is really big.

宇宙

We live in a galaxy, the Milky Way Galaxy.

是浩瀚的

There are about a hundred billion stars in the Milky Way Galaxy.

我們生活在宇宙中的一個星系裡 也就是銀河系

And if you take a camera

銀河系裡大約有上千億的恆星

and you point it at a random part of the sky,

如果你拿一個照相機

and you just keep the shutter open,

隨便對著天空的某一個角落

as long as your camera is attached to the Hubble Space Telescope,

打開快門

it will see something like this.

如果你的照相機連接著一個哈勃天文望遠鏡的話

Every one of these little blobs

你就會看到這樣一幅景象

is a galaxy roughly the size of our Milky Way --

這一團一團的

a hundred billion stars in each of those blobs.

都是是跟銀河系差不多大小的星系

There are approximately a hundred billion galaxies

就是說這枚一團裡都有上千億的恆星

in the observable universe.

並且在我們可以觀測到的宇宙範圍內

100 billion is the only number you need to know.

存在着上千億這樣的團

The age of the universe, between now and the Big Bang,

上千億——你記住這個數字就可以了

is a hundred billion in dog years.

宇宙的年齡 也就是從宇宙大爆炸至今

(Laughter)

大概是一千億“狗年”（一狗年約等於八年）

Which tells you something about our place in the universe.

（笑聲）

One thing you can do with a picture like this is simply admire it.

我這麼說也是為了告訴你我們在宇宙中的位置

It's extremely beautiful.

對於這樣一幅圖像 我們能做什麼呢？也就是純粹的欣賞景仰吧

I've often wondered, what is the evolutionary pressure

多麼美麗的圖像

that made our ancestors in the Veldt adapt and evolve

我時常會想 在星系的圖像還不存在的時候

to really enjoy pictures of galaxies

是什麼進化壓力 促使我們非洲大草原的的祖先

when they didn't have any.

不斷地適應 進化

But we would also like to understand it.

並開始欣賞星系？

As a cosmologist, I want to ask, why is the universe like this?

但是我們還想試圖理解它們

One big clue we have is that the universe is changing with time.

作為一個宇宙學家 我的疑惑是 為什麼宇宙是這樣的？

If you looked at one of these galaxies and measured its velocity,

一個線索就是宇宙是隨著時間而不斷變化的

it would be moving away from you.

如果你關注一個星系 並度量這個星系變化的速率

And if you look at a galaxy even farther away,

你會發現這個星系是不斷離你遠去的

it would be moving away faster.

如果你再去關註一個更遠距離以外的星系

So we say the universe is expanding.

這個星系會以更快的速度離你遠去

What that means, of course, is that, in the past,

所以我們說宇宙是在不斷膨脹的

things were closer together.

這是什麼意思呢？就是說 在過去

In the past, the universe was more dense,

物質是很緊密的聚合在一起的

and it was also hotter.

在過去 宇宙是相對更加緊實的

If you squeeze things together, the temperature goes up.

並且其溫度也更高

That kind of makes sense to us.

你把東西擠壓到一起 溫度自然會升高

The thing that doesn't make sense to us as much

這個很好理解

is that the universe, at early times, near the Big Bang,

難以令人理解的是

was also very, very smooth.

宇宙在非常早期的時候 也就是大爆炸之後不久

You might think that that's not a surprise.

也是非常非常平滑的

The air in this room is very smooth.

你可能想 這沒什麼可驚訝的

You might say, "Well, maybe things just smoothed themselves out."

這個房間裡的空氣就很平滑

But the conditions near the Big Bang are very, very different

你也許會說 “可能物質自然就平滑了”

than the conditions of the air in this room.

但是要知道宇宙大爆炸之後初期的情況

In particular, things were a lot denser.

與現在這個屋子裡的情況是非常不一樣的

The gravitational pull of things

具體來說 就是物質都很緊密

was a lot stronger near the Big Bang.

這些物質之間的引力

What you have to think about

在大爆炸之後初期比現在要強烈得多

is we have a universe with a hundred billion galaxies,

你可以這樣想

a hundred billion stars each.

我們這個宇宙裡 有上千億的星系

At early times, those hundred billion galaxies

每個星系裡 有上千億的恆星

were squeezed into a region about this big --

在很早期的時候 這樣的上千億的星系

literally -- at early times.

被擠壓到了僅僅這樣大小的一個空間裡

And you have to imagine doing that squeezing

真的是這樣的 在早期的時候

without any imperfections,

並且 你要知道 這樣的擠壓必須

without any little spots

以一種完美的方式發生

where there were a few more atoms than somewhere else.

