Abstract
We investigate the possibility of gravitationally generated particle production via the mechanism of nonminimal torsion–matter coupling. An intriguing feature of this theory is that the divergence of the matter energy–momentum tensor does not vanish identically. We explore the physical and cosmological implications of the nonconservation of the energy–momentum tensor by using the formalism of irreversible thermodynamics of open systems in the presence of matter creation/annihilation. The particle creation rates, pressure, and the expression of the comoving entropy are obtained in a covariant formulation and discussed in detail. Applied together with the gravitational field equations, the thermodynamics of open systems lead to a generalization of the standard ΛCDM cosmological paradigm, in which the particle creation rates and pressures are effectively considered as components of the cosmological fluid energy–momentum tensor. We consider specific models, and we show that cosmology with a torsion–matter coupling can almost perfectly reproduce the ΛCDM scenario, while it additionally gives rise to particle creation rates, creation pressures, and entropy generation through gravitational matter production in both low and high redshift limits.
Highlights
General Relativity has been established as the theory of gravitational interactions for over a century, being consistent with all experiments and being able to describe a huge set of observational results
The main motivation for considering this formalism is related to the possibility of the interpretation of the nonconservation of the standard matter energy–momentum tensor in theories with torsion–matter coupling, as describing particle creation on a cosmological scale, a process that would naturally require the use of the irreversible thermodynamics of open systems [25,26,27,28,29,30]
We have explored the significance of the nonvanishing divergence of the matter energy–momentum tensor by adopting the theoretical perspective of the thermodynamics of irreversible processes, as introduced and developed in [25,26,27,28]
Summary
General Relativity has been established as the theory of gravitational interactions for over a century, being consistent with all experiments and being able to describe a huge set of observational results. One can as well, start from the equivalent gravitational description in terms of torsion, namely from the teleparallel equivalent of general relativity [10,11,12,13], and extend the corresponding Lagrangian, given by the torsion scalar T In this way, we can obtain f (T) gravity [14,15], f (T, TG) gravity [16], scalar-torsion theories [17], f (T, B) gravity [18], etc. One can further extend the torsional formulation in theories with nontrivial couplings between em em gravity and the matter sector, such as in f (T, T ) (where T is the trace of the matter energy–momentum tensor) [19], or in theories with a nonminimal coupling between the torsion scalar and the matter Lagrangian [20] These theories prove to have interesting cosmological applications [20,21,22,23,24]. We can replace the derivatives with respect to the time with the derivatives with respect to z according to the rule
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