Compared to human experience, the Universe
has been around a long time: nearly 14 billion years. That’s a soul-crushingly long time. That’s
older than Earth, older than the Sun, older than, well, everything. Pretty much by definition. And yet, when you think about time itself,
how long is 14 billion years, really? It seems like a long time to us, now. But time keeps
on slippin’ into the future, and every day the Universe is a little bit older. I mean
sure, duh, of course it is. But have you ever really thought about what
that means? Someday the Universe will be 20 billion years old. 30. Then 50 billion, a
hundred billion, a trillion… and even then the clock still ticks. Those numbers sound
abstract, but those days will inevitably come. Time runs long. What will the Universe look
like in that far, far future? Different. It’ll look different. We’re about to go on a long journey. Literally the
longest possible. We’re going to the end of the universe. First, a bit of a content warning: The stuff
I’m going to show you today is incredibly interesting, incredibly cool… and, well…maybe
a wee bit distressing to some people. But I want to say that it’s not all darkness
and dissolution. Well, to be honest, it’s mostly darkness
and dissolution, but there is some light, almost literally, at the end of the long,
long tunnel. I’m going to have to use some pretty big
numbers in this episode, and by big, I mean BIG. Probably bigger than any numbers you’ve
come across before! I’ll have to use scientific notation, which is a shorthand way of expressing
numbers using powers of ten. For example, the number 100 is equal to 10
x 10, so we say it’s “ten squared,” or “ten to the power of two.” 1000 is
10 x 10 x 10, so it’s ten cubed, ten to the third power. This might seems silly for smaller numbers,
but it gets useful for much larger ones. A million is 10 to the 6th, a billion is 10
to the 9th, and so on. Mind you, every time the power, the exponent, goes up by one, the
actual number is ten times bigger. So 10 to the 18th isn’t twice as big as 10 to the
9th, it’s a billion times bigger! At different times in the life of the Universe,
different objects dominate. For example, in the current Universe, NOW, you can make the
case that stars are the dominant objects; they produce the most amount of energy. Before
stars, something else was. Dispersed plasma, I suppose, generally speaking. Astronomers Fred Adams and Greg Laughlin looked
at what will happen to the Universe on very long time scales, and divided it into five
broad epochs. Then wrote about it in their book, The Five Ages of the Universe, which
is a pretty good read. Their divisions aren’t official in any way, but I think they did
a good job given what we know, so let’s use them. First came the Primordial Era, which we already
went over in the last episode. It’s basically everything from the moment the Universe big
banged up to when the first stars formed, about 400 million years later. The second era is the one we live in now,
where stars rule the night. This is the Stelliferous Era. We’re about 13.4 billion years into
it. But how long will it last? Or to phrase it
another way, when will the last star die? The lower the mass of a star the longer it
will “live,” steadily fusing hydrogen into helium at a very slow rate. Models indicate
the lowest mass red dwarf can do this for about a trillion years. That is a long time. A red dwarf that formed
when the Universe was young would only now have used up about 1% of its hydrogen, and
has about 99% of its life ahead of it. In human terms, they’re still infants. Right now, galaxies are merrily churning away,
converting nebular gas into stars. But eventually that gas will run out. Estimates vary, but
star formation in most galaxies will start to peter out in a few billion years. Fewer
stars will be born, and the ones already born will start to die off. Galaxy collisions and
other events may trigger star formation after that, so maybe that’ll extend things a bit.
But even if it lasts another 50 billion years, or a hundred, it hardly matters. When the longest star sticks around for a
trillion years, why quibble about a few billion here and there? As this happens, galaxies will change color
and fade. Most spiral galaxies have disks that are a vibrant blue in color, the massive
hot luminous stars dominating their emission. But as these stars die and the fainter, lower
mass stars take over, the galaxy will redden and dim. A few billion years after the nebular
gas runs out the only stars left will be those long-lived red dwarfs. In a trillion years, this is what awaits us.
