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Walking through the timeline of the universe with Katie has been quite the journey for me.
It's hard to believe that we started at the very beginning with the Big Bang and now, just 10 episodes later, we've finally arrived at the future of the universe.
And while we don't know exactly what that looks like, at least not with unwavering certainty we have a pretty good idea of what type of things to expect.
Some that fill me with dread, others with hope.
So, Dr. Mack, we have reached the present moment.
We have.
We've had a little bit of time to reflect on some of the questions that have come up in my mind and in the minds of our listeners as a result of learning about the history of the universe.
But we're not finished.
No, no.
There is so much more.
Because we know something about the future.
Yeah, yeah.
I mean, obviously there's limits to that, right?
Like, we can't know for sure what's going to happen in the future in any sense.
But we do have ways to extrapolate, based on what we know of how the universe has evolved so far and, more directly, how
You know, we see stars and planets out in the cosmos changing and how we see galaxies changing.
And so we can, we can apply some of that to our local environment and we can figure out kind of what's going to happen next for us and for the galaxy and for the universe as a whole.
And, you know, again, there's uncertainty in all that.
There are things that are going to be random or unpredictable.
But you know, we have a pretty good understanding of just the basics of like how stars work and how solar system dynamics works and how matter evolves over time in an expanding universe.
So there's a lot that we really do know.
So what does that mean, like, For just to just to begin very locally.
What does that mean for our little planet?
Well, OK, so it depends on on what we're talking about in terms of timescale.
There are some things that we can say about the future of our planet in the sort of human scale, near term, just based on what we see in terms of trends like how we're affecting our environment and how that might change in the future.
And you know, if we don't change how we're dealing with fossil fuels, what's going to happen to the global temperature, all of that kind of stuff.
There are things that we can predict in the sort of short term based on our observations of our environment.
Now right,
But there are also things that we can kind of anticipate for the planet as a whole.
And some of these are in the category of like random things that might happen at various times that we can't fully say, like that's going to happen now or that's definitely going to occur.
And then there are things that are like this is just how a planetary system evolves.
So there are some things that we know will eventually happen.
Exactly.
Exactly.
Yeah.
Just based on like how the sun is burning through its fuel and how that's going to affect the Earth.
There are a lot of things that we don't know for sure about.
So like there's stuff that happens to planets in kind of random ways, like asteroid impacts, stuff like that,
Like we're probably going to be hit by another big asteroid or comet at some point in, you know, the next decade.
I don't know tens of thousands of years or millions of years.
Something's going to hit us because things hit planets right.
And we know that there have been a lot of impacts in the past.
Some of them have been very large, and we know that there will be some in the future.
Whether or not we will have technology to deflect impacts in the future, I don't know.
There's a lot of cool research being done, a lot of cool demonstrations being done of that can alter the movements of space rocks.
It's really cool.
Like the DART mission recently and stuff like that.
So there are ways that we might be able to mitigate some of these possibilities, but stuff like that will happen.
There will be space rocks that will come for us at some point.
But we don't know the when or how of that.
It's not something we can currently predict, and it's hard to imagine us being able to predict it With much accuracy, although we should be able to see them coming right.
Yeah, so we're doing pretty well with surveys right now of the space rocks out there.
We need more attention, more funding for a lot of that.
But there are a lot of surveys now under the general category of planetary protection or planetary defense, it's sometimes called, where we can catalog and survey all of the kind of near-Earth objects.
And at the moment there's nothing that we think is like a major threat that's very likely to.
There are a few things where we're like you know, in several hundred years there's a chance that that'll hit us.
And we're going to until we get better data about the orbit, we're not sure.
So there are a few objects that we're monitoring that are kind of near Earth objects that are potentially hazardous.
They're called potentially hazardous objects.
It's a good name.
Yeah, yeah.
But there's nothing where we're like, that's definitely going to hit us, right?
And sometimes we do get some warning.
There have been a few impacts in the last I don't know couple of decades where there has been a little bit of warning for like a very small impact.
And astronomers knew about it like a few hours ahead of time and then it hit in the atmosphere and then maybe like a bit of rock, hit the Earth, but it was nothing major.
But there have been other things that we didn't see coming, like the one that hit over Russia yesterday.
Okay.
So in the short run I shouldn't be particularly worried, but in the long run I should be worried to the extent that it's like a near guarantee.
Yeah.
I mean, phrasing it in terms of worry, I'm not sure.
But in terms of, like, is the imminent impact of a near-Earth object likely?
No.
Is the eventual impact of an object likely?
Extremely?
Yes.
Right.
Right.
So it depends on kind of the timescale in terms of when you think this is going to happen.
But then there's that wild card of like maybe we'll learn how to get rid of these things pretty well and maybe we'll survive long enough to figure a lot of that stuff out.
So to use an analogy, I'm probably not going to die tomorrow, but I very likely will die.
Yes.
Yes, exactly.
Got it.
Exactly.
OK.
So there's stuff like that.
And then there's also like random stuff like how the sun is going to fluctuate.
Like there's solar flares.
We had just recently a big solar storm.
There were a lot of cool aurora all over the continental United States in a way that is very uncommon.
Right.
That was a big coronal mass ejection kind of phenomenon.
Well, I don't know if it was a coronal mass ejection.
It was a solar flare.
But there are events called coronal mass ejections, where there's like a really big burst of material that comes from the sun.
And some of those can be damaging.
There was one in 1859 that knocked out a bunch of telegraphs called the Carrington event.
It was a really big solar storm.
And if something like that happened—
You know, in the modern day, it would disrupt communication and cause big problems on Earth.
So stuff like that will happen randomly.
You know, and then there are other random things that could happen that we can't really predict.
Like there could be supernovae that go off as the sun is kind of, you know, wandering around the galaxy and getting closer to different planets.
Clumps of stars.
