One of the great remaining mysteries of modern physics is the accelerating expansion of the universe. It has been proposed that dark energy is responsible for this phenomenon. In order to understand dark energy, it is necessary to compute the energy density of the vacuum that arises from the fundamental theories that describe the universe. The vacuum energy density appears in Einstein’s theory of general relativity as the famous cosmological constant, and was first introduced in order to establish a static universe. General relativity would have otherwise predicted an expanding or contracting universe, and it was generally believed at the time that the universe was not changing in size. Later, when Hubble discovered that the universe was indeed expanding, Einstein discarded the constant. However, there is nothing in general relativity that forbids the introduction of the cosmological constant, and today, it is clear that its value must be found if one is to use general relativity to explain dark energy.

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• Physics