Death from the Skies! (36 page)

We don’t know for sure how much Andromeda is moving to the side. At its current distance from us, even a transverse velocity of hundreds of miles per second translates to a very tiny shift as seen by a telescope. However, it’s safe enough to assume that the transverse velocity is roughly the same as its velocity directly toward us, and some theoretical models back that up. That’s not enough for it to totally miss us.

So, given enough time, Andromeda and the Milky Way are due for a train wreck. What will happen?

Two astronomers decided to find out. T. J. Cox and Abraham Loeb at the Harvard-Smithsonian Center for Astrophysics modeled the interaction between the two giants over several billion years. What they found out doesn’t bode all that well for us.

The two galaxies accelerate toward one another as they close in. Faster and faster they approach, until they finally physically collide about two billion years from now. The collision is almost ethereal—stars are so far apart that in essence the two galaxies will pass right through one another. The odds of any two stars getting close enough to physically collide are practically zero.

In most galaxy collisions we observe today, the victims are suffering a burst of star formation.
¹¹¹
This is because gas clouds, unlike stars, are very large, so in a typical galaxy collision the chance of a
cloud
collision—of
many
collisions—is a virtual certainty. When the clouds collide, they collapse and form stars. Many of these stars are massive and hot, so they light up the gas around them. Galaxy collisions in the Universe today advertise their presence by lighting up like neon signs.

However, according to the model created by Cox and Loeb, by the time the Milky Way and Andromeda merge a few billion years from now, much of the gas currently existing in the two will have already been used up to make stars. Unlike other galaxy collisions, our own won’t be accompanied by a starburst. This makes the collision safer for us; no starburst means no giant clusters of massive stars irradiating their environment, and no wave of supernova explosions destroying everything around them.

That doesn’t mean there’s no drama, however. During the collision, the shapes of the galaxies get distorted. Currently the Milky Way and Andromeda are both “grand design” spirals, with majestic spiral arms. But imagine you are a star on the galaxy’s edge, on the side facing Andromeda. As the other galaxy nears, you start to feel a gravitational tug from it, and eventually that pull is equal to the force you feel from your home galaxy. A star on the far side of the Milky Way, however, feels a greatly reduced pull since it is so much farther away from Andromeda. This has the effect of stretching out the galaxies, pulling them apart like taffy, forming long tentacles called
tidal streams.

Over millions of years the two galaxies pass each other, whipping around in a curving path (depending on the amount of transverse velocity). The two long tails of stars, gas, and dust pulled out from the galaxies curve along with them, forming glowing tentacles hundreds of thousands of light-years long. From some distant galaxy, the two would look like some weird pair of marine creatures fighting to the death (or perhaps mating).

While the two galaxies pass through each other, they don’t have enough velocity to escape each other’s grasp. After about another billion years they fall back toward one another, repeating the sequence, and then again in less than another billion years. Finally, about five billion years from now, the two galaxies will have merged. Their cores will coalesce, and the matter ejected into the long tails will settle into a stable orbit. Instead of two spirals, the resulting merger will yield a single giant galaxy that is elliptical in shape—Cox and Loeb have dubbed it
Milkomeda
(I suppose
Andromeway
sounded too much like the name of some sort of pharmaceutical). In fact, many of the giant elliptical galaxies seen in the sky may be the result of such massive mergers; they are the junk heaps of cosmic collisions.

But what of the Sun? What happens to us during all this?

Interestingly, this whole event transpires during the lifetime of the Sun. While the Sun may be a red giant by the time it all ends (see chapter 7), it’ll still be around. Maybe.

Cox and Loeb’s model can make some predictions about the Sun’s fate. They find that after the first passage of the two galaxies, the Sun has a large chance of staying within the Milky Way’s disk. However, there is a small chance (about 12 percent) that it will be ejected into one of the long tidal tails. There is no danger from this, and in fact (as we’ll see in a moment) this may be the safest place for us to be. And the view! From that vantage point, we’ll be looking down on the collision with very little dust to obscure the scene. We’ll have box seats to one of the most colossal events in the Universe.

The chance of the Sun’s getting tossed out of the Milky Way becomes greater with each passage of the two galaxies. By the time the cores merge, the odds of the Sun’s being farther than 100,000 light-years from the center of the merger remnant are about 50 percent (and we’re better than 3 to 2 to be at least 65,000 light-years from the center). We’re currently about 25,000 light-years from the Milky Way’s center, so that’s a significant change.

In fact, during the merger there is a small chance (less than 3 percent) that we’ll swap sides, becoming bound to the Andromeda galaxy! While these are long odds, it’s an amazing idea. Stars tend not to be fickle in collisions, and stick with the ones who brought them, but a few will change allegiance given the chance.

There is also another possibility: there is a small chance—less than 1 percent, but it’s there—that the Sun will actually drop toward the center of the system. If this were to happen, then the Sun could actually get within a few thousand light-years of the merged cores of the two galaxies, and this would be very, very bad.

Remember, all large galaxies have supermassive black holes in their centers. Andromeda is no exception: at its heart lurks a black hole much larger than ours, weighing in at 30 million times the mass of the Sun (ours is only about 4 million). When the cores coalesce, the two monster black holes will merge, creating a single black hole with 34 million solar masses.
¹¹²
Even a 1 percent chance of getting dropped near such a monster is a little higher percentage than I’d like. Still, if we can manage to escape getting swallowed by the black hole, there’s yet another problem: gas.