That's in really long timeframes.
We don't know of any stars that could go supernova in a way that could harm us anytime soon, where soon is like astronomically soon.
So like a million years?
I'm not sure if a million years, but at least like thousands of years.
We don't think anything is likely.
Great.
Great.
That's super good to hear.
Yeah, yeah.
But eventually, as things kind of mix around in the galaxy, we might get close to something that could go supernova or gamma ray burst and that could affect the Earth.
And then there are, you know, a bunch of sort of geological things that can happen, like glaciation events or axial tilt changes or weird plate tectonics.
So, like, complicated things are going to change on the Earth.
Like, the layout of the continents is not going to stay as it is now.
Like, we know that it was different in the past.
It'll be different in the future.
The magnetic field is not going to stay as it is now.
We know that the magnetic field fluctuates and changes.
It reverses sometimes.
So, stuff like that will change.
So You know, oftentimes when we think about the future on a kind of celestial sense, we think about like the Earth as it is, and then a bunch of things change in the sky.
But it's really, it's both, right?
The Earth is going to change in interesting ways, in complicated ways and in some ways that we can't predict.
So I was really worried about this episode, but so far it's been super encouraging.
It seems like we've got it all figured out.
We're going to be here forever.
Like the continents will change.
Maybe the magnetic field will change, but we are going to be fine forever.
Um, I mean, I mean, I'm still, I'm still on fairly short timescales, cosmologically speaking, right now.
Like I'm I'm at like millions of years still.
Well, I mean, just the fact that we have a chance at millions of years is encouraging.
We've only had 250,000, 300,000 so far.
So that could put us still in the first quarter of humanity.
Yeah, yeah.
Which would be amazing.
But you're about to tell me that it's not going to be forever, I suspect.
I mean, none of the things I've talked about so far are guaranteed to kill us on timescales of millions of years, right?
So that's good, right?
Like some things could massively harm the Earth, like comet impacts and things like that, or glaciation events.
Like that could get bad, although we've had glaciation events during human history, right?
Like it's not the end of the world necessarily.
I will get to the end of the world.
There will be one.
There will be one.
Yeah, yeah.
But there are a lot of these kind of like interesting changes that would massively affect civilization if they happened at a short timescale.
But you know, won't necessarily, you know, destroy everything.
And then there are a lot of cool things, too, with like the Earth-Moon dynamic.
So I don't know if you're aware, but like the moon is moving away from us, right?
Yeah.
Yes, I'm very aware.
I've been thinking about how we could slow that down, how we could sort of rope the moon to us so that we can just stay right where we are forever, because I don't like it when things change.
Well, but also like when the moon gets too far away, we won't get total solar eclipses anymore, which will be such a bummer.
Oh, that would be a bummer.
Because it'll get farther away.
It'll be, you know, smaller in appearance on the sky and it won't be able to cover up the sun completely anymore.
It'll only be annular eclipses at some point, which is really sad.
Wow.
That is sad.
Because solar eclipses are like one of the coolest things in the existence, right?
Yeah, I got to experience totality recently.
Isn't it amazing?
It was about the closest I've had to a full blown Mysterium Tremendum religious experience.
Yeah.
Yeah.
In a couple decades.
It was incredible.
No, same.
I mean, I saw one in 2017 and like.
I was so shaken.
I mean, I knew what was happening.
I knew that it was just the motions and the apparent size.
But I had to sit down on the ground.
It was really emotionally affecting in a way that I did not really expect.
My seven-year-old nephew, about one minute into totality...
Everyone was completely silent and just in a state of absolute awe.
And then my nephew shouted suddenly, amazement.
Perfect.
Very appropriate.
Yeah.
Yeah.
Yeah.
So we're going to lose that at some point.
I mean, the moon is currently moving away at like...
I think it's 3.78 centimeters per year, something like that.
So it's, you know, it's moving away more than an inch a year and it's going to keep going.
And part of that dynamic though, like part of what's making it move away, is also what's making the Earth's rotation slow down on average.
So there's this sort of tidal force connection thing between the Earth and the moon.
The dynamics of that are sort of dissipating the angular momentum in such a way that, like the moon is moving away and the Earth is slowing down.
And Essentially, you can think about it as we're sort of evolving toward a tidally locked system.
So, you know, the moon is tidally locked to us.
We always see the same face of the moon, right?
Right.
We've never seen the dark side of the moon.
The far side, yeah.
The lunar far side is always facing away from us.
Right.
But the moon doesn't always see the same face of the earth.
But there are systems of sort of closely rotating binary things like Pluto and Charon, where both objects always see the same side of each other.
So Pluto and Charon are fully tidally locked.
They only see the same side of each other.
They're always facing together.
And our system is kind of evolving toward being like that.
We're not going to get there, which I'll explain.
But the Earth is slowing down.
The moon is moving out.
Those are connected things.
And so the Earth is getting slower.
The length of the day is increasing.
Not very fast.
It's like...
18 milliseconds per century on average, although it's really complicated exactly how that length is changing.
And I could go into a really long discussion about all the things that affect the length of the day and how sometimes it speeds up and sometimes it slows down.
Even climate change seems to be having some impact on the length of the day, because the rotation of the Earth depends on just so many things.
It depends partially on how the atmosphere is doing, but also internal dynamics of the Earth and the plate tectonics.
And then there's the tidal forces from the moon and just super complicated, super interesting.
There's things called leap seconds that get thrown in sometimes to try to correct for this.
It's cool stuff, but we do not have time to get into it.
But anyway, we know that the rotation of the Earth is slowing down.
And one of the ways we know this, which is just so cool, is that we have records going back centuries, like dozens of centuries.
I think I read 27 centuries of records.
And you'd wonder like how do you know exactly how fast the Earth was rotating across that kind of timeframe?
Like, right, what kind of records would you use?
You use records of eclipses.
