SOLVING THE "WHY NOW?" PUZZLE

A variety of observations indicate that the universe is dominated by dark energy with negative pressure, one possibility for which is a cosmological constant. If the dark energy is a cosmological constant, a fundamental question is: Why has it become relevant at so late an epoch, making today the only time in the history of the universe at which the cosmological constant is of order the ambient density. Ideas from Causal Set theory and Unimodular Gravity provide an answer, leading to a time dependent cosmological "constant" term of the right magnitude. I will briefly review the motivation behind the model, the ansatz that yields this fluctuating cosmological term, which is always of order the ambient density and summarize the results of the computer simulations based on the model.