While there is not enough gas left over during and after the merger to form new stars, it takes far less gas falling into an SMBH to create an active galaxy. While not explicitly calculated by Cox and Loeb, it is implied in their models that some mass will drop toward the center of the merger, where it can form an accretion disk and be consumed by the black hole there. As you may recall, many active galaxies blasting out copious amounts of radiation and matter seem to have odd shapes, implying they recently suffered collisions.

If this were the case, then once again our galaxy—well,
Milkomeda
—will become active. Beams of matter and energy will blast out of the supermassive black hole in the core, and, if the Sun is in the wrong place at the wrong time . . . well, you know what happens to us: chapter 5 discussed these beams from a black hole. Now imagine them being a thousand times more powerful, with us in their path. If the Sun drops toward the core of the new galaxy and the supermassive black hole there decided to throw a fit, we’re in for a very bad ride. However, if the Sun is ejected off to 100,000 light-years away from the core, then the odds of intersecting one of those beams is rather small . . . and the work of Cox and Loeb indicates we have a far better chance of heading out, not in.

Of course, we’re talking about a time maybe five billion years from now. All politics is local, they say, and if we’re still around we’ll probably be contending with a star on its way to becoming a red giant and white dwarf. When your own small town’s politics are so messed up, who has time to worry about the big-city slickers and what they’re doing so far away?

You may be familiar with what’s called exponential or scientific notation: using exponents to represent very large or very small numbers. So instead of writing out 10,000,000,000, it’s easier to refer to it as 10
¹⁰
: a 1 followed by ten zeros. Similarly, very small numbers are written using a negative exponent: 10
⁻⁷
= 0.0000001 (the 1 is seven places to the right of the decimal point). This chapter is rife with scientific notation because the numbers involved will get staggeringly large very quickly. However, this introduces a slight prejudice in our barely evolved human brains that can trick even those of us familiar with the notation.

The number 10
¹²
looks like it is only a little bit bigger than 10
¹¹
, but it’s actually ten times larger (1 trillion versus 100 billion). Worse, 10
²⁰
looks
only twice as large as 10
¹⁰
, but it’s actually
10 billion
times larger! Even for someone experienced in this notation, it can be difficult to appreciate at a glance. Right now, the Universe is a little over ten billion years old: 10
¹⁰
. But far, far in the future, when it’s 10
²⁰
years old, the length of time spanned by 10 billion years will be a tiny, tiny fraction of the total age of the Universe. Keep this in mind, because by the end, even 10
²⁰
will be an infinitesimal amount of time compared to the length of our journey.

So far, we have examined a series of singular events that wreak havoc on our little planet: exploding stars of different flavors, catastrophic impacts, the death of the Sun.

Certainly, these events are exciting, and of course it’s the big flaming explosions that make headlines. A tree getting hit by lightning and burning to the ground might make the local newscast, but one that simply decays from within and falls over from rot after fifty years won’t even get noticed.

But while we might never get hit by an asteroid or fried by a GRB, the Earth is aging.
Everything
is aging. Even if we manage to survive the death of the Sun, how do we survive the aging of
the Universe itself
?

The answer may be bleak indeed:
we don’t.
While you’ve been reading this book, the Universe has aged. Maybe a week, maybe a few days if you’re a fast reader, and during that time the Sun has eaten through a few trillion tons of hydrogen, stars have exploded, and the volume of the Universe has increased. We’re all a little older, and so is the cosmos. After you’re done with this book and you put it on a bookshelf, it will age. It will
always
age. It’s inevitable: it will be a year older, a decade, a millennium. It will have decayed into dust by then, no doubt, but the atoms in that dust will age too. Someday they will be millions of years older, billions.
Trillions.

And even that is a microscopic drop in the ocean of time. Time may very well go on forever, and a trillion years will be like the blink of an eye. The Universe will continue to age, and as it does, it will change. This change will be profound: it is more than just massive stars dying and galaxies colliding; the very nature of the Universe and the things in it will change fundamentally over stretches of time so long we don’t even have names for them.

What will the Universe look like in a trillion trillion
trillion
years? How about a trillion times that age?

Different.
It’ll be different. But we’ll never know: we won’t be around to see it. And by that I mean nothing like humans at all, nothing like life as we know it. Not even matter as we know it will survive to see this stage of the cosmos.

Time and tide wait for no man. But deep time waits for
nothing.
Not even matter.

The only way to understand this look forward is to first take a look back, all the way back, to the beginning of the Universe. It may seem like a long time ago, but I promise you, soon it will seem like yesterday afternoon.

In the beginning, there was nothing.

Then there was everything.

Maybe that’s a little
too
brief. The details turn out to be important.

How we understand the beginning of the Universe—and its fate—depends on how we see it now. By carefully examining the clues coming down from space through our telescopes, we actually know a surprising amount about what the Universe has been doing the past few billion years. Perhaps even more surprising is how much we can extrapolate about what it will do in the future . . . and so far into the future that billions of years will seem as but a whisper, a mere tick of the cosmic clock.