A Dark energy question (based on Blandford's talk at SSI)

I was just browsing through transparencies of this year's SLAC summer
school transprancies on Cosmology which are online at
http://www-conf.slac.stanford.edu/ssi/2003
anyhow I was through Blandford's talk on transparency 4 
which is at http://www-conf.slac.stanford.edu/ssi/2003/lec_notes/blandford/p1pg04.html
I find that for a darl energy/dark atter dominated universe
j= a'''a^2/(a')^3 = constant

I had never seen this before. can anyone provide a reference where
this has
been derived or discussed? also is this true for any  dark energy eqn
of state?
Thanks