  A time-varying cosmological "constant" Lambda is consistent with Einstein's
equation, provided matter and/or radiation is created or destroyed to
compensate for it. Supposing an empty primordial universe endowed with a very
large cosmological term, matter will emerge gradually as Lambda decays.
Provided only radiation or ultrarelativistic matter is initially created, the
universe starts in a nearly de Sitter phase, which evolves towards a FRW regime
as expansion proceeds. If, at some cosmological time, the cosmological term
begins increasing again, as presently observed, expansion will accelerate and
matter and/or radiation will be transformed back into dark energy. It is shown
that such accelerated expansion is a route towards a new kind of gravitational
singular state, characterized by an empty, conformally transitive spacetime in
which all energy is dark.
