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Looking up at the night sky,

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we are amazed by how it seems to go on forever.

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But what will the sky look like

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billions of years from now?

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A particular type of scientist,

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called a cosmologist,

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spends her time thinking about that very question.

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The end of the universe is intimately linked

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to what the universe contains.

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Over 100 years ago,

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Einstein developed the Theory of General Relativity,

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formed of equations that help us

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understand the relationship

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between what a universe is made of

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and its shape.

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It turns out that the universe

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could be curved like a ball or sphere.

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We call this positively curved or closed.

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Or it could be shaped like a saddle.

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We call this negatively curved or open.

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Or it could be flat.

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And that shape determines

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how the universe will live and die.

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We now know that the universe is very close to flat.

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However, the components of the universe

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can still affect its eventual fate.

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We can predict how the universe

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will change with time

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if we measure the amounts or energy densities

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of the various components in the universe today.

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So, what is the universe made of?

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The universe contains all the things that we can see,

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like stars, gas, and planets.

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We call these things ordinary or baryonic matter.

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Even though we see them all around us,

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the total energy density of these components

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is actually very small,

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around 5% of the total energy of the universe.

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So, now let's talk about what the other 95% is.

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Just under 27% of the rest

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of the energy density of the universe

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is made up of what we call dark matter.

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Dark matter is only very weakly interacting with light,

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which means it doesn't shine or reflect light

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in the way that stars and planets do,

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but, in every other way,

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it behaves like ordinary matter --

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it attracts things gravitationally.

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In fact, the only way we can detect this dark matter

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is through this gravitational interaction,

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how things orbit around it

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and how it bends light

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as it curves the space around it.

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We have yet to discover a dark matter particle,

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but scientists all over the world are searching

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for this elusive particle or particles

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and the effects of dark matter on the universe.

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But this still doesn't add up to 100%.

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The remaining 68%

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of the energy density of the universe

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is made up of dark energy,

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which is even more mysterious than dark matter.

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This dark energy doesn't behave

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like any other substance we know at all

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and acts more like anti-gravity force.

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We say that it has a gravitational pressure,

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which ordinary matter and dark matter do not.

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Instead of pulling the universe together,

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as we would expect gravity to do,

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the universe appears to be expanding apart

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at an ever-increasing rate.

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The leading idea for dark energy

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is that it is a cosmological constant.

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That means it has the strange property

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that it expands as the volume of space increases

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to keep its energy density constant.

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So, as the universe expands

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as it is doing right now,

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there will be more and more dark energy.

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Dark matter and baryonic matter,

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on the other hand,

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don't expand with the universe

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and become more diluted.

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Because of this property

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of the cosmological constant,

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the future universe will be more and more dominated

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by dark energy,

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becoming colder and colder

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and expanding faster and faster.

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Eventually, the universe will run out of gas

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to form stars,

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and the stars themselves will run out of fuel

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and burn out,

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leaving the universe with only black holes in it.

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Given enough time,

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even these black holes will evaporate,

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leaving a universe that is completely cold and empty.

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That is what we call the heat death of the universe.

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While it might sound depressing

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living in a universe

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that will end its lifetime cold

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and devoid of life,

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the end fate of our universe

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actually has a beautiful symmetry

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to its hot, fiery beginning.

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We call the accelerating end state

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of the universe a de Sitter phase,

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named after the Dutch mathematician

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Willem de Sitter.

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However, we also believe

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that the universe had another phase

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of de Sitter expansion

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in the earliest times of its life.

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We call this early period inflation,

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where, shortly after the Big Bang,

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the universe expanded extremely fast

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for a brief period.

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So, the universe will end

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in much the same state as it began,

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accelerating.

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We live at an extraordinary time

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in the life of the universe

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where we can start to understand

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the universe's journey

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and view a history

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that plays itself out on the sky

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for all of us to see.