Entropy in the Universe

          @misc{ pirsa_PIRSA:11070006,
            doi = {10.48660/11070006},
            url = {https://pirsa.org/11070006},
            author = {Page, Don},
            keywords = {Cosmology},
            language = {en},
            title = {Entropy in the Universe},
            publisher = {Perimeter Institute},
            year = {2011},
            month = {jul},
            note = {PIRSA:11070006 see, \url{https://pirsa.org}}
          }
          

Abstract

"A positive cosmological constant allows arbitrarily many different quantum states, but apparently only if there can be big bangs and/or big crunches. Without any big bang or big crunch, the entropy may be limited by the Gibbons-Hawking entropy of pure deSitter, and the matter entropy might even more limited by a value roughly the three-fourths power of the Gibbons-Hawking entropy. A classical analogue of an upper limit on the entropy is the finite canonical measure for nonsingular cosmologies. Inflation by itself does not explain the arrow of time and seems to require that not only a small region have suitable initial conditions. A special quantum state, such as the Symmetric-Bounce one, with a suitable measure, such as volume averaging and Agnesi weighting, appears possible to explain the arrow of time and other observations, such as high order that would not be expected for Boltzmann brain observations, distant stars that would not be expected if we were fluctuations of empty de Sitter spacetime, and positive cosmological constant that appears not to dominate many other measures."