Because there are really good records.
People write it down when the moon is eclipsing the sun.
To the minute.
Yeah, to the second.
That's a really notable event.
And so we know, based on the solar system dynamics, based on the motion of the earth and the moon, which are more regular, exactly when the eclipses should have happened.
And then we know from human records...
Ancient Babylonian Chinese Islamic, like all of these ancient records, where exactly people were on Earth, which part of the Earth was directly underneath the eclipse.
And so it's this really cool collaboration between these, like ancient astronomers and, you know, current solar system dynamics researchers.
It's very, very cool.
Wow.
That is beautiful.
Yeah, it's really lovely.
Yeah, so we can see exactly how much the length of the day has been increasing.
The Earth has been slowing down in its rotation.
That's something that's going to continue on average in sort of chaotic ways.
But on average the Earth is going to slow down some more and the day is going to get longer over time.
You know we also have reason like to.
We can extrapolate that it was like 19 hours long, like a billion and a half years ago, something like that.
So like the rotation, like that's the kind of timescale.
So it really changes, but it just changes on such a large timescale.
Right, exactly.
Exactly.
Planetary defense, potentially hazardous objects, coronal mass ejections all phrases that can feel a little bit science fiction but are in fact quite real.
And what's also real is the constant of change.
Things can feel so stable to us.
The position of the continents, our distance from the moon, the length of a day.
All of those things though, are actually in flux on such long time scales that humans can't perceive them.
But I'm grateful for what we can perceive like total solar eclipses, which will eventually no longer occur due to shifting tidal forces, and astronomers' records from centuries ago that help us understand Earth's rotation today.
I mean, that's enough to cause some amazement.
Okay, so the days are getting longer.
The Earth's rotation is slowing down.
The moon is getting further away from us.
And that's obviously not going to matter on the scale of centuries or even millennia, but it will eventually shape Earth life and Earth experience.
Yeah, yeah.
It'll change Earth experience.
But the thing that's going to be most important for the experience of life on Earth is going to be the way the sun is changing right.
Right.
So our sun is a kind of ordinary star, and so it goes through ordinary stellar evolution.
It's currently about a middle-aged, sort of smallish star, okay?
Just like me.
Right.
Halfway home.
Right, right.
Yeah.
So it's currently fusing hydrogen into helium in its core.
We call that being on the main sequence.
And that has to do with, like the way that we used to put stars on diagrams or sometimes or still put stars on diagrams in astronomy.
But anyway, we call it a main sequence star.
It's just in like the middle of its life.
It's doing its sort of standard midlife, ordinary star things.
Yeah, yeah.
And stars all do this.
They fuse hydrogen into helium in their cores as part of their life cycle, right?
And so that's continuing.
And if our star were significantly more massive, then it would burn through all of its hydrogen and then it would start burning helium and higher elements inside in its core and in layers and go through a different process.
But our star being.
You know the mass that it is when it gets through its hydrogen.
That's kind of
That's kind of it.
It can't really do a lot of other nuclear burning.
There will be some things that happen with helium eventually, but that kind of defines its lifespan is when it gets through that hydrogen burning.
So it won't have a lot of life beyond the hydrogen burning phase.
And do we know about how much hydrogen it has?
Yeah, we have really good stellar models.
So we can really well predict the life cycle of the sun and the future of the sun and, to some degree, how it has evolved in the past.
And so we can say in some number of billions of years how much brighter it'll get.
We can talk about how it'll change in sort of structure.
Basically what happens is, As it's burning through that hydrogen, it's going to gradually get brighter and it's going to sort of puff out.
And so it'll evolve toward what's called a red giant.
So it'll get brighter and bigger and sort of puffier and redder.
And the redder is because actually the surface of it will be cooler, but it'll be overall brighter.
And so the effect on things around it will be worse, right?
It'll feel warmer on Earth.
Yeah.
And so it's going to go through that process.
So it takes a while, but we're about halfway through the sun's life.
And then so the sun is about 5 billion years old.
So you might think that we've got a good number of billions of years where things are okay.
But as it's burning through the hydrogen, as it's getting brighter, it doesn't have to get very much brighter for things to get real bad on Earth.
Okay.
Okay.
So in about a billion years, the sun will be something like 10% brighter, somewhere around there.
That doesn't seem like that much to me.
It doesn't, but it turns out that that's really actually catastrophic.
That's going to be a big problem.
Just 10%.
Yeah, so this 10 brighter, that's enough to make this sort of enhanced greenhouse effect on the Earth, and it's actually enough to start boiling the oceans.
You wouldn't think that it would be that bad, but it actually is that bad.
10% more light from the sun, boiling the oceans.
Boiling the way that I boil a pot of water with all the steam coming off of it?
Yeah, the surface temperature of the Earth will be exceeding 100 degrees Celsius.
The oceans will begin to boil.
That's in about a billion years.
So life on Earth that relies on livable surface temperatures and liquid water.
On the surface we've got something like a billion years.
Well, I would think something less than a billion years.
Well, yeah, less than a billion years.
Before the oceans boil, it would be a real problem for me personally.
Yeah.
I mean, once the oceans become like kind of a hot soup, you're in a problem.
But in a billion years, your point is that it won't just be bad for me.
It'll be bad for cockroaches and crabs and all kinds of things that have been very resilient.
Yeah.
Yeah.
And when you read about this stuff, it's like, when does all life on Earth totally end?
Because like there are places on Earth that are deeply inhospitable in various ways.
And we often find life there.
Anyway, you know, like these hydrothermal vents and you know these weird things, acidic places, all of that.
But it seems like at some point, maybe in about four billion years, it'll get hot enough that we'll get this kind of runaway greenhouse like Venus.
And then that's even worse in terms of, in terms of sort of the temperatures, because Venus is massively inhospitable.
You cannot live on Venus.
