Do you ever think about what would happen if the world were a little bit different?
如果世界发生一些变化，会有什么不同？
How your life would be different if you were born 5,000 years from now instead of today?
如果你出生在5000年前，而不是现在，你的生活会发生什么变化？
How history would be different if the continents were at different latitudes
如果各个大洲在不同的维度，历史会发生什么变化，
or how life in the Solar system would have developed if the Sun were 10 percent larger.
或者说如果太阳的体积比现在大10%，太阳系的生命会怎样演化？
Well, playing with these kinds of possibilities is what I get to do for a living but with the entire universe.
研究这种类型的问题是我的工作，只不过，我研究的是整个宇宙。
I make model universes in a computer.
我在电脑中做了一些宇宙的模型。
Digital universes that have different starting points and are made of different amounts of different kinds of material.
这些数字宇宙模型有着不同的起始时间和不同数量与种类的材料。
And then I compare these universes to our own to see what it is made of and how it evolved.
然后我拿它们与我们的宇宙相比较，想看看我们宇宙的主要成分，以及宇宙是怎样发展与进化的。
This process of testing models with measurements of the sky has taught us a huge amount about our universe so far.
这个用测量天空来测试模型的过程教会了我们很多关于我们的宇宙的知识。
One of the strangest things we have learned is that
我们所发现的最奇怪的事情之一，
most of the material in the universe is made of something entirely different than you and me.
就是宇宙中的大多数物质是由不同于构成你我的东西组成的。
But without it, the universe as we know it wouldn't exist.
但是如果没有它，我们所知道的宇宙就不会存在。
Everything we can see with telescopes makes up just about 15 percent of the total mass in the universe.
我们能用显微镜看见的所有物质只占全宇宙所有物质的15%。
Everything else, 85 percent of it, doesn't emit or absorb light.
余下85%的物质不会吸收或发射光线。
We can't see it with our eyes, we can't detect it with radio waves or microwaves or any other kind of light.
我们不能用肉眼观察它们，也不能用无线电波、微波或其他任何的可见光探测它们。
But we know it is there because of its influence on what we can see.
但我们知道它们就在那里，因为它们影响了我们所能看见的世界。
It's a little bit like,
这就有一点像，
if you wanted to map the surface of our planet and everything on it using this picture of the Earth from space at night.
你想用一张在夜里从太空中拍摄的照片，来给我们的地球画一张包含所有东西的地图。
You get some clues from where the light is,
你能够从有亮光的地方得到一些线索，
but there's a lot that you can't see, everything from people to mountain ranges.
但还有很多你看不见的东西，从人群到山脉等等。
And you have to infer what is there from these limited clues.
你必须通过有限的线索去推测那是什么。
We call this unseen stuff "dark matter."
我们称这些看不见的东西为“暗物质”。
Now, a lot of people have heard of dark matter,
有很多人都听说过暗物质，
but even if you have heard of it, it probably seems abstract, far away, probably even irrelevant.
但即使你听说过，它对你来说也很有可能是抽象的、遥远的，甚至跟你的生活完全无关。
Well, the interesting thing is, dark matter is all around us and probably right here.
有趣的是，暗物质就在我们身旁，很有可能就在这儿。
In fact, dark matter particles are probably going through your body right now as you sit in this room.
事实上，你坐在这里的时候，暗物质粒子很可能正在穿过你的身体。
Because we are on Earth and Earth is spinning around the Sun,
因为我们在地球上，地球围着太阳转，
and the Sun is hurtling through our galaxy at about half a million miles per hour.
太阳又以超过80万公里的时速穿过银河系。
But dark matter doesn't bump into us, it just goes right through us.
但是暗物质并不会撞向我们，它们只会穿过我们。
So how do we figure out more about this?
那么我们怎么去研究它呢？
What is it, and what does it have to do with our existence?
