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# System prepended metadata

title: Theoretical Cosmology

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# Theoretical Cosmology 

## Inflation 

1. constrains on isocurvature fluctuations (Planck 2018 Inflation)
Single-field inflation with a canonical kinetic term gives rise to primordial super-Hubble comoving curvature perturbations, R. In this case the relative number densities of the various particle species are spatially constant, i.e., the perturbations are adiabatic. Typically photons are chosen as a reference species. Then adiabaticity implies that for every particle species with number density ni the quantity $δ(n_i/n_γ)$ vanishes. However, in addition to R, multi-field inflation can stimulate isocurvature modes, $I_i$, where at primordial times $n_i/n_γ$ varies spatially.