General relativity (GR) is the gravitational framework that underpins the standard model of cosmology, the ΛCDM model; its predictions have been widely tested at astrophysical and cosmological scales, often with remarkable precision. However, our inability to directly observe the constituents of this model’s so-called ‘dark sector’ (i.e., dark matter and dark energy), along with the tensions characterising some of its parameters, prompts us to question the validity of GR at cosmological scales. That implies substituting GR with a modified gravity theory (MG) that can, for instance, explain the observed accelerated expansion of the universe without the need to introduce a cosmological constant (Λ). How can we test if a MG cosmology is sustained by current observations?
Because MG theories are numerous, we can turn to general, model-independent parameterizations of gravity, able to capture the phenomenology of several classes of deviations from GR at once. In this talk, I will present two such parameterizations: the μ/Σ framework and the growth index γ. I will show how they can be used to solve a discrepancy within different cosmic microwave background measurements (CMB) known as the ‘lensing anomaly’, while also explaining the apparent MG detection reported by the Planck CMB telescope as a collateral effect of the same anomaly.