Name: A Brief History of the Universe: Crash Course Astronomy #44
Uploaded: 2016-02-05T05:53:52.000Z
Duration: 12 min 36 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Hey folks, Phil Plait here, and for the past few episodes, I’ve been going over what

we know about the structure, history, and evolution of the Universe, and how we know it.

Now it’s time to put that into action. We can use this knowledge of physics, math, and

astronomy to figure out what the Universe was like in the past, going all the way back

to literally the first moment after it was born.

So, here you go: a brief history of the Universe:

In the beginning there was nothing. Then there was everything.

It may seem a little weird to suppose that we can understand how the Universe got its

start. But it’s much like any other field of science: We have clues, observations, based

on what we see going on now. Knowing the rules of physics we can then run the clock backwards

and see what things were like farther and farther into the past.

For example, as I’ve talked about in the past couple of episodes, the Universe is expanding.

That means in the past it was denser, more crowded, and hotter. At some point it was

hotter than the surface of a star, hotter than the core of a star, hotter than the heart

of a supernova. And as we push the timer back even farther we find temperatures and densities

that make a supernova look chilly and positively rarefied.

A lot of what we know about the early Universe comes from experiments done in giant particle

colliders. When the cosmos was very young and very hot, particles were whizzing around

at high speeds and slamming into each other, creating other subatomic particles in the

process. That’s exactly what colliders do! The higher energy we can give our colliders,

the faster we can whack particles together, and the earlier phase of the Universe we can

investigate. That’s one of the main reasons why we keep making ‘em bigger and more powerful,

to test our ideas of what the young cosmos was like.

So let’s wind the clock back. Looking around us, peering into the Universe both near and

far, what can we say about the beginning of everything?

When the Universe got its start, it was unfathomably hot and dense. It was totally different then

than it is today, because when you pump more energy into something, the way it behaves,

even its fundamental physical nature, changes.

If you take a snowball and heat it up, it’ll melt. We call that a phase change, or a change

of state. Heat it more and it vaporizes, changing into a gas. It’s still water, still composed

of water molecules, but it looks and acts pretty differently, right?

When you heat something up what you’re doing is giving it more energy. In a solid this

means the atoms wiggle around more and more until they break free of their restrictive

bonds with each other, and the solid melts. The atoms are still bound by other forces,

but if you heat them more they break free of those, too, and the liquid becomes a gas.

Heat them more, give them more energy, and the atoms whiz around faster and faster. Heat

them to millions or billions of degrees, and the atoms themselves fall apart. They collide

so violently they can overcome the hugely strong forces holding their nuclei together,

and you get a soup of subatomic particles; electrons, neutrons, and protons.

Heat them more and even protons and neutrons will collide hard enough to shatter into their

constituent subatomic particles, which are called quarks. And as far as we know, quarks

and electrons are basic particles, so they can’t be subdivided any more.

Maybe you can see where I’m going here. As we wind the clock backwards, the Universe

gets denser and hotter. At some point in the past it was so hot that atoms wouldn’t have

been able to hold on to their electrons. A little farther back and it was so hot that

nuclei couldn’t stay together, and the Universe was a small, ultra dense ball of energy mixed

Go a wee bit farther back and even that changes. Neutrons and protons couldn’t form, because

the instant they did they’d whack into each other hard enough to fall apart. The Universe

The cosmos was a bizarre, unfamiliar place back then. Even the basic forces we see today

— gravity, electromagnetism, and the two nuclear forces responsible for holding atomic

nuclei together as well as letting them disintegrate in radioactive decay — were all squeezed

Like the snowball melting and vaporizing, each of these moments in the history of the

Universe was like a phase change. The very nature of reality was changing, its laws and

behavior different. At some point, we go so far back, so close

to that first moment in time, that our laws of physics… well, they don’t break down

so much as say, “Here Be Dragons.” We just don’t understand the rules well enough

to be able to say anything about that first razor thin slice of time.

How far back are we talking here? If we call the instant of the Big Bang “time zero,”

then our physics cannot describe what happens in the first 10-43 seconds.

Now let me just say, only semi-sarcastically, that that’s not so bad. The Universe is

13.82 billion years old, so being able to go back to that very first one-ten-millionth

of a trillionth of a trillionth of a trillionth of a second is a massive triumph of physics!

What happened after that fraction of a second is better understood. The Universe expanded

and cooled, the four forces went their separate ways, and the first basic subatomic particles were

able to hold themselves together. This all happened in the very first second of the Universe’s existence.

Three minutes later — yes, three minutes — the Universe cooled enough that these

subatomic particles could start to stick together. For the next 17 minutes, the Universe did

It was still ridiculously hot, like the core of a star, but it’s at those temperatures

that nuclear fusion can occur. For a few minutes the particles smashed together, forming deuterium,

an isotope of hydrogen, helium, and just a smattering of lithium. A little bit of beryllium

was made as well, but it was radioactive, and rapidly decayed into lithium.

Then, at T+20 minutes, the Universe cooled enough that fusion stopped. When it did, there

was three times as much hydrogen as helium in the Universe. This primordial ratio is

still pretty much true today. When we measure the Sun’s elemental abundance, we see it’s

At this point the Universe is still hotter than a star’s surface, but it’s also still

expanding and cooling. As it does, structures start to form as the gravity of matter can

overcome the tremendous heat. These will become the galaxies we see today. This is important

and a bit weird, so I’ll get back to it in a minute.

The next big event happened when the Universe was at the ripe old age of about 380,000 years.

Up to this point, electrons couldn’t bond with the atomic nuclei zipping around; every

time they did it was so hot that random photons would blow them off again. The Universe was ionized.

But then, after 380 millennia, it had cooled enough that electrons could combine with protons

and helium nuclei, becoming stable neutral atoms for the very first time. We call this

This was an important event! Free electrons are really really good at absorbing photons,

absorbing light. When the Universe was still ionized, prior to recombination, it was opaque.

A photon couldn’t get very far before an electron sucked it up.

But after recombination, the photons were free to fly. The Universe became transparent!

Because the light emitted at this time is what we see as the cosmic microwave background

today! Those neutral atoms emitted light; they were as hot as a red dwarf star. Those

photons have been traveling ever since, fighting the expansion of the Universe, redshifting

into the microwave part of the spectrum, and seen today all over the sky. That background

glow predicted by the Big Bang model has been on its journey to Earth for almost 13.8 billion years!