I have decided to give a little, basic lesson on vacuum energy because some people round here don't have the first clue about it. Or what a real vacuum means in quantum mechanics. A real vacuum in classical mechanics does treat the vacuum as pure empty space. This is also known as the Newtonian vacuum. Since then, we have made some amazing advances in our understanding of the vacuum, largely thanks to quantum mechanics. No longer can we view the vacuum as pure space. It is in fact, a dynamic sheet of virtual particles which often jump into and out of existences. Let me give you some idea how we know this. The vacuum of spacetime is absolutely filled to the brim with quantum activity. Even if you could try and create a perfect vacuum by removing all the tiny bits of matter, there would still be a residual background noise of radiation. This has a better name called the zero point field. Even when you think you are dealing with no energy, you will still find a vibration equivalent to \(\frac{1}{2}\hbar \omega\). This is why in fact, the mathematical way we write a zero vacuum state \(|0>\) isn't actually accurate or real. This is all confirmed for the avid reader in the quantum mechanics section http://en.wikipedia.org/wiki/Free_space#Quantum_mechanics
"Filled to the brim" is a bit strong, though. All it is is a manifestation of quantum indeterminacy. Zero point energy is by definition unavailable to us. However, interestingly, I gather vacuum fluctuations are responsible for permitting the spontaneous emission from excited states of atoms and molecules to occur.
