The search for dark matter -- and what we've found so far | Risa Wechsler

譯者: Lilian Chiu 審譯者: Helen Chang

Do you ever think about what would happen

你可曾想過如果世界有 一點點不同，會怎麼樣？

if the world were a little bit different?

How your life would be different

你的人生會有何不同，

if you were born 5,000 years from now

如果你不是活在現代

而是距離現代五千年的時代？

instead of today?

How history would be different

歷史又會有何不同，

if the continents were at different latitudes

如果各大陸在不同的緯度，

or how life in the Solar system would have developed

或者，太陽系的生命 會有怎樣的發展，

if the Sun were 10 percent larger.

如果太陽的尺寸大了 10% 呢？

Well, playing with these kinds of possibilities

玩弄這些可能性

is what I get to do for a living

就是我的謀生工作，

but with the entire universe.

不過我玩的是整個宇宙。

I make model universes in a computer.

我在電腦上做宇宙模型。

Digital universes that have different starting points

這些數位宇宙有不同的起始點，

and are made of different amounts of different kinds of material.

用不同量、不同的材料製造而成。

And then I compare these universes to our own

接著，我拿這些宇宙 和我們自己的宇宙做比對，

to see what it is made of and how it evolved.

來看看它是什麼形成的， 以及它如何演化。

This process of testing models with measurements of the sky

這種用天空的測量值 來測試模型的過程，

has taught us a huge amount about our universe so far.

目前為止讓我們學到了 大量關於我們宇宙的知識。

One of the strangest things we have learned

我們所學到最奇怪的事情之一

is that most of the material in the universe

就是宇宙中的大部分材料

is made of something entirely different than you and me.

都由一種和你我 完全不同的成份組成。

But without it,

但若沒有它，

the universe as we know it wouldn't exist.

我們所認識的宇宙就不會存在。

Everything we can see with telescopes

我們用望遠鏡能看見的一切

makes up just about 15 percent of the total mass in the universe.

僅佔宇宙全部質量的 15%。

Everything else, 85 percent of it,

其餘的 85%

doesn't emit or absorb light.

不會發光或吸收光。

We can't see it with our eyes,

我們的肉眼無法看見，

we can't detect it with radio waves

我們無法用無線電電波、微波，

or microwaves or any other kind of light.

或任何其他類型的光來偵測它們。

But we know it is there

但我們知道它在那裡，

because of its influence on what we can see.

因為它會影響到 我們能看見的東西。

It's a little bit like,

這有點像你想要把地球的表面

if you wanted to map the surface of our planet

and everything on it

和表面上的一切都畫成地圖，

using this picture of the Earth from space at night.

但你的依據只有夜晚從太空中 拍攝的一張地球照片。

You get some clues from where the light is,

你能從光的所在得到一些線索，

but there's a lot that you can't see,

但有很多是你看不見的，

everything from people to mountain ranges.

從人到山脈都有。

And you have to infer what is there from these limited clues.

你得要用這些有限的線索 來推斷地面上還有什麼。

We call this unseen stuff "dark matter."

我們把這個看不見的東西 叫做「暗物質」。

Now, a lot of people have heard of dark matter,

很多人聽過暗物質，

but even if you have heard of it,

但即使你聽過它，

it probably seems abstract,

可能也覺得很抽象、

far away, probably even irrelevant.

很遙遠，甚至可能很不重要。

Well, the interesting thing is,

有趣的是，

dark matter is all around us

我們身邊都有暗物質，

and probably right here.

可能這裡就有。

In fact, dark matter particles

事實上，暗物質粒子

are probably going through your body right now

可能在你坐在這間房間時，

就正在穿過你的身體。

as you sit in this room.

Because we are on Earth

因為我們在地球上， 地球繞著太陽轉動，

and Earth is spinning around the Sun,

and the Sun is hurtling through our galaxy

而太陽正在飛馳穿越我們的銀河，

at about half a million miles per hour.

速度大約是每小時五十萬英里。

But dark matter doesn't bump into us,

但暗物質不會撞上我們，

it just goes right through us.

它會直接穿過我們。

So how do we figure out more about this?

所以我們要如何對它有更多了解？

What is it,

它是什麼？它和我們的 存在又有什麼關係？

and what does it have to do with our existence?

Well, in order to figure out how we came to be,

為了搞懂我們是如何出現的，

we first need to understand how our galaxy came to be.

