scholarly journals Phantom-like dark energy from quantum gravity

2021 ◽  
Vol 2021 (12) ◽  
pp. 040
Author(s):  
Daniele Oriti ◽  
Xiankai Pang
Keyword(s):  
Dark Energy ◽  
Quantum Gravity ◽  
Mean Field ◽  
Accelerated Expansion ◽  
De Sitter ◽  
Effective Equation ◽  
Phantom Divide ◽  
Gravity Effects ◽  
Expansion Of The Universe ◽  
De Sitter Universe

Abstract 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.

scholarly journals A novel teleparallel dark energy model

2016 ◽  
Vol 25 (02) ◽  
pp. 1650025 ◽  
Author(s):  
Giovanni Otalora
Keyword(s):  
Dark Energy ◽  
Scalar Field ◽  
Teleparallel Gravity ◽  
Accelerated Expansion ◽  
Late Time ◽  
De Sitter ◽  
Sitter Group ◽  
Current State ◽  
Expansion Of The Universe ◽  
The Universe

Although equivalent to general relativity, teleparallel gravity (TG) is conceptually speaking a completely different theory. In this theory, the gravitational field is described by torsion, not by curvature. By working in this context, a new model is proposed in which the four-derivative of a canonical scalar field representing dark energy is nonminimally coupled to the “vector torsion”. This type of coupling is motivated by the fact that a scalar field couples to torsion through its four-derivative, which is consistent with local spacetime kinematics regulated by the de Sitter group [Formula: see text]. It is found that the current state of accelerated expansion of the universe corresponds to a late-time attractor that can be (i) a dark energy-dominated de Sitter solution ([Formula: see text]), (ii) a quintessence-type solution with [Formula: see text], or (iii) a phantom-type [Formula: see text] dark energy.

scholarly journals Origin of Universe accelerated expansion and prediction of the right deceleration parameter

2019 ◽  
Author(s):  
Paolo Di Sia
Keyword(s):  
Dark Energy ◽  
Deceleration Parameter ◽  
Ad Hoc ◽  
Accelerated Expansion ◽  
Dense Layer ◽  
De Sitter ◽  
Dark Energy Density ◽  
Particle Velocities ◽  
De Sitter Universe ◽  
The Right

The filament (f) theory implies initial isotropic particle-velocities with uniform value-distribution between zero and the speed of light c. That leads to a universe boundary coinciding with the events horizon of its centre. The very dense layer of particles and antiparticles expanding with almost c observed at retarded times attracts the internal particles thus implying for them an accelerated expansion. There is no need for an "ad hoc" dark energy implying repulsion. The predicted negative deceleration parameter q corresponds to a dark energy density 95.5 % of the critical value. If the red-shift of far galaxies was due to the only Doppler-Fizeau effect, the standard value 73 % is obtained. The past and future q is predicted. The q value was a negative maximum just after the primordial annihilation of the particles with the antiparticles and will vanish at about 3TH (where TH denotes the Hubble time) after the present time t. For t larger than 3TH, it will be positive, tending to 1/2, typical of the Einstein-De Sitter universe.

scholarly journals Holographic Ricci dark energy as running vacuum

2016 ◽  
Vol 31 (13) ◽  
pp. 1650075 ◽  
Author(s):  
Paxy George ◽  
Titus K. Mathew
Keyword(s):  
Dark Energy ◽  
Vacuum Energy ◽  
Additive Constant ◽  
Late Time ◽  
De Sitter ◽  
Energy Status ◽  
Ricci Dark Energy ◽  
Future Singularity ◽  
Expansion Of The Universe ◽  
De Sitter Universe

Holographic Ricci dark energy (DE) that has been proposed ago has faced problems of future singularity. In the present work, we consider the Ricci DE with an additive constant in its density as running vacuum energy. We have analytically solved the Friedmann equations and also the role played by the general conservation law followed by the cosmic components together. We have shown that the running vacuum energy status of the Ricci DE helps to remove the possible future singularity in the model. The additive constant in the density of the running vacuum played an important role, such that, without that, the model predicts either eternal deceleration or eternal acceleration. But along with the additive constant, equivalent to a cosmological constant, the model predicts a late time acceleration in the expansion of the universe, and in the far future of the evolution it tends to de Sitter universe.

scholarly journals Barrow HDE model for statefinder diagnostic in FLRW universe

2021 ◽  
pp. 2150030
Author(s):  
Anirudh Pradhan ◽  
Archana Dixit ◽  
Vinod Kumar Bhardwaj
Keyword(s):  
Dark Energy ◽  
Cosmological Parameters ◽  
State Parameter ◽  
Accelerated Expansion ◽  
Density Parameter ◽  
Phantom Divide ◽  
Phantom Divide Line ◽  
Expansion Of The Universe ◽  
Flrw Universe ◽  
The 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.

scholarly journals Does holographic equipartition demand a pure cosmological constant?

2020 ◽  
pp. 2050334
Author(s):  
P. B. Krishna ◽  
Titus K. Mathew
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Dark Energy Model ◽  
State Parameter ◽  
De Sitter ◽  
Entropy Maximization ◽  
Expansion Of The Universe ◽  
The Law ◽  
De Sitter Universe ◽  
The Universe

The spacial expansion of the universe could be described as a tendency for satisfying holographic equipartition which inevitably demands the presence of dark energy. We explore whether this novel idea proposed by Padmanabhan gives any additional insights into the nature of dark energy. In particular, we obtain the constraints imposed by the law of emergence on the equation of state parameter, [Formula: see text]. We also present a thermodynamic motivation for the obtained constraints on [Formula: see text]. Further, we explicitly prove the feasibility of describing a dynamic dark energy model through the law of emergence. Interestingly, both holographic equipartition and the entropy maximization demand an asymptotically de Sitter universe with [Formula: see text], rather than a pure cosmological constant.

scholarly journals Late Time Acceleration of the Universe from Quantum Gravity

2022 ◽  
Author(s):  
Xiankai Pang
Keyword(s):  
Quantum Gravity ◽  
Mean Field ◽  
Field Approximation ◽  
Mean Field Approximation ◽  
Late Time ◽  
De Sitter ◽  
Effective Equation ◽  
Late Time Acceleration ◽  
Sitter Spacetime ◽  
The Universe

We show that the accelerating expansion phase of the universe can emerge from the group field theory formalism, a candidate theory of quantum gravity. The cosmological evolution can be extracted from condensate states using mean field approximation, in a form of modified FLRW equations. By introducing an effective equation of state w, we can reveal the relevant features of the evolution, and show that with proper choice of parameters, w will approach to −1, corresponds to the behaviour of cosmological constant, results in a late time acceleration and leads to de Sitter spacetime asymptotically.

scholarly journals Phase space analysis and singularity classification for linearly interacting dark energy models

2020 ◽  
Vol 80 (2) ◽  
Author(s):  
Muhsin Aljaf ◽  
Daniele Gregoris ◽  
Martiros Khurshudyan
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Cosmological Parameters ◽  
State Parameter ◽  
Late Time ◽  
De Sitