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What Shape Does The Universe Take? | Unveiled

VOICE OVER: Ashley Bowman WRITTEN BY: Dylan Musselman
What shape is the universe? In this video, Unveiled unpicks the multiple theories about what the universe actually looks like... If you were to somehow view the universe from the outside, what would you see? And would it be an open or closed system?
Transcript

What Shape Does the Universe Take?


When we look up at the night sky, we generally see two recurring shapes. Stars, planets, moons, and many asteroids are spherical in nature, while solar systems, galaxies, and even black holes tend to be flattened discs. These shapes are natural incarnations of the effects of gravity on massive objects. But what shape does the most massive structure of all take? If we could see our entire universe from an outside perspective, what would it look like?

This is Unveiled, and today we’re answering the extraordinary question: What Shape Does the Universe Take?

Human beings have guessed at the shape of our universe for millennia. We have an intrinsic desire to classify the world we live in and make some order out of the chaos of uncertainty. To this end, one of our most central desires is to know what it looks like. Religious texts have answered this question in different ways throughout history. Jain texts describe the universe as similar to the shape of a human body, with a narrow top that broadens out in the middle and bottom. The ancient Israelites saw the universe as a flat disc floating on a body of water, with the heavens high above and the underworld resting below. The philosopher Aristotle believed our universe to be a sphere, as the sphere is the most perfect shape. And Hindu texts envision the universe as a giant cosmic egg with everything contained inside. But thanks to advancements in our understanding of the world, we can actually calculate the shape of the universe and arrive at a scientific answer for the first time in history.

Previously, there was simply no way to know what shape the universe took, but that changed when Albert Einstein introduced his theory of General Relativity. General Relativity was important because it showed that mass curves the fabric of space, and that therefore the shape of the universe, and its eventual fate, can be inferred by the amount of matter it contains. It also allowed us to calculate a “critical density” - the amount of mass necessary for the expansion of the universe to stop after an infinite time. Scientists can compare the actual amount of mass in the universe to the critical density to determine what shape the universe takes and how it will eventually end. As it turns out, there are three likely shapes that it can take.

If the density of the universe is less than the critical density, then there just isn’t enough matter in the universe to counteract the expansion and halt it, and our universe will expand forever. In this scenario, the universe is considered open or hyperbolic and takes on the shape of a saddle. When speaking of the shape of the universe, however, it’s important to remember that our universe actually exists in four dimensions, so any shapes we talk about are lower dimensional shapes that we can understand, but we’re really talking about a four-dimensional saddle. In this scenario, though, light would never return to where it originated but would continue on forever and grow more distant from its origin as time continued. And because it’s expanding forever, it would reach a point where matter would begin to be stretched and pulled apart into pieces, causing our universe to end in what’s known as a Big Rip.

If the density of the universe is greater than the critical density, then the universe will eventually stop expanding. This would make our universe a closed one, and it would take the shape of a four-dimensional sphere. In this case, the universe is finite and has no discernible edge. But it also means that any light will eventually end up where it started after it travels around the universe. A spherical universe ends in something called a Big Crunch, where all matter begins to be pulled in on itself and pulverized in a reverse Big Bang. Although this isn’t the widely accepted view of the shape of the universe, recent research points to this being the case. A 2019 study published in Nature Astronomy concluded that based on Planck Satellite measurements of the cosmic microwave background radiation, there’s a 99% probability that the universe is a closed one. The paper argues that the standard cosmological model is based on unknown physics, as nobody has ever actually seen inflation, dark energy, or dark matter, and that we may need to discover a different set of physics entirely to understand the shape of our universe.

Despite this, the most widely accepted theory of the universe is that its actual density is equal to the critical density, and that the universe is flat like a sheet of paper and infinite in extent. Scientists arrived at this conclusion using data from the Wilkinson Microwave Anisotropy Probe (WMAP). If the universe was flat, then microwave background fluctuations would only be about one degree across, and this was shown to be the case. As of 2013, NASA declared that the universe is flat with only a .4% margin of error. This means that the overall density of the universe is about 5.9 protons per cubic meter, with 4.6% of all mass being atoms, 24% being dark matter, and 71% being dark energy. The end of the universe is similar to the end of a saddle universe, with all matter being torn apart in the Big Rip.

Although a saddle, sphere, and sheet of paper are the most probable shapes that the universe can take, that doesn’t mean there aren’t other theories of its geometry. As previously stated, much of the physics dealing with inflation is unknown, so it could be that our universe takes a completely different shape. In addition, when referring to shape, scientists are referring to topology, and objects in this situation can be treated like they’re made of rubber and can be malleable as long as there is no cutting or pasting. Because of this, the universe could be shaped like many different things. There are theories that it’s shaped like a donut, or “torus”, where the universe is flat but connected in multiple places. It could also be shaped like a trumpet if there’s negative curvature, where one end is very flared and wide and the other end is very narrow. If this were the case, someone living in the narrow end would only experience two dimensions. Some shapes that the universe can take are even nonorientable, meaning that if you were to travel along such a loop you would arrive back on Earth with your heart on the wrong side of your body and your watch ticking counter clockwise, although you would see everyone else as being mirrored. Other possible shapes include a cube, a hexagonal prism, a chimney, or even a twisted chimney.

Somewhat surprisingly, our advanced scientific views loosely resemble the predictions made about the shape of our universe in ancient times. The Hindus believed that the universe was shaped like a cosmic egg, which isn’t too far off from it being a sphere. Jainism predicted that the universe was narrow at the top and widened out at the bottom like a human body, or even somewhat like a trumpet. And ancient Israelites predicted a flat disc universe, which is close to the flat universe that scientists theorize. Overall, there’s consensus that the universe is most likely shaped like a piece of paper, but that could one day change as we learn more about the world we live in, and perhaps even develop new physics.

And that’s what shape the universe takes.
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