我們首先要了解 我們的銀河系怎麼出現的。

This is a picture of our galaxy, the Milky Way, today.

圖上的是現今我們的銀河系。

What did it look like 10 billion years in the past

在一百億年前，它是什麼樣子？

or what would it look like 10 billion years in the future?

或者，在一百億年後， 它會是什麼樣子？

What about the stories

至於我們已經透過對天空 進行大型研究的方式所描繪出來的

of the hundreds of millions of other galaxies

that we've already mapped out with large surveys of the sky?

其他數十億個銀河， 又有什麼樣的故事？

How would their histories be different

它們的歷史又會有什麼不同，

if the universe was made of something else

如果宇宙是由其他東西構成，

or if there was more or less matter in it?

或者宇宙中物質的量 比較多或比較少？

So the interesting thing about these model universes

所以，這些模型宇宙的 有趣之處在於，

is that they allow us to test these possibilities.

它們讓我們可以 去測試這些可能性。

Let's go back to the first moment of the universe --

咱們回到宇宙的最初——

just a fraction of a second after the big bang.

大爆炸之後的那一瞬間。

In this first moment,

在這最初的一刻，

there was no matter at all.

完全沒有任何物質存在。

The universe was expanding very fast.

宇宙擴張的速度非常快。

And quantum mechanics tells us

而量子力學告訴我們，

that matter is being created and destroyed

物質時時刻刻都會

被創造出來和消滅掉。

all the time, in every moment.

At this time, the universe was expanding so fast

此時，宇宙擴張的速度非常快，

that the matter that got created couldn't get destroyed.

快到被創造出來的物質 來不及被消滅。

And thus we think that all of the matter was created during this time.

因此，我們認為所有物質都是 在這個時候被創造出來的。

Both the dark matter

包括暗物質

and the regular matter that makes up you and me.

以及形成你我的一般物質。

Now, let's go a little bit further

現在，再向快轉一點

to a time after the matter was created,

到物質被創造出來之後，

after protons and neutrons formed,

質子和中子形成之後，

after hydrogen formed,

氫形成之後，

about 400,000 years after the big bang.

也就是大爆炸後的約四十萬年。

The universe was hot and dense and really smooth

宇宙又熱又密實，且相當平滑，

but not perfectly smooth.

但沒有到完全平滑的程度。

This image, taken with a space telescope called the Planck satellite,

這張影像是用「普朗克衛星」 太空望遠鏡拍攝的，

shows us the temperature of the universe

呈現出宇宙的溫度，

in all directions.

各方向都有。

And what we see

我們可以看見

is that there were places that were a little bit hotter

有些地方比較熱，

and denser than others.

也比其他地方更密實。

The spots in this image

影像上的點

represent places where there was more or less mass in the early universe.

代表在早期宇宙中有比較多 或比較少質量的地方。

Those spots got big because of gravity.

因為引力，那些點會變大。

The universe was expanding and getting less dense overall

在過去 138 億年間，

over the last 13.8 billion years.

整體宇宙一直在擴張且降低密度。

But gravity worked hard in those spots

但在那些點上，引力十分賣力，

where there was a little bit more mass

因此會有比較多一點質量，

and pulled more and more mass into those regions.

並把更多質量拉到那些區域中。

Now, all of this is a little hard to imagine,

這些都有點難想像，

so let me just show you what I am talking about.

所以，我先讓大家看看 我在談的是什麼。

Those computer models I mentioned allow us to test these ideas,

我剛剛提到的那些電腦模型 讓我們可以測試這些想法，

so let's take a look at one of them.

咱們來看看其中一個模型。

This movie, made by my research group,

這支影片由我的研究團隊製作，

shows us what happened to the universe after its earliest moments.

呈現出宇宙在最初的時刻 之後發生了什麼事。

You see the universe started out pretty smooth,

各位可以看見， 宇宙一開始十分平滑

but there were some regions

但有一些區域有比較多一點材料。

where there was a little bit more material.

Gravity turned on and brought more and more mass

引力發動，把更多質量帶到

into those spots that started out with a little bit extra.

那些一開始相對有 額外多一點質量的點上。

Over time,

隨時間過去，

you get enough stuff in one place

在一個地方有足夠的量之後，

that the hydrogen gas,

本來和暗物質完全 混合在一起的氫氣，

which was initially well mixed with the dark matter,

starts to separate from it,

開始和暗物質分離，

cool down, form stars,

冷卻下來，形成星體，

and you get a small galaxy.

就得到了一個小銀河系。

Over time, over billions and billions of years,

經過了數十億年的時間，

those small galaxies crash into each other

那些小銀河系彼此相撞，

and merge and grow to become larger galaxies,

合併、成長，成為更大的銀河系，

like our own galaxy, the Milky Way.

就像我們自己的 銀河系，本銀河系。

Now, what happens if you don't have dark matter?

如果沒有暗物質，會怎麼樣？

If you don't have dark matter,

如果沒有暗物質，

those spots never get clumpy enough.

那些點就永遠不夠變成塊狀。

It turns out, you need at least a million times the mass of the Sun

結果發現，在單一高密度的區域，

需要有太陽一百萬倍的質量，

in one dense region,

before you can start forming stars.

才能開始形成星體。

And without dark matter,

若沒有暗物質，

you never get enough stuff in one place.

永遠無法在一個地方 聚集這麼多東西。

So here, we're looking at two universes, side by side.

在這裡，我們可以 看到兩個宇宙並排。

In one of them you can see

各位可以看到，在其中一個宇宙裡

that things get clumpy quickly.

很快就會形成塊狀。

In that universe,

在那個宇宙裡，

it's really easy to form galaxies.

銀河系很容易形成。

In the other universe,

在另一個宇宙裡，

the things that start out like small clumps,

一開始的那些小塊狀

they just stay really small.

就會一直維持那麼小。

Not very much happens.

沒有發生什麼。

In that universe, you wouldn't get our galaxy.

在那個宇宙中，不會 產生出我們的銀河系。

Or any other galaxy.

或任何銀河系。

You wouldn't get the Milky Way,

不會產生本銀河系，

you wouldn't get the Sun,

不會產生太陽，

you wouldn't get us.

不會有我們。

We just couldn't exist in that universe.

我們無法存在於那個宇宙中。

OK, so this crazy stuff, dark matter,

好，所以這瘋狂的東西，暗物質，

it's most of the mass in the universe,

它幾乎是宇宙的全部質量，

it's going through us right now, we wouldn't be here without it.

現在正在穿過我們，

若沒有它，我們就不會在這裡。

What is it?

它是什麼？

Well, we have no idea.

我們不知道。

(Laughter)

（笑聲）

But we have a lot of educated guesses,

但我們有很多根據經驗做出的猜測，

and a lot of ideas for how to find out more.

以及許多點子可以再深入了解。

So, most physicists think that dark matter is a particle,

大部分的物理學家 認為暗物質是粒子，

similar in many ways to the subatomic particles that we know of,

和我們所知道的亞原子粒子 在許多方面都很相似，

like protons and neutrons and electrons.

就像光子、中子、電子。

Whatever it is,

不論它是什麼，

it behaves very similarly with respect to gravity.

它的行為與重力非常相似。

But it doesn't emit or absorb light,

但它不會發光或吸收光，

and it goes right through normal matter,

且它會直接穿過一般物質，

as if it wasn't even there.

好像它們不存在一樣。

We'd like to know what particle it is.

我們想要知道它是什麼樣的粒子。

For example, how heavy is it?

比如，它有多重？

Or, does anything at all happen if it interacts with normal matter?

或者，如果它和一般物質 發生交互作用之後會如何？

Physicists have lots of great ideas for what it could be,

物理學家對此有很多很棒的想法，

they're very creative.

他們非常有創意。

But it's really hard,

但困難之處在於

because those ideas span a huge range.

那些想法橫跨的範圍非常廣。

It could be as small as the smallest subatomic particles,

暗物質有可能和最小的 亞原子粒子一樣小，

or it could be as large as the mass of 100 Suns.

或者也有可能大到 和一百個太陽的質量一樣大。

So, how do we figure out what it is?

所以，我們要如何知道它是什麼？

Well, physicists and astronomers

物理學家和天文學家 有很多方法可以尋找暗物質。

have a lot of ways to look for dark matter.

One of the things we're doing is building sensitive detectors

我們會做的其中一件事就是 建造敏感度高的偵測器，

in deep underground mines,

放在很深的地下礦井中，

waiting for the possibility

等待著，看有沒有可能

that a dark matter particle, which goes through us and the Earth,

會有一個能穿過我們 和地球的暗物質粒子

would hit a denser material

會擊中一種密度較高的材料，

and leave behind some trace of its passage.

