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The wCDM 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 rather than characterizing dark energy as a cosmological constant in Einstein's field equation, it presumes dark energy has a constant cosmological equation of state parameter (w), which is the universe's ratio of pressure to energy density:
w = p / ρ
Some kind of dark energy is considered necessary to explain observation, but various cosmological models have been developed, of which Lambda-CDM is the current favorite, adopted for explaining a lot of observation data without excess complication. Other models also presume general relativity (GR) or something very close to it, and certain amounts of dark matter and dark energy, but differ regarding the rules that determine the amount of dark energy over time; wCDM is one such alternative.
Another model, w0waCDM, presumes w evolves in a particular way: w0 indicates the current value of w, and a constant wa (termed the state evolution parameter) is the ratio of any past w's rate-of-change with the difference between a at that time and that of the present (i.e., from 1).