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I am new to dark matter and modified gravity so excuse and inform me if I am wrong.

If changes are made in the Friedman equations then there wouldn't always be an underlying action action principle. Is this not a major issue in Modified gravity? (again inform me if this has been solved).

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    $\begingroup$ As far as I know, action principle is good to have but not necessary. There are quite a few modified gravity theories whose starting point is not from a Lagrangian. On the other hand, there is really a lot of modified gravity theories which have fully consistent Lagrangian and even have exactly the same linearized gravitons (e.g. Lovelock gravity). A recent paper (arxiv.org/abs/1812.11187) even shows that one can do nice cosmological model without adding extra matter fields by just modifying the gravity sector alone. $\endgroup$ – Everiana Mar 20 at 3:11

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