It may take longer; I’ve seen some calculations that show the very lowest mass stars MIGHT
last ten times that long. Whatever. I’m not going to worry too much over factors of
ten here or there. As the Universe ages, those are essentially statistical fluctuations. But when that does happen, when the last star
dies, it means that the only objects left in the Universe generating appreciable energy
will be the corpses of stars: white dwarfs, neutron stars, and black holes. And, to be
fair, brown dwarfs as well, those objects intermediate in mass between stars and planets. With the exception of black holes, these objects
are supported by various forms of degeneracy pressure. That means, in a few trillion years,
it will be the end of the Stelliferous Era, and the start of the Degenerate Era. The Universe will be dark. Dark to human eyes,
at least, assuming we have eyes in a trillion years, or that we’re even around. In the
infrared things are a bit brighter; many of these objects will be glowing at those wavelengths
since they’ll be warm. Well, lukewarm. Neutron stars and white dwarfs
are born very hot, and fade over time. How long that takes depends on how massive they
are and other factors, but it’s safe to assume that they’ll have cooled to room
temperature at best in a few trillion years. And the deeper we get into the Degenerate
Era, the cooler they’ll be. They’ll have their moments though, because
over trillions of years, binary white dwarf orbits will decay, and the stars will merge
and explode as supernovae. Binary neutron stars will merge and form gamma-ray bursts
too, explosions so bright they’ll outshine a thousand galaxies in this far-flung future. Briefly. These are short-lived events, and
soon thereafter the Universe will return to darkness. Interestingly, brown dwarfs are a better bet
as an energy source. Binary brown dwarfs could merge to form a relatively normal if low-mass
star that could shine for hundreds of billions of years. But again, time is long. After a
quadrillion years, this too shall pass. Stars just ain’t what they used to be. In fact, in the Degenerate Era, neither is
the Universe. In our dark energy episode, I mentioned that
as the Universe expands, our view of it will shrink. At the same time, all the galaxies
in the Local Group will collide and merge, forming one big elliptical galaxy. By deep
into the Degenerate Era, all we’ll be able to see is our own bloated, dark galaxy; the
rest of the Universe will be forever cut off from us. Not that there’ll be all that much to see
anyway. And I hate to say this, but even THAT won’t
last. Our models of how subatomic particles behave
predicts that over very long periods of time, protons — the positively charged particles
in the nuclei of atoms — will decay. The half-life of such an event is brain-hurtingly
long, at least 1034 years, and it’s almost certainly longer. But the Degenerate Era is longer yet. As protons
decay, one by one, matter itself will disintegrate. White dwarfs, neutron stars, brown dwarfs,
planets: All of them will dissolve as their constituent protons decay. There’s an upside,
kinda. Adam and Laughlin calculate that proton decay, which releases a tiny bit of energy,
will cause white dwarfs to radiate heat with about the energy of 400 Watts. Mind you, the
microwave oven in my kitchen generates more power, and it’s a lot smaller than a white
dwarf. But comparatively, in 1038 years, this’ll look like a bright star does to us now. By 1040 or so years from now, even degenerate
stars will be gone. The only big objects that will be left are black holes. Thus begins the Black Hole Era. Black holes don’t generate a lot of energy
unless they happen to suck down a large amount of material that can be torn apart and turned
into an accretion disk. Otherwise, basically they just sit there. However, there is a way black holes can make
energy: They evaporate. I KNOW. Most people think that black holes
can only eat stuff, and once it’s in there, that’s where it is forever. Oh, but that’s a pesky word: “forever.”
When you’re talking 1050, 1060, 1090 years, what does “forever” mean? Back in the 1970s, physicist Stephen Hawking
worked out the math of black holes combined with quantum mechanics, and discovered that
under some circumstances they can emit particles. This is a very weak effect, and has extraordinarily
complicated physical mechanisms behind it — check the dooblydoo for links that explain
it. But the end result is that black holes can very slowly leak mass, and the more massive
they are, the slower the leak. How slow? A black hole with three times the
Sun’s mass, the minimum size for one created in a supernova, will take about 1068 years
to evaporate. That’s a ridiculously long time… but the Universe can wait. Even supermassive black holes in galactic
centers evaporate. It takes them — get this — 10^92 years. 10 to the 92nd power. That number is so huge,
so colossally ridiculous, that I can’t even come up with an analogy for it. It’s a 1
followed by 92 zeroes. There aren’t even that many subatomic particles in the entire
Universe. See what I mean? Ridiculous. That’s the
length of time we’re talking here. But it’ll happen. Eventually. As black holes
lose mass they emit particles and energy faster, so each time one evaporates completely it’ll
emit a flare of light like a small bomb going off. During the Black Hole Era, that’ll
be the only source of energy in the Universe. Eventually, they’ll all go away. And…
that’ll be it. There won’t be anything else in the Universe except subatomic particles
and photons, and they’ll all be so low energy they might as well not exist. That’ll happen
pretty much by 1092 or 93 years from now. At this point, the Universe, it’s safe to
