VACUUM ENERGY

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.

VACUUM ENERGY AND THE HEISENBERG UNCERTAINTY PRINCIPLE

One of the key concepts in quantum mechanics is known as the Heisenberg Uncertainty Principle, which states that in quantum physics certain related physical properties, such as energy and time, can never both be known precisely in a quantum system. The more precisely that you know one of the physical properties, the less precisely you know the other property.
Consider what this means for empty space. A region of "empty space," completely devoid of any matter, would seem to have precisely zero energy, but the fundamental uncertainty codified in the Heisenberg Uncertainty Principle means that you can never have a region that has precisely zero energy.

VACUUM ENERGY AND THE COSMOLOGICAL CONSTANT

When Albert Einstein developed his theory of general relativity, he identified a cosmological constant within his equations that represented the negative pressure of empty space, causing the universe to expand.

His reasoning for incorporating this value, rather than setting the constant equal to 0, was that he believed the universe was eternal and static. By having an expansion factor that perfectly counter-balanced the attractive pull of gravity, this could be achieved. The discovery of the expanding universe by Edwin Hubble in the 1920's changed his mind.

If the universe was already expanding, and not static, he felt that there was no reason to include this cosmological constant.

THE HISTORY AND IMPLICATIONS OF VACUUM ENERGY

In 1934, the physicist Georges Lemaitre (who also developed the big bang theory) combined these ideas to suggest that this vacuum energy might be the cause of the pressure described by the cosmological constant. Experimental evidence in support of this claim came in 1948, in the form of the Casimir effect.

THE VACUUM CATASTROPHE

The idea that the vacuum energy and the cosmological constant are linked has resulted in what is sometimes called the "vacuum catastrophe." From observation of the expansion of the universe, the upper limit on the cosmological constant (also known as dark energy when talking about the expansion of the universe) is found to be about 10^-9 joules per cubic meter.
Unfortunately, theoretical calculations of what one would expect on if one extrapolated the value from quantum electrodynamics and related approaches arrive at a prediction on the order of 10¹¹³ joules per cubic meter. This is difference by a factor of 10¹²² between theoretical expectations and observed reality is problematic, to say the very least.

Trying to find a theoretical explanation for the value of the cosmological constant in our universe is a rich area of physics research.