Emergent cosmos in Einstein–Cartan theory

Abstract

Based on Padmanabhan's proposal, the accelerated expansion of the universe can be driven by the difference between the surface and bulk degrees of freedom in a region of space, described by the relation dV /dt = N sur − N bulk where N sur and N bulk = −N em + N de are the degrees of freedom assigned to the surface area and the matter-energy content inside the bulk such that the indices "em" and "de" represent energy-momentum and dark energy, respectively. In the present work, the dynamical effect of the Weyssenhoff perfect fluid with intrinsic spin and its corresponding spin degrees of freedom in the framework of Einstein-Cartan (EC) theory are investigated. Based on the modification of Friedmann equations due to the spin-spin interactions, a correction term for Padmanabhan's original relation dV /dt = N sur +N em −N de including the number of degrees of freedom related with these spin interactions is obtained through the modification in N bulk term as N bulk = −N em + N spin + N de leading to dV /dt = N sur + N em − N spin − N de in which N spin is the corresponding degrees of freedom related with the intrinsic spin of the matter content of the universe. Moreover, the validity of the unified first law and the generalized second law of thermodynamics for the Einstein-Cartan cosmos are investigated. Finally, by considering the covariant entropy conjecture and the bound resulting from the emergent scenario, a total entropy bound is obtained. Using this bound, it is shown that the for the universe as an expanding thermodynamical system, the total effective Komar energy never exceeds the square of the expansion rate with a factor of 3 4π .

Static vacuum solutions on curved space–times with torsion

“…Since its reemergence in the late 1950s there have been renewed interests in ECKS theory in recent years. As the theory is still considered viable and remains an active field of research, much attempt have been made to generalize ECKS gravity in order to incorporate torsion into novel quantum theories and therefore providing possible extensions of GR to theories of micro-physical interactions [37,38,39,40], exploring cosmological implications of torsion [41], emergent universe scenario [42], gravitational collapse [43,44] and higher dimensional gravity theories [45,46,47]. Work along this line has been carried out in black hole physics [48,49,50,51,52,53] where possible effects of torsion can be investigated from astrophysical viewpoint.…”

Generalized thermodynamic constraints on holographic-principle-based cosmological scenarios

“…In this section, a holographic equipartition model is introduced, in accordance with previous studies [29,30], based on the original work of Padmanabhan [20], and other related research [21][22][23][24][25][26][27][28]. Although the assumption of equipartition of energy used for this model has not yet been established in a cosmological spacetime [30], we herein assume the scenario to be viable.…”

“…( 15) indicates that the difference between the degrees of freedom is assumed to lead to the expansion of cosmic space [20]. This is the so-called holographic equipartition law proposed by Padmanabhan and has been examined from various viewpoints [21][22][23][24][25][26][27][28][29][30]. In the present paper, we assume Eq.…”

“…The attracted scenario is Padmanabhan's holographic equipartition model [20]. In this model, cosmological equations can be derived from the expansion of cosmic space due to the difference between the degrees of freedom on the surface and in the bulk [21][22][23][24][25][26][27][28]. However, an extra driving term for the accelerated expansion is not derived from the Bekenstein-Hawking entropy [45], which is usually used for the entropy on the horizon of the universe [29,30].…”

Bouncing cosmological solutions from $$f(\mathsf{R,T})$$ f ( R , T ) gravity

“…In the context of matter bounce scenarios, many studies have been performed using quintom matter [86][87][88], Lee-Wick matter [89], ghost condensate field [90], Galileon fields [91,92] and phantom field [93][94][95][96][97]. Cosmological bouncing models have also been constructed via various approaches to modified gravity such as f (R) gravity [98][99][100][101][102], teleparallel f (T) gravity [103,104], brane world models [105], Einstein-Cartan theory [106][107][108][109][110][111][112][113], Horava-Lifshitz gravity [114], nonlocal gravity [115,116] and others [117]. There are also other cosmological models such as Ekpyrotic model [118,119] and string cosmology [120][121][122][123][124] which are alternatives to both inflation and matter bounce scenarios.…”