What is the density parameter, $\Omega$, compared against to classify cosmic geometry?

Answer

The critical density ($\Omega_0$) required to exactly balance expansion

The density parameter ($\Omega$) compares the actual average energy density of the universe to the critical density ($\Omega_0$), which is the value needed for expansion to be perfectly balanced.

What is the density parameter, $\Omega$, compared against to classify cosmic geometry?
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