Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing "Hoax" - Philip Plait (2002)
Part III. Skies at Night Are Big and Bright
Chapter 16. When the Universe Throws You a Curve: Misunderstanding the Beginning of It All
stronomy sometimes has a way of making people feel small. L J For most of our history we humans have been pretty selfimportant. We believe that the gods pay special attention to us, even intervening in our daily affairs. We claim territory for ourselves, and ignore what goes on outside those borders. Why, we've even said the whole universe revolves around us!
But the universe is under no obligation to listen to our petty boasts. Not only are we not at the center, but also there really isn't a center at all. To see why, we need to look into the past a bit, back into our own history.
For thousands of years it was thought that the Earth was the center of the universe and the heavens spun around us. Certainly, observations support that belief. If you go outside and look up for even a few minutes, you'll see that the whole sky is moving. But you don't feel any movement, so clearly the Earth is fixed, and the sky moves.
Even today, when we know better, we still talk as though this is the way things are: our vocabulary reflects the geocentric universe. "The Sun rose at 6:30 this morning" is less accurate that saying, "From my fixed location on the surface of the spherical Earth, the horizon moved below the apparent position of the Sun at 6:30 this morning." But it is easier to say.
This Earth-centered model was fine-tuned by the Greek astronomer Ptolemy around A.D. 150 or so. People used it to predict planet positions, but the planets stubbornly refused to follow the model. The model was "refined"-that is, made more complicated-but it never quite made the grade.
Eventually, a series of discoveries over the centuries removed the Earth from the center of the universe. First, Nicolaus Copernicus presented a model of the solar system in which the Earth went around the Sun, rather than vice-versa. His model wasn't really all that much better than Ptolemy's model at figuring out where the planets would be. But then Johannes Kepler came along a few centuries later and tweaked the model, discovering that the planets orbit in ellipses instead of circles, and things were a lot better.
So with Copernicus's model it looked like the Sun was the center of the universe. That's not as good as having the Earth there, but it's not too bad.
Then around the turn of the twentieth century, Jacobus Kapteyn tried to figure out how big the universe was. He did this in a simple way: he counted stars. He assumed that the universe had some sort of shape, and that it was evenly distributed with stars. If you saw more stars in one direction, then the universe stretched farther that way.
He found an amazing thing: the Sun really was the center of the universe! When he mapped out the stars, the universe was blobby, like an amoeba, but it seemed to be fairly well centered on the Sun. Maybe the ancients were right after all.
Or not. What Kapteyn didn't realize is that space is filled with gas and dust, which obscures our view. Imagine standing in the middle of a vast, smoke-filled room, like an airplane hangar. You can only see, say, 20 meters in any direction because smoke blocks your vision. You have no idea what shape the room is; it might be a circle, or a square, or a pentagram. You don't even know how big it is! It could have walls just a meter beyond your vision, or it could stretch halfway to the Moon. You can't tell just by looking. But no matter how big or what shape, the room will look like it is about 20 meters in radius and centered smack dab on you.
That was Kapteyn's problem. Because he could only see out to a few hundred light-years before gas and dust blocked his view, he thought the Milky Way, which was then considered to be the whole universe, centered on us. However, observations by another astronomer, Harlow Shapley, in 1917 revealed that we are not at the center of the Milky Way, but indeed displaced quite a bit from the center.
Do you see the pattern? First the Earth was the center of everything-hurrah! Then, well, ahem. Maybe the Sun still is-yay! But then, yikes, actually we're way out in the suburbs of the Galaxy. Well, this was getting downright insulting.
But the worst humiliation was yet to come. Kapteyn's universe, as it was called, was about to collapse. Or, more aptly, explode.
Observations by Edwin Hubble, after whom the space telescope is named, showed that our Milky Way galaxy was just one of thousands and perhaps millions of other galaxies. What was thought to be the whole universe was really only just a single island of stars floating in space. Instead of being at the center of everything, we were just another face in the crowd.
When Hubble analyzed the light given off by these other galaxies, he got what may be considered the single biggest surprise ever sprung on a scientist. He found that almost all these myriad galaxies were rushing away from us. It was as if we were a cosmic pariah, and everything else in the universe was falling all over itself trying to get away from us.
Make no mistake: this is really weird, and completely unexpected. The universe was thought to be static, unchanging. Yet Hubble found that it's on the move. It's hard to underestimate the impact of these observations. And there was more: Hubble found that not only were galaxies all rushing away from us, but also the ones farther away were moving faster than the ones near us. The tools of the time didn't let him look at galaxies that were terribly far away, but more recently, as bigger and more sensitive telescopes have come on line, we have found that Hubble was right. The farther away a galaxy is, the faster it appears to recede from us.
It didn't take long for people to realize that this was characteristic of an explosion. If you blow up a bomb, then take a snapshot of the explosion a few seconds later, you see how shrapnel that's farther from the center must be moving faster. The fastest bits move the most in a given time, while slower bits haven't moved out as far.
This implies that the universe started in a gigantic explosion. You can think of it this way: if all the galaxies are moving away from us as time goes on, then they must have been closer in the past. If you reverse time's arrow and let it run backwards, there must have been a time in the past when everything in the universe was crushed into a single point. Let time run forward again, and BANG! everything is set in motion.
