- #1
jcap
- 170
- 12
The first Friedmann equation for a flat Universe is given by:
$$\bigg(\frac{\dot{a}(t)}{a(t)}\bigg)^2 = \frac{8 \pi G}{3} \rho(t)$$
The energy density ##\rho(t)## is given by:
$$\rho(t) \propto \frac{E(t)}{a(t)^3}$$
where ##E(t)## is the energy of the cosmological fluid in a co-moving volume.
Is the energy ##E## the energy measured by a local observer at time ##t## or is it the energy measured with respect to a (global) reference observer at the present time ##t_0## where ##a(t_0)=1##?
$$\bigg(\frac{\dot{a}(t)}{a(t)}\bigg)^2 = \frac{8 \pi G}{3} \rho(t)$$
The energy density ##\rho(t)## is given by:
$$\rho(t) \propto \frac{E(t)}{a(t)^3}$$
where ##E(t)## is the energy of the cosmological fluid in a co-moving volume.
Is the energy ##E## the energy measured by a local observer at time ##t## or is it the energy measured with respect to a (global) reference observer at the present time ##t_0## where ##a(t_0)=1##?