Unifying the dark sector through a single matter fluid with nonzero pressure

Dunsby P.K.S. Luongo O. Muccino M.
15 January 2024 American Physical Society

Physical Review D
2024 #109 Issue 2

We explore a generalized unified dark energy model that incorporates a nonminimal interaction between a tachyonic fluid and an additional scalar field. Specifically, we require that the second field possesses a vacuum energy, introducing an ineliminable offset due to a symmetry-breaking mechanism. After the transition (occurring as due to the symmetry-breaking mechanism of the second field), the corresponding equation of state (EoS) takes the form of a combination between a generalized Chaplygin gas (GCG) component and a cosmological constant contribution. We reinterpret this outcome by drawing parallels to the so-called Murnaghan EoS, widely-employed in the realm of solid-state physics to characterise fluids that, under external pressure, counteract the pressures effect. We examine the dynamic behavior of this model and highlight its key distinctions compared to the GCG model. We establish parameter bounds that clarifies the models evolution across cosmic expansion history, showing that it, precisely, exhibits behavior akin to a logotropic fluid that eventually converges to the ΛCDM model in the early universe, while behaving as a logotropic or Chaplygin gas at intermediate and late times respectively. We explain our findings from a thermodynamic perspective, and determine the small perturbations in the linear regime. At very early times, the growth factor flattens as expected while the main departures occur at late times, where the Murnagham EoS results in a more efficient growth of perturbations. We discuss this deviation in view of current observations and conclude that our model is a suitable alternative to the standard cosmological paradigm, introducing the concept of a matterlike field with nonzero pressure.

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Department of Mathematics and Applied Mathematics, University of Cape Town, Rondebosch, Cape Town, 7701, South Africa
Cosmology and Gravity Group (CGG), University of Cape Town, Rondebosch, Cape Town, 7701, South Africa
South African Astronomical Observatory, Observatory, Cape Town, 7925, South Africa
Centre for Space Research, North-West University, Potchefstroom, 2520, South Africa
Università di Camerino, Via Madonna delle Carceri 9, Camerino, 62032, Italy
SUNY Polytechnic Institute, Utica, 13502, NY, United States
Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, Perugia, 06123, Italy
INAF - Osservatorio Astronomico di Brera, Milano, 20121, Italy
NNLOT, Al-Farabi Kazakh National University, Al-Farabi avenue 71, Almaty, 050040, Kazakhstan

Department of Mathematics and Applied Mathematics
Cosmology and Gravity Group (CGG)
South African Astronomical Observatory
Centre for Space Research
Università di Camerino
SUNY Polytechnic Institute
Istituto Nazionale di Fisica Nucleare
INAF - Osservatorio Astronomico di Brera
NNLOT

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