Phantom-like dark energy from quantum gravity

We analyse the emergent cosmological dynamics corresponding to the mean field hydrodynamics of quantum gravity condensates, in the group field theory formalism. We focus in particular on the cosmological effects of fundamental interactions, and on the contributions from different quantum geometric modes. The general consequence of such interactions is to produce an accelerated expansion of the universe, which can happen both at early times, after the quantum bounce predicted by the model, and at late times. Our main result is that, while this fails to give a compelling inflationary scenario in the early universe, it produces naturally a phantom-like dark energy dynamics at late times, compatible with cosmological observations. By recasting the emergent cosmological dynamics in terms of an effective equation of state, we show that it can generically cross the phantom divide, purely out of quantum gravity effects without the need of any additional phantom matter. Furthermore, we show that the dynamics avoids any Big Rip singularity, approaching instead a de Sitter universe asymptotically.

Matter production effects and interacting scenario within a reconstructed mimetic cosmology for late times

In this work we explore two possible scenarios that can be considered to extend a recent proposed model by the authors known as reconstructed mimetic cosmology. This study is complemented with an statistical analysis for each case. The first scenario considers the inclusion of matter production as a possible source of cosmic expansion in the reconstructed mimetic model, at effective level was found that this construction can cross the phantom divide, the model evolves from quintessence to phantom dark energy. The second scenario corresponds to a construction of an interacting scheme for the dark sector which is described by the unified mimetic model. The resulting interaction term (not imposed by an Ansatz), Q, exhibits changes of sign leading to the violation of the second law along the cosmic evolution and non adiabaticity; the temperatures for the components of the dark sector are computed and such components are shown to be out of thermal equilibrium.

Barrow HDE model for statefinder diagnostic in FLRW universe

We have analyzed the Barrow holographic dark energy (BHDE) in the framework of flat FLRW universe by considering the various estimations of Barrow exponent △. Here, we define BHDE, by applying the usual holographic principle at a cosmological system, for utilizing the Barrow entropy rather than the standard Bekenstein–Hawking. To understand the recent accelerated expansion of the universe, consider the Hubble horizon as the IR cutoff. The cosmological parameters, especially the density parameter [Formula: see text], the equation of the state parameter [Formula: see text], energy density [Formula: see text] and the deceleration parameter [Formula: see text] are studied in this paper and found the satisfactory behaviors. Moreover we additionally focus on the two geometric diagnostics, the statefinder [Formula: see text] and [Formula: see text] to discriminant BHDE model from the [Formula: see text]CDM model. Here we determined and plotted the trajectories of evolution for statefinder [Formula: see text], [Formula: see text] and [Formula: see text] diagnostic plane to understand the geometrical behavior of the BHDE model by utilizing Planck 2018 observational information. Finally, we have explored the new Barrow exponent △, which strongly affects the dark energy equation of state that can lead it to lie in the quintessence regime, phantom regime and exhibits the phantom-divide line during the cosmological evolution.

Cosmological aspects of unified theories

Στη παρούσα διατριβή, μελετάμε κοσμολογικές πτυχές ορισμένων από τα γνωστά μοντέλα σκοτεινής ενέργειας, επεκτείνοντας την έρευνά μας στις Ενοποιημένες Θεωρίες. Εξετάζουμε την ύπαρξη γεωδαισιακά πλήρων ανωμαλιών στα quintessence και βαθμοτανυστικά quintessence μοντέλα χρησιμοποιώντας αναλυτικές εκφράσεις για τον συντελεστή κλίμακας και το βαθμωτό πεδίο. Στη συνέχεια, χρησιμοποιώντας την εντροπία Tsallis και εφαρμόζοντας τον πρώτο νόμο της θερμοδυναμικής κατασκευάζουμε νέα κοσμολογικά σενάρια σκοτεινής ενέργειας. Το Σύμπαν εμφανίζει τη συνηθισμένη θερμική ιστορία, με την ακολουθία της εποχής της ύλης και σκοτεινής ενέργειας, και σύμφωνα με την τιμή της μη εκτατής παραμέτρου δ η καταστατική εξίσωση σκοτεινής ενέργειας μπορεί να είναι quintessence, phantom-like, ή να βιώνει το phantom-divide crossing κατά τη διάρκεια της εξέλιξης. Στην περίπτωση όπου έχουμε απουσία της κοσμολογικής σταθεράς, το σενάριο μπορεί να μιμηθεί πολύ καλά την ΛCDM Κοσμολογία, σε εξαιρετική συμφωνία με τα παρατηρησιακά δεδομένα Supernovae Ia. Τέλος, επανεξετάζουμε τον πληθωρισμό με μη κανονικά βαθμωτά πεδία. Ένα σημαντικό πλεονέκτημα αυτής της προσέγγισης είναι ότι οι ελεύθερες παράμετροι, όπως ο εκθέτης μη κανονικότητας δεν χρειάζεται να λαμβάνει αφύσικες τιμές. Το σενάριο που προκύπτει μπορεί να οδηγήσει σε πληθωριστικές παρατηρήσιμες ποσότητες οι οποίες βρίσκονται σε αξιοσημείωτη συμφωνία με τις παρατηρήσεις.

