Modified gravity and massive neutrinos: constraints from the full shape analysis of BOSS galaxies and forecasts for Stage IV surveys

Understand

We constrain the growth index $\gamma$ by performing a full-shape analysis of the power spectrum multipoles measured from the BOSS DR12 data.

• We adopt a theoretical model based on the Effective Field theory of the Large Scale Structure (EFTofLSS) and focus on two different cosmologies: $\gamma$CDM and $\gamma \nu$CDM, where we also vary the total neutrino mass.
• We explore different choices for the priors on the primordial amplitude $A_s$ and spectral index $n_s$, finding that informative priors are necessary to alleviate degeneracies between the parameters and avoid strong projection effects in the posterior distributions.
• Our tightest constraints are obtained with 3$\sigma$ Planck priors on $A_s$ and $n_s$: we obtain $\gamma = 0.647 \pm 0.085$ for $\gamma$CDM and $\gamma = 0.612^{+0.075}_{-0.090}$, $M_\nu < 0.30$ for $\gamma \nu$CDM at 68\% c.l., in both cases $\sim 1\sigma$ consistent with the $\Lambda$CDM prediction $\gamma \simeq 0.55$.