A Friedmann–Lemaître–Robertson–Walker model is a model in cosmology. It is a solution to Einstein's equations describing an homogeneous and isotropic expanding or contracting spacetime. Hence these are solutions used as models in cosmology. Indeed, an FRW-model is part of the standard model of cosmology.
The plain FRW model parameterizes a homogenous and isotropic spacetime after diffeomorphism gauge fixing with a single parameter , the scale factor of the universe depending on coordinate time (fixed by some gauge condition).
The equations of motion of the FRW model are then
is called the Hubble parameter?;
Moreover, the ratio
is part of the experimental/phenomenological input into the model, which describes which kind of matter/radiation is assumed to fill spacetime
|w||source of energy-density filling spacetime|
|1/3||radiation or relativistic matter|
The first equation may be rewritten as
where the density parameter consists of the contribution
Jorge L. Cervantes-Cota, George Smoot, Cosmology today – A brief review (2011)(arXiv:1107.1789)
Wikipedia, Friedmann–Lemaître–Robertson–Walker metric