在通過時留下一些痕跡。

We're looking for dark matter in the sky,

我們也在天空中尋找暗物質，

for the possibility that dark matter particles

看有沒有可能 暗物質粒子會撞到彼此，

would crash into each other

and create high-energy light that we could see

創造出高能光，讓我們能

with special gamma-ray telescopes.

從特殊的伽瑪射線望遠鏡看見。

We're even trying to make dark matter here on Earth,

我們甚至嘗試 在地球上製造暗物質，

by smashing particles together and looking for what happens,

做法是用瑞士的大型強子對撞機

using the Large Hadron Collider in Switzerland.

讓粒子對撞，看看會怎麼樣。

Now, so far,

目前，

all of these experiments have taught us a lot

所有這些實驗讓我們學到很多

about what dark matter isn't

暗物質「不是」什麼。

(Laughter)

（笑聲）

but not yet what it is.

但尚未了解它是什麼。

There were really good ideas that dark matter could have been,

關於暗物質可能是什麼東西， 有一些很棒的想法，

that these experiments would have seen.

若想法為真，實驗就看得到， 但並沒有看到。

And they didn't see them yet,

so we have to keep looking and thinking harder.

所以我們得要繼續尋找， 更努力思考。

Now, another way to get a clue to what dark matter is

想要了解暗物質是什麼， 還有另一個方法，

is to study galaxies.

就是研究銀河系。

We already talked about

我們已經談過

how our galaxy and many other galaxies wouldn't even be here

若沒有暗物質，我們的銀河系 和其他銀河系就不會存在。

without dark matter.

Those models also make predictions

那些模型也能針對銀河的 許多其他面向做預測：

for many other things about galaxies:

How they're distributed in the universe,

它們在宇宙中如何分佈、 它們如何移動、

how they move,

how they evolve over time.

它們如何隨時間演化。

And we can test those predictions with observations of the sky.

我們可以透過觀察天空 來測試那些預測。

So let me just give you two examples

讓我舉兩個例子，

of these kinds of measurements we can make with galaxies.

說明我們能用銀河系 做什麼樣的測量。

The first is that we can make maps of the universe with galaxies.

第一，我們可以用銀河系 來畫出宇宙的地圖。

I am part of a survey called the Dark Energy Survey,

我是「暗能量巡天」研究的一員，

which has made the largest map of the universe so far.

該研究製作出了目前 最大的宇宙地圖。

We measured the positions and shapes of 100 million galaxies

我們測量了八分之一的天空中

一億個銀河系的位置和形狀。

over one-eighth of the sky.

And this map is showing us all the matter in this region of the sky,

這張地圖讓我們看到在天空中 這個區域裡的所有物質，

which is inferred by the light distorted from these 100 million galaxies.

其推論根據是來自 這一億個銀河系的扭曲光線。

The light distorted from all of the matter

光線會被那些銀河系和我們之間的

that was between those galaxies and us.

所有物質給扭曲。

The gravity of the matter is strong enough to bend the path of light.

物質的引力強到足以 彎曲光走的路徑。

And it gives us this image.

就得出了這張影像。

So these kinds of maps

所以，這類地圖可以告訴我們 那裡有多少暗物質存在，

can tell us about how much dark matter there is,

they also tell us where it is

以及存在的位置，

and how it changes over time.

和其隨時間的改變。

So we're trying to learn about what the universe is made of

所以，我們是從非常宏觀的角度

在試圖了解宇宙的組成。

on the very largest scales.

It turns out that the tiniest galaxies in the universe

結果發現，宇宙中最小的銀河系

provide some of the best clues.

提供了一些最好的線索。

So why is that?

為什麼會這樣？

Here are two example simulated universes

這裡是用兩種不同的暗物質

with two different kinds of dark matter.

模擬出來的兩個宇宙範例。

Both of these pictures are showing you a region

這兩張圖都呈現出 一個銀河系周圍的區域，

around a galaxy like the Milky Way.

它很類似我們的本銀河系。

And you can see that there's a lot of other material around it,

各位可以看到， 它周圍有許多其他材料，

little small clumps.

