Albert Einstein took a bold step forward just two years after proposing his theory of general relativity when he decided to apply it to the universe as a whole. He made use of his theory, which described gravity as the curvature of spacetime around a mass, to determine the shape of the universe.
The theory postulates that the more massive a body is, the more warped the geometry surrounding it and the slower time moves. The theory of general relativity is not exclusive to one part of the universe as it measures it as a whole.
Simplifying the Universe
Einstein understands how controversial his ideas might be, but his proposal paved the way for a new era in cosmology that started with the application of general relativity to the universe as a whole and allowed researchers to study the origin of the cosmos.
However, finding solutions to the complex theory of general relativity needs to impose simplifications. According to Big Think, this happens often in physics, especially in linear problems. Prior to computers solving nonlinear systems, physics offered effective approximations. But it could still not solve the problem in its full complexity.
On the other hand, Einstein thought that he only needed to simplify the universe and fit it into a version of his equations that is solvable by hand. During that time, no one thought that the universe is expanding. But time has proven that this view changed after scientists noticed some local displacements of stars.
So what shape is the universe? Since there are less data available, scientists are freer to speculate. Einstein, like most people of his time, believed that the cosmos is static and that most matter is part of the Milky Way galaxy. Then in 1924, it became clear that Milky Way is just one of the billions of others, as per Hubble.
Einstein believes that the universe is spatially bounded and finite. Its geometry is determined by its total mass and/or energy as a result of special relativity. Einstein's first simplification known as the cosmological principle says that the universe on average looks the same in all directions.
It is homogeneous, and there is no preferred point or direction at large volumes. On a smaller scale, it will show that stars do not spread. Much like walking on a crowded beach, there is a lot of variation with some empty spots here and there, but from afar, it looks homogenous.
Einstein's Universe Is Spherical
After all those things are considered, it is easier to solve the equations and reveal that Einstein's universe is spherical with a geometry determined by the radius of the universe.
A similar report by the Space Academy says that Einstein assumed a finite, spherical, and static universe with a geometry characterized by a 3D generalization of a sphere. Since his theory suggests that matter bends geometry, scientists believe that it becomes clear that his equation points to a spherical universe.
But if the universe is finite and static and gravity is an attractive force, it means that matter will collapse on itself unless a negative pressure is present.
Therefore, to keep the universe static, Einstein added the cosmological constant into his theory. Dubbed as a negative pressure, the cosmological constant detracts from the formal beauty and simplicity of Einstein's original equations of 1915. Since the only reason for the cosmological constant's existence is to secure a static and stable finite universe, Einstein no longer wants to look much further.
But Russian physicist Alexander Friedmann expanded this geometry in 1922. When Einstein visited Hubble in California in 1933, he accepted cosmic expansion and discarded his vision of a static universe. Nonetheless, his equations provided a richer universe.
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