We live in an unusual age, the age when the stars still shine.

We should count ourselves lucky.

Nearly all of future history will be dark.

But events will still unfold in that long-cooling blackness, and civilizations may endure.

So how will the universe and its far-future denizens spend eternity?

[THEME MUSIC] In 100 trillion years, the last star in the universe will expand, the final atoms of hydrogen fuel and settle quietly into a dim white dwarf before slowly fading to black as it radiates away its remaining heat.

The Era of Stars will be over.

That 100 trillion years is 10,000 times the current age of the universe.

And so the days of starlight and warmth have a way to go.

But even when they are done, the universe will be young in comparison to the long, dark ages to follow.

In fact, our universe will spend almost all of its infinite time in darkness, slowly crawling towards maximum entropy and ultimate heat death.

But that doesn't mean that stuff won't happen.

There are many fascinating ways in which the universe can still decay to increasingly less interesting states-- or, scientifically, many ways for the universe to cool down, dissipate energy, and gain entropy.

Far-future civilizations may be able to harness those mechanisms and so cling to existence through uncountable eons-- uncountable, but not incalculable.

Life and structure can only exist as long as the universe is not in perfect equilibrium, what we call heat death.

Today, we're going to figure out how long before the universe reaches its final maximum entropy, minimum interesting state, and answer the questions, how long before nothing ever happens again?

And what will happen to the universe as it approaches that moment?

But first, let's recap our mounting doom.

In some previous episodes, we looked at the long series of ends of the world and of the galaxy that are in store for us, from disasters that will almost certainly befall the Earth, to the heating and death of the sun, to the merger with the Andromeda Galaxy, and, finally, to the death of the last stars in the galaxy.

This left us in a sorry state-- the merged Milky Way-Andromeda Galaxy comprised of nothing but stellar remnants, the ultradense neutron stars and black holes from long-extinct massive stars, as well as the white dwarfs left from lower-mass stars, including the recently extinguished red dwarfs.

Those white dwarfs will fade to black in only several billion years, far shorter than the several trillion-year lives of those stars.

And what happens to life in that era?

Civilizations may have persisted or even started from scratch in the Red Dwarf Era.

But they'll have lost all connection with the greater universe before it ends.

Long before the last red dwarf fades out, the accelerating expansion of the universe will have dragged all galaxies beyond the Virgo Supercluster outside of the cosmic event horizon.

That's the boundary of our patch of the universe beyond which no new light can reach us.

The greater universe will fade from view, and even the cosmic microwave background will also dim to undetectability within the era of red dwarfs.

There'll be no evidence of a universe beyond the local galaxy and no evidence that there was ever a Big Bang.

So at this point in the universe's future history, the Age of Stars has passed and no starlight will ever shine again.

The universe contains nothing but cold, dark nuggets of superdense matter.

We have entered the Degenerate Age, not to be confused with the 1970s.

But neutron stars and white-- or, by now, black-- dwarfs are made of degenerate matter.

This is matter that is fully collapsed in a quantum mechanical sense.

It's so densely packed that all possible quantum states are completely filled and no further collapse is possible, short of becoming a black hole.

Believe it or not, many of those black dwarfs will still have planetary systems from their days as regular stars.

It's hard to imagine civilizations persisting on those pitch-black worlds, but it's not impossible.

Yet even if they did, their days are numbered.

The next destructive event is that every planetary system in the galaxy will eventually be disrupted by close encounters between stellar remnants.

As stars randomly pass close to each other, planets are flung into the blackness.

It'll take something like 1,000 trillion years, 10 to the power of 15 years, for, essentially, all planetary systems to be obliterated in this way.

The next calamity to befall the combined Milky Way-Andromeda Galaxy and, in fact, every galaxy will be its complete dissolution.

As dark star remnants rotate through countless galactic orbits, they interact with each other gravitationally.

The remnants, mostly black dwarfs but also the diaspora of frozen planets and similar substellar objects, will be flung out of the galaxy.

Heavier bodies, mostly neutron stars and black holes, sink towards the center.

According to an estimate by Freeman Dyson, 90% to 99% of our galaxy's stars will be scattered into the void in something like 10 to the power of 18 years when the universe is a million times older than the age of the last stars' death.

Around 10 times longer still, and the entire megagalaxy will either have dispersed or fallen into the massive black hole at the galactic center.

What happens after the Age of Galaxies depends on a critical question.

Do protons decay?

That's a cool bit of physics that deserves its own episode.

But in short, protons have the most stable composite particles.

According to the standard model of particle physics, they should last forever.

But there are several beyond the standard-model mechanisms that would allow them to decay into positrons, neutrinos, and gamma ray photons.

This decay has never been detected.