沒有任何一處疏鬆

Because if there had been, they would have collapsed under the gravitational pull

即使僅僅是某一處比另一處多了幾個原子

into a huge black hole.

因為假如有任何疏鬆或不平均的話 整個宇宙就會在引力的作用下垮塌

Keeping the universe very, very smooth at early times

從而變成一個巨大的黑洞

is not easy; it's a delicate arrangement.

在宇宙早期保證宇宙處於一個非常平滑的狀態

It's a clue

是很不容易的 需要精心的安排

that the early universe is not chosen randomly.

這就從一個側面說明了

There is something that made it that way.

早期的宇宙不是一個隨機的組合

We would like to know what.

有某種原因的存在導致了這樣的組合的發生

So part of our understanding of this was given to us by Ludwig Boltzmann,

我們研究就是為了明白這種原因

an Austrian physicist in the 19th century.

路德維希 波茲曼 一個生活在19世紀的奧地利物理學家

And Boltzmann's contribution was that he helped us understand entropy.

向我們提供了部分的解釋

You've heard of entropy.

波茲曼的貢獻就在於他幫我們理解了“熵”這個概念

It's the randomness, the disorder, the chaoticness of some systems.

你應該聽說過熵

Boltzmann gave us a formula --

熵是指一種存在於某些系統中的隨意性 無序性和混亂性

engraved on his tombstone now --

波茲曼給出了一個方程式 ——

that really quantifies what entropy is.

這個方程式現在是刻在他的墓碑上的——

And it's basically just saying

來給熵定量

that entropy is the number of ways

這個方程式大概就是說

we can rearrange the constituents of a system so that you don't notice,

熵是一個值 用來描述在你未發現一個系統

so that macroscopically it looks the same.

產生了變化的前提下 可以對系統進行重組的方式的數目

If you have the air in this room,

這裡的變化是指外部大體上的變化

you don't notice each individual atom.

比如說這個屋子裡的空氣

A low entropy configuration

你不會看到每個原子

is one in which there's only a few arrangements that look that way.

一個低熵值的形態 是指一個

A high entropy arrangement

只能有少數的組合方式的變化來保證外在沒有變化的形態

is one that there are many arrangements that look that way.

一個高熵值的組合 是指在這種形態裡

This is a crucially important insight

可以有很多種排列組合的方式並不影響其外在沒有變化

because it helps us explain

這是一個非常重要的見解

the second law of thermodynamics --

因為這個見解可以幫助我們來解釋

the law that says that entropy increases in the universe,

熱力學第二定律

or in some isolated bit of the universe.

這個定律告訴我們 宇宙的熵值在不斷提高

The reason why entropy increases

或是說宇宙各部分的熵值在各自提高

is simply because there are many more ways

熵值升高的原因很簡單

to be high entropy than to be low entropy.

那就是高熵值的情況下比在低熵值的情況下

That's a wonderful insight,

存在有更多的可能性

but it leaves something out.

這是一個很好的觀點

This insight that entropy increases, by the way,

但是卻不盡全面

is what's behind what we call the arrow of time,

順便說一下 這個熵值不斷提高的說法

the difference between the past and the future.

就是我們說的單向時間軸 時間之箭

Every difference that there is

也就是過去與未來的區別

between the past and the future

之所以有這個過去

is because entropy is increasing --

與現在之間的區別

the fact that you can remember the past, but not the future.

原因就是不斷上升的熵值

The fact that you are born, and then you live, and then you die,

你能記住過去的事情 卻不能對未來的事情有印象

always in that order,

你出生 生活 然後死亡

that's because entropy is increasing.

這些事情都是依次發生的

Boltzmann explained that if you start with low entropy,

原因都是熵值在不斷提高

it's very natural for it to increase

博茨曼解釋說 如果一個初始狀態是低熵值

because there's more ways to be high entropy.

很自然的這個狀態會升高到高熵值

What he didn't explain

因為這樣就提供了更多種的存在可能性

was why the entropy was ever low in the first place.

但是博茨曼沒有解釋

The fact that the entropy of the universe was low

為什麼低熵值是一個初始狀態

was a reflection of the fact

宇宙早期的熵值很低

that the early universe was very, very smooth.

這就反映了

We'd like to understand that.

早期的宇宙是非常平滑的

That's our job as cosmologists.

我們需要理解的就是這一現象

Unfortunately, it's actually not a problem

我們宇宙學家就是做這個的

that we've been giving enough attention to.

可惜的是 我們並沒有給予

It's not one of the first things people would say,

這個問題足夠的重視

if you asked a modern cosmologist,

如果你問一個宇宙學家

"What are the problems we're trying to address?"

