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A gravitational entropy that grows as stars and galaxies form would make the second law consistent from the Big Bang onward.","posed_since":"1979","precise":"Construct a coarse-grained entropy functional $S_{\\mathrm{grav}}$ for classical self-gravitating systems or perturbed Friedmann spacetimes (for example built from the Weyl tensor, the tidal part of spacetime curvature, following the Weyl curvature hypothesis) that is nondecreasing under gravitational clustering and reduces to the Bekenstein-Hawking entropy $A/(4\\ell_{P}^{2})$ for black holes. 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