 As remarked above, Penrose (1979) started this suite of ideas when he proposed the Conformal Curvature Hypothesis: that an entropy should be attributed to the gravitational field proportional to some measure of “purely gravitational” degrees of freedom, with a low entropy attibuted to homogeneous and isotropic gravitational fields. Some work in subsequent decades has been done, primarily based on Goode and Wainwright (1985) and Newman (1993a, 1993b), to try to generalize Penrose's proposal and make it rigorous. Almost all this work has focused on the behavior of conformal singularities (characterized at the end of  section 1.3)  which are, in a natural sense, “early” cosmological singularities, such as the Big Bang, and on the behavior of various measures of gravitational degrees of freedom moving to the future away from such singularities. (There has been some work, such as Rudjord et al. 2008, attempting to link the Conformal Curvature Hypothesis directly to black-hole entropy.) The idea is that the initial cosmological singularity, in accord with Penrose's Conformal Curvature Hypothesis, had extraordinarily low entropy, thus compensating the high entropy of the homogeneous ordinary matter present then, making the early universe a state of low total entropy. As the universe develops over time, and matter clumps into individual system (stars, galaxies, clusters and super clusters of galaxies, etc.), the entropy of ordinary matter seems to drop, but, again, that is more than compensated for by the enormous increase in gravitational entropy, thus saving the Generalized Second Law. 