It's hot enough to melt lead.
Like it's just it's real bad there.
So if we start to look like that, that's that's not good.
And correct me if I'm wrong here, but it seems like the cool forms of life, by which I don't mean like physically cool, I mean like interesting.
And I don't want to judge, but I do think that you know, Tuatara are a little more interesting than bacteria.
Right, right, yeah.
And I think we're a little more interesting than Tuatara.
Not to create a hierarchy.
I mean.
Okay, well, maybe not.
I think we are.
Some Tuatara are fairly cool, but yeah, yeah, sure.
Yeah, no, they're amazing.
They've been around, you know, a thousand times longer than we have.
Obviously, they can do things that we can't do.
I would just submit that they don't know what's keeping the stars apart.
Right.
That is true.
So in terms of the really interesting life that becomes, I would imagine, unless there's some really significant evolutionary shift and maybe with that much pressure there would be that becomes really challenging at a billion-year lifespan.
Yeah.
Yeah, absolutely.
You don't want to be on the earth in a billion years, right?
But a billion years is a long time.
Even a million years is a long time.
It's not inconceivable that humanity could be living somewhere else by then, right?
You can be optimistic, right?
We can figure maybe we'll go to other parts of the solar system.
Now, one thing that people sometimes bring up is is that as the sun gets brighter, the habitable zone of the solar system is going to move right.
So the habitable zone is like the range of distance from the sun where liquid water can exist on the surface of a planet.
Technically, right now we have like three planets in this habitable zone, Venus, Earth, and Mars.
And as the sun gets a little brighter, like Mars will get a little warmer, right?
And sometimes people are like, oh, we can move to Mars when the sun gets too bright for the Earth.
And that's not really true because yeah, Mars will get a little bit warmer, but it still won't have any atmospheres.
Like, or not very much atmosphere.
It still won't have enough pressure for liquid water to exist on the surface.
Like there are parts of Mars at various times a year that are above freezing, but there's no liquid water because the pressure is so low that the ice just sublimates right.
So unless you can actually terraform Mars, like seriously change the atmosphere of Mars, you're not going to move there, even when it's, like you know, 70 degrees Fahrenheit outside right.
It's not a good backup.
But in principle, humanity could move to other places when the Earth becomes inhospitable.
So let's imagine that a billion years from now, I mean first off right, if we made it a billion years, that would be astonishing.
It would, yes, absolutely.
Because very few forms of life so far have survived for a billion years.
Yeah, yeah.
And nothing as complex as us.
And so that in and of itself would be a big win, I think.
Yeah, for sure.
For sure.
That would put us, instead of being in the first quarter of human history, that would put us in like the first 1 of human history, which you know.
Right now I don't feel like we're in the first 1 of human history, but of course I don't.
Right now, it feels to me like 2024, 2025 has some fourth quarter vibes, but that's just vibe based.
That's not reality.
It's important to note.
But we could theoretically be on Europa or we could terraform Mars.
I mean in the time span of many millions of years, which is orders of magnitude longer, or an order of magnitude longer than we've been here so far.
Yeah.
We could figure out a lot of stuff that we haven't figured out yet.
Yeah.
Yeah.
We could be in lots of places in the solar system.
But even then, even then, we're still going to have an issue.
Yeah, yeah.
Because the sun is going to keep getting brighter, right?
And it's going to keep expanding.
And, like you know, I'm skeptical about the idea of terraforming but like yeah, you know, maybe there could be habitats or something like that.
But the sun is going to become a problem.
Like in about maybe 7 billion years, it'll be fully in its red giant phase.
And at that point, it's going to start doing things like... like eating the inner planets.
Like when it gets that big, it's going to engulf Mercury and Venus almost certainly.
There's possibilities that like those could be like sort of kicked around or something, but pretty likely it's going to eat Mercury and Venus, like just fully engulf them.
Like it will literally suck them in and they'll just be gone?
Like they will fall into the red giant sun, yes, and be obliterated.
Will there be any record of them?
Any way to know that they were Yeah.
So let me tell you this, just a slight detour here.
A few years back I was at a sort of morning coffee discussion thing back at NC State when I was a professor there.
And we have these morning coffee things where we sit and we talk about the papers that have come out.
You know, over the last couple of days,
And there was a paper that we were discussing that was about a polluted white dwarf.
That Okay, so just skipping ahead.
After our sun becomes red giant, it'll become a white dwarf.
I'll talk more about what that is, but it's a sort of compact core of a star after it goes through the red giant phase.
So this paper was about this polluted white dwarf.
So by polluted in this case they meant that they looked at the spectrum of the light coming from the white dwarf and they saw signatures of heavy elements, of like silicon and, like you know, like iron or something like rocky kind of elements that make up things like rock, right?
And they saw these signatures, these heavy elements.
And you wouldn't expect to see that in a white dwarf.
Yeah, those would not be in the core of a star, right?
Those would not be in the white dwarf.
You would not expect to see that level of contamination in the spectrum of a white dwarf.
And so the implication was that this was a star that ate some planets, right?
Wow.
And so we saw that contamination in that white dwarf spectrum because that star ate some planets.
And they ate rocky planets, right?
Like it ate presumably terrestrial type planets, right?
And I was sitting there at this coffee thing and I was like, that star ate its planets.
Like, what if there were people on those planets?
And the only record we have now is...
Of those planets is that the white dwarf is polluted.
Yeah.
Yeah.
There's a little gunk in the spectrum of this white dwarf that we're seeing from, like you know, hundreds of light years away.
Right.
So eventually Mercury and Venus will be pollution in the sun's white dwarf.
Yeah.
And probably more planets, too.
You mean like ours?
Like ours.
Yeah.
Yeah.
So after it eats Mercury and Venus, chances are the Earth will fall in too.
It's not certain because of the kind of changes in the gravity as things are moving around.
It's possible that we'll get kicked out of the solar system, but more likely, I think, that we'll fall in.
Anyway.
So when I was at this coffee thing I was thinking like someday that's going to be us right.
Yeah.
Yeah.
We're going to be like a little bit of gunk in the spectrum of a white dwarf seen by some other astronomers hundreds or thousands of light years away.
That's what's going to be left of human existence is this polluted white dwarf spectrum.
And they're going to write a paper about it and everybody's just going to shrug it off and nobody's going to have an existential crisis at coffee.
And I mean, it was...
I had, I don't know.
I had a moment like nobody else at the coffee break seemed to be, you know, sort of emotionally affected by this, but I was like the star ate its planets.
Yeah, that's, I mean, I'm emotionally affected by Dr. Mac, if that makes you feel any better.
I am.
Yeah.
I just had to swallow really hard.
Anyway, so after a while, the sun will kind of lose its outer atmosphere.
So after it's a red giant after about maybe eight billion years, the way that that works is it kind of sloughs off its outer atmosphere.
So it kind of blows out a lot of the outer atmosphere.
And then what's left is this dense core, this white dwarf.
So the core of the sun will be this very dense sort of carbon and oxygen filled body.
And that's called a white dwarf.
And it'll be Yeah, it'll be about the size of the Earth, but with most of the mass of the sun in there.
So it's a very dense object, a white dwarf.
So the sun will become the size of the Earth, but with more density.
Mm-hmm.
And having eaten us, probably.
Having eaten Mercury and Venus, almost certainly.
Maybe having eaten Mars.
And all of that will be inside of an Earth-sized object that's just much, much denser than Earth.
Yeah, yeah.
It'll be somewhere around the size of the Earth.
But that's kind of the scaling, where you know there are different kinds of what we call compact objects in astronomy.
So there are objects that are sort of a different kind of matter.
So white dwarfs are one of them.
They're a compact object where you get something about the mass of the sun and something about the size of the Earth.
If the sun were much, much bigger, more massive, then it could collapse into what's called a neutron star, which is where you, you have a different kind of very compact material, and that is about the about the mass of the sun in something about the size of a city.
So really yeah, so there are different scales for this weird kind of matter.
It's called degenerate matter, but the the white dwarf kind is is the kind where you take something about the mass of the sun and you compress it to about the size of the earth.
A white dwarf is called degenerate matter.
Yeah, electron degenerate matter.
Yeah.
So wait, so you're telling me that in the distant future, Earth will be pollution inside of degenerate matter.
Yeah, yeah.
I mean, I have to say I like the name.
Yeah.
So all that we ever were or did will be pollution inside of degenerate matter.
I'm sure there are worse fates.
I just can't imagine any of them.
At least there will be for a long while anyway, some remnant of us.
And more importantly, we don't need to matter to the universe in order to matter to each other.
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So to recap, our sun is currently in the middle of its lifespan.
As it continues to burn through its hydrogen, it will become brighter, puffier, and redder in color.
And while this isn't happening rapidly, small changes in the brightness of the sun could drastically affect humanity.
In a billion years time, the sun will have become 10 brighter, which It may not sound that bad, but it's actually very bad, like boiling oceans bad.
In terms of the Sun, we have less than a billion years of habitability on Earth remaining.
This isn't to say we won't have the technology to move humanity elsewhere by then, although Mars is very likely out of the question.
To survive we'll have to go somewhere far from home, because in seven billion years time the Sun will be fully in its red giant phase and start eating the inner planets, possibly including Earth.
The only record of its entire existence would be some heavy metal pollution inside a white dwarf.
And while that is a little upsetting, the Sun isn't the only threat to humanity.
We'll need to consider
I'll let Katie explain.
All right.
So now we're 8 billion years out.
The sun is degenerate matter.
The earth is no more.
Or maybe it is, in which case it's been ejected out of the solar system, which might, by the way, be even worse.
Yeah.
Well, it'd be very cold.
Yeah.
I might rather be pollution inside of degenerate matter than be a rogue planet just hurtling through the universe waiting to hit something.
Yeah, but okay, so we should make a note however, that at this stage, this is where things get really very pretty.
Okay.
Okay, because when the sun sloughs off those outer layers and becomes a white dwarf, those outer layers turn into something really beautiful.
It's called a planetary nebula.
It has nothing to do with planets, but a planetary nebula is...
Something where, after a star like our sun goes through the red giant phase, loses its atmosphere.
That atmosphere creates this beautiful nebula.
That's like glowing in amazing colors.
And so we have.
If you look up planetary nebula online, you can see images of these beautiful like bubbles of light popping around, these sort of dying white dwarf stars.
And there are lots of different shapes.
There's some that have like sort of bulbous shapes or kind of like starburst kind of looking things.
But generally speaking you get kind of a roundish like haze of glowing stuff around this.
You know sort of slowly cooling white dwarf star in the center.
It's a very cool, it's a very beautiful thing.
So the remnants of our solar system will be beautiful eventually.
Is there any chance that we or some descendant of us will be able to observe that beauty in any way?
I mean, if we do leave, you know, if we get enough technology to like, leave the solar system, then yeah, we could look back and see it.
Sure.
But I think the thing I want to emphasize is that like, Even though we want to imagine – and like I'm doing this as we're talking,
Like I'm thinking about this in an extremely human-centric way, where it's like how bad is this for us?
How bad is that for us?
How bad is this next step for us?
And each step is pretty bad, if we even get to these steps, which itself is phenomenally unlikely, I know.
But even then –
There is still yet more problems to come.
The galaxy itself has problems.
Even if we make it to a star, that's say, four light years away, which is incomprehensible today.
Even if we could somehow do that, there's still problems.
Because there's a fundamental thing about everything we've observed in the universe, which is that it ends.
Yeah.
And even before we get to like the universe ending, you know, beyond our solar system being destroyed, like we have another problem coming, which is the Andromeda galaxy.
So the Andromeda galaxy is two and a half million light years away right now.
And I mentioned before.
You know, as the universe is expanding, we see that other galaxies are moving away from us right,
We see that their light is red shifted, meaning that it's being stretched out by the expansion of the universe.
It's looking redder.
And as things move away from us, they look redder.
And that's the case for most of the galaxies in the universe that we can see.
But there are a handful whose light is blue shifted, which means the light is being compressed as they come closer to us.
They're getting closer to us.
So the universe is expanding, but these galaxies are getting closer to us because of gravity.
Because of gravity, yeah.
So on sort of smaller scales, on scales of you know sort of millions of light years, there can be gravitational binding.
So we, our galaxy, is part of what's called the local group, which is a group of galaxies in our sort of, you know, local area of the universe, and it includes the Andromeda galaxy, and these are galaxies that are gravitationally bound.
We're all kind of bound together, like just because we formed in similar places, like we formed close together in the early universe, and so the Andromeda galaxy is bound to us and it's moving toward us, it's coming, it's coming for us at about 110 kilometers a second.
That feels like a lot.
It's a lot faster than the moon is moving away from us.
Exactly.
Yeah, yeah.
I mean, it's far away now.
It's two and a half million light years away, but it's going to get here.
And it'll take probably something like four billion years, maybe a little less, to get here.
Okay.
And so, you know, all this stuff is happening in the sun.
The timescale on that is like five to eight billion years, right?
But the Andromeda galaxy will kind of get here first.
Yeah.
And when the Andromeda galaxy collides with our galaxy, like in galaxy collisions, it's really unlikely for stars themselves to hit each other just because there's so much empty space.
Like the stars are generally just going to miss.
They're going to pass right by because even in a galaxy collision like, there's still a lot of space between stars.
So you're not going to have stellar collisions, but you will have like just a massive mess of gravity, right?
Because, like all this stuff's going to come in, it's going to get like thrown around, churned around, like there's going to be tidal forces.
You know, when we see galaxy collisions out in the universe, we see these like long streamers of stars as collisions happen and it sort of stretched these stellar streams out.
And we see in our own galaxy streams of stars from when our galaxy has eaten littler ones, right?
So when Andromeda, which is sort of a similar mass to the Milky Way, maybe about 10 times more massive, when it comes and hits us, it's going to be this really major merger event where it's going to sort of mess up everything.
So some things are going to get flung out of the galaxies.
And our sun might be something that gets flung out of the galaxy in this process.
We don't know.
We can't calculate it that carefully.
But some of the stars are just going to get flung out.
Some are going to get, you know, kind of mixed up in the center of the new forming galaxy as these things come together.
And then a bunch of things are going to happen when those galaxies collide.
So each galaxy has some gas in it.
And that gas, when it collides, can form bursts of star formation.
Now, by that time in, you know, 4 billion years...
Most of the gas in both the Milky Way and Andromeda will be used up from just the regular process of star formation.
But there will probably be some bursts of new star formation.
So you're going to have stars being born.
You're going to have supernovae.
Each of our galaxies has a supermassive black hole in the center.
So ours is about 4 million times as massive as the sun.
Andromeda's is about, I think it's about 100 million times as massive as the sun.
And so those black holes will merge.
And when that happens, that could also eject a bunch of stars or tear apart stars, right?
Because when you have a bunch of supermassive black hole, things happening like that can have stars come in and get torn up and engulfed by the black holes.
So there's going to be a lot of, like you know, chaos.
That's going to happen when Andromeda hits us.
And then eventually, over like seven billion years or something, the two galaxies will settle into sort of an elliptical galaxy.
So just a kind of big, big elliptical mess of stars.
But there will be multiple billions of years where things are pretty chaotic in our neck of the woods.
Yeah, yeah, exactly, exactly.
And where stars are being ejected or, you know, when the black holes collide, it could create these jets of radiation that come out of that, from the material that gets kind of caught up in that.
So we could be a little bit of a quasar for a bit.
I don't know.
Oh, wow.
So stuff like that could happen.
We don't really know for sure.
And if the sun gets ejected from the galaxy?
If a star gets ejected from a galaxy, what does that mean for the star and its solar system?
I mean it's probably not good for the solar system just because, like the sort of gravitational forces could probably mess up all those orbits a lot.
But yeah, I mean, you just have you know, depending on whether the planets get ripped off or not, it would just be out there, not in a galaxy.
And we know that there are stars that are outside of galaxies, right?
Okay.
Yeah.
That's not terribly uncommon that not every star is like nicely embedded in a galaxy.
There are some that are just kind of out there.
Some of them have been ejected from galaxies.
I guess it's mostly things that have been ejected from galaxies, but you could have stars forming kind of out in the field that are, you know, in little clumps of matter that happen to be not in the biggest clumps of matter.
You can have stars that are not inside galaxies and it doesn't matter really to the star whether it's inside a galaxy or not.
In terms of like, The star is kind of its own little contained system.
Depending on where in a galaxy you are.
You might be more dangerous inside a galaxy, because if you're in a place where there's a lot of supernovae going off, then that could be quite dangerous.
But then again, like...
You need a certain number of supernovae to go off in the vicinity before your star forms in order for it to have enough heavy elements to make planets.
And so if something's really isolated, it probably doesn't have enough metals to make planets and stuff.
But if it's ejected after it's already formed the planets and stuff, then it could just be out there.
If it keeps its planets which I worry probably wouldn't be able to do that
But in principle, it could just be out there.
It doesn't really need to be inside a galaxy.
So, while the Sun is working its way toward becoming a red giant, the Andromeda Galaxy is going to come in and shake things up.
It will collide with the Milky Way in four billion years and this collision will create a big mess in terms of gravity.
