Looking up at the night sky, we are amazed by how it seems to go on forever.
仰望夜空，我们为其永无止境的广阔而感到惊讶。
But what will the sky look like billions of years from now?
然而数十亿年前的天空又是什么样子的呢？
A particular type of scientist, called a cosmologist, spends her time thinking about that very question.
有一种科学家，被称为宇宙学家，她们就专门思考这样的问题。
The end of the universe is intimately linked to what the universe contains.
宇宙的尽头与其自身的构成有着密切的联系。
Over 100 years ago, Einstein developed the Theory of General Relativity,
一百多年前，爱因斯坦提出了广义相对论，
formed of equations that help us understand the relationship between what a universe is made of and its shape.
帮助我们以方程的形式来理解宇宙的组成和其形状之间的关系。
It turns out that the universe could be curved like a ball or sphere.
结果表明，宇宙可能是呈球状或半球状的曲线。
We call this positively curved or closed.
我们称之为正弯曲或正关闭。
Or it could be shaped like a saddle.
或者它也可能呈马鞍形。
We call this negatively curved or open.
我们称之为负弯曲或负开放。
Or it could be flat.
又或者它也可能是扁平状。
And that shape determines how the universe will live and die.
这样不同的形状决定着宇宙将会怎样存在和灭亡。
We now know that the universe is very close to flat.
如今我们认为宇宙最接近扁平状。
However, the components of the universe can still affect its eventual fate.
然而组成宇宙的各个部分同时也会影响其最终的命运。
We can predict how the universe will change with time
我们能够预测宇宙如何随着时间而发生改变，
if we measure the amounts or energy densities of the various components in the universe today.
这需要我们对目前组成宇宙的各要素的数量及其能量密度进行测量。
So, what is the universe made of?
那么，宇宙是由什么构成的呢？
The universe contains all the things that we can see, like stars, gas, and planets.
宇宙中包含着我们肉眼所能看到的一切，比如恒星、气体和行星。
We call these things ordinary or baryonic matter.
我们称这些为普通物质或重子物质。
Even though we see them all around us,
尽管它们就在我们的周围，
the total energy density of these components is actually very small, around 5% of the total energy of the universe.
但这些组成部分的总能量密度却十分渺小，它们仅占整个宇宙总能量的5%左右。
So, now let's talk about what the other 95% is.
那么现在我们就来探讨组成宇宙另外95%的部分是什么。
Just under 27% of the rest of the energy density of the universe is made up of what we call dark matter.
其他近27%的宇宙能量密度是由暗物质构成的。
Dark matter is only very weakly interacting with light,
暗物质与光的相互作用很弱，
which means it doesn't shine or reflect light in the way that stars and planets do,
也就是说暗物质本身并不发射或反射光线，这与其他恒星和恒星不同，
but, in every other way, it behaves like ordinary matter -- it attracts things gravitationally.
但从其它方面来讲，暗物质与普通物质是相似的--暗物质通过重力吸引物体。
In fact, the only way we can detect this dark matter is through this gravitational interaction,
事实上，我们唯一能够探测暗物质的方法就是通过暗物质与其它物质的重力相互作用，
how things orbit around it and how it bends light as it curves the space around it.
比如物体如何围绕暗物质运行或暗物质如何扭曲光线使其周围空间呈显曲线状。
We have yet to discover a dark matter particle,
目前我们还没能发现一块暗物质颗粒，
but scientists all over the world are searching for this elusive particle or particles and the effects of dark matter on the universe.
但全世界的科学家们都在探索这个或这些神秘的物质以及它对宇宙所产生的影响。

But this still doesn't add up to 100%.
可是这样的组成结构还是未能达到100%。
The remaining 68% of the energy density of the universe is made up of dark energy,
剩余68%的宇宙能量密度来自于暗能量，
which is even more mysterious than dark matter.
这种物质比暗物质更加神秘。
This dark energy doesn't behave like any other substance we know at all and acts more like anti-gravity force.
这种暗能量的性质与我们所熟知的其他物质完全不同，它更像是反重力。
We say that it has a gravitational pressure, which ordinary matter and dark matter do not.
暗能量具有一种重力压力，这是普通物质和暗物质所不具有的。
Instead of pulling the universe together, as we would expect gravity to do,
宇宙并没有因为重力而向一起聚集，这与我们所预想的不同，
the universe appears to be expanding apart at an ever-increasing rate.
相反宇宙却似乎是在不断膨胀，并且这种膨胀还处于一种持续增长的速度。
The leading idea for dark energy is that it is a cosmological constant.
有关暗能量的主导观点认为它是一个宇宙常量。
That means it has the strange property that it expands as the volume of space increases to keep its energy density constant.
那意味着暗能量具有一种特殊属性，它能够使暗能量随着宇宙体积的增长而扩大，从而使其能量密度保持不变。
So, as the universe expands as it is doing right now, there will be more and more dark energy.
因此，随着宇宙不断膨胀，正如此时此刻，暗能量也会随之不断增长。
Dark matter and baryonic matter, on the other hand, don't expand with the universe and become more diluted.
然而暗物质和重子物质，从另一方面来看，则不会随宇宙而增长，反而他们会不断地减弱。
Because of this property of the cosmological constant,
因此，由于宇宙常量所具有的性质，
the future universe will be more and more dominated by dark energy, becoming colder and colder and expanding faster and faster.
未来的宇宙中更多充斥的将是暗能量，宇宙将会变得越来越冷，其膨胀速度也会越来越快。
Eventually, the universe will run out of gas to form stars,
最终，宇宙将会由于缺少气体而无法形成恒星，
and the stars themselves will run out of fuel and burn out, leaving the universe with only black holes in it.
同时恒星自身也会燃烧殆尽，宇宙中留下的只会是一个个黑洞。
Given enough time, even these black holes will evaporate, leaving a universe that is completely cold and empty.
如果经历足够的时间，即使是这些黑洞也会消失，那时宇宙将会是一片寒冷空虚。
That is what we call the heat death of the universe.
那就是我们所说的宇宙热寂说。
While it might sound depressing living in a universe that will end its lifetime cold and devoid of life,
尽管这听起来让人觉得生活在宇宙之中十分沮丧，因为它将在寒冷和了无生气中走向灭亡，
the end fate of our universe actually has a beautiful symmetry to its hot, fiery beginning.
但宇宙最终冰冷的消亡却完美地与其火热的发端相对称。
We call the accelerating end state of the universe a de Sitter phase, named after the Dutch mathematician Willem de Sitter.
我们称宇宙的加速最终状态为德西特阶段，这是根据荷兰数学家威廉·德西特命名的。
However, we also believe that the universe had another phase of de Sitter expansion in the earliest times of its life.
然而，我们也相信宇宙还存在另一个德西特膨胀阶段，它发生在宇宙形成的初期。
We call this early period inflation, where, shortly after the Big Bang,
我们称其为早期膨胀阶段，那是在大爆炸后，
the universe expanded extremely fast for a brief period.
宇宙急速膨胀的短暂时期。
So, the universe will end in much the same state as it began, accelerating.
因此宇宙将会以与其产生时相同的状态结束，加速。
We live at an extraordinary time in the life of the universe
我们生活在一个对于宇宙生命来讲意义非凡的时代，
where we can start to understand the universe's journey and view a history that plays itself out on the sky for all of us to see.
在这样的时代我们能够去了解宇宙的历程，同时也能够去了解它那呈现在天空中为我们每个人所展示的历史。