w0waCDM

The w0waCDM (actually, w₀w_aCDM) model is a cosmological model that includes the Big Bang, dark matter, and dark energy, the model being much like the Lambda-CDM model (such as the inclusion of cold dark matter (CDM)), but does not characterize dark energy as a cosmological constant (Lambda or Λ) in Einstein's field equation. It is also unlike wCDM, i.e., it does not presume dark energy is such that the cosmological equation of state parameter (w, "cosmological EOS parameter") remains constant. Instead, it presumes a dark energy such that w evolves with the Hubble expansion in a particular way:

w = w0 + wa(1-a)
  where
w = p/ρ

• w - cosmological EOS parameter.
• w₀ - current value of w.
• w_a - state evolution parameter, which is the ratio of the change in w to the change in the scale factor.
• a - scale factor.
• p - pressure.
• ρ - energy density.

Some kind of dark energy is considered necessary to explain observation, but various cosmological models have been developed, of which Lambda-CDM has been the favorite, explaining a lot of observation data without excess complication. As further observation data is collected, Lambda-CDM may be inadequate. Various similar models also presume general relativity (GR) or something very close to it, along with certain amounts of dark matter and dark energy, but differ regarding the amount of dark energy that has existed at each point in time. wCDM and w0waCDM are two such alternatives. wCDM presumes the effect of dark energy is such that the cosmological EOS (w) is constant. w0waCDM is identical to wCDM if w_a is taken as zero (making w constantly w₀). Furthermore, wCDM is identical to Lambda-CDM if w is taken as -1. Parameters of interest generally have w and w₀ not straying far from -1 over much of the age of the universe.