Power Density as a Universal Biosignature: An Application to the Stellivore Hypothesis
Power density is a central measure underlying a common thread in cosmic evolution. It measures the rate at which free energy transits in a complex system of a given mass. We argue that it can be seen as a universal signature, and thus a strategy of ''following high power density'' can inform both biosignature and technosignature searches. We use known binary star masses and accretion rates to compute power densities of cataclysmic variables, neutron stars and transient black holes, and show that these accreting binaries have high power densities. We compare this result with other accreting systems in astrophysics as controls: young stellar objects (YSOs) and supermassive black holes (SMBHs). We review possible natural explanations, and then explore the possibility that these accreting binary systems might be instances of Type II stellar civilizations on Kardashev's scale, civilizations feeding on stars, or $\textit{stellivores}$. Besides power density, we present additional living clues, such as their variety, their existence far from thermodynamic equilibrium, their apparent homeostasis, the need to fine-tune models to explain them, the apparent existence of energy reserves, accretion control and ejection control, as well as other potential living systems functions. We summarize this case using living systems theory and discuss implications for big history.