Some of our stars will be moved, others will be ejected from our galaxy entirely, black holes will merge and eventually, after much chaos, Andromeda and the Milky Way will settle into an elliptical galaxy.
But now that the Earth is either pollution in degenerate matter or a rogue planet hurtling through space, what's next?
So, on a timescale of say, five or six billion years, we've got a sun problem, but we've also got this galaxy collision problem which, from a macro perspective, might be a bigger issue, because it's not like you could just move to a younger sun.
Right.
I mean, I guess, I don't know.
It's possible that our sun could get caught up in one of these stellar streams and it would just be kind of a gentle movement into another part of this larger chaotic galaxy.
I mean, it wouldn't necessarily be a big deal.
It depends on where our sun ends up in that whole collision, right?
If it ends up near the center, then that's a big problem.
If it ends up near the edges...
Maybe it's just, instead of being at the edge of a spiral galaxy, now it's at the edge of an elliptical galaxy.
It's not necessarily worse than the sun getting bright.
I think the sun getting bright, we're sure that's going to happen.
What's going to happen with Andromeda, we don't know for certain.
Right.
Okay.
So at any rate, we're a white dwarf.
We're pollution inside of a white dwarf.
How long does that white dwarf hang around?
Right.
Oh, white dwarfs can live a really long time.
They can live billions and billions of years.
Okay.
So what happens with a white dwarf is it just cools, right?
So it's this very dense matter.
It's got this heat from when it had all of that.
You know nuclear fusion happening from the compressed matter in there.
And then it just cools over billions and billions and billions of years.
I don't even know how long.
Like, eventually, it evolves into what's called a black dwarf.
That's the theory behind it.
Just because it cools down.
But I don't think that anything has ever lived that long.
I don't think the universe is old enough to have black dwarfs in it.
I'd have to check, you know, exactly.
Oh, so theoretically, we will become a black dwarf at some point.
But like, that'll be so distant from now that like, we haven't even seen it yet.
Yeah.
There are no known black dwarfs to my knowledge.
Okay.
So a white dwarf can continue to sort of vaguely glow as a white dwarf for tens or hundreds of billions of years.
And our universe is 13.8 billion years old.
Wow.
Yeah.
Yeah.
So that's going to be like this just kind of slow fade of the sun.
And then you know, as that's happening, and after the Andromeda collision, people call it Milkdromeda.
This is the new galaxy that's...
Cool.
Like mixing together two celebrities' names when they hook up?
Yeah, yeah.
Oh, great.
I love that.
Milkdromeda.
That's good.
That's pretty good.
It's not as good as degenerate matter, but it's good.
Right, right.
Yeah.
So Milkdromeda will be hanging out and the sun will be this cooling white dwarf.
There will be some new star formation when the collision occurs, but not a whole lot, because there won't be a whole lot of gas left.
And then, you know, the star formation is going to kind of slow down, right?
And the stars that are already formed.
Some of them are going to go supernova and make neutron stars.
Some of them are going to go supernova and make black holes.
A lot of them are just going to turn into white dwarfs and fade.
And so over billions and billions and billions of years, the Milky Way, or Milkdromeda, will just get dimmer right.
It'll get redder and it'll get dimmer.
You'll have more of these white dwarfs.
The really, really low-mass stars, the red dwarfs, will last the longest.
And so the stars that are shining will be most of these very low-mass red stars.
And the stars are going to kind of just fade over time, right?
I don't know if I ever told you this statistic, but I think maybe we talked about this when we talked about cosmic noon, where we can calculate the amount of star formation at different times in the history of the universe.
And based on like how much gas there is in the galaxies and stuff like that, and how much galaxies are colliding and all of that.
And we can calculate that.
You know there was a peak of star formation back, you know, several billion years ago.
And since then the amount of star formation has been huge, dropping off, and we can calculate, like based on extrapolating into the future, how much star formation that will be in the future.
We can say that of all the stars that ever have been or ever will be, about 90 or 95 percent have already been born.
Whoa yeah, so so from now until we're almost done yeah yeah, from now until the end of time we're we're working on the last five or ten percent Oh,
Yeah.
I just got to take that in for a second.
Yeah.
Yeah.
I mean, you know when you talk about like, are we in the you know, final quarter or whatever in terms of the formation of stuff in the universe?
We're in the last.
The last few minutes, right.
Like.
Yeah.
Last five or 10%.
And that's just a matter of like, you know, the use of the gas, right?
The formation of stars from gathering gas and gravitational collapse.
Now I will say on the hopeful side of things, at least in soccer, a lot can happen after the 85th minute.
That's true.
You know it sometimes is the most exciting part, but that is astonishing to me that over 90 of the stars that have ever or will ever lived Mm-hmm.
Have already happened.
Have already been born.
Yeah.
Yeah.
Yeah.
Wow.
Yeah.
Wow.
Yeah.
I was blown away when I first saw that statistic.
But, yeah, that's... So we're not at the beginning.
Mm-mm.
No.
No.
I mean, there's a lot of time left in the universe.
I mean, I think next time we're going to talk about the end of the universe, but there's a lot of time left before you know you can say like the universe is over.
But there's not a lot of stuff happening in the future really in terms of like how much has happened so far?
Right,
At least if you talk about like things like formation of structure.
But before we get to the end of the universe and before we get to the end of the episode, I just want to get into one more thing, one more sort of milestone in this future evolution.
So, you know, we know the universe is expanding.
Galaxies are, on average, getting farther apart, you know, notwithstanding some of them coming close together, like Andromeda and us, because they were already born together.
But the universe is expanding and that expansion is speeding up.
Yeah.
And if we extrapolate that into the future in a way that's reasonably straightforward, then there will be a point when everything outside of our galaxy, outside of Milftromeda, is moving away from us so quickly that we won't be able to see it anymore.
There will be stars in the sky but we won't be able to see.
Like if we put a JWST out in space, we wouldn't be able to see any galaxies.
Exactly, exactly.
So in about 100 billion years we could put up a space telescope, we could point it out into the universe and we will see nothing.
And we would never know, we would have no way of knowing that there are other galaxies.
Yeah, exactly.
And this is the part that's more unsettling to me than just the fact that we won't be able to see stuff.
I mean.
So we won't be able to see stuff, partially because it'll be far away, but mostly because it'll be stretched.
The light will be stretched out so much that it'll be sort of lost in the sort of static of the universe, right?
It'll be too red to see.
But also the same thing will happen to the cosmic microwave background.
Like the cosmic microwave, background will be stretched out to the extent that we will not be able to see that.
We won't be able to glimpse the Big Bang anymore.
So we won't be able to see the Big Bang or other galaxies.
We won't have any evidence that the universe is expanding.
Right.
Because it will have expanded so fast.
And we won't have any evidence of the beginning of the universe.
We'll have no information about the early universe.
We'll be in this dying elliptical galaxy that seems entirely isolated in this big empty cosmos.
And we will think, based on what we can observe, that the universe is that galaxy.
Yeah, yeah.
There will be no way to know that there's anything past that galaxy.
And so all we will ever be able to know. is the history of that galaxy.
Right.
Exactly.
Exactly.
Yeah.
And, you know, maybe there will be records, like written records or something, or maybe there will be some way to make inferences about the past that we haven't thought of yet.
But essentially you know, when you get to that sort of 100 billion years-ish mark, we'll have no information about the wider universe or our own past.
That makes me wonder...
About all the things that we can't know.
All the things that happened on the other side of whatever horizon we can't see past.
Yeah, yeah.
I mean, if we think about, like, you know, we talked about inflation, cosmic inflation.
If that really happened, like...
That really wiped out information from before that.
For all practical purposes we don't have information from before inflation, because it sort of expanded the universe so much that our own little observable universe became very isolated in the sense that we can't see beyond that.
Yeah, so there's information we can't get from... you know, before that stage in our universe.
But we can see.
I mean that's, we're talking about a tiny tiny, tiny fraction of a second after whatever the beginning was.
We can see a lot right now.
Like, we have a lot of information about the beginning now.
But yeah, we'll never see beyond our cosmic horizon.
We'll never see, you know, beyond the, you know, the epoch of inflation, if that's what happened.
And in 100 billion years they won't have information about the history of the universe at all.
They won't have any way to know about what we know.
Which is kind of wild, because you sort of imagine that just information, you know, knowledge increases right.
Right.
That's exactly what I was thinking.
Yeah.
I mean you imagine a world where we, a universe where we accumulate information and accumulate knowledge and do a better and better job of passing it down.
And then there will come a time where actually we can't Pass it down.
Unless there are written records somehow.
But like, I mean, most likely, you know, where we observe less.
Right.
Like, we might have records, but we won't have any evidence, any observational evidence.
Yeah.
They'll have to believe us that we could see the beginning of the universe.
Yeah.
And it'll be this like ancient text, right?
And it'll be a matter of faith at some level, right?
Yeah.
Yeah.
I mean, I don't know.
It's not certain.
There are possibilities for what might be happening with dark energy that could change that story to some degree.
If dark energy changes in some way within the next 100 billion years, it could change what we can or can't see beyond the galaxy.
But based on how things are looking right now, it does look like we will lose that information, which is kind of wild.
Yeah.
It's sort of heartbreaking, but it's also really beautiful because...
We're in a special moment.
Yeah, it makes me feel really lucky that we have all this information right now.
You know that we can learn about the history of the universe.
We can make inferences about the future that you know and living right now on this planet, where you know the sun is not boiling the oceans.
And that's great, right?
And Andromeda has not flung us into a supermassive black hole.
Like, these are good things.
We've got a good time going right now.
Yeah, and of course, we could only be here if we had a good time going.
Yeah, there's an anthropic argument there, right?
Yeah, of course.
But it's also worth remembering that we've got a good time going in the scheme of things and that we have a reasonably long potential good time still to come.
Right, right.
Not on astronomical scales, maybe, but on human scales.
No, but on human scales, yeah.
Longer than we have existed on the Earth, right?
Like.
We can be reasonably assured that the Earth can be existed upon for longer than we've been here already, assuming that we take care of it and pay attention to what's coming from the sky and all of that.
But we have an opportunity to carry on in this wonderful place for a long, long time.
So eventually the sun will fade, as will Milkdromeda.
In the grand scheme of things, we're at the tail end of the star formation in our universe.
And in a hundred billion years, whatever's left of humanity will have no way of knowing much of what we know now.
How the universe started, that there are other galaxies, that we once had records of such things.
What might we not know from the time before that theoretical cosmic inflation, when the universe expanded exponentially?
That question is, of course, impossible to answer.
But we're lucky to be living in a time when we can know other things and when there is still a lot of good to come.
Okay, so now that we know what our future looks like, it's time to discuss the stage of this that concerns me the most the end of everything, astrophysically speaking.
Next time.
In our final episode, Katie will explain the ways this all might end, and I'll be prepared to cope with that conversation and all that it entails.
This show is hosted by me, John Green, and Dr. Katie Mack.
This episode was produced by Hannah West, edited by Linus Obenhaus, with music and mix by Joseph Tuna-Medish.
Special thanks to the Perimeter Institute for Theoretical Physics.
Our associate script editor is Annie Fillenworth.
Our editorial directors are Dr. Darcy Shapiro and Megan Modafferi.
And our executive producers are Heather DiDiego and Seth Radley.
This show is a production of Complexly.
If you want to help keep Crash Course free for everyone forever, you can join our community on Patreon at patreon.com slash crashcourse.