But failed attempts to spot it tell us it must take at least 10 to the power of 34 years for a proton to have a 50% chance of decaying.

Theory says that if protons decay at all, this half-life could be up to 10 to the power of 37 years.

So in something like 10 to the power of 39 or 10 to the power 40 years, all protons in the observable universe will be gone.

That means all of the planets, black dwarfs, space dust, everything.

The universe will contain only photons, electrons, and black holes.

And so we would enter the Black Hole Era.

If protons decay, black holes would be the only mass of bodies left in the universe after 10 to the power 40 years.

Some will be the remnant black holes of individual stars that were flung from galaxies long ago.

But most of the black hole mass will be in supermassive black holes.

These monsters once grew in the cores of the now-forgotten galaxies.

Now they are all that's left of those galaxies.

Some have grown to masses of up to 100 trillion suns, having swallowed good-sized bites from entire galaxy clusters.

But all black holes evaporate over time via Hawking radiation, something we've discussed in detail.

They slowly leak away their mass as a cool heat [INAUDIBLE] of random particles for the most part faint radio light.

The small black holes, say, around 10 times the mass of the sun, completely evaporate in around 10 to the power of 67 years.

The largest supermassive black holes that might ever form will take a little longer, up to 10 to the power of 106, or a million googol, years.

During the long Black Hole Era, there is still energy to be had for an enterprising, super-advanced civilization.

Black holes themselves can be used as engines through Hawking radiation, as we've also discussed previously, but also via other mechanisms that deserve episodes of their own.

If life manages to master this energy source, then its future history could be as ridiculously long as the Black Hole Era.

By the way, dark matter will probably also be long gone by now.

Even though we don't know exactly what it is, dark matter particles will likely either annihilate themselves as they collide with each other or be captured by dense stellar remnants during the Degenerate Age and, ultimately, end up in black holes.

But even black holes must end.

Occasional flashes of gamma rays will light up the darkness as black holes reach that last explosive stage of their evaporation.

And after that, just particles and light, now not even bound gravitationally.

The last stuff in the universe will become more and more diffuse and dim as the accelerating expansion of space continues and the infinitely long progression to absolute heat death is all that remains.

Now, before we finish, we have to take a step back and ask, what if protons don't decay?

In that case, there will be structure in the universe for a very, very long time.

Black holes evaporate.

But smaller stellar remnants, ancient planets, asteroids, et cetera, could persist.

The fate of these depends on quantum mechanisms.

Over infinite time, nothing is truly stable.

By a process called quantum tunneling, everything eventually reaches the lowest possible energy state.

For the remaining matter in the universe, quantum tunneling allows the elements lighter than iron to fuse together, while elements heavier than iron decay.

In the end of this scenario, every atom in the universe must fuse or decay into iron, the most stable element on the periodic table.

Black dwarfs decay into iron stars-- still degenerate and insanely dense, but now perfect balls of iron.

This will take something like 10 to the power of 1,500 years.

But even iron stars can't last forever.

The same process of quantum tunneling eventually transport a star's material toward its center.

Iron stars evolve into neutron stars.

That would take an unthinkable 10 to the power of 10 to the power of 76 years.

But we may not have to wait that long for oblivion.

Quantum tunneling may actually bring on the Black Hole Era much earlier.

It depends on how small black holes can really be.

If small, stable black holes are possible, then quantum tunneling should allow small regions within larger bodies to collapse into black holes, which would then consume the rest of the surrounding body.

This possibility was also pointed out by Freeman Dyson.

He figures that the most likely minimum black hole mass is the Planck mass of 20 micrograms.

If that's the case, then all matter larger than a dust grain will collapse into a black hole in around 10 to the power of 10 to the power of 26 years, then promptly evaporate by Hawking radiation.

This same process will nail all of the neutron stars too.

Again, that's if protons don't decay.

If they do, then matter is kaput much earlier.

So after somewhere between a million googol and 10 to the power of 10 to the power of something ridiculous, the universe will be nothing but an increasingly diffuse void of elementary particles with maybe a bit of dust if you're lucky.

At this point, there's really no hope for extracting useful energy from the universe.

It approaches heat death in which all energy is perfectly distributed, entropy has peaked, and there's nothing for any future civilization to cling to, assuming they don't have technology so advanced that they arrest the expansion of the universe itself or develop a portal gun-- not likely.

The end of the universe will probably be this eternally expanding, cooling nothingness.

Or it could be more dramatic.

Dark energy may tear space to shreds in the Big Rip.

Vacuum decay may drop the universe to an even lower energy state, wiping out the laws of physics as we know them.

Or quantum fluctuations may spawn new universes from the void.

We'll explore these extreme futures of spacetime time in the near future of "Space Time."