Wednesday, February 18, 2015

just two equations?

why are there exactly two Friedmann equations and not more (or less)?


  1. Because of the cosmological principle. Under the assumption of isotropy and homogeneity, the only independent Einstein equations are the time-time and space-space parts, and those give rise to the two Friedmann equations. If the assumptions are relaxed (for example in LTB models), one gets more relations (for example separate radial and angular Friedmann equations).

  2. thanks a lot Steffen, for your answer, to which I agree fully. I just would like to add that this expresses the fact that the Einstein field equation is of second order, hence acceleration or deceleration $\ddot{a}$ are natural phenomena.