The events described below will seem like science fiction, I suppose, since things get pretty weird at very short times after the Universe formed, and in the mind-numbingly long stretches of future time. But as far as we can tell, this is science
fact,
based on solid evidence. Like any examples of conjecture, there may be pieces we are missing that affect what really happened, or what really will happen. That is the nature of science; more observations and more information always lead to a refinement of the results. Science asymptotically approaches reality, and it’s hard to say just how far up the curve we are now.
But even with that caveat, the future of the cosmos is fascinating, if bleak. But as is the case in science and in stories, we have to start at the beginning.
Oh, and I better warn you: the very dawn and the very dusk of the Universe are times when things are completely different from what we see around us now. Be prepared to stretch your mind a bit.
The Universe is many, many things—it’s literally everything—but it’s also a damn odd place.
IN THE BEGINNING
Some 13.7 billion years (plus or minus 200 million years) ago, the Universe exploded into existence.
This mere statement causes a substantial amount of confusion. Astronomers refer to this event as the Big Bang, or, more accurately, we use the term
Big Bang
as a
model
for what we think happened. What’s the distinction? Well, for one, the event was neither Big nor was it a Bang. When it popped into being, the Universe was smaller than the size of a proton, so it wasn’t terribly big.
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And there wasn’t a bang either: it was more of a pop, or a snap.
By observing the Universe as it is now, we can run the clock backward and figure out what it was like in the past. What we discovered is that in the past, the Universe was hotter and more dense. The farther back in time you go, the hotter and denser it gets (the reverse is true as well: the older the Universe gets, the less dense and the cooler it gets). It gets smaller too: the Universe is expanding now (more on this in a moment), so in the past it was smaller. Eventually, you go back far enough to a time when the Universe was basically just a singular point: an infinitely small, infinitely hot, infinitely dense object.
Well, that’s
weird.
And it’s probably not even technically correct. As we turn the clock back, the Universe shrinks. At some point we see it as the size of a present-day galaxy, and then a star, and then a planet, and then a grapefruit, and then an atom. When it gets smaller than an atom, the weird world of quantum mechanics rears its head once again. One of the most fundamental rules of QM is that many characteristics of objects are related, and the more you know about one the less you know about another. The more carefully you measure an electron’s position, for example, the less you know about its velocity. The more you nail down one aspect of an object ruled by quantum mechanical processes, the slipperier another property becomes. It’s almost as if there is a cosmic censorship going on, happening at very teeny-tiny scales. The closer you look, the fuzzier things get.
In practical terms—and I’m not sure what the word
practical
even means on scales like this!—this tells us that when the Universe was really, really small, there is very little we can know about what it was truly like. Our equations and understanding of physics tell us a great deal about the state of the Universe when it was a day old, an hour old, a second old . . . even a tiny fraction of a nanosecond old. But if you go back far enough in time, to when the Universe was literally 10
−43
second old,
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our physics breaks down. There is a veil hiding the true beginnings of the Universe, farther back than which we can never directly see.
That’s another reason scientists prefer not to call this event the Big Bang. We don’t know much at all about the event itself, whether it was a bang or not. We can only figure out what happened right after it.
Still, you might wonder what happened
before
the Big Bang. This is a natural question, and there are two ways to think of it. One of them is that the question is meaningless. That may sound like a cop-out, but let me ask you this: what’s north of the north pole?
That question has no meaning, right? If you travel north, you get to the north pole, and you’re done. There’s no more north to go.
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Now remember that time itself was created in the Big Bang. Before then, there was no time, so there was no “before.” The question has no meaning.
That’s pretty strange, even for cosmology and quantum mechanics. It’s also unsatisfying. We’re used to things existing for a finite stretch of time, embedded in a bigger stretch of time. A symphonic concert might start at 7:00 p.m. and end at 8:24. But there existed things before the symphony started (the orchestra arrived at the concert hall, warmed up, filed on stage), and things continue after (the brass section empties their spit valves, the members leave the stage, they go home and watch reruns of
Gilligan’s Island
). So how can there be a beginning of time, a point on the timeline before which there is nothing?
There may be a way out of this conundrum. There are some theories that say that the Universe is not really all there is. There may be some sort of meta-Universe out there, some framework from which we are forever locked away, and our Universe is just a subset of it. This Universe existed before ours did and is much like ours, with the same or similar laws of physics controlling its behavior, including quantum mechanics. In the chapter on black holes we saw how particles can pop into existence spontaneously. There is a possibility that a tiny blip in the fabric of this other Universe’s space-time suddenly came into being, something like the creation of particles ex nihilo. Under some conditions, this herniated region of space and time would quickly collapse, but it’s also possible, in the realm of quantum mechanics, for this region to
grow.
Something like a black hole, it’s disconnected from the greater Universe around it, and becomes its own entity, its own Universe. Space and time and energy and matter simply spring into existence inside. After a few billion years it expands to the point where stars form, galaxies take shape, and on a planet somewhere lost in all that volume of space, a person reading a book is scratching his head and thinking the book’s author has lost his mind.
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At the moment, we don’t know for sure whether there was anything before our Universe existed, or if that idea even has meaning at all. But since the Big Bang theory was first postulated, we’ve learned quite a bit about what happened after that first 10
−43
second.
