The True Shape of the Universe Is Flat, According to Science—And Ancient Geometry Proves It
Here’s what you’ll learn when you read this story:
- Mathematical evidence indicates that our universe is flat, but it could be infinitely big, as well.
- You can figure out if an object is flat with a simple pencil-and-paper geometry exercise. To ascertain the shape of the universe, scientists did a similar calculation, but at huge scales.
- The evidence supports the idea that our universe will keep expanding forever. But there may be a different reality that’s too far away for us to see.
The universe is vast. As far as we can see in any direction, we are in a small portion, or bubble, of the whole entire universe. That bubble is about 90 billion light years across.
Yet, as contradictory as it sounds, the universe is flat. In fact, all evidence indicates that our universe is as flat as can be, even as it could be infinitely big. We’re even able to measure this flatness to a precision of less than one percent, which is pretty impressive.
This observation leads to a pretty strange outcome. We used to think that the shape of the universe was all we needed to know about the future of the cosmos. For example, a flat shape would mean slow and steady expansion, while curved would mean that it would eventually re-collapse to a super-dense, super-hot singularity, like a rubber band snapping back to its unexpanded form. This is the kind of state that led to the Big Bang about 13.8 billion years ago. But then we discovered dark energy, a still-mysterious force that could explain our accelerating cosmic expansion. That means the universe is flat and is expanding faster and faster, a bizarre combination of circumstances that we thought couldn’t exist!
How is this possible? It all comes down to geometry.
First, we need to understand how we can say one object is flat while another is round. What’s the real difference? There are two very easy tests you can apply to check if something is flat or curved, and the Earth itself is a great example.
The first test involves triangles. If you take a sheet of paper and draw three points on it, then connect those points with three lines to make a triangle, you’ll find that the interior angles of that triangle all add up to exactly 180 degrees. This is the geometry that we all learned in high school and stretches all the way back to the time of Euclid in ancient Greece.
But if you take a globe and pick three random cities, you get something different. If you draw straight lines connecting those cities to make a triangle on the surface of the globe, you will find that the interior angles of that triangle add up to more than 180 degrees. If you are adventurous enough, you can attempt this in real life and demonstrate that the Earth is indeed curved.
Another test involves parallel lines. On a flat piece of paper lines that start off parallel will remain parallel forever. Again, this is high school geometry. But on curved surfaces this isn’t always the case. For example, lines of longitudes start off at the equator being perfectly parallel. But as they march northwards they end up intersecting at the North Pole. This means if you and a friend start off on a journey together at the equator and always have your compass pointing North, and never turn left or right, you will end up intersecting your friend at the North Pole.
When it comes to the universe, we just have to apply these kinds of geometric tests at very large scales. One of the easiest ways to do this is with the cosmic microwave background, or CMB. This is light that was emitted when our universe was only 380,000 years old and when it expanded and cooled from a plasma state. That light has been sailing through the cosmos for over 13 billion years before eventually hitting our detectors on Earth. And since light rays always move in straight lines, we can use the appearance of the CMB on the sky to test the curvature of the universe.
We can do this test because the cosmic microwave background isn’t perfectly uniform. There are small variations, with hot spots and cold spots of temperature differences no more than one part in a 100,000. But we understand the physics that generated the CMB so well that we can predict how big those splotches on the sky should be. So, in a flat universe where light rays stay perfectly parallel and triangles add up to 180 degrees, the splotches should be the same size as we predict. If they’re larger or smaller, then we know that the light rays are getting curved, and therefore our universe has a different shape.
What does this mean for the overall structure of the universe? Unfortunately, we don’t get to see the whole entire universe. It’s only been around for so long—scientists estimate 13.8 billion years—and light can only move fast (186,000 miles per second), which is why we only get to see a small part. What’s outside of that bubble is anybody’s guess, although we strongly suspect it’s simply more stars and galaxies and all the usual stuff that makes up a universe. It could be that the universe is infinitely big and is simply flat everywhere throughout its entire infinite expanse.
But it’s also possible that beyond the boundaries of our observable bubble, the universe is curved. That would mean it only appears flat to us because we only get to see a small portion of it. This is like trying to measure the curvature of the Earth from your own backyard. Even if you have a grand estate, it will be very difficult for you to measure the curvature of the entire globe based on data you can only take from a very small portion of it.
But again, we do not know—and perhaps can never know—the true extent or shape or structure of the universe. We are locked inside of our observable bubble, and as far as we can tell, it’s flat. And we just have to live with that.
Contributor, science educator and theoretical cosmologist
Paul M. Sutter is a science educator and a cosmologist at Johns Hopkins University and the author of How to Die in Space: A Journey Through Dangerous Astrophysical Phenomena and Your Place in the Universe: Understanding Our Big, Messy Existence. Sutter is also the host of various science programs, and he’s on social media. Check out his Ask a Spaceman podcast and his YouTube page.
