 The second main avenue of testing focuses on the SM’s account of structure formation, which describes the evolution of small perturbations away from the background FLRW geometry in terms of a small number of parameters such as the tilt \(n_s\) and the scalar to tensor ratio \(r\). Observations from different epochs, such as temperature anisotropies in the CMB and the matter power spectrum based on galaxy surveys, can be used as independent constraints on these parameters as well as on the background parameters (indeed such observations turn out to give the best constraints on the background model parameters). These two routes to testing almost FLRW spacetime geometry are closely linked because the background model provides the context for the evolution of perturbations under the dynamics described by general relativity.