A Thermodynamic Approach towards Understanding the Dark Universe

We present a thermodynamic approach in modeling the evolution of the universe based on a theory that space consists of energy quanta and is the cosmic fluid component of the universe. It provides an insight on the nature of dark energy and dark matter, as well as a rationale for the accelerated expansion of the universe. The universe started from an atomic size volume of an ideal gas at very high temperature and pressure. Upon expansion and cooling, phase transitions occurred resulting in the formation of fundamental particles, and matter. These nucleate and grow into stars, galaxies, and clusters with the aid of gravity. From the cooling curve of the universe we constructed a thermodynamic phase diagram of cosmic composition, from which we obtained a correlation between dark energy and the energy of space. Using Friedmann’s equations, our model fits well the WMAP data on cosmic composition with an equation of state parameter, w = −0.7. The dominance of dark energy started at 7.25 × 10⁹ years, in good agreement with BOSS measurements. The expansion of space is attributed to Quintessence associated with a quantum space field. Dark Matter is identified as a plasma form of matter similar to that which existed during the photon epoch, prior to recombination. The thermodynamics of expansion of the universe was adiabatic and decelerating during the first 7 billion years after the Big Bang; it became non-adiabatic and accelerating thereafter. The latter maybe due to an influx of energy from a source outside the universe, if it is open. If it is closed, thermodynamics requires that the pressure of space be negative. Said pressure would cause the accelerated expansion of the universe in accordance with the theory of General Relativity, and the law of conservation of energy. We provide a mechanism to explain this. The acceleration should not be interpreted as due to a repulsive form of gravity. Our Quantum Space model fits well the behavior of the observable universe.