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You have all of this bumping and interaction

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and because of a bump or some energy release or because

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of the heat temperature, if a photon is released,

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it'll immediately be absorbed by something else.

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If something gets- if some energy gets released

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it'll immediately be absorbed by something else, beacuse

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the universe is so dense, especially with charged particles.

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Here, all of a sudden, it's not that dense.

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So over here, things that were being emitted could not travel long distances.

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They would immediately bump into something else.

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While you go over here, the universe is starting to

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look like the universe we recognize.

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All of a sudden, if one of these really hot (and it's still nowhere near as hot as

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this universe over here) but if one of these hot

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atoms emits a photon and they would, because they are at 3,000 Kelvin,

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if they emit a photon, all of a sudden there's actually space for that photon to travel.

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So for the first time in the history of the universe, 380,000 years

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after the Big Bang, you now have photons.

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You now have electromagnetic radiation.

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You now have information that can travel over long, long distances.

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So given that this happened (it's still roughly 13.7 billion years ago)

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380,000 years is not a lot when you talk about 13.7 [billion years].

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It still wouldn't really even change the dial,

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Because we're talking of the hundreds of thousands

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or 700 million years.

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So this is actually a very small number.

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So it's still approximately 13.7 billion.

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It's really 13.7 billion minus 380,000 years;

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but given that this was the first time that information could travel,

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that photons could travel through space, without most of them

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having to bump into something, especially something that's probably charged,

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the other interesting thing is that these atoms that form are now neutral.

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What could we expect to see today?

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Well, let's think about it.

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These left- these photons were emitted 13.7 billion years ago.

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And they were emitted from every point in the universe.

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So this is every point in the universe.

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The universe was a pretty uniform place at that time.

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Very minor regularities.

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But you could see, because it was this white-hot thing

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just began to condense. It hadn't formed a lot of the structures

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that we now associate with the universe. It was just kind of a fairly uniform

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spread of; at that time, reasonably hot hydrogen atoms.

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So this is every point in the universe. So let's think about what's going on here.

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Let me draw another diagram.

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Let's say that- so we're talking about this point in the universe

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right over here- the universe, even 380,000 thousand years

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after the Big Bang, still much much much smaller than the universe today;

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But let's say that this is- let's say that this is the point in the universe

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where we happen to be now.

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At that- at this point in time, there was no earth, there was no solar system,

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there's no Milky Way it was just a bunch of hot hydrogen atoms.

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Now if we were at this point in the universe, there must have been points

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in the universe at that exact same time-

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that were emitting this radiation.

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Actually, every point in the universe was emitting this radiation.