The Cosmic Equation of State

Understand

The cosmic spacetime is often described in terms of the FRW metric, though the adoption of this elegant and convenient solution to Einstein's equations does not tell us much about the equation of state, p=w rho, in terms of the total energy density rho and pressure p of the cosmic fluid.

• LCDM and the R_h=ct Universe are both FRW cosmologies that partition rho into (at least) three components, matter rho_m, radiation rho_r, and a poorly understood dark energy rho_de, though the latter goes one step further by also invoking the constraint w=-1/3.
• This condition is required by the simultaneous application of the Cosmological principle and Weyl's postulate.
• Model selection tools in one-on-one comparisons favor R_h=ct with a likelihood of ~90% versus only ~10% for LCDM.