Cosmology and entropy: in search of further clarity
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The concept of cosmic entropy, and the purported need to explain the initial conditions of Friedmann-Robertson-Walker 'Big Bang cosmology', continue to cause confusion within the scientific community. David Wallace's 2010 paper went some way towards disentangling this confusion, but left a number of significant issues unaddressed. The purpose of this paper is to define and resolve these issues. The paper begins by making a clear distinction between the entropy density and the entropy of a comoving volume. The different behaviour of these two quantities in Big Bang cosmology is explained and identified. A second distinction is drawn between the different behaviour of radiative entropy and the entropy of matter, and a third distinction is made between actual entropy and maximum possible entropy. The paper then devotes some attention to the particular issues associated with the entropy of matter, and its relationship to the existence of life and complexity. Wallace's account of Big Bang nucleosynthesis is endorsed, albeit in the context of a more general line of argument, demonstrating that the expansion of the universe generates information in both the radiation content as well as the matter content, and does so without the need for any 'clumping' of matter. The role of stars and galaxies in the cosmic entropy budget is then expounded, the argument concurring and extending that provided by Wallace. In particular, attention is drawn to the eventual evaporation of gravitationally bound systems. However, whilst Wallace accepts black-hole entropy as something of a special case, this paper takes a more sceptical approach to black hole thermodynamics, endorsing and extending recent arguments from Dougherty and Callender. The paper concludes by analysing the role of entropy within inflationary cosmology, identifying the differences and similarities with Big Bang cosmology.