What kind of galaxies have the highest entropy?
Late at night, waiting for a hyped-up rocket launch, I stumbled upon an old attempt to calculate the entropy of my simulated dark matter ensemble while idly scrolling my old code from my first year of PhD. It wasn’t used in any published result, other than confirming that my simulation indeed had the correct arrow of time. Still, now, years later, I decided to take a leap of faith, don the dusty eyeglass of Boltzmann et al. and paint a picture of galaxies through the language of thermodynamics.
For a long time, galaxies were viewed as glittering groups of stars orbiting one another, but modern cosmology has revealed a much more profound picture. Galaxies — the glowing bits — are lively participants in a complex environment shaped by the presence of dark matter halos, and stars themselves are the result of complex physics concerning the motion and evolution of interstellar medium. The evaluation of a galaxy’s entropy is a complicated task, requiring an understanding of all relevant physical processes. I think a rigorous computation takes a PhD project, maybe several.
In this mini-series, I aim to provide an undergraduate-friendly overview of the basic thermodynamic description of gravitationally bound particle ensembles, as well as the isothermal sphere model, and some astrophysics concerning the possible life histories of elliptical galaxies. While the topic may seem daunting, I believe it is fascinating to explore the relationship between entropy and the complexity of galactic structures.
Join me as I dive into this exciting field, drawing parallels between coffee-mixing and dark matter’s role in the cosmos.