”宇宙學界現在在試圖解決的哪些問題？“

One of the people who did understand that this was a problem

這個問題不會是他最先給你的答案之一

was Richard Feynman.

認識到這個問題的人之一

50 years ago, he gave a series of a bunch of different lectures.

便是理查德·費曼

He gave the popular lectures

五十年前 他給了一系列講座

that became "The Character of Physical Law."

他面向大眾的講座

He gave lectures to Caltech undergrads

被編成了一本書 叫做『物理理論的特性』

that became "The Feynman Lectures on Physics."

他給加州理工本科生做的講座

He gave lectures to Caltech graduate students

變成了『費曼物理講座』一書

that became "The Feynman Lectures on Gravitation."

他給加州理工研究生做的講座

In every one of these books, every one of these sets of lectures,

被編成了『費曼引力講座』一書

he emphasized this puzzle:

在每本書 每組講座裡

Why did the early universe have such a small entropy?

費曼都強調了這個難題

So he says -- I'm not going to do the accent --

為什麼宇宙早期有如此低的一個熵值？

he says, "For some reason, the universe, at one time,

他說——我就不學他的口音了——

had a very low entropy for its energy content,

他說”出於某種原因 宇宙曾經

and since then the entropy has increased.

有一個很低的熵值

The arrow of time cannot be completely understood

而從那時起 熵值在不斷的升高

until the mystery of the beginnings of the history of the universe

如果宇宙初期歷史這個謎團

are reduced still further

沒有被我們從簡單的揣測

from speculation to understanding."

帶入到理解這個層次

So that's our job.

我們便無法去完全理解時間之箭”

We want to know -- this is 50 years ago, "Surely," you're thinking,

我們就是要解決這個問題

"we've figured it out by now."

我們想解決——從五十年前開始　你可能會想 ”那肯定

It's not true that we've figured it out by now.

這個問題現在肯定已經解決了”

The reason the problem has gotten worse,

但事實並非如此

rather than better,

這個問題現在更難解決了

is because in 1998

而不是更加容易

we learned something crucial about the universe that we didn't know before.

原因就是在1998年

We learned that it's accelerating.

我們對於宇宙有了一個突破性的發現

The universe is not only expanding.

我們了解到了 宇宙是在加速擴張的

If you look at the galaxy, it's moving away.

宇宙不僅是在擴張而已

If you come back a billion years later and look at it again,

如果你看著那個星系 他在離你遠去

it will be moving away faster.

如果你隔了10億年後再回來看

Individual galaxies are speeding away from us faster and faster

這個星系會離你更遠

so we say the universe is accelerating.

每個星係都是以加速度不斷離我們遠去的

Unlike the low entropy of the early universe,

所以我們說宇宙是在加速擴張的

even though we don't know the answer for this,

與早期宇宙低熵值的狀態不同的是

we at least have a good theory that can explain it,

儘管我們不知道為什麼宇宙是在加速擴張的

if that theory is right,

我們有一個還沒有確認的理論

and that's the theory of dark energy.

如果這個理論是正確的話

It's just the idea that empty space itself has energy.

這就是暗能量理論

In every little cubic centimeter of space,

根據這個理論 一個空的空間裡也是有能量的

whether or not there's stuff,

在空間的每一個立方厘米裡

whether or not there's particles, matter, radiation or whatever,

不管有沒有東西存在 都是有能量的

there's still energy, even in the space itself.

不管有沒有粒子 物質 放射 還是別的什麼

And this energy, according to Einstein,

空間 就因為空間本身 也是有能量的

exerts a push on the universe.

這個能量 愛因斯坦認為

It is a perpetual impulse

會對於宇宙施加一個推力

that pushes galaxies apart from each other.

這個推理是一個永存的推力

Because dark energy, unlike matter or radiation,

推動著星系互相遠去

does not dilute away as the universe expands.

這個暗能量 與物質或者放射性不同的是

The amount of energy in each cubic centimeter

不會因為宇宙的擴張而被稀釋

remains the same,

每立方厘米裡能量

even as the universe gets bigger and bigger.

是不變的

This has crucial implications

即使宇宙變得越來越大

for what the universe is going to do in the future.

這個對於解釋宇宙走向何方

For one thing, the universe will expand forever.

有很關鍵的意義

Back when I was your age,

首先 宇宙將會永遠擴張

we didn't know what the universe was going to do.

當我想你們這個年級的時候

Some people thought that the universe would recollapse in the future.

我們不知道宇宙將會怎樣

Einstein was fond of this idea.

有人說宇宙將會又一次垮塌

But if there's dark energy, and the dark energy does not go away,

愛因斯坦很喜歡這個理論

the universe is just going to keep expanding forever and ever and ever.

但是 如果有暗能量的話 並且它不會消失的話

14 billion years in the past,

這個宇宙將不斷 永遠的擴張下去

100 billion dog years,

在過去與的140億年裡

but an infinite number of years into the future.

也就是一千億狗年

Meanwhile, for all intents and purposes,

但是我們不能估計未來還有多少年

space looks finite to us.

同時 不管怎麼看

Space may be finite or infinite,

空間對於我們都看起來是有限的

but because the universe is accelerating,

空間可能是有限的 也可能是無限的

there are parts of it we cannot see

但是因為宇宙是加速度擴張的

and never will see.

有一部分宇宙 我們是看不到的

There's a finite region of space that we have access to,

並且永遠都看不到

surrounded by a horizon.

我們能夠接觸到的宇宙只是很有限的一部分

So even though time goes on forever,

在一個有限的視線範圍之內

space is limited to us.

所以即使時間不斷前行

Finally, empty space has a temperature.

空間對於我們來說仍然是有限的

In the 1970s, Stephen Hawking told us

最後 空的空間裡是有一個溫度的

that a black hole, even though you think it's black,

在上世紀70年代 史蒂芬·霍金告訴我們

it actually emits radiation

黑洞 即使你認為它是黑壓壓的一片

when you take into account quantum mechanics.

也是有放射性的

The curvature of space-time around the black hole

如果你從量子力學的角度來考慮問題

brings to life the quantum mechanical fluctuation,

黑洞周圍的時間-空間曲率

and the black hole radiates.

帶來了量子力學意義上的波動

A precisely similar calculation by Hawking and Gary Gibbons

並且黑洞開始具有放射性

showed that if you have dark energy in empty space,

霍金跟加利 吉布森有一個很類似的計算

then the whole universe radiates.

這個計算顯示 如果你在一個空的空間裡有暗能量

The energy of empty space

那麼整個宇宙都是會有放射性的

brings to life quantum fluctuations.

空的空間裡的能量

And so even though the universe will last forever,

帶來量子波動

and ordinary matter and radiation will dilute away,

並且即使宇宙永遠存在

there will always be some radiation,

普通物質跟放射性被稀釋

some thermal fluctuations,

宇宙中總是會有一些放射性存在的

even in empty space.

還有熱波動

So what this means

即使是在這樣一個空的空間裡

is that the universe is like a box of gas

這就說明

that lasts forever.

宇宙就像一個永遠存在的

Well what is the implication of that?

装满的气体的盒子

That implication was studied by Boltzmann back in the 19th century.

那麼這說明了什麼呢

He said, well, entropy increases

博茨曼在19世紀就對此進行了研究

because there are many, many more ways

他說 熵值增加

for the universe to be high entropy, rather than low entropy.

因為相比起低熵值的狀態

But that's a probabilistic statement.

宇宙有更多方式達到一個高熵值的狀態

It will probably increase,

但這是一個概率問題

and the probability is enormously huge.

熵值很有可能升高

It's not something you have to worry about --

並且這個可能性是非常巨大的

the air in this room all gathering over one part of the room and suffocating us.

但是你不用擔心說

It's very, very unlikely.

這個房間裡所有的空氣將聚集到房間的一個角落 讓我們窒息

Except if they locked the doors

這是非常非常不可能的

and kept us here literally forever,

除非門被鎖上

that would happen.

我們真的永遠的在這里呆下去

Everything that is allowed,

剛才所說的情況才能有可能發生

every configuration that is allowed to be obtained by the molecules in this room,

所有可能發生的情形

would eventually be obtained.

所有這個屋子裡的原子所可能形成的所有的組合

So Boltzmann says, look, you could start with a universe

最後總會被實現

that was in thermal equilibrium.

所以博茨曼說 你看 宇宙可以從一個

He didn't know about the Big Bang. He didn't know about the expansion of the universe.

熱平衡的狀態開始發展

He thought that space and time were explained by Isaac Newton --

他不知道宇宙大爆炸理論 更不知道宇宙擴張理論

they were absolute; they just stuck there forever.

他以為牛頓對於時間空間的解釋是正確的 那就是

So his idea of a natural universe

時間空間是絕對的 永遠不變的

was one in which the air molecules were just spread out evenly everywhere --

所以博茨曼的自然宇宙的理論

the everything molecules.

認為空氣分子就是平均分散在各處的

But if you're Boltzmann, you know that if you wait long enough,

構成各種東西的分子

the random fluctuations of those molecules

但是如果你是博茨曼 你會想 如果你足夠耐心