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Hank gives us an inclusive overview of how everything in the universe is thought to have begun, and how cosmologists predict it will all come to an end. Now get happy!

Hosted by: Hank Green
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You know all this is gonna end, right? Like, you, and your guinea pig, and the bacteria living in your bathtub, and the Real Housewives franchise -- all those things are gonna end, and of course, you know that, but also, big, permanent-seeming things are gonna end, like the earth, and the sun, and this galaxy, and that galaxy over there, and the universe. The end's not gonna happen anytime soon, but give it some time and even the universe will change into something different, something we wouldn't even recognize, and then -- PFTTFFTTT. Gone. How exactly that's gonna happen is, of course, up for some serious debate because what we don't know about the universe could fill, like, a universe.

But astrophysicists, cosmologists, and particle physicists are pretty smart people, and they think about this all the time, so at this point, what do they think is gonna happen to earth and to the solar system and the universe in the far distant future?

It's good stuff to know, you know, just in case you happen to run out of other stuff to worry about.

(Theme music plays)

 The History of the Universe

The universe is about thirteen point seven billion years old, give or take about one hundred and thirty million years, but it hasn't always been the way it is now. It's believed that it was born with the big bang as an unfathomably hot, dense point called the singularity. The big bang didn't occur as an explosion like you would think, even though it's called "The Big Bang", and the universe didn't expand into space because space didn't exist before the universe was formed. Instead, think of the big bang as the simultaneous appearance of space everywhere in the universe, you know, if you could picture that. I, for one, cannot, but the people who study this stuff say that the universe hasn't expanded from any one spot since the big bang, but that space itself has been stretching and carrying matter with it. 

So, the universe started at a single, hot, dense point, smaller than an electron or something at the ripe old age of a hundredth of a billionth of a trillionth of a trillionth of a second. This point began to expand faster than the speed of light, in a process called inflation. It's thought that in the same instant the universe doubled in size, at least ninety times, growing from the size of a subatomic particle to the size of a golf ball -- Tada! Whole universe. From there, it continued to grow a little more slowly, and as it expanded, it cooled, and matter began to form.

For instance, just one second after the big bang, the fundamental building blocks of matter, called quarks, start to assemble into subatomic particles, like protons and neutrons. And for the next three hundred and eighty thousand years or so, as the universe continued to cool and become less dense, these particles began to structure themselves in a way that we would recognize. They started colliding to form atoms -- protons and neutrons inside a nucleus, tied up together with a tiny orbit of electrons. Here, some of the first, lightest elements, like Helium, Lithium, and of course, Hydrogen, began to appear. At this point, the universe transformed from a dense fog of particles into something transparent, but it also went completely black since no stars had yet formed.

So the period between three hundred and eighty thousand years to about four hundred million years after the big bang is what we call "The Cosmic Dark Ages", since there weren't any big objects emitting visible light yet. But for the next half billion years after that, clumps of gas began to collapse enough to form the first stars and galaxies. The expansion of the universe gradually slowed as gravity tried to pull matter back in on itself, but then, about three point five billion years after the big bang, something weird started to happen.

A force, stronger than gravity, began to overpower this pull, and accelerate the expansion of the universe. This force, called "dark energy", is a leading candidate for the tidal of least understood thing in physics. All we really know is that it's a repulsive force that causes the universe to expand at an ever-increasing rate.

 The Predicted Future of the Universe

So at this point, you probably get why the fate of the universe is so tough to predict. Our information is profoundly incomplete. We keep finding out about stuff, like dark energy, and we're like, "oh, okay, that's gonna take a minute to process." Anyway, a little south of nine billion years after the big bang, our solar system was born and then our little adorable planet showed up. And here we are. But what's gonna happen to us?

Because everything ends. I mean, the second law of thermodynamics tells us that as time passes, entropy, that is, disorder in general, increases on all levels. Stars, including our sun, exhaust their supplies of fuel. Planets come untethered from their stars and galaxies break apart. We're not a hundred percent sure about anything regarding the true nature of the universe, so the end could happen in a different way. In fact, a lot of really smart people think it will happen differently. But one thing pretty much everyone agrees on: there's almost certainly going to be an end.

But here on earth, we've got some things besides cold, black nothing to worry about. Even if the end of everything does happen, you won't be around for it. Neither will I, and neither will our great, great, great, great, great, great, great, great, great, great, great, grand kids. They'll be past caring one way or another. Because on earth, things are always changing. It's what makes life possible. But it can also be kind of unconducive to immortality, if you know what I mean.

In the next one hundred thousand years, Niagara Falls will erode away into Lake Eerie, the moon will slow down the rotation of the earth enough that we'll have to add a leap second to our clocks every day, and if global warming doesn't tamper with it, we're due for another ice age. Oh, and the movement of the stars will render most of the constellations we see from earth unrecognizable, which will make astrologers presumably really confused.

