How is vacuum energy related to dark energy?

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Examine the observable universe's place within the whole universe — Cosmology Learn about defining and measuring the observable universe within the “whole” universe.

In cosmology, vacuum energy is considered the simplest and oldest theoretical explanation for dark energy.

Mathematically, vacuum energy is equivalent to the cosmological constant originally introduced by Albert Einstein. In the framework of quantum field theory, vacuum energy is understood as an energy density inherent to empty space, arising from the totality of quantum fluctuations—virtual particle-antiparticle pairs that constantly appear and annihilate.1

However, a significant discrepancy exists between theory and observation: the value of vacuum energy density predicted by quantum field theory is approximately $10^{120}$ times larger than the observed density of dark energy. This massive gap remains one of the most formidable challenges in modern physics, leading researchers to explore other possibilities, such as dynamical fields known as "quintessence" or potential modifications to general relativity.1