Einstein’s Theories of Relativity (1917)
Whereas gravity is traditionally understood as the relation between two objects, Einstein suggested that massive objects such as the Earth actually bend time and space, and that smaller objects follow that curved path around them (such as the Moon rotating around the Earth). Since what scientists understood to be the galaxy and universe at the time was clearly spread out and not a giant ball of matter, Einstein proved the cosmological constant, a sort of anti-gravity in space, that kept the universe unchanging. This led to the concept of a static (and infinite) universe without a “center.”
Hubble Discovers the Universe Is Expanding (1929)
When Edwin Hubble began his career at the Mount Wilson Observatory in 1919, it was still widely believed that the Milky Way Galaxy was the entire universe. By measuring the variation in star brightness over the next ten years, he proved that some stars we see are much farther away that the boundaries of the Milky Way. Further studies based on his work used the Doppler shift of different galaxies to prove that the universe is definitely expanding.
Observable Universe (2011)
Of course, much of our understanding of the universe comes from concepts and theories, but in reality, we only “know” what we can actually see. That’s why we mostly deal with the observable universe, that is, the galaxies, matter, and beyond that we can see from Earth since light from these regions have reached us at this present time (but is not to be confused with the so-called “visible” universe).
Given our terrestrial viewpoint, the observable universe is spherical and expands equally around us in all directions, with an estimated diameter of 93 billion light years. Andrew Colvin made this rendering to help us understand this concept.