I'm a little confused about the density [tex]\rho [/tex] in the equation:(adsbygoogle = window.adsbygoogle || []).push({});

[tex] H^2 = \left(\frac{\dot{a}}{a}\right)^2 = \frac{8 \pi G}{3}\rho - \frac{kc^2}{a^2} [/tex]

Measuring [tex]\rho [/tex] at a single instant in time seems easy. But [tex]\rho [/tex] changes with time. The time dependence of [tex]\rho [/tex] is given as [tex]\rho=\frac{M}{a(t)^3} [/tex] where M is a constant. But to determine M from a measurment of [tex]\rho [/tex], doesn't one have to know a(t), which is what the equation is trying to find?

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# Density in the friedmann equation

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