它是什么，它与我们的存在又有什么关系？
Well, in order to figure out how we came to be, we first need to understand how our galaxy came to be.
为了了解我们是怎么来的，首先要了解银河系的起源。
This is a picture of our galaxy, the Milky Way, today.
这是今天的银河系的一张照片。
What did it look like 10 billion years in the past or what would it look like 10 billion years in the future?
它在100亿年前或者100亿年以后会是什么样子？
What about the stories of the hundreds of millions of other galaxies that we've already mapped out with large surveys of the sky?
我们已经探测到的其他数亿个星系又是怎样的呢？
How would their histories be different if the universe was made of something else or if there was more or less matter in it?
如果它们是由其他物质组成，由更多或更少的物质组成，那么它们的历史又会有什么不同？
So the interesting thing about these model universes is that they allow us to test these possibilities.
有意思的是，这些宇宙模型可以让我们测试这些可能性。
Let's go back to the first moment of the universe -- just a fraction of a second after the big bang.
让我们回到宇诞生的最初--宇宙大爆炸后不到一秒的时刻。
In this first moment, there was no matter at all. The universe was expanding very fast.
在这个时刻，什么物质也没有。宇宙膨胀得非常迅速。
And quantum mechanics tells us that matter is being created and destroyed all the time, in every moment.
量子力学告诉我们，物质无时无刻不在被创造与毁灭。
At this time, the universe was expanding so fast that the matter that got created couldn't get destroyed.
这时候宇宙膨胀得太快，以至于所创造的物质来不及被毁灭。
And thus we think that all of the matter was created during this time.
因此我们认为所有的物质都是在这个时候创造的。
Both the dark matter and the regular matter that makes up you and me.
包括暗物质和构成了你我的普通物质。
Now, let's go a little bit further to a time after the matter was created,
现在，让我们往后退一步，来到物质被创造之后的时间，
after protons and neutrons formed, after hydrogen formed, about 400,000 years after the big bang.
在质子与中子形成以后，在氢元素形成以后，大约在大爆炸发生的40万年以后。
The universe was hot and dense and really smooth but not perfectly smooth.
那时的宇宙温度高，密度大，很平滑，但又不是非常平滑。
This image, taken with a space telescope called the Planck satellite,
这张由普朗克天文望远镜拍摄的照片
shows us the temperature of the universe in all directions.
展示了宇宙中所有方向上的温度。
And what we see is that there were places that were a little bit hotter and denser than others.
我们可以看到，有些地方要比其它地方温度更高、密度更大。
The spots in this image represent places where there was more or less mass in the early universe.
图片中的这些斑点代表了在宇宙早期物质质量不均匀的分布状况。
Those spots got big because of gravity.
这些点会因为引力而变大。
The universe was expanding and getting less dense overall over the last 13.8 billion years.
宇宙会膨胀到比过去138亿年的密度都要小。
But gravity worked hard in those spots where there was a little bit more mass
但质量更大的地方引力更强，
and pulled more and more mass into those regions.
会拉动更多的质量到这个地方。
Now, all of this is a little hard to imagine, so let me just show you what I am talking about.
这可能有点难以理解，让我来详细解释一下。
Those computer models I mentioned allow us to test these ideas, so let's take a look at one of them.
我们可以用刚才提到的那些电脑模型来测试这些想法，让我们来看一下其中的一个。
This movie, made by my research group, shows us what happened to the universe after its earliest moments.
这是我的研究团队制作的一个影片，展示了宇宙诞生之后的一些事。
You see the universe started out pretty smooth, but there were some regions where there was a little bit more material.
我们可以看见宇宙开始的时候非常平滑，但是有些地方有更多的物质。
Gravity turned on and brought more and more mass into those spots that started out with a little bit extra.
这些点开始出现重力，并不断带来越来越多的质量。
Over time, you get enough stuff in one place that the hydrogen gas, which was initially well mixed with the dark matter,
慢慢的，这里有了足够的物质，最初与暗物质相混合的氢气，
starts to separate from it, cool down, form stars, and you get a small galaxy.
开始分离出来，冷却并形成恒星，形成了一个小的星系。
Over time, over billions and billions of years, those small galaxies crash into each other and merge
再后来，数十亿年之后，这些小星系互相撞击、融合，
and grow to become larger galaxies, like our own galaxy, the Milky Way.
形成了更大的星系，就像我们的银河一样。
Now, what happens if you don't have dark matter?
如果没有暗物质会发生什么？
If you don't have dark matter, those spots never get clumpy enough.
如果没有暗物质，这些点就会不够重。
It turns out, you need at least a million times the mass of the Sun in one dense region, before you can start forming stars.
这表示在形成恒星之前，你需要在一个稠密的地方获得至少100万倍于太阳的质量。
And without dark matter, you never get enough stuff in one place.
没有暗物质，就永远不会在一个地方得到足够的质量。
So here, we're looking at two universes, side by side.
现在我们看一下并排的这两个宇宙。
In one of them you can see that things get clumpy quickly.
其中一个质量增加得很快。
In that universe, it's really easy to form galaxies.
在这个宇宙中更容易形成星系。
In the other universe, the things that start out like small clumps, they just stay really small.
在其他宇宙中，在那些开始时只有很小块状物质的宇宙，它们只会保持原有的很小的状态。
Not very much happens. In that universe, you wouldn't get our galaxy. Or any other galaxy.
很少会改变。在那样的宇宙，你不会发现像我们这样的星系，或其他任何的星系。
You wouldn't get the Milky Way, you wouldn't get the Sun, you wouldn't get us.
不会有银河系，不会有太阳，也不会有我们这样的人类。
We just couldn't exist in that universe.
那个宇宙根本不会有生命。
OK, so this crazy stuff, dark matter, it's most of the mass in the universe,
这种令人匪夷所思的暗物质是宇宙中的主宰。

it's going through us right now, we wouldn't be here without it.
它正在穿过我们的身体，没有它我们也不会存在。
What is it? Well, we have no idea.
它究竟是什么？我们也不知道。
But we have a lot of educated guesses, and a lot of ideas for how to find out more.
但我们有一些合理的猜测，还有进一步解开谜题的想法。
So, most physicists think that dark matter is a particle,
大多数的物理学家认为暗物质是一种粒子，
similar in many ways to the subatomic particles that we know of, like protons and neutrons and electrons.
在很多方面与我们所知的亚原子很像，比如质子、中子和电子。
Whatever it is, it behaves very similarly with respect to gravity.
无论它是什么，它都和重力的性质非常相似。
But it doesn't emit or absorb light, and it goes right through normal matter, as if it wasn't even there.
但它并不发出或吸收光线，并且它会穿过正常物质，好像它不存在一样。
We'd like to know what particle it is. For example, how heavy is it?
我们想知道它到底是哪种粒子。例如，它有多重？
Or, does anything at all happen if it interacts with normal matter?
或者它与正常物质相互作用的时候会有什么变化？
Physicists have lots of great ideas for what it could be, they're very creative.
物理学家对于它是什么有很多大胆又有创造力的想法。
But it's really hard, because those ideas span a huge range.
但这非常困难，因为这些想法范围很广。
It could be as small as the smallest subatomic particles, or it could be as large as the mass of 100 Suns.
它可以小得像最小的亚原子，也可以大如100个太阳。
So, how do we figure out what it is?
我们应该怎么研究它呢？
Well, physicists and astronomers have a lot of ways to look for dark matter.
物理学家和天文学家有很多方法去寻找暗物质。
One of the things we're doing is building sensitive detectors in deep underground mines,
我们所做的一件事，就是在很深的矿井里建造灵敏的探测器，
waiting for the possibility that a dark matter particle, which goes through us and the Earth,
等待穿过我们和地球的暗物质粒子
would hit a denser material and leave behind some trace of its passage.
