Death from the Skies! (39 page)

Interestingly, in this late stage of the Stelliferous Era, some even lower-mass stars will still be able to shine. Because high-mass stars create heavier elements like iron and magnesium, stars forming later get imbued with these materials. Heavier elements make a star hotter (they absorb the light from the star, trapping the heat in), so lower-mass stars—perhaps even as lightweight as 0.04 solar mass—will be able to get fusion started in their cores. But again, we have to consider the span of time: even if these stars stave off turning into white dwarfs for 15 trillion years,
that time will still eventually come.
At some point, all stars in the Universe will be gone, having become white dwarfs, neutron stars, or black holes.

The tiny white dwarfs fade with time (neutron stars cool even faster). Eventually, the galaxy contains no stars actively fusing elements in their core at all. Over the next few trillion years these stars fade too. By the time the Universe is 100 trillion years old, the galaxies—and therefore the Universe itself—will be dark.

In the distant future, not only will the Universe be darker, it’ll look a whole lot emptier as well.

Standing on a beach looking toward the horizon, you can see only so far out. The Earth curves downward, hiding more distant objects from view. The visible horizon is only a few miles away, and you cannot see things any more distant.

The Universe has a horizon too. Since it’s 13.7 billion years old, we cannot see any objects more than 13.7 billion light-years away. The Universe might be bigger than that, but the light emitted by any objects farther away than that distance has not had enough time to reach us, so we don’t see them.

In fact, it’s actually worse than that. The Universe is expanding; the fabric of space is literally being stretched out. Objects farther away appear to be receding from us at greater speeds. If you look out to a great enough distance, galaxies appear to be receding from us at the speed of light. We cannot detect such galaxies: their light is approaching us at the same speed that space is expanding. Like running on a treadmill, that light can’t get anywhere, so it never reaches us.
¹²³

And it’s even worse than that. In 1998, it was discovered that not only is the Universe expanding, but its expansion is
accelerating.
Not only is the Universe getting bigger, the
amount
it’s getting bigger is bigger every day.

This has a rather depressing outcome for the distant future. Because the Universe is accelerating, galaxies that are currently inside our horizon (because they are receding from us at less than the speed of light) will eventually move outside our horizon (because they will accelerate relative to us to beyond the speed of light).
¹²⁴
This means that over time distant galaxies will fade as the expanding Universe sweeps them out of our view. As time creeps ever forward, galaxies that are closer to us now will slip away, and the cosmic horizon will close around us like a noose.

However, it won’t tighten too much. Space expands, but this expansion can be counteracted by gravity. You might expect that, say, two stars orbiting each other will get farther apart as space expands between them. However, that’s not the case. Since the two objects have gravity, and they are bound to each other—that is, their gravity holds them together—
space doesn’t expand between them.
It’s just another peculiar outcome of relativity and the way space-time behaves.

This means that even though the Universe is expanding, and even accelerating, the cosmic horizon won’t continue to shrink forever. The Local Group of galaxies—the Milky Way, Andromeda, and a dozen or two smaller galaxies—are gravitationally bound to each other. We know that by 10 billion years from now we will have merged with Andromeda, and over time we’ll gobble up all the remaining smaller galaxies as well. Some calculations show that the cosmic horizon will shrink down to encompass just the Local Group’s volume of space in something like 100 billion years, still during the Stelliferous Era. By that time, the Local Group will be one giant elliptical galaxy.
¹²⁵
From our point of view, we will see closer galaxy groups like the Virgo Cluster fall over the horizon, but our own will always be visible.

Eventually, our view will be extremely limited: as far as we’ll be able to tell, the entire Universe will consist of our one massive galaxy with literally nothing outside of it. Any new species that evolves during this time will have no clue that, once upon a time, a Universe far vaster than their own once teemed with galaxies and stars.

What will their cosmology be like?

The tightening of the horizon will occur far sooner than all the stars in the galaxy will die out, hundreds of billions of years compared with tens of trillions. Still, it’ll be a harbinger of things to come, of a Universe growing increasingly darker.

It should be noted that the acceleration of the cosmic expansion does mean one thing for sure: the Universe will not recollapse. Before the acceleration was discovered, it was still a matter of some debate whether the Universe would expand forever or whether the combined gravity of all the matter in it would slow, stop, and eventually reverse the expansion. But the discovery of the acceleration pretty much put an end to that debate. The Universe will expand forever, ever faster, while (somewhat ironically) our view of it will get smaller and smaller, until we have our own private Universe just a few million light-years across.
¹²⁶

What does this mean for us, for humans? To a good approximation, it means that we have about 100 trillion years to get our affairs in order. After that, we won’t have enough light to read our books by. Things’ll get boring.

Assuming that anything resembling humans still exists a thousand times the current age of the Universe from now, there
are
ways to extend the stars’ reign. Physically colliding stars—literally smacking them into each other—to make new ones will help. But how long can you do that? If you decide you need a star like the Sun, you can smush together a few dwarfs and get a star that shines for another few billion years. Remember, though, that the Universe is
trillions
of years old by this point. A billion years is a pittance in comparison. When the Universe is 100 trillion years old, our descendants will be out of fuel, out of stars, and out of luck.

The time scales here are forbidding. When we reach this point in the age of the Universe, galaxies will have lived the vast majority of their lifetime populated only by dwarfs. Think of it this way: currently, our galaxy has only been around a tiny fraction of its potential life span. Right now, as you read this, despite the Universe being over 13 billion years old,
99.9
percent of the galaxy’s life still lies ahead of it.

We think of the Universe as being relatively unchanging, but in fact we live in a very special epoch compared with the dim future. By the time the last dwarf fades away, the galaxy will look back at the time stars like ours could exist in the same way you look back at the time you were a month old.

And after all that, we’re only just getting started. We’re about to enter a realm where even 100 trillion years is a single breath of time.

When the last normal, fusing stars die, the only objects left in the Universe that can generate energy will be white dwarfs, neutron stars, black holes, and degenerate low-mass objects that lack the capability to fuse hydrogen in the first place, called
brown dwarfs.
¹²⁷
Because the Universe is dominated by these objects, this time period is called the Degenerate Era.

In visible light, the Universe will be pretty dark at this point. However, it won’t be completely dark, since there
will
be a few scant sources of light.

White dwarfs will fade; when they are at about 10,000 degrees Fahrenheit they will shine with the same color as the Sun, getting redder as they age. When they reach a temperature of about 800 degrees Fahrenheit they radiate mostly in the infrared and will become invisible.

Every now and again a black hole may pass close enough to a white dwarf, neutron star, or brown dwarf to shred it and consume the debris. An accretion disk will form and shine brightly, but only as long as the black hole eats. Once the meal is gone, the light source shuts off (this may provide a temporary source of energy for any future beings looking to stay alive, but it really is only a short-term solution).

Brown dwarfs will have their moments as well. These failed stars give off visible light for a short time after they form because of their internal heat, but their lack of core fusion means they have no ongoing source of energy. Eventually they cool and glow faintly in the infrared.

But they still can get a second chance. Collisions between stars are incredibly rare in the present-day Universe because stars are so small compared to the distances between them. However, the word
rare
has less meaning as time stretches on. Something that has an incredibly small chance of happening in 13.7 billion years may become a virtual certainty over 100 trillion.

The Degenerate Era will actually last much longer than this, in fact, so collisions between stars will happen frequently once you grasp that time scale. When two brown dwarfs merge, their mass will be just above the fusion limit, so a relatively normal star could result. In fact, if the collision is a little bit off-center, then matter from the two objects could be stripped off, forming a disk around them. It’s entirely possible that planets could form from this material; is it too hard to imagine life forming under such circumstances? Their view of the Universe would be far, far different from ours. Their skies would be entirely dark except for the one sun burning during the day. No stars, no galaxies, no ribbon of milky gas streaming across the sky. What myths and legends would arise on such a planet?
¹²⁸

At any one time, perhaps a hundred or so of these odd stars will shine in a galaxy. But again, these new stars would shine briefly, then suffer the same fate as the Sun did all those forbidding trillions of years in the past.

There will be other, brief flashes of light. A collision between two white dwarfs could result in an object whose mass is so high that it collapses into a neutron star or even a black hole. A Type I supernova may result, which to any denizens of this future era would be even more blindingly bright than to us: there will be literally nothing against which to compare it.

It’s also possible that two low-mass white dwarfs could merge to form an odd type of “normal” star, much as the colliding brown dwarfs will, but again, this is a short-lived object (a mere few billion years!) and will fade with time.

If two neutron stars collide, then they will form a black hole with a gamma-ray burst to announce the merger (see chapter 4). But this fades within days, and the black hole itself will be dark, one among many millions of others orbiting a dark galaxy.

And it will get even darker. Time piles up. After trillions, quadrillions,
quintillions
of years, even the brown dwarfs go away. They merge to form normal stars that eventually die, or they get ejected from the galaxy entirely. In fact, after this length of time, the galaxy will have a hard time holding itself together. In the far distant future, the galaxy itself will evaporate.

Stellar collisions
¹²⁹
are the culprit for this next stage of galactic evolution. A moving object has energy, and this energy can be transferred to another object (which allows us to do things like play pool, throw a baseball, hold a book, and so on). When two stars pass close to each other, they can exchange energy by interacting gravitationally. In general, what happens to two stars as they pass each other depends on their mass (it also depends on the sizes, shapes, and directions of their orbits, but we’re being general here). The higher-mass object gives away some of its orbital energy to the lower-mass object. An orbit with lower energy is smaller, so the higher-mass star will sink closer to the center of the galaxy, while the lower-mass star will move outward. Over many such encounters, lower-mass stars “evaporate” away; they get ejected from the galaxy to wander the depths of intergalactic space.

The higher-mass stars drop to the center of the galaxy, where an unpleasant host awaits them: a supermassive black hole (see chapter 8). Eventually, all the higher-mass stars in the galaxy will get eaten by the black hole.
¹³⁰

This process has been seen on much smaller scales: globular clusters—gravitationally bound spherical collections of roughly a million stars—are packed tightly enough with stars that collisions of this sort are more frequent. In all globular clusters, even after only a few billion years, the more massive stars tend to be closer to the cluster center, with lighter stars farther out.

The time scale for galactic evaporation is about 10
¹⁹
to 10
²⁰
years (10 quintillion to 100 quintillion years), making this process currently undetectable in galaxies.

But the Universe is still young. Patience.

Incidentally, over this length of time, the odds of a star getting extremely close to the Sun go up, and close in on 100 percent. Even by the beginning of the Degenerate Era (T + 10
¹⁵
years), it’s likely another star will have passed close enough to the Sun to dislodge the Earth from its orbit and eject it from the solar system (of course, any star that passes that close is likely to eject the outer planets as well—and by this time, Mercury and Venus will have been swallowed by the red-giant Sun, so
Earth
will be the innermost planet). Given enough time, planets even closer to their stars will go; even by halfway through this era, it’s very unlikely that
any
planet orbiting
any
star
anywhere
will not have been ejected from its system. By the time the galaxy itself has evaporated through stellar collisions, there may be ten times as many planets as stars roaming intergalactic space, frozen to their cores and utterly uninhabitable.