Death from the Skies! (38 page)

Thus the Big Bang model was formed.

Over the years, the model has been reworked, refined, with parts added and others taken away. When an astronomer uses the term
Big Bang,
she doesn’t just mean that singular point in time 13.7 billion years ago; she is also implying a vast amount of other work done to make the model fit what is observed about the Universe. And, in fact, it is one of the most successful scientific theories of all time.
¹¹⁹

One critical factor in confirming the Big Bang model is the finite speed of light. That may sound weird, but it’s this finite speed that allows us to see what the Universe was doing in the past. Imagine that the speed of light were infinitely fast. If we looked at a galaxy 10 billion light-years away, we’d see it as it is
right now,
at this very moment. It would probably look a lot like ours, and there’s not a whole lot about the Universe we could learn from it.

Instead, though, we have a wonderful characteristic of the Universe: light is not infinitely fast. It’s
pretty
fast, covering 186,000 miles every second (about a foot per nanosecond, if that helps you any), but the Universe is so big it takes a long time for that beam of light to make it here from some distant galaxy.

What this means is that we don’t see galaxies as they are right now; we see them as they were when they were younger. Telescopes are very much like time machines in this regard—the farther away we look in space, the farther back we look in time. How do we find out what the Universe was like five billion years ago? Easy: find galaxies that are five billion light-years away and take a look.

And why stop there? Our telescopes are huge, and our detectors sensitive. We have seen galaxies well over 12 billion light-years away, so we’re seeing them as they were when the Universe itself was about a billion years old. Because of this, we can actually see what galaxies looked like when they were young, and discover what happens when they age.

We can also detect and analyze the gas that lies between galaxies in the distant Universe, which in turn tells us even more about early conditions. In fact, radio telescopes tuned to the microwave part of the spectrum have detected a uniform hiss coming from all over the sky. This hiss is not noise: in a very real way it’s the cooled light from the fireball of the birth of the Universe. After about 100,000 years, the Universe had expanded and cooled enough that the matter became transparent to light, meaning that light could travel freely through it. Before then, a photon wouldn’t get very far before being absorbed by some bit of matter. This light, free to move across space, has since “cooled” as the Universe expanded, and has been able to travel to our waiting instruments.

These characteristics—and many, many more—have provided to us a wonderful series of clues on the way the Universe behaves. Because of this, we have a fairly good grasp on what the Universe was like almost all the way back to its birth, nearly 14 billion years ago.

But what about its future? Is it possible to take what we know about physics and astronomy and extrapolate the eventual fate of the cosmos?

Yes, it is. We can get a fairly good idea of what the Universe will be like in the next few billion years (for example, our local neighborhood will look surprisingly different pretty quickly). As we look farther into the future, though, our crystal ball gets cloudier, but given what we see and know we can guess in broad terms what will happen.

I’ll give it to you straight: things don’t look good for us. If we want to survive into the far future—and I mean
far
—we’ll have to change ourselves in such fundamental ways that I wouldn’t even consider the result to be human anymore. And even then, escape from the Universe’s eventual demise may be impossible.

And yet there is still hope. Maybe not for
us,
exactly, but for whoever comes next. Maybe there won’t be anyone next, but the Universe may yet get another chance to try.

As the Universe ages, it changes profoundly. The time scales of these overall changes themselves change, getting longer as the Universe ages. When the Universe was young it changed rapidly. For example, one of the first big changes occurred just 10
⁻³⁵
second after it was created. Before this time, all the forces of the Universe—gravity, electromagnetism, and the nuclear forces—were combined in one unified force, and in fact this is called the Grand Unification Epoch. Today, these forces are as different as they can be, but when the Universe was incredibly hot and dense, they were indistinguishable. But just after that razor’s slice of time following the cosmic birth, the temperature and density dropped enough that the forces started acting differently.

It’s an incredibly short length of time, 10
⁻³⁵
second. But it was enough for the Universe to change profoundly. It went through many such changes: dropping in temperature and density such that particles like protons and electrons could form, and then dropping again such that these could come together to make more complicated elements, and then form stars, galaxies, and finally us.

There are lots of ways to divide up the Cosmic Epochs. A good way is to look at what’s producing the most amount of energy at that time. Right now, that would be stars. However, the stars will all eventually die out, and the current age will end. What then?

Astronomers Fred Adams and Greg Laughlin looked into this idea in great depth in their book
The Five Ages of the Universe
. As the title suggests, they found five ways to divvy up time in the Universe. Up until stars formed was the Primordial Era, which we just toured above. The current era of stars they dubbed the Stelliferous Era. After that is the Degenerate Era, then the Black Hole Era, and finally—forbiddingly—the Dark Era. The time covered by these eras is staggering, and difficult to grasp. While reading their book I had to constantly take a step back and laugh at the numbers. Maybe that was a defense mechanism on my part, like whistling past a graveyard.

Actually, that analogy is a little more on the mark than I’d like.

That’s just a gentle warning. We’re about to take the longest journey you’ve ever been on. It’ll last so long that even using scientific notation gets overwhelming. You’d better sit back and relax. You’re going to be reading this chapter for a long,
long
time.

The era of stars began with the birth of the first stars. It’s not known precisely when that happened, but the best estimates put it at about 400 million years after the birth of the Universe. Theoretical models show that it wasn’t until roughly then that the gas distributed throughout the Universe was cool and dense enough to collapse under its own gravity.

