One of the most startling cosmological discoveries made recently is of course the realization that the rate of expansion of the universe is increasing. this realization gave a new life to the cosmological constant that einstein once called one of the biggest blunders he made. suppose that future astrophysicists make yet another startling discovery that the rate of expansion of the universe stopped increasing and turned around to start decreasing. in such a circumstance, make a sound physical argument about the nature of the cosmological constant based on this hypothetical new discovery. (hint: what are the critical quantities that determine the rate of expansion of the universe? )