1.   Apart from notations for the Hubble parameter, the matter density parameter and the vacuum energy parameter, there is no further explanation.

2.   Einstein first proposed such a force, or negative pressure produced by vacuum energy, to correct what he thought was a flaw in his relativity theory.

3.   He suspects that a constant vacuum energy may not be the force behind cosmic acceleration.

4.   Instead, he suggests that a force that weakens over time is responsible, perhaps an exotic type of vacuum energy called quintessence.

5.   One suggestion is that empty space has a kind of vacuum energy that pushes galaxies apart.

6.   The gravity of matter would tend to decelerate expansion, but by acting against that gravity the vacuum energy of the constant might accelerate the expansion by now.

7.   Thus as the universe expands, the total vacuum energy, and the repulsive force associated with it, would grow larger with time.