Learn more about the shape of the universe from Laura Mersini-Houghton and other scientists on the Science Channel, Wednesday, June 15, 2011, in a new episode of Through the Wormhole with Morgan Freeman: “Is there an Edge to the Universe?”
As part of my long-term project to Be a Better Science Writer, lately I’ve been reading Brian Greene. You may know him as a prominent string theorist, or as the author of several popular books on cosmology. Or maybe just as “that eleven dimensions guy.”
I mainly know Brian Greene because my college roommate, Sarah, had a crush on him. (Sarah: “You know, when Brian Greene looks into that video camera and tells me there are eleven dimensions…I believe him!”) Anyone who can win the hearts of college-aged women while discussing quantum physics in a PBS documentary must, I figured, know something about the right way to explain science.
So as I’m reading Greene, I’ve been trying to figure out what makes him such a good science writer. Some of it is what you’d expect: funny analogies, Simpsons references. (This may have also been part of what won Sarah over, since she’s the kind of person who talks half in quotations from The Simpsons.) But he also employs some tricks that absolutely crack me up, and that I don’t think I could ever get away with. I’m paraphrasing here, but he says a lot of stuff like: You may have to read this part twice. It’s pretty difficult. Or: I know this material is boring, but don’t be tempted to skip ahead to the black holes. Or, when he’s talking about space-time geometry (this one’s a direct quote): “Feel free to go into skim mode if at any point this section gets too heavy.”
By the time I was halfway through The Fabric of the Cosmos, I was pretty impressed, both with Greene and with myself. He was explaining heavy physics, and I was understanding it! Gravity and acceleration are equivalent—check. A particle in one place can mysteriously affect a particle in another place without sending it any kind of signal—check. All the order we see in the universe is a leftover from the extremely low-entropy state of the early universe—check. The shape of the universe might be like a video game—wait. What?
That’s right, Greene told me. A video game. It’s like we’re all characters moving across the screen of a flat universe, and if we go off the right side of the screen, we’ll reappear on the left. Or maybe the shape of the universe is more like a sphere. Or a Pringle.
A Pringle?! Surely the universe couldn’t really be anything like bad snack food, or like a Pac-Man game. There’s gotta be something more to this, I thought.
So I called up Laura Mersini-Houghton, a UNC theoretical physicist and cosmologist who’s spent a lot of time thinking about the shape of the universe. (Or the universes, plural. More on that later. But don’t skip ahead, okay?)
Mersini-Houghton never talks about Pringles, which makes this easier for me. Instead, she describes one of the possible universe-shapes as a hyperbola growing with time. If that seems like a crazy shape for the universe to be, think about it this way: it just reflects the idea of space-time having negative, or concave, curvature. The opposite of this theory is that the universe has positive, or convex, curvature—that it’s like a sphere.
What exactly does this idea of curved space mean? It means that if you made a slice through space, at a fixed moment in time, and tried to draw a triangle on a really huge scale (millions of light-years across), it wouldn’t be a proper, two-dimensional triangle. It would either be weirdly bloated (if the universe has positive curvature), or weirdly sunk in (if the universe has negative curvature). That’s what you can see in the first two shapes in the image at the top of this page.
The third option is that space-time isn’t curved—it’s flat. Triangles are really triangles, however big you draw them. The universe-as-video-game scenario depicts a flat universe. But the idea that our universe is flat and loops around like Pac-Man got ruled out a long time ago, Mersini-Houghton tells me. That universe just wouldn’t have enough dimensions. (To be fair, Greene says the same thing, ten pages later. But that was ten pages of me picturing all of us as Pac-Man characters!) But there’s another possible flat-universe scenario: that space-time just goes on, flatly, forever: a tabletop with no edges, as Greene describes it.
So are triangles really triangles—is the universe flat, or curved? Scientists have suspected for a while now that it’s flat, Mersini-Houghton says. “There was about a 95 percent chance that the universe is flat, and a 5 percent chance that it’s like a hyperbola,” she says. (At this point, none of the evidence favors the spherical-universe idea.) Then a NASA spacecraft, WMAP, measured an angle formed by two “sides” comprised of the distance microwave background radiation has traveled since the Big Bang—and confirmed that the resulting triangle is flat, or darn close to it. Scientists are watching to see if the Planck satellite, which can take even more accurate measurements than WMAP’s, confirms the flatness. “We’ll know for sure in a year, when the data from Planck are published,” Mersini-Houghton says.
It looks like we’ve got a flat universe on our hands. Flat, and infinite. There’s just one problem, according to Mersini-Houghton: our understanding of the Big Bang suggests that the universe isn’t infinite. “We know how big the universe is,” she says. “In meters, it’s a ten with twenty-five zeros after it. That’s really big, but not so big that we can’t imagine it.” That’s the distance that light from the edges of the universe has traveled in the fourteen billion years since the Big Bang. How could anything possibly exist outside that range?
Easily, Mersini-Houghton says, if it were created by a different Big Bang. That is, if it were part of a different universe.
The idea that there might be many universes isn’t anything new: physicists have being finding hints of it in their equations since the 1950s. But many of them ignored the possibility of a multiverse because, well, it seemed kind of crazy.
But in 2006, Mersini-Houghton and colleagues started making testable predictions of what neighboring universes might be doing to our own. They said another universe is pulling some of our matter toward it, creating a void in space. They predicted where this void would be; eight months later, scientists found it. Since then, there have been more findings that suggest that the multiverse might be for real. The most compelling was a 2008 finding that huge numbers of galaxies are moving the wrong way, independently of the path laid out for them by the expansion of space that’s been happening ever since the Big Bang.
“It can’t be something like a black hole doing it, because that would affect only the galaxies in the neighborhood of the black hole,” Mersini-Houghton says. Whatever’s making these galaxies move is influencing a huge amount of mass. Supporters of the multiverse idea think this means the galaxies are being drawn to something outside our universe.
So our theory becomes a flat, infinite multiverse, with finite universes scattered across it like cookies on an endless cookie sheet. (That’s my simile, so we’re using my preferred snack food.) I like this theory, and not just because it’s tastier than Pringles. But if you’re not satisfied yet, or you want to learn more about the history of the cosmos, Mersini-Houghton and other scientists are going to be on the Science Channel on Wednesday, June 15, answering the question, “Is there an edge to the universe?”
Also, Greene’s most recent book is about the possibility of a multiverse or parallel universes. I haven’t read it yet, but I hear Greene has moved on from The Simpsons to South Park. (Fans of Kenny should be pleased: it’s Cartman whose life is risked to explain potential and kinetic energy.)