Untying the growth index to relieve the $\sigma_8$ discomfort

Understand

The matter fluctuation parameter $\sigma_8$ is, by model construction, degenerate with the growth index $\gamma$.

• Here, we study the effect on the cosmological parameter constraints by treating each independently from one another, considering $\sigma_8$ as a free and non-derived parameter along with a free $\gamma$.
• We then try to constrain all parameters using three probes that span from deep to local redshifts, namely the CMB spectrum, the growth measurements from redshift space distortions and the galaxy cluster counts.
• We also aim to assess the impact of this relaxation on the $\sigma_8$ tension.