In the next one million years, our planet is due for a good ole fashioned catastrophic extinction event. These have happened periodically over the past four billion years of earth's history. It could be the eruption of a super volcano, which have been frequent enough to change the game in terms of the history of life on earth. Think of an explosion so major that sulfuric acid mixed with volcanic ash blocked the sun all over the world for like, ten years, and sends the planet into a volcanic winter. It's happened before, lots of times, we even survived one before, although barely. Some scientists think that the eruption of the Toba Super Volcano in Indonesia seventy thousand years ago left only about one thousand breeding pairs of Homo sapiens to repopulate the earth. And did they ever.

Now, sometime within the next hundred million years, we're likely to be hit by a meteorite, similar to the one that possibly killed off the dinosaurs, because things in space are not so far apart that they don't run into each other occasionally. And big space rocks have collided with the earth for as long as there's been an earth. In fact, that's probably what caused most of the major extinction events so far. Of course, some people think that we could destroy one of those space rocks before it punches us in the face, but who knows where humans will be, if anywhere, a hundred million years from now.

Two hundred and fifty million years from when you watch this, we will make a full circuit around the milky way, in which time, all the continents on earth may have fused into a single super-continent.

And now, we've gotta start thinking about the sun getting bigger, because that's what's gonna happen next. In six hundred million years, the sun will begin to enter the red giant phase -- basically a star's midlife crisis. A red giant has fused all its hydrogen into helium, causing it to heat up and expand to more than one hundred times its original size. It'll take a few billion years for the sun to actually swallow the earth, but its growing luminosity will probably cause enough climatic changes on earth, like all the oceans evaporating, for instance, that in one point three billion years, all plants and animals will have gone extinct. And the only living things will be bacteria and Archaea hanging out in little pockets of liquid water at the earth's poles.

In one point six billion years, even those extremophiles will be having a tough time of it. In about three point five billion years, the surface of the earth will resemble that of Venus, and the earth's orbit will widen with the growing, yet less massive sun, until about seven point nine billion years from now, when the sun will swallow up Mercury, Venus, and Earth.

Some researchers have suggested that if humankind has figured out how to survive that long, maybe we can lasso a passing asteroid and get it towing to wider orbit, or, or we could move on to one of Saturn's moons, like Titan, because at that point, it might be a perfect place for us.  Seven point nine billion years, though, we are so not gonna be around -- but, I love the optimism.

Anyway, eight billion years from now, the sun will become a white dwarf, which, by that point, will be about half its present mass, and by fourteen point four billion years from now, it will be cool enough to be invisible to human eyes. It's been a good sun, though. You better appreciate while still keeping you warm. 

 The End of the Universe

After that, the solar system is pretty much dead. Now, it's kind of hard to say what will happen to the universe -- every astrophysicist's got ideas, of course, and each idea has a kind of painful nickname: "the big rip", "the big crunch", "the big freeze". Which scenario will win out mostly depends on dark energy, which we don't know diddly about, if you don't remember.

Let's start with "the big rip". Here, the rate of expansion driven by dark energy keeps growing and growing without limit, becoming so extreme that it actually overwhelms all of the fundamental forces of physics, from the gravitation between objects to the strong and weak forces that hold subatomic particles together, in which case, not only will galaxy clusters break apart and star systems dissipate, but every form of matter, right down to molecules and atoms, will be shredded into oblivion. Some physicists say that this could happen as soon as seventeen billion years from now. 

Or another possibility is that, oh, like a hundred billion years from now, the strength of dark energy will diminish as time goes on, and the expansion of the universe will stop accelerating. In this case, if dark energy weakens, gravity might ultimately win the tug of war and pull all the matter of the universe back on itself. That result is referred to as "the big crunch".

But these days, a lot of bets are being placed on "the big freeze". In this scenario, dark energy keeps the universe flying apart until everything gets so far flung that there's simply no thermodynamic energy left. There'll be no motion, no heat, no life, stars will burn out, new stars will stop forming, and the cosmos will dissipate into cold, dark nothingness. This party isn't expected to start for about a hundred trillion years, though, so that's good news.

But these aren't the only ideas, they're just the most popular ones. Some cosmologists think that as entropy in the universe increases, it will configure itself into something so complicated it will become a self-aware entity. So yeah, I'm with the guy who thinks the universe can become a giant brain! That sounds cool.

But that's what's great about this stuff: cosmologists are constantly bickering over questions like "is the universe even real?", "are we real?", "how many universes are there?", "is the universe gonna recur over and over again in an endless cycle of big bangs?", "why can't you unscramble an egg?". Because when you're trying to figure out the nature of the universe from a tiny rock in deep space, there are no stupid questions.


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