And what a big bang it was, starting up the universe and sending it flying. Could this be right? Did the universe start out as a single point that exploded outwards? Perhaps no single scientific theory has stirred people, incited their anger, their confusion and, indeed, their awe more than the Big Bang theory. I suspect that even Darwin's observations on evolution may have to take back seat to the biggest bang of them all.
But it does have one comforting aspect: it says we are at the center, because everything is rushing away from us . . .
... or does it? Let's use an analogy. Imagine you are sitting in a movie theater, and the seats are packed together so closely that they are touching. Furthermore, the seats are all on movable tracks. I hit a button, and suddenly every seat moves so that there is now one meter separating each chair. Your nearest neighbors are all one meter away, in front of you, behind you, on your left, and on your right. The next seats over are all two meters away, and the next ones from those are three meters away, and so on. But wait! That's true for any seat in the house. If you got up and moved into a seat a couple of rows up, and we repeated this experiment, you would see exactly the same thing. The next seats over would be one meter away, and the ones past that would be two meters away, and so forth.
So no matter where you sit, it looks like all the seats are rushing away from you. It doesn't matter if you are actually in the center seat or not!
Also, the seats farthest away from you appear to be moving the fastest. The seats next to you moved one meter when I hit the button, but the next ones moved two meters, and so on. Again, no matter where you sit, you'd see the same thing: it looks like all seats are moving away, and that the ones that are farther away move the fastest.
That is exactly what Hubble found. Shakespeare said, "All the world's a stage," not realizing that, in a way, all the universe is a movie theater. Scientists studying Hubble's observations quickly realized that the universal expansion may be real, but it gives the illusion that we are at the center, when we may not be at the center at all.
And if that's not weird enough, the universe still has some tricks up its sleeve.
With stuff this bizarre going on, it's no surprise to find Einstein lurking somewhere nearby. Einstein was busily pondering the universe in the years before Hubble's shocking discoveries. He was applying some pretty hairy math to the problem, and came across a difficulty. The universe, he discovered, should not be here. Or, more precisely, that something was supporting it against its own gravity. Left to itself, the universe's gravity would cause all the galaxies to attract each other, and the universe would quickly collapse like a souffle after the oven door is slammed. Before Hubble, remember, it was thought that the universe was unchanging. Something must be counteracting gravity, so Einstein decided to add a constant to his equation that would be a sort of antigravity. He didn't know what it was, exactly, but he figured it had to be there.
Or so he thought. When he found out along with the rest of the world that the universe was expanding, he realized that the expansion itself would counteract gravity, and he didn't need his cosmological constant. He discarded it, calling it "the biggest blunder of my life."
It's too bad, really. As astronomer Bob Kirshner once pointed out to me, given what Einstein knew at the time, he could have actually predicted the expansion of the universe. Why, he'd have been famous!
Anyway, what Einstein came to understand in later years is that the universe is a peculiar place. First, he realized that space is a thing. What that means is, it was always thought that space was just a place in which stuff existed, but space had no real presence itself. It was just space. But Einstein saw that space was a tangible thing, like a fabric into which the universe was woven. Gravity could distort that fabric, bending space itself. A massive object like a planet or a star (or, on a smaller but no less real a scale, a lobster or a toothbrush or a nail) warps space.
A common analogy compares our three-dimensional space to a two-dimensional rubber sheet. Stretched out, that sheet represents space. If you roll a tennis ball across it, the ball will move in a straight line. But if you put, say, a bowling ball on it, the sheet will get a funnel-shaped depression. If you then roll the tennis ball near the bowling ball, the path of the tennis ball will bend, curving around the bowling ball. That's what happens in the real universe: a massive object warps space, and the path of an object will bend when it gets near it. That warping is what we call gravity.
If space is itself a thing, then it's possible for space to have a shape. Indeed, the mathematics of cosmology strongly imply that space has some sort of shape to it. It's hard for us mere humans to wrap our brains around such a concept, so once again the twodimensional analogy is pretty useful.
Imagine you are an ant, and you live on a flat sheet that extends infinitely in every direction. To you, there is no up or down; all there is is forward, back, left, and right. If you start walking, you can walk forever and always get farther from where you started.
But now I'm going to play a trick on you. I take you off the sheet and put you on a basketball. You can still only move in back or forth, ahead or back. But now, if you start walking straight, eventually you'll get back to where you started. Surprise! If you have a good grasp of geometry, you might realize that maybe your two-dimensional space is only a part of another, higher dimension. Furthermore, you can guess a bit about the shape of your space because your walk returned you to your starting point. That kind of space is closed, because it curves back onto itself. There is a boundary to it; it's finite.
Open space would be one that curves the other way, away from itself, so it takes on a saddle shape. If you lived in open space, you could walk forever and never get back to where you started.
These three spaces-open, closed, and flat-have different properties. For example, if you remember your high school geometry, you'll recall that if you measure the three inside angles of a triangle and added them together, you get 180 degrees. But that's only if space is flat, like a page in this book. If you draw a triangle on the surface of a sphere and do the same thing, you'll see that the angles always add up to more than 180 degrees!
