0:00 For about the first second of its life, the entire universe we know was over a trillion degrees hot.0:07 Yeah, and that’s probably an understatement.0:10 It was, um, incomprehensibly hot and dense.0:13 Okay, so in Episode 19 we talked about gravity pulling everything together.0:19 But this sounds like the ULTIMATE case of that.0:23 Are we just running the clock backward on the whole universe?0:27 That is EXACTLY what we’re doing.0:30 The Big Bang theory isn't really about a bang, like an explosion.0:34 It’s about expansion.0:36 Imagine you see a video of a shattered vase.0:39 If you run the video backward, you see all the pieces flying back together to form the vase, right?0:47 Right, back to a single point.0:49 Astronomers did that with the universe.0:52 They saw that galaxies are all moving away from each other.0:56 So they ran the clock back, and back, and back… for about thirteen-point-eight billion years.1:03 And everything comes together into one… spot?1:07 One incredibly hot, dense, tiny point.1:09 A singularity.1:10 Everything—all the matter, all the energy, even space and time itself—was crushed into this single point.1:18 Wait, so what was before that?1:21 What was the point in?1:22 Ah, that's the trick.1:24 The question itself might not make sense.1:27 Since time and space began with the Big Bang, there was no "before" and no "in." The point didn't expand into space; space itself began to expand.1:38 Whoa.1:38 Okay.1:38 So it's not a firecracker going off in a big empty room.1:43 Exactly.1:43 It’s the room itself suddenly getting bigger, everywhere, all at once.1:48 Think of a balloon with dots drawn on it.1:51 As you inflate the balloon, all the dots move away from each other, but no single dot is the "center" of the expansion.2:00 Okay, I get that.2:02 So this tiny, hot universe starts expanding.2:05 What happens next?2:06 Is it just a soup of… stuff?2:08 A soup of pure energy and fundamental particles.2:12 It's so hot that atoms can't exist.2:15 Even protons and neutrons can't hold together at first.2:19 But as the universe expands, it cools down.2:22 Like steam condensing into water.2:24 A perfect analogy.2:26 After about three minutes, it was cool enough for the first atomic nuclei to form—mostly hydrogen and helium.2:34 But it was still a foggy, opaque plasma.2:37 Light couldn't travel freely.2:39 So the universe is a hot, expanding fog of basic ingredients.2:44 How does it become… well, THIS?2:46 Stars and planets and us?2:48 You have to wait.2:49 For about three hundred and eighty thousand years, actually.2:53 At that point, the universe cooled enough for electrons to finally combine with those nuclei and form the first stable, neutral atoms.3:03 And what changed then?3:05 Suddenly, the fog cleared!3:07 For the very first time, light could travel across the universe without being scattered.3:14 And that moment is SO important.3:16 Why?3:16 What did it do?3:17 It gave us the proof!3:19 We can still see the light from that moment today.3:23 It's been traveling for over thirteen billion years, and as the universe expanded, that light has stretched out into microwaves.3:32 We call it the Cosmic Microwave Background radiation.3:36 Hold on.3:37 You're saying there's an echo of the Big Bang that we can still detect?3:42 Yes!3:42 It's everywhere.3:44 No matter where you point a radio telescope in the sky, you find this faint, uniform glow.3:50 It's the oldest light in the universe.3:53 It’s the baby picture of the cosmos.3:56 Wow.3:56 So that's not just a theory, it's something we can actually measure.4:01 It's the single most powerful piece of evidence for the Big Bang.4:06 That, and the fact that the amount of hydrogen and helium we see in the oldest stars perfectly matches what the theory predicts should have been made in those first few minutes.4:19 So the universe expands and cools, the fog clears, and then gravity—our old friend—starts doing its job?4:27 Now you've got it.4:28 Gravity starts pulling those clouds of hydrogen and helium together.4:33 Over millions of years, they form the first stars and galaxies.4:38 And inside those stars, heavier elements are forged, which eventually make up planets… and us.4:45 Exactly.4:46 Every atom in your body, besides the very oldest hydrogen, was created inside a star that wouldn't exist if the universe hadn't started in that hot, dense state and expanded just the way it did.5:00 So we’re not just looking back at the start of the universe.5:05 We’re looking at the start of… everything that makes us, us.5:09 We are, quite literally, made of stardust.5:12 But that stardust is made from the leftovers of the Big Bang.5:17 It all connects.5:18 It's one single, unbroken story.
