The Cosmological Constant

Remember, Einstein didn't like a expanding/contracting Universe, so he put it a term (called the cosmological constant) to counteract this and produce a static universe. When Hubble discovered the expansion of the Universe, Einstein called the cosmological constant his "biggest mistake."

Well, it's back.

If you go back to the Einstein field equations and work out the most general form of the dynamical equation, you get

With this term, the Friedmann equation looks like this:

What is this Lambda term? It corresponds to an additional form of energy in the Universe, which provides an outwardly directed force on the Universe, like a pressure (which could in principle balance the force of gravity).

But what is it????

The cosmological constant

... seems to be a manifestation of the energy density of empty space...
... may(?) come from the energy associated with virtual particles...
... affects the dynamics and curvature of the Universe...
... seems like it could be real (based on observations of the universe)!

Sounds like a great problem for the physicists! Models to explain the cosmo constant abound, including some very creative ones. However, predictions for the magnitude of the cosmological constant are currently 50-100 orders of magnitude off from the current (tentative!) observed value... Doh!