A successful scientific theory is one that makes precise and accurate predictions. Scientists are even happier when two distinct theories make predictions that agree with one another. Thus, physicists are a bit chagrined when they use their two best theories to predict the simplest possible quantity, and the result is that they disagree spectacularly enough that it is often called “the worst prediction in the history of science.”
Empty space is, well, empty. Containing nothing, it would seem that calculating the energy of empty space would be simple and the prediction would be zero. However, that expectation is not correct.
The two theories that, when combined, underlie all of modern physics are called the theory of general relativity and the standard model of particle physics. General relativity describes the behavior of the force of gravity and applies to large structures in the Universe. In contrast, the standard model of particle physics is used to explain all other forces and governs the quantum world of the very small.
Both theories can be applied to empty space. So, what happens when the two theories are used to calculate the energy density of a true vacuum?