A LITTLE AFTER THE BEGINNING: T + 10
−
43
SECOND TO TODAY
We know that the Universe went through several different phases of its life prior to the one we live in now. In the very early Universe, when it was still unfathomably hot and dense, it consisted of a stew of bizarre subatomic particles held in sway by unfamiliar forces. As the Universe expanded and cooled, different types of particles were able to form and become stable (whereas before it was just too hot for them to exist, like an ice cube won’t last long on a frying pan). To make it easier on themselves, physicists divide the timeline of the Universe into different slices, different eras, based on what particles were present and what forces dominated at the time.
After just one microsecond (10
−6
second), things had settled down enough for protons and neutrons to form from the thick soup of subatomic particles called
quarks.
After one second, one tick of the clock, was the period of
nucleosynthesis,
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when conditions were similar to those in the core of a star. The heat and density allowed some protons and neutrons to come together and form stable nuclei. For about three minutes after the nucleosynthesis period started, the subatomic particles smashed into each other and created an entirely new kind of matter: helium (two protons plus two neutrons). Even a trace of lithium (three protons and three or four neutrons) was formed, but nothing heavier than that—the more complex reactions needed to make carbon and neon never got a chance to occur.
This left all the matter in the entire Universe divided into roughly 75 percent hydrogen, 25 percent helium, and a trace of lithium.
There was no calcium, no iron, no oxygen. For that matter (pun intended) there were no stars, no planets, no galaxies. Everything was pretty simple at this point, just a lot of extremely hot gas strewn into long filaments and ripples: fluctuations in the cosmic matter distribution created by fluctuations in the explosion of the Big Bang itself.
These streamers would soon begin to collapse under their own gravity. Under conditions still not fully understood, the matter would form the first stars at about T + 400 million years. Galaxies themselves formed around the same time, collecting along the matter filaments, creating a vast spongelike network of fantastic galactic clusters streaming throughout the Universe.
And so, after 13.7 billion years, here we are.
HOW WE KNOW WHAT’S SO
All of this is probably a little overwhelming. It may even strike you as ridiculous! It’s so far outside our comfort zone, our usual thought processes, that it might seem as if scientists are just making it all up.
I promise, we’re not. There is a logical series of steps that lead to our understanding of the early Universe.
One of the very first people to think about the Universe as a whole was a German astronomer named Heinrich Olbers. In the early 1800s, when Olbers was studying the sky, it was assumed that the Universe was infinitely old, and that it was infinite in extent. There was no reason to think otherwise. But as Olbers realized, this raises a problem. If the Universe is infinite, and populated with stars throughout its extent, then no matter what direction you looked eventually you’d be seeing the surface of a star. No matter how teeny-tiny a section of the sky you chose, a line drawn from you out into space in that direction must hit a star at some point. It might be a bazillion light-years away, but if the Universe is indeed infinite that is but a mere stroll compared to that infinity.
And that’s the problem. The apparent size of a star gets smaller as it gets more distant, of course, so it also appears fainter. But the drop in size and brightness is compensated by the Universe being literally infinite. The number of stars increases with distance, and in fact the number of stars
increases
at the same rate at which the brightness
decreases.
The two cancel out.
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So if the sky appears full of stars, literally, with no apparent space whatsoever between them, then the entire sky will glow with the same brightness as a star itself. To any observer inside such a Universe, it would be as if the sky were as bright as the Sun,
everywhere you look.
Obviously, such a Universe would be uninhabitable. Also just as obviously, our Universe doesn’t behave that way.
This is what Olbers pointed out, and the conundrum is now known as Olbers’s paradox. This problem baffled people for some time, and the answer to the paradox came from a somewhat surprising source: Edgar Allan Poe.
Yes,
that
Poe. Besides writing scary stories and depressing poems like “The Raven,” he was quite a deep thinker. It occurred to him that perhaps the problem lay not in the Universe, but in our underlying assumption: what if the Universe were
not
infinite in space and/or time? If the Universe were finite in space, then you’d run out of stars at some distance from Earth. And if it were finite in time—that is, it had a beginning—then there simply hasn’t been enough time for the light from very distant stars to reach us. Paradox solved.
In fact, Poe was right. In his 1848 work
Eureka,
he wrote:
Were the succession of stars endless, then the background of the sky would present us an uniform luminosity, like that displayed by the galaxy—since there would be no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all. That this may be so, who shall venture to deny? I maintain, simply, that we have not even the shadow of a reason for believing that it is so.
This was radical thinking for its time. While it was common in mid- to late nineteenth-century society to assume that the Universe had a beginning because it said so in the Bible, this was somewhat unsatisfying to a scientist. Poe changed that.
Less than a century later, the astronomer Edwin Hubble, together with other astronomers such as Vesto Slipher and Ellery Hale, made one of the most shocking discoveries in the history of science: essentially every galaxy they could observe appeared to be rushing away from us. This was so difficult to believe that it took years before there were enough observations to convince everyone, but the evidence was undeniable: the Universe itself is expanding.
This had profound implications. If galaxies were moving away from us, then they grew more distant as time went on. That in turn means they were all closer together in the past. If you run the cosmic clock backward far enough, then, at some point in the past, every galaxy,
every bit of matter and energy in the Universe, would have all been in the same spot.
This meant the Universe had a beginning, a point in time when it all began. Matter and energy rushed outward from that point in time, expanding evermore. Albert Einstein had already been working out the general equations that govern the behavior of time and space when Hubble and his team discovered the cosmic expansion, and the news of the discovery electrified him. It was soon accepted by scientists that Einstein’s work was correct, and that the Universe itself could be described using mathematics.