Imagine: take a globe. Start at the north pole and draw a line straight down to the equator through Greenwich, England. Then go due west to, say, San Francisco. Now draw another line back up to the north pole. You've drawn a triangle, but each inside angle is 90 degrees, which adds up to 270 degrees, despite what your geometry teacher taught you. Actually, your teacher was just sticking with flat space; closed and open space can be quite different. In open space, the angles add up to less than 180 degrees.
So that ant, if it were smart enough, could actually try to figure out if its space is open, closed, or flat just by drawing triangles and carefully measuring their angles.
This is all well and good if you're an ant, but what about us, in our three-dimensional space? Actually, the same principles apply. Since space itself is warped, it can take on one of these three shapes, also called geometries. And, just like the ant, you could try taking a walk to see if you come back to where you started. The problem is that space is awfully big, and even the fastest rocket we can imagine would take billions or even trillions of years to come back. Who has that kind of time?
There's an easier way. Karl Friedrich Gauss was a nineteenthcentury mathematician who worked out a lot of the math of the geometry of the universe. He actually tried to measure big triangles from three hilltops, but was unable to tell if the angles added up to more or less than 180 degrees.
There are still other ways. One is to look at incredibly distant objects and carefully observe their behavior. Using complicated physics, it's possible to determine the universe's geometry. At the moment, our best measurements show that the universe is flat. If it curves at all on large scales, it's very difficult to see.
Now let's imagine again that you're an ant, back on the ball. As a fairly smart ant, you might ask yourself: If my universe is curved, where is the center? Can I go there and look at it?
The answer is no! Remember, you're stuck on the surface of the ball, with no real concept of up or down. The center of the ball isn't on the surface, it's inside, removed into the third dimension, which you cannot access. You can search all you want, but you'll never find the center, because it's not in the universe as you know it.
The same can be said for own 3-D universe. If it has a center, it might not be in our universe at all, but in some higher dimension.
As it happens, even this might not be the case. Gauss showed mathematically that, as bizarre as it sounds, the universe can be curved without curving into anything. It just exists, and it's curved, and that's that. So it's not that we are curved into the fourth dimension, if there is such a thing. The fourth dimension may not exist at all, and our universe simply may not have a center.
This is the worse humiliation of all. To be removed from the center of the universe is one thing, and it's another to have it appear that we are at the center, only to realize that anywhere in the universe can make that claim. But then, to be told there isn't any center at all is the ultimate insult.
Maybe in a way it's the perfect equalizer. If we can't occupy the center of everything, at least no one else can, either.
And yet we are still not done.
Einstein was just getting started when he realized that space was a tangible thing. Time, he found, was a quantity that in many ways was like space. In fact, space and time were so intertwined that the term space-time continuum was coined to describe the union.
He also realized that the moment of creation, the Big Bang, was more than just a simple (though all-encompassing) explosion. It was not an explosion in space, it was an explosion of space. Everything was created in the initial event, including space and time. So asking what there was before the Big Bang really has no meaning. It's like asking, where was I before I was born? You were nowhere. You didn't exist.
But time was created in the event as well. So asking what happened before the Big Bang is what we call an ill-posed question, another question with no meaning. The physicist Stephen Hawking likens it to asking, "What's north of the north pole?" Nothing is! The question doesn't even make sense.
We want it to make sense, because we are used to things happening in a sequence. I get up in the morning, I ride my bike to work, I make my coffee. What did I do before I woke up? I was sleeping. Before that? I got into bed, and so on. But face it, at some point there was a first event. In my case, it was a moment in January 1964, which probably happened because it was a cold night and my future parents decided to snuggle a bit.
But there was something even before that, and before that. Eventually, we run out of thats. There was a first moment, a first event. The Big Bang.
In television documentaries it's very common to show an animation of the Big Bang as an explosion, a spherical fireball expanding into blackness. But that's wrong! Since the explosion was the initial expansion of space itself, there isn't anything for the universe to expand into. The universe is all there is. There is no outside, any more than there was a time before the Big Bang. What's north of the north pole?
The illusion of living in a big expanding ball persists. I have a hard time shaking it myself. You would think that there was some direction to the center of the universe, and if you looked that way you'd see it. The problem is, the explosion is all around us. We are part of it, so it's everywhere we look: the biggest movie theater of them all.
Still confused? That's okay. I sometimes think even cosmologists get headaches trying to picture the fourth dimension and the curvature of space, though they'd never admit it. There's an expression in astronomy: cosmologists are often wrong, but never unsure of themselves.
Yet we continue to try to understand this vast universe of ours. Maybe Albert himself put it best: "The most astonishing thing about the universe is that we can understand it at all."
I cannot leave this topic without one final note. Historians studying medieval astronomy are beginning to come to the conclusion that, to the medieval astronomers, being at the center of the universe was not all that privileged a position to occupy. It was thought that all the detritus and other, um, waste products of the heavens fell to the center, making up the Earth. So instead of being an exalted position, the center of the universe was actually a rather filthy place to be. In the end, maybe not even having a center is better than the alternative.