Significance
One number per ingredient, and it decides everything. Feed w into the fluid equation and the dilution law falls out: matter thins as a⁻³ (volume alone), radiation as a⁻⁴ (volume plus redshift), and dark energy with w = −1 does not thin at all — which is why the early universe was radiation-dominated, the middle matter-dominated, and the present is being taken over by a constant. The expansion accelerates once the total w falls below −1/3. Whether dark energy's own w is exactly −1 is the open question of the subject: Planck's 2018 combination gave −1.03 ± 0.03, indistinguishable from a cosmological constant, and the baryon-acoustic-oscillation surveys since have pulled toward a w that changes with time. Treat any single number here as dated. The relation is Lemaître's, from the fluid equation of 1927; w as a parameter one quotes a value for is a much later convention, which is why no name sits on this entry.