Expanding Universe in Milne Coordinates
If we have a particle (X,Y,Z,T) = (a(T),a(T),a(T),T) in comoving coordinates, in Milne coordinates this is: t=T cosh( a(T)*sqrt(3) ) x=T/sqrt(3) * sinh( sqrt(3)*a(T) ) which gives a parametric equation in T. We can plot these curves. e.g. for linear expansion we have the lines a(T) = v.T for various constants v.