A model for the cosmic creation of nuclear exergy

Abstract

The exergy¹ of a physical system is the maximum amount of mechanical work that can be extracted from that system so that a system in thermodynamic equilibrium has zero exergy. An old theme in cosmic thermodynamics is that the second law implies that the Universe is running down (running out of exergy) and approaching thermodynamic equilibrium, ‘heat death’. However, in the standard model of the early Universe², at 0.01 s after the big bang the Universe consisted of ordinary matter (nucleons), electrons and positrons, neutrinos, photons and gravitons. The gravitons were decoupled, but the others were in thermodyamic equilibrium. Thus the Universe had already reached the state of heat death, and its exergy was zero. The main problem is, therefore, not to describe the running-down of the Universe, but to understand its revival. When and how was exergy created, in particular the nuclear exergy, which is transformed into life-supporting light in our Sun? We have studied a model which should represent the nucleon gas of the early Universe quite well. We find that the main creation of nuclear exergy started around 10 s after the big bang, and most of the exergy was created during the first few minutes, 85% during the first hour, and that the process was essentially completed during the first 24 h. The final value of the exergy was 7.72 MeV per nucleon. The consumption of this exergy started much later and takes place in the stars over a time scale of hundreds and thousands of millions of years.