Observational evidence has mounted for this date as well. Although we have never directly detected these pioneer stars—they would be so distant now that directly observing them would be nearly impossible—they had an effect on their environment, and that
can
be detected. These stars would have been made entirely of hydrogen and helium (and again, a trace of lithium), making them relatively simple compared to modern stars. Such a chemical composition made it possible for the early stars to be much more massive on average than current ones (the heavier elements in modern stars make them hotter, so they “switch on” at a much lower mass). Some models put these stars at well over 100 times the mass of the Sun. They flooded space with ultraviolet light, which ionized the hydrogen atoms around them, tearing the electrons off.

These electrons
polarized
the light from the stars: in effect, this means the waves of light coming from the stars were all aligned, like people in a room all facing the same direction.
¹²⁰
This polarization effect can still be detected today, and the observations agree with the theoretical models on the time when stars first appeared.

Also, at the ends of their short lives, these stars would have exploded as massive supernovae, scattering the Universe’s first heavy elements into the surrounding environs, from which the next generation of stars would form. The first stars probably created gamma-ray bursts when they exploded; these might yet be detected too.

We still live in the Stelliferous Era. Stars are the dominant feature of the Universe, and produce most of the energy we detect. As we saw in the last chapter, the available source of gas in the Milky Way to make stars will run out in the next few billion years, although some galaxies may use up their gas more slowly. But one way or another, the gas will eventually run out, and essentially no more stars will be born anywhere in the Universe.
¹²¹

We know the Sun will last as a normal star for several billion more years before turning into a red giant, frying the Earth, losing its outer envelope, and then “retiring” as a white dwarf (chapter 7). But the length of time a star lives depends almost entirely on its mass. A star with a lot more mass than the Sun eats through its fuel far faster, and may only live a few million to a billion years. However, stars with
less
mass will live longer.

The lowest-mass star that can currently exist has about 0.08 times the mass of the Sun. Below that limit, the core isn’t hot enough or under enough pressure to fuse hydrogen into helium. This type of star is small (one-tenth the Sun’s diameter), dim (one one-thousandth the Sun’s luminosity), cool (with a temperature of about 5,000 degrees Fahrenheit), and red. Not surprisingly, these stars are called
red dwarfs.

Imagine you take a large rock and hit it with a sledgehammer, shattering it. If you look at the pieces that remain you might see a few large pieces, a few more that are smaller, and a lot of little pebbles and shards. That is a natural size distribution in stars as well: when a cloud collapses and forms stars, only a few will be really big, some will be smaller, and more smaller yet. The vast majority will be the smallest type; it’s estimated that 75 percent of all the stars in the Universe are red dwarfs.

Although they have a small fraction of the mass of the Sun, red dwarfs are incredibly miserly with their fuel and can last far, far longer. A very low-mass dwarf can reasonably expect to shine for the next several
trillion
years.

This is longer than any other kind of star in the Universe. If we let the cosmic clock run forward, we see the last stars being born in a few hundred billion years. Very rapidly after that all the massive stars will be gone, since they don’t live long. The last core-collapse supernova in the Universe may occur only a hundred million years after the last massive star is born. This is a tick of the clock compared to how much time has elapsed in the Universe at that point.

Sometime not long after that, somewhere in the Universe, a star just barely too low-mass to explode will age and die, expanding into a red giant, blowing off its outer layers, and fading away as a white dwarf. It is part of a long, long line of such events: there are 100 billion galaxies in our Universe, each with an average of about 100 billion stars.

As time goes on, trillions of stars with lower and lower mass fade away and die. Stars with the lowest mass will take the longest, but they’ll all cross the finish line at some point.

If we wait a sufficiently long time—oh, say, a trillion years—all stars like the Sun will be long gone, and only the lowest-mass dwarfs will remain. You might think galaxies would be dim and red then, only illuminated by the tiny stars. Interestingly, though, galaxies may be as bright at that time in the distant future as they are today. We saw in chapter 7 that the Sun steadily increases in brightness as it ages. All stars do this, even red dwarfs. Calculations done by
The Five Ages of the Universe
authors Adams and Laughlin, together with their colleague Genevieve Graves, indicate that a star with one-tenth the Sun’s mass will live for about 10 trillion years. As it ages, it gets brighter and slightly hotter. What they found in their models, after adding up all the light from all the stars in the galaxy and then letting the galaxy age, is that the amount of light given off in toto by dwarfs increases roughly as quickly as light from the more massive stars fades as they die. In other words, the total light emitted by a galaxy will stay roughly constant for several hundred billion years, with the ever-brightening dwarfs picking up the slack as massive stars die off.

As the red dwarfs heat up, they will change color too. A hotter star gets bluer, and so too will red dwarfs. It’s possible that for a few dozen billion years the galaxy will shine with a demonic red hue, and then this will slowly morph to a vibrant blue.

But all good things . . . , as they say. Even dwarf stars eventually die. Unlike the Sun, which can only fuse fuel in its core, the smallest red dwarfs circulate their fuel. Like hot air rising and cool air sinking,
¹²²
the helium created in the core circulates upward and mixes with the rest of the star. As the hydrogen falls into the core it can fuse, forming more helium, which then mixes more with the star.

Eventually, the star runs out of hydrogen—and unlike the Sun, which just runs out of
available
hydrogen in its core, the dwarf
totally
runs out. Gone. Kaput. All that is left in the star is helium, and it lacks the mass to fuse it into carbon. The star cools, the helium contracts, and it becomes a pure helium degenerate white dwarf (see chapter 7 for details on this odd quantum state).

In seven or eight trillion years’ time, in the Milky Way (well, Milkomeda, after we collide with the Andromeda galaxy, and probably consume all the smaller galaxies in the Local Group as well) the last dwarf star will become a white dwarf. For trillions of years the galaxy will have glowed a beautiful blue, but that too shall pass.