The first Friedmann equation for a flat Universe is given by:(adsbygoogle = window.adsbygoogle || []).push({});

$$\bigg(\frac{\dot{a}(t)}{a(t)}\bigg)^2 = \frac{8 \pi G}{3} \rho(t)$$

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##?

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# Definition of Energy in Friedmann equations?

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