撞击密度更大的物质，并留下一些痕迹的可能性。
We're looking for dark matter in the sky,
我们正在寻找天空中的暗物质，
for the possibility that dark matter particles would crash into each other and create high-energy light
寻找暗物质粒子相互碰撞并产生高能光的可能性，
that we could see with special gamma-ray telescopes.
我们能用特殊的伽马射线望远镜观察到它。
We're even trying to make dark matter here on Earth, by smashing particles together and looking for what happens,
我们甚至尝试将粒子撞击在一起，看是否能产生暗物质，
using the Large Hadron Collider in Switzerland.
使用的是瑞士的大型强子对撞机。
Now, so far, all of these experiments have taught us a lot about what dark matter isn't but not yet what it is.
目前为止，所有的这些研究让我们排除了各种疑似暗物质的情况，但还是没有告诉我们什么是暗物质。
There were really good ideas that dark matter could have been, that these experiments would have seen.
有一些很好的观点认为暗物质是存在的，而且能够通过实验观察到。
And they didn't see them yet, so we have to keep looking and thinking harder.
但实验并没有证明这一点，因此我们还需持续努力的去寻找。
Now, another way to get a clue to what dark matter is is to study galaxies.
了解什么是暗物质的另一种方法是研究星系。
We already talked about how our galaxy and many other galaxies wouldn't even be here without dark matter.
我们已经讨论过，如果没有暗物质，我们的银河系和许多其他星系将不复存在。
Those models also make predictions for many other things about galaxies:
这些模型还对银河系的许多其他方面做出了预测：
How they're distributed in the universe, how they move, how they evolve over time.
它们如何在宇宙中分布，如何移动，如何随着时间演变。
And we can test those predictions with observations of the sky.
我们可以通过对天空的观察来检验这些预测。
So let me just give you two examples of these kinds of measurements we can make with galaxies.
那么，让我提供两个我们可以对星系进行此类测量的例子。
The first is that we can make maps of the universe with galaxies.
首先我们可以用星系绘制宇宙图。
I am part of a survey called the Dark Energy Survey, which has made the largest map of the universe so far.
我加入了一个名为暗能量调查的项目，该组织绘制了迄今为止最大的宇宙图。
We measured the positions and shapes of 100 million galaxies over one-eighth of the sky.
我们测量了跨越八分之一天空的近1亿个星系的位置和形状。
And this map is showing us all the matter in this region of the sky,
这张地图向我们展示了天空这一区域中的所有物质，
which is inferred by the light distorted from these 100 million galaxies.
这是由这1亿个星系扭曲的光线推断出来的。
The light distorted from all of the matter that was between those galaxies and us.
光线在穿过这些星系和我们之间的所有物质时扭曲了。
The gravity of the matter is strong enough to bend the path of light. And it gives us this image.
物质的引力足够强到可以弯曲光路。于是我们得到了这张图片。
So these kinds of maps can tell us about how much dark matter there is,
这种类型的地图可以告诉我们暗物质的数量，
they also tell us where it is and how it changes over time.
也可以告诉我们暗物质的位置以及其随时间的变化。
So we're trying to learn about what the universe is made of on the very largest scales.
因此，我们试图在大尺度上了解宇宙的构成。
It turns out that the tiniest galaxies in the universe provide some of the best clues. So why is that?
事实证明，宇宙中最微小的星系提供了一些好的线索。为什么呢？
Here are two example simulated universes with two different kinds of dark matter.
这是两个带有两种不同暗物质的宇宙模型。
Both of these pictures are showing you a region around a galaxy like the Milky Way.
这两个图片都展示了像银河那样的星系周围的区域。
And you can see that there's a lot of other material around it, little small clumps.
你会发现周围还有很多其他物质，那些小的亮点。
Now, in the image on the right, dark matter particles are moving slower than they are in the one on the left.
