By Andrew Zimmerman Jones

Vacuum energy is the energy created by empty space by the constant creation and destruction of particles by the quantum fields that fill the universe.

Of all of the weird influences of quantum physics, probably one of the strangest is that there's no such thing as a truly "empty space," because these regions are filled will quantum fields that are constantly fluctuating, creating pairs of virtual particles that instantly annihilate each other.

This activity generates a background energy, known as the vacuum energy.

1. The problem of quantum gravity: Combine general relativity and quantum theory into a single theory that can claim to be the complete theory of nature.
2. The foundational problems of quantum mechanics: Resolve the problems in the foundations of quantum mechanics, either by making sense of the theory as it stands or by inventing a new theory that does make sense.

1. The unification of particles and forces: Determine whether or not the various particles and forces can be unified in a theory that explains them all as manifestations of a single, fundamental entity.
2. The tuning problem: Explain how the values of the free constants in the standard model of particle physics are chosen in nature.
3. The problem of cosmological mysteries: Explain dark matter and dark energy. Or, if they don't exist, determine how and why gravity is modified on large scales. More generally, explain why the constants of the standard model of cosmology, including the dark energy, have the values they do.