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as I try to understand Einstein's Field Equations, I realize that I don't understand the meaning of its components.

I gather that Rtt=Ttt+Txx+Tyy+Tzz is the dominating parameter in cosmological models. (Sorry, I also have problems with tex.)

Ttt means "mass energy density". Is this "relativistic mass" per unit volume?

Txx etc. means pressure. Is this momentum flow per unit area? The net flow should be 0 (in a comoving frame), how do I calculate Txx.

Example:

A constant matter density gives Rtt=Ttt=rho in the comoving frame.

What if all that matter annihilates? What remains constant, Rtt or Ttt?

Rtt=rho+3*rho/3 or

Rtt=rho/2+3*rho/6?

Is Rtt equal to some total energy density?

Thanks.