Cosmological constant

In general relativity, the matter-energy content of an object, will determine the curvature of the surrounding spacetime. Einstein asked the question: can the vacuum of empty space contain energy? This vacuum energy is known as the cosmological constant. Einstein, who found it unappealing, tried to find ways to work around including it in his equations, however, he couldn't. In fact, Einstein, found that his equations worked for an expanding universe. However, the accepted view at the time was for a static universe. Thus, Einstein introduced a small cosmological constant to balance it out.

We know today, that the cosmological constant is very close to zero. However, this problem came onto the scene again, this time, in particle physics. In the Standard Model, when symmetries are broken, the vacuum should gain some significant amount of energy. This amount of energy should be about 10^100 times the amount that we observe in experiment. This is the prediction of a large vacuum energy when symmetry is broken. This is the biggest divergence from experiment to theory in physics. It is a massive discrepancy.