Research on sentencing: The search for reform : Volume I, edited by Alfred Blumstein, Jacqueline Cohen, Susan E. Martin, and Michael H. Tonry. National Academy Press (2101 Constitution Avenue, Washington D.C. 20418), 1983, 315 pp., softcover—$19.95.

Abstract

The variation of dark energy density with redshift, ρX(z), provides a critical clue to the nature of dark energy. Since ρX(z) depends on the dark energy equation of state wX(z) through an integral, ρX(z) can be constrained more tightly than wX(z) given the same observational data. We demonstrate this explicitly using current type Ia supernova (SN Ia) data [the Tonry/Barris sample], together with the Cosmic Microwave Background (CMB) shift parameter from CMB data (WMAP, CBI, and ACBAR), and the large scale structure (LSS) growth factor from 2dF galaxy survey data. We assume a flat universe, and use Markov Chain Monte Carlo (MCMC) technique in our analysis. We find that, while wX(z) extracted from current data is consistent with a cosmological constant at 68% C.L., ρX(z) (which has far smaller uncertainties) is not. Our results clearly show the advantage of using ρX(z), instead of wX(z), to probe the variation of dark energy.