在右侧的图像中，暗物质粒子的移动速度比左侧的慢。
If those dark matter particles are moving really fast,
如果这些暗物质粒子的移动速度非常快，
then the gravity in small clumps is not strong enough to slow those fast particles down.
则小区域中的重力不足以使它们减速。
And they keep going. They never collapse into these small clumps.
它们会继续前进，不会撞向这些小点。
So you end up with fewer of them than in the universe on the right.
因此，最终左侧宇宙中的暗物质含量少于右侧的宇宙。
If you don't have those small clumps, then you get fewer small galaxies.
如果没有这些小的亮点，那么得到的小星系就会更少。
If you look up at the southern sky, you can actually see two of these small galaxies,
如果我们仰望南方的天空，实际上可以看到两个这样的小星系，
the largest of the small galaxies that are orbiting our Milky Way, the Large Magellanic Cloud and the Small Magellanic Cloud.
它们环绕着银河系，分别是大麦哲伦星云和小麦哲伦星云。
In the last several years, we have detected a whole bunch more even smaller galaxies.
在过去的几年中，我们发现了更多甚至更小的星系。
This is an example of one of them that we detected with the same dark energy survey that we used to make maps of the universe.
这是我们利用制作宇宙图所用的暗能量检测发现的很多非常小的星系中的一部分。
These really small galaxies, some of them are extremely small.
其中一些星系非常小。
Some of them have as few as a few hundred stars, compared to the few hundred billion stars in our Milky Way.
有的只有几百颗恒星，而我们的银河系有几千亿颗。
So that makes them really hard to find.
因此，真的很难找到它们。
But in the last decade, we've actually found a whole bunch more of these.
但是在过去的十年中，我们实际上发现了更多的小星系。
We now know of 60 of these tiny galaxies that are orbiting our own Milky Way.
现在，我们知道有60个小星系，正在绕着我们的银河系旋转。
And these little guys are a big clue to dark matter.
这些小家伙是暗物质的重要线索。
Because just the existence of these galaxies tells us that dark matter can't be moving very fast,
因为仅这些星系的存在就告诉我们：暗物质不可能很快移动，
and not much can be happening when it runs into normal matter.
当碰到正常物质时不会发生太多改变。
In the next several years, we're going to make much more precise maps of the sky.
在接下来的几年中，我们将制作更加精确的巡天图。
And those will help refine our movies of the whole universe and the entire galaxy.
这些将有助于完善我们对整个宇宙和整个银河系的理解。
Physicists are also making new, more sensitive experiments
物理学家们还在做新的更灵敏的实验，
to try to catch some sign of dark matter in their laboratories.
试图在他们的实验室中发现暗物质的迹象。
Dark matter is still a huge mystery. But it's a really exciting time to be working on it.
暗物质仍然是一个巨大的谜，但研究它的过程同样让人非常激动。
We have really clear evidence it exists. From the scale of the smallest galaxies to the scale of the whole universe.
我们有明确的证据表明它存在。从最小的星系到整个宇宙。
Will we actually find it and figure out what it is? I have no idea.
我们会真正找到它并弄清楚它是什么吗？我不知道。
But it's going to be a lot of fun to find out.
但是找到它的过程会很有趣。
We have a lot of possibilities for discovery,
我们有很多发现的可能性，
and we definitely will learn more about what it is doing and about what it isn't.
我们肯定会了解到更多有关它的行为以及它不是什么的知识。
Regardless of whether we find that particle anytime soon,
不管我们什么时候发现该粒子，
I hope I have convinced you that this mystery is actually really close to home.
我都希望我已经使各位相信，这个谜团的答案实际上离我们很近。
The search for dark matter may just be the key to a whole new understanding of physics and our place in the universe. Thank you.
对暗物质的研究很可能是我打开对物理学的全新理解，并探究我们在宇宙中位置的钥匙。谢谢。