Okay, so you have read the Title. We know the shape of an egg, we know the the shape of the EARTH, we know the shape of the SUN and the MOON.
But It's the kind of question that messes up with your brain. When you even start to think that HOW TO THINK ABOUT IT: WHAT IS THE SHAPE OF UNIVERSE ?
Religious text have answered this in different ways throughout history
JAIN texts describe the universe as similar to 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 advancement in our understanding of the world, we can actually calculate the shape of the universe and arrive at a scientific answer.
Previously, there was no simply way to know what shape the universe took, But that changed when Albert Einstein introduced his theory of General Relativity.
General Relativity is important because it shows that mass curves the fabric of space and that therefore the shape of universe, and it's 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.
Critical density :- It is the value at which the Universe is at balance, and expansion is stopped. This value is estimated as (1-3)×10^-26 kg/m³ and it’s calculated when you take the matter-energy density of the universe and divide it by the matter-energy density of the universe that is required to achieve that balance.
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.
So really when we talk about the shape of the universe we're talking about the density of the universe. And it's important to remember that our universe actually exists in Four dimensions (4D), any shape we talk about are lower dimensional.
It turns out that there are 3 likely shapes that it can take.
THE FLAT UNIVERSE, which isn't flat like a 2 dimensional piece of paper, it's flat because it has no curvature this is the shape the universe would take if it has the exact amount of mass to stop the universe expanding after an infinite time. It will have no bounds and expand forever, slowing but never stopping.
THE 4D SPHERE UNIVERSE, If the universe contains more mass - more than the critical mass or the density of the universe is greater than the critical density then its eventually enough to stop expansion of the universe. It will have a positive curvature. And in this scenario the universe is closed and looks like a 4D sphere. It would be finite but without begining or end. Eventually, the mass will cause the universe to collapse back in to itself. (Any light will eventually end up where it started after it travels around.)
A spherical universe ends in something called a BIG CRUNCH, where all the matter Begins to be pulled in on it self and pulverized in a reverse BIG BANG.
OPEN OR HYPERBOLIC, SHAPE LIKE A SADDLE UNIVERSE, if the universe has insufficient mass to stop it's expansion or we can say that the if 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 hault it and it will have no bounds and expands forever in an open shape sort of like a saddle.
We are talking about 4 dimensional saddle and in this scenario, though, light would never return to where it originated but would continue on forever and grow more distant from it's 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.
A 2019 study published in Nature Astronomy concluded that based on Planck satellites measurements of the cosmic microwave background radiation, there's a 99% probability that the universe is a closed one.
Despite this the most of the cosmological observation say the universe should expand forever, so the positive curvature isn't likely. But it also doesn't seem to have the right condition for negative curvature.
The expansion rate of universe is 82.4 kilometers per second per megaparsec, which is faster than light itself.
MEGAPARSEC :- It's an unit of length. It is equal to 3.26 Light-Years.
The universe rate of expansion is accelerating, but that Rate is balanced by the mass of dark matter.
In short, the universe is likely ever so nearly flat. But it's possible that's the observable universe. Because the speed of light is fixed value we can only see 13.8 billion years back. It's possible we just can't detect the complete curvature with the instruments we have, or in ways of laws of physics will allow.
Scientists arrived at this conclusion using data from the Wilkinson microwave anisotropy probe (WMAP). If the universe was flare, then microwave background fluctuations would only be about one degree across.
As of 2013, NASA declared that the universe is flat with only a 0.4% margin of error. This conclude 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
But this doesn't mean there aren't other theories of it's geometry. As much of the physics we deal with inflation is unknown, so it could be our universe takes a completely different shape.
For example:- There are theories that it's shaped like a DONUT, or “TORUS”, where the universe is flat but connected in multiple places, it can be shaped like a TRUMPET, a CUBE, a HEXAGONAL PRISM, a CHIMNEY, or even a TWISTED CHIMNEY.
So, as the scientists Everytime contradict them selves or their own theories still overall, we all know the Earth isn't flat. But there's consensus that the universe almost is.
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