Albert Einstein's theory of relativity is a mathematical framework that revolutionized our understanding of space, time, and gravity. It consists of two major components: the special theory of relativity and the general theory of relativity.
The special theory of relativity, published by Einstein in 1905, introduced two fundamental principles: the principle of relativity and the constancy of the speed of light. The principle of relativity states that the laws of physics are the same for all observers in uniform motion relative to each other. The constancy of the speed of light postulates that the speed of light in a vacuum is constant and independent of the motion of the source or observer. These principles led to remarkable consequences, including time dilation, length contraction, and the equivalence of mass and energy (expressed by the famous equation E=mc²).
The general theory of relativity, developed by Einstein in 1915, extended the principles of the special theory to include gravity. According to general relativity, gravity is not a force as described by Newtonian physics but rather a curvature of spacetime caused by mass and energy. In this theory, massive objects like stars and planets distort the fabric of spacetime, and other objects move along the curved paths dictated by this distortion.
Regarding the expansion of the universe, Einstein's equations of general relativity predicted that the universe could be either expanding or contracting. Initially, Einstein believed in a static universe and introduced a mathematical term called the cosmological constant to counterbalance the gravitational attraction and maintain a static state. However, in 1929, Edwin Hubble's observations of distant galaxies revealed a relationship between their recession velocities and their distances. This relationship, known as Hubble's law, indicated that galaxies were moving away from each other, suggesting an expanding universe.
Einstein later acknowledged his initial assumption of a static universe as a mistake, referring to it as his "greatest blunder." He realized that his equations of general relativity indeed allowed for an expanding universe without the need for the cosmological constant.
In summary, Einstein's theory of relativity provided a new understanding of space, time, and gravity, and while it didn't directly prove the expansion of the universe, it set the foundation for the understanding of how the universe can evolve and change over time, including the possibility of an expanding universe.