*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)!