The vacuum or the any idea of ultimate nothingness has bewilldered curious minds for long. According to the current status in particle physics, what is going on in the vacuum is following. Several pairs of particles and anti-particles annihilate themselves, get created again, then annihilate, then create…and so on…thus in consequence, the net energy is conserved, because the particles created were also annihilated themselves. But the vacuum is not like a calm ocean. rather it is overwhelmed by constant ebb and flow, only that for each ebb there is a corresponding flow, precisely.
Adding any amount of energy to this vacuum is equivalent to introducing a cosmological constant. This fact relates Quantum Field Theory to space-time geometry. But the determination of this constant is a bane, which constitutes the cosmological constant problem.
Einstein believed in a static universe. According to his general relativity, universe was going to collapse. This fact is easy to understand by the virtue of Newton’s law of universal gravitation. Every object in the universe is attracting every other object. So to avoid this collapse, Einstein introduced a cosmological constant Lambda to have an anti-gravity effect.
The interplay between Quantum field theory and cosmological constant, and how this causes the vacuum energy, fascinates me a lot.
The cosmological constant problem is deemed to be among the most fundamental problems of the modern physics. Dr. Edward Witten opines that progress in String theory can’t be made with this problem lurking.
[E. Witten (2000) ‘ The cosmological constant problem from the viewpoint of string theory ‘, hep-th/0002247 ]