Contents

Contents

Idea

The Friedmann–Lemaître–Robertson–Walker models (often FRW-models) are class of models in cosmology. These are solutions to Einstein's equations describing a spatially 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. (In contrast to inhomogeneous cosmology.)

Details

The plain FRW model parameterizes a homogenous and isotropic spacetime after diffeomorphism gauge fixing with a single parameter $t \mapsto a(t)$, the scale factor of the universe depending on coordinate time $t$ (fixed by some gauge condition).

The equations of motion of the FRW model are then

1. $H^2 \coloneqq (\dot a / a)^2 = 2 \rho - k/a^2$

2. $\ddot a/ a = -(\rho + 3 p)$

3. $\dot \rho + 3 H(\rho + p ) = 0$

where

• $H \coloneqq \dot a/a$ is called the Hubble parameter?;

• and

• $p$ is the pressure

• $\rho$ is the density

of a “perfect fluid” of matter and radiation filling the universe.

Moreover, the ratio

• $w \coloneqq p/\rho$

is part of the experimental/phenomenological input into the model, which describes which kind of matter/radiation is assumed to fill spacetime

wsource of energy-density filling spacetime
0dust matter
1stiff fluid
-1cosmological constant

The first equation may be rewritten as

$\Omega \coloneqq \Omega_R + \Omega_M + \Omega_\Lambda = 1 + \frac{k}{a^2 H^2}$

where the density parameter $\Omega$ consists of the contribution

• $\Omega_R = 2 \frac{\rho_r}{a^2}$ of radiation;

• $\Omega_M = 2 \frac{\rho_r}{a^2}$ of matter;

• $\Omega_\Lambda = \frac{\Lambda}{a^2}$ of cosmological constant;

References

Named after Alexander Friedmann?, Georges Lemaître, Howard Robertson?, Arthur Walker.

Introduction and survey: