Despite the success Einstein had thanks to, among others, the theories of general and special relativity, Einstein rejected to accept the answer that came out from mathematics. In fact, just as Friedman and Lemaître, Einstein realised that the equations of general relativity showed that the universe couldn’t be static. On the contrary, the fabric of space could expand of contract, but could not maintain a fixed size.
In the process of resisting to accept this, as he and the scientists of his epoch were convinced that the universe was static, he introduced a new term to the equations of general relativity that he named cosmological constant.
What does the cosmological constant consist on?
As it is already known, the matter and radiation scattered through space exert an attractive force, which makes an object approach to any other. By interpreting his equations in this way, Einstein realised that the universe had to be contracting. It is for this reason that Einstein introduced a new factor in the equations, that would provoke a contrary effect to gravity, and would cause a repulsive force. In other words, this new factor counteracted the gravitational force.
Choosing precisely the value of the new term, Einstein discovered that he could equilibrate perfectly the attractive gravitational force with the repulsive gravitational force that he had introduced to the equations. In this way, Einstein managed to calibrate these two forces to make the universe be static.
Despite that in that epoch scientists thought the universe was static, all changed in 1929. In this year, Hubble published his observations of faraway galaxies, demonstrating that all of them (except the closer ones) move away from us. In addition, he also observed that the further away a galaxy is from us, the faster it moves away. In this way, Hubble demonstrated, with astronomical observations, that the universe is expanding. I explain all this in this article.
When Einstein knew that the observations of the universe showed the opposite of what he had defended and calculated until that moment, he qualified the inclusion of the cosmological constant in his equations as his greatest mistake.
The cosmological constant brought back to life
Hubble’s observations negated for some time the need for a cosmological constant. However, this changed when a group of astronomers at the end of the 1990s discovered that the universe was not only expanding, but it did it at an accelerated rate. They named this strange form of anti-gravitational force (similar to the cosmological constant, and that forms more than 70% of the universe) as dark energy.