What is the shape of the universe? The universe is everything that we can observe, so we can’t stand outside it to see if it’s shaped like a ball or a potato chip or something else entirely. That doesn’t mean cosmologists aren’t trying to figure it out, though. It’s an important question, though it forces us to expand our ways of thinking about shape. As it turns out, the answer to the question relates to what the universe is made of and how it began. The issue got some public attention recently when three cosmologists claimed the universe curls back on itself, which contradicts many other observations. So who’s right?
The Shape of Everything
First, what does it even mean for the universe to have a shape if it stretches out forever in every direction?
While Earth’s roundness is most obvious in pictures taken from space, people have known the shape of our planet since antiquity. (Fun fact: the myth that Christopher Columbus discovered Earth is round was invented by Washington Irving, the “Legend of Sleepy Hollow” guy. Maybe don’t trust that dude’s historical anecdotes.) In other words, we don’t have to go to space to know our planet’s shape: We just do measurements that would come out differently if the planet were flat, without ever leaving the ground.
Think of how longitude lines look on a globe: They start off parallel to each other at the Equator but converge at the poles. If we had two airplanes fly north from the Equator starting in, say, Ecuador and Kenya, they would get closer and closer to each other until they eventually arrived at the same point. Cosmologists say Earth has “positive curvature” everywhere, which is a fancy way to say “spherical.”
The same thing goes for the universe: We can observe various properties that would appear differently depending on its shape. Instead of airplanes, we talk about light, which follows the straightest possible path it can. If two initially parallel rays of light converge, the universe has positive curvature like a sphere, and we say it is “closed.” If those rays diverge, the universe is “open” and has negative curvature. (This one’s harder to visualize: It’s a potato-chip-like geometric shape called a “hyperboloid.”) If they stay parallel, the cosmos is flat.
One way to check for convergence or divergence is in the cosmic microwave background, or CMB: light coming from all directions that are from 380,000 years after the Big Bang. That means those photons have been traveling for 13.8 billion years. Fluctuations in the CMB would appear different if the universe were closed, open, or flat since the photons would have to travel along paths determined by cosmic geometry.
So which is it? Well, many CMB observations, including the balloon-carried BOOMERanG telescope and the WMAP space observatory, measured these fluctuations and found the universe is — drumroll, please! — flat. That might sound boring, especially if you’re a photon traveling for 13.8 billion years in one direction, but it’s actually what cosmologists expected.
As it turns out, flatness is like balancing a coin on its edge: It stays like that if you set it up that way, but even a slight nudge will dislodge it. More scientifically speaking, if the universe started off closed or open, the effects of those geometrical beginnings would become more pronounced over time. We don’t see that, so astronomers suspected the universe was flat even before the CMB measurements showed it was so.
Flatness helps us determine the relative amounts of matter — both regular atoms and the mysterious invisible dark matter that holds galaxies together — and dark energy, which makes the expansion of the cosmos accelerate. A flat cosmos means those things are on a seesaw: The more dark energy in a given volume, the less matter there can be, and vice versa.
All this is why a recent article in Nature Astronomy made waves. The authors claim discrepancies between measurements can be explained if the universe is ever-so-slightly closed. Keeping the universe almost but not quite flat is a difficult trick, and the wealth of evidence in favor of flatness makes most cosmologists skeptical.
The flatness of the universe is one of the stranger facts about it. A little weirdness never stopped scientists, though, and explaining why the cosmos started off flat requires extra physics, such as “inflation”: the hypothetical extremely rapid expansion of the universe during the first split-second of time after the Big Bang. Many of the next generation cosmology observations are dedicated to understanding those first moments of time, so the story of how the universe came to be flat is far from over.