$\Omega_{\text{m}}$ and $\Omega_{x}$, but only up to $\Omega_{\text{m}0}$ [@Kunz:2007rk], since one can compensate for any change of $\Omega_{\text{m0}}$ with a modification of the DE model. Of course, if we parameterize the evolution of $\Omega_{x}$ with a simple equation of state, we can break the degeneracy with $\Omega_{\text{m}0}$, as is usually done in analyses of SNIa data, but that is exactly what we are trying to avoid in this work.

The same result is valid if instead of pure pressureless matter one includes further components (e.g. massive neutrinos) that evolve with an effective equation of state $w_{\text{m}}(z)$, provided $w_{\text{m}}(z)$ can be inferred from other observations (e.g. knowledge of the neutrino masses).

Linear Perturbation Observables {#sec:LinPerts}
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The linear perturbation observables are the correlations of positions, velocities and shapes (ellipticities) of sources (i.e. galaxies, Lyman-$\alpha$ lines, clusters, background radiation) in angular separation and redshift. Given knowledge of $D(z)$, these can be converted to the more usual dependence on wavenumber $k$ and redshift.

Let us first discuss the clustering of matter. We denote the root mean square of the correlation of galaxy number counts in Fourier space as $\delta_{\text{gal}}$ (i.e. $\delta_{\text{gal}}\equiv P_{\text{gal}}^{1/2}(k,z)$ where $P_{\text{gal}}$ is the galaxy power spectrum). We define from now 