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It is easy to dismiss the idea that the Earth is flat, but did you know that cosmologists also refer to our universe as possibly flat? However, this describes a specific geometric calculation and does not mean that the universe is flat, as in pancake-shaped.
This is a theory associated with Alan Guth and others, developed in the 1980s. It aims to explain how the universe follows Euclidean geometry, such as parallel lines never meet, triangles have angles summing to 180°, and straight paths stay perfectly straight.
What Does “Flat” Really Mean in Space?
When cosmologists say the universe may be flat, they are referring to its curvature or lack thereof. This simply means that it follows the Euclidean geometry laws, such as when you draw a triangle across space, its three angles will add up to 180°.
Because imagine it in this way, when you draw a triangle in a piece of flat paper, the interior angles always sum up to 180°, but when you draw it in a balloon or sphere, the total will deviate.
Additionally, on these curved spaces, parallel lines that are supposed not to meet could be converging or diverging. Measurements from missions like WMAP and Planck confirm that the universe’s curvature parameter is nearly zero at ≲ 0.005. In other words, it behaves like a flat Euclidean sheet even on the grandest of scales.
Read more: Space.com
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How Do Scientists Know the Universe Is Flat?
The strongest evidence came from the CMB or cosmic background radiation. These are basically ancient light generated by the Big Bang explosion. The scientists measured the angular size of temperature patterns in the CMB, testing if the universe could be flat, a sphere, or even torus or donut-shaped.
They essentially performed trigonometry at the largest scale possible, checking if the Euclidean geometric laws apply. WMAP’s nine-year data found a curvature value consistent with a flat universe, and Planck further supported that precision.
In simpler terms, parallel lines do not meet, triangles always sum up to 180°, and straight lines stay perfectly straight. These findings from NASA strongly support a flat geometry, ruling out notable curvature. Everything we explained is a simplification of the idea, but it stands.
Flat Doesn’t Mean Endless — It Could Wrap Around
A flat universe does not mean that the cosmos is infinite. It could also wrap around itself like an endless loop on a donut-shaped universe, where, if you travel far enough, you might end up in the same place.
There is no currently firm evidence yet as to what our universe looks like on the grandest scale; however, the possibility is still there, based on our understanding of physics and geometry, that the universe may be flat but wrapped up in a complex, finite shape.
Read more: Astronomy Magazine
Why It Matters That the Universe Is Flat
The flatness of the universe will tell us a lot about its beginnings and possible end. If it is true that the universe is flat and follows Euclidean geometrical laws, then it must have been flat just after the Big Bang, too. The straightening of the curvature might be what is causing the cosmic inflation or the expansion of the universe.
A flat, inflationary universe is also tied to mysterious forces like dark energy, the primary force thought to be driving the expansion of the universe. This expansion of space-time could be how the cosmos meets its end, like collapsing into itself or ripping it all apart. The flatness of the universe will play a role in whether it expands forever, rips apart, or loops back.
Author's Final Thoughts
The universe is not flat like a pancake or a sheet of paper, but instead it is flat in a geometric sense. It follows Euclidean rules even in cosmic scales, where the triangles add to 180° and parallel lines never meet. This is one of the weirdest and confusing descriptions of our universe, but so far, all the evidence we have gathered points to it as being precise.
Read next: Scientists Explain: If the Universe Is Expanding, What Exactly Is It Expanding Into?
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