Einstein's Cosmological Considerations of the General Theory of Relativity
- Einstein's paper Cosmological Considerations of the General Theory of Relativity was, yet another, key paper by Einstein which changed our view of the universe forever.
Albert Einstein, in 1917, published a paper entitled Cosmological Considerations of the General Theory of Relativity.
No longer was he concerned with the way gravity affects starlight or causes the precession of planetary orbits. Instead, he turned his attention to the role of gravity on the largest cosmic scale.
The Cosmological Principle
Einstein made his task easier by making the assumption now known as Einstein's cosmological principle.
- Einstein's cosmological principle states that the universe is more or less the same everywhere. That is, it is homogenous and isotropic.
A homogeneous universe is one whose composition is the same everywhere. One conclusion to draw from this is that the earth does not have a privileged position. The same sort of elements, space-time parameters, and other physical entities, are the same on and around the Earth as they are anywhere else in the universe.
Isotropic means looks the same in every direction. An isotropic universe looks the same in every direction.
Homogeneity does not imply isotropy, e.g. if galaxies were arranged in north to south lines the universe would look very different if you tilted your head to one side, but everywhere in the universe could have the same curious arrangement of galaxies.
Isotropy does not imply homogeneity. Just because the density of galaxies looks the same in every direction from Earth does not imply such symmetry applies elsewhere (although it would be very strange if it did not).
The number of distant galaxies is observed to be the same in every direction - the universe is isotropic on the large scale. Measurements of the microwave background radiation also justify the presumption of isotropy.
Einstein's gravity formula, like Newton's universal law of gravitation implies that every object in the universe is pulled to every other. This might eventually lead to a big crunch. But in 1917 Albert Einstein and the scientific establishment believed the universe was static. So Einstein changed his gravity formula to include a cosmological constant that "imbued empty space with an inherent pressure that pushed the universe apart". Singh p.148.
Big Bang by Simon Singh provides more detailed discussion of Albert Einstein's cosmological principle and other cosmological considerations of the general theory of relativity.