Role of collisional matter in the framework of extended teleparallel theory

The purpose of this work is to examine the cosmic evolution in the presence of collisional matter (CM) with and without radiations in a modified Teleparallel theory involving a generic function [Formula: see text] which depends on the scalar torsion [Formula: see text] and the boundary term associated to the divergence of torsion [Formula: see text]. We select seven novel [Formula: see text] models including power law, logarithmic models and exponential models, some of these reported in [S. Bahamonde, M. Zubair and G. Abbas, Phys. Dark Univ. 19 (2018) 78; S. Bahamonde and S. Capozziello, The Eur. Phys. J. C. 77 (2017) 107; C. Escamilla-Rivera and J. L. Said, Class. Quantum Grav. 37 (2020) 165002] and discuss the evolutionary scenario. The behavior of deceleration parameter [Formula: see text], Hubble parameter [Formula: see text], Equation-of-state (EoS) for dark energy (DE) and effective EoS is presented. [Formula: see text]CDM epoch and crossing of phantom divide line (approaching to phantom era) is observed in scenarios like noncollisional matter (NCM) with radiation, CM with and without radiation. Results are found to be adequate with recent cosmic observations.

Interacting phantom cosmology via Noether symmetry approach

In this paper, we have presented a cosmological model where a phantom scalar field is minimally coupled to dark matter component. Noether symmetry method was applied both to investigate the cosmological solution and to find out what is the form of the potential of scalar field and the unknown function in the considered model. By using this method, these forms are resulted as trigonometric functions. Also, the obtained cosmological solutions are compatible with observations describing the accelerated expansion of the Universe. Furthermore, it turns out that the effective equation of state parameter in the model can cross the phantom divide line.

Tsallis holographic dark energy with hybrid expansion law

In this work, we explore the Tsallis holographic dark energy (THDE) model with IR cutoff as Granda–Oliveros horizon describing the Universe experiencing an accelerating expansion phase in the framework of flat Friedmann–Lemaître–Robertson–Walker (FLRW) Universe. The Universe evolution from earlier decelerated to the current accelerated phase is exhibited by the deceleration parameter acquired in the THDE model. By the value of the Tsallis parameter [Formula: see text], the equation of state (EoS) parameter for the THDE model represents the rich behavior of the Cosmos as, the quintessence era ([Formula: see text]), crossing the phantom divide line and phantom era ([Formula: see text]). The squared sound speed [Formula: see text] also suggests that the THDE model is classically stable at present. Also, the correspondence with the quintessence and phantom scalar field for the THDE model is analyzed to describe the accelerated expansion of the Universe.

Dynamics of a quintom dark energy model non-minimally coupled with a mixed kinetic geometric term

Within this work, a new dark energy model it is proposed, by taking into account a non-minimally mixed kinetic geometric coupling between the two fields which construct a quintom model. By considering numerical analysis of the corresponding field equations, the evolution of the Universe in the present model has been analyzed, taking into account different viable potentials. For exponential potentials, the model presents an accelerated expansion, and the dark energy equation of state exhibit the phantom divide line crossing. However, for a steeper potential, in the case of a mixed potential, it is observed that the Universe can manifest Big Crunch singularities in the distant future, and the moment for which the singularity occur is sensitive to the value of the mixed coupling strength embedded into the η coeffcient. The results show that an increase of the $\eta$ parameter determine a delay in the the Big Crunch time in the distant future. In the present context, the choice of the potential function play a fundamental role in the evolution of the dynamical system, leading to very distinct cosmological scenarios.