say, is dead. Kaput. Done. We have entered the ominously named Dark Era. It’ll stretch
to infinity, if time even has any meaning by then. That’s awful. I mean, seriously, writing
about this and talking about it is hard because it’s not fun to think about this stuff. I mean, it IS,
kinda, but when you internalize it it’s bleak. There IS another idea that would prevent this
potentially eternal darkness from happening, but you won’t like it much. Dark energy is pumping up the Universal expansion,
causing it to accelerate. We don’t know much about dark energy; we don’t even know
what it is. We do know that the cosmic horizon, how far out we can see into the Universe,
is shrinking as the expansion accelerates. One idea (among many) about dark energy is
that its influence will get stronger and stronger. As it does, the horizon will shrink ever faster,
closing in on us more and more rapidly. But that’s not some illusion, it’s a physical
limit to the Universe, a stretching of spacetime. No force — not gravity or electromagnetism
or anything — can cross it. It will be as if that part of the Universe across the horizon
is ripped away from us, stretched beyond breaking. Over time, according to this hypothesis, the
cosmic horizon will eventually shrink until it’s smaller than our galaxy, smaller than
the nearest stars, smaller than the solar system, smaller than our planet… and it’ll
keep shrinking until it’s smaller than a subatomic particle! When that happens it’ll
be as if the Universe is torn apart at the most fundamental quantum level. Astronomers call this the Big Rip. We have
no idea if it will actually happen or not, but if it does it’ll be many billions of
years from now, long after the Sun dies but long before the Dark Era. I’m not particularly comforted by this idea,
but the good news is that this idea isn’t really much more than conjecture. As we learn
more about dark energy we’ll learn more about its eventual influence on the fate of
the cosmos. Yay? So. Is there any reprieve? Maybe. It’s possible, though by no means
proven, that our Universe is one of many Universes. A multiverse, if you will. Those other Universes
may be ticking along just fine long after ours has wound down. Of course, we can’t
get to them, but still. Good on them. And there’s another idea. It’s a little
far-fetched, but not completely outside the realm of physics as we know it. We think of the vacuum of space as being devoid
of energy, empty. But there’s an idea in quantum mechanics that this might be an illusion.
There might yet be a lower energy state we don’t see. Think of it like a staircase.
You’ve been standing on what you think is the bottom step, but then you find out you’re
actually one step up from that. Our Universe may be narrowly balanced on this
second-to-lowest step. It’ll stay there, but if something bumps it, down it goes. It’s possible that somewhere, out in the
dark, dead Universe, for whatever reason, after a gazillion years, some small bit of
space will quantumly jostled, and drop down to that next lower state, the true vacuum.
What happens when it does? It’ll bump the regions around it, and they’ll
drop. And so on, and so on. Here’s the weird part — well, it’s all weird, but here’s
the really weird part: Inside this region, the laws of physics get rewritten. How? We
don’t know. As far as we know, we can’t know what happens in there. But in a sense
it will erase space and time inside it. Like, poof. Gone. Everything changes, at some
fundamental level we can’t even understand. This wave of destruction expands at the speed
of light, engulfing all of what remains of the Universe. What is left behind in its wake
is... something new. Something different. We literally can’t know. This idea actually gives me hope. Think of
it as a cosmic reboot. The Universe has led a long, long life, and lingered an unimaginably
slow death. This gives it a new lease, a chance to start over again. Maybe this is similar
to how our Universe came into being in the first place, as a quantum fluctuation in some
other Universe, budding off from there to create everything we know. Maybe this has happened before and will happen
again. Over and again, an infinite number of times. I have to say, I like this idea.
If it’s true — and we don’t know, it’s just conjecture at this point — but if it
IS true, then it’s not the death of our one Universe. It’s the opportunity for the
birth of an infinite number more. And that is perhaps the single most hopeful
thing I know. Today you learned that our Universe’s days
are numbered. Stars will die out after a few trillion years, protons will decay and matter
will dissolve after a thousand trillion trillion trillion years, black holes will evaporate
after 1092 years, and then all will be dark. But there’s hope that a new Universe will
be born from it. There’s always hope. Crash Course Astronomy is produced in association
with PBS Digital Studios. Head over there before the end of the universe to watch even
more awesome videos. This episode was written by me, Phil Plait. The script was edited by
Blake de Pastino, and our consultant is Dr. Michelle Thaller. It was directed by Nicholas
Jenkins, edited by Nicole Sweeney, the sound designer is Michael Aranda, and the graphics
team is Thought Café.
I love Cosmology but sometimes, if caught in the right/wrong mindframe, the issue of time gives me anxiety.
This is one of the first astronomy videos in a while to really hit me hard. When I saw that "1092 - ∞" time period for the final stage of the universe's existance, I audibly sighed.
Nice reference to this song @ 0:25.
Does the false vacuum theory have any particularly strong evidence to support it? I tried researching it on the Internet but couldn't find anything apart from conjecture.