小小的塊狀物。

Now, in the image on the right,

在右邊的影像中，

dark matter particles are moving slower than they are in the one on the left.

暗物質粒子移動的速度 比在左邊的影像中更緩慢。

If those dark matter particles are moving really fast,

如果那些暗物質粒子移動得很快，

then the gravity in small clumps is not strong enough

那麼，小塊狀物的引力不夠強，

to slow those fast particles down.

無法讓那些快速粒子慢下來。

And they keep going.

粒子會持續移動， 永遠不會崩塌成小塊狀物。

They never collapse into these small clumps.

So you end up with fewer of them than in the universe on the right.

所以，最後在左圖 宇宙中的數量就比右圖少了。

If you don't have those small clumps,

如果沒有那些小塊狀物，

then you get fewer small galaxies.

產生的小銀河系就會比較少。

If you look up at the southern sky,

抬頭看南方的天空，

you can actually see two of these small galaxies,

你可以看到兩個 像這樣的小銀河系，

the largest of the small galaxies that are orbiting our Milky Way,

這些小銀河系中最大的兩個 在繞行我們的本銀河系，

the Large Magellanic Cloud and the Small Magellanic Cloud.

大麥哲倫星系和小麥哲倫星系。

In the last several years,

在過去幾年，

we have detected a whole bunch more even smaller galaxies.

我們偵測到一大堆更小的銀河系。

This is an example of one of them

這是其中一個例子，

that we detected with the same dark energy survey

是我們在製作宇宙地圖時所做的

that we used to make maps of the universe.

暗能量巡天研究中偵測到的。

These really small galaxies,

這些非常小的銀河系當中

some of them are extremely small.

有一些小到極點。

Some of them have as few as a few hundred stars,

當中有些只有幾百個星體，

compared to the few hundred billion stars in our Milky Way.

相較之下，我們的本銀河系 有數千億個星體。

So that makes them really hard to find.

因此很難找到這些小銀河系。

But in the last decade,

但，在前十年間，

we've actually found a whole bunch more of these.

我們真的發現了 不少這種小銀河系。

We now know of 60 of these tiny galaxies

我們現在知道有六十個小銀河系

that are orbiting our own Milky Way.

繞行我們的本銀河系。

And these little guys are a big clue to dark matter.

這些小傢伙是了解 暗物質的大線索。

Because just the existence of these galaxies tells us

因為，光是這些銀河系的 存在就告訴我們

that dark matter can't be moving very fast,

暗物質移動的速度不快，

and not much can be happening when it runs into normal matter.

且當它碰上了一般物質時， 不太會發生什麼現象。

In the next several years,

在接下來幾年，

we're going to make much more precise maps of the sky.

我們將會繪製更精確的天空地圖。

And those will help refine our movies

那些地圖會協助我們再改善

of the whole universe and the entire galaxy.

我們為整個宇宙及整個 銀河系所做的影片。

Physicists are also making new, more sensitive experiments

物理學家也在進行 敏感度更高的新實驗，

to try to catch some sign of dark matter in their laboratories.

試圖在他們的實驗室中 捕捉暗物質的徵兆。

Dark matter is still a huge mystery.

暗物質仍然是個大謎團。

But it's a really exciting time to be working on it.

但能投入研究它是很讓人興奮的。

We have really clear evidence it exists.

我們有清楚的證據能證明它存在。

From the scale of the smallest galaxies

從最小型銀河系的規模

to the scale of the whole universe.

到整個宇宙的規模。

Will we actually find it and figure out what it is?

我們將來會找到它 並搞懂它是什麼嗎？

I have no idea.

我不知道。

But it's going to be a lot of fun to find out.

但試著找出答案的過程會很好玩。

We have a lot of possibilities for discovery,

我們有很多發現的可能性，

and we definitely will learn more about what it is doing

且我們肯定會更了解 它做什麼及不是什麼。

and about what it isn't.

Regardless of whether we find that particle anytime soon,

不論我們是否能 在近期找到那種粒子，

I hope I have convinced you

我希望我已經說服各位，

that this mystery is actually really close to home.

這個謎團其實離我們的家園很近。

The search for dark matter

尋找暗物質之旅

may just be the key to a whole new understanding of physics

有可能是個關鍵，能讓我們 了解一種全新物理學

and our place in the universe.

及我們在宇宙中的地位。

Thank you.

謝謝。

(Applause)

（掌聲）