Cosmological perturbation theory is a mathematical framework used to analyze small deviations from a homogeneous and isotropic universe described by the Friedmann-Lemaître-Robertson-Walker (FLRW) metric. It helps in understanding how initial fluctuations in density and gravitational fields evolve over time, impacting the formation of large-scale structures in the universe. This theory is crucial for studying modified gravity theories as it examines how these alternative theories of gravity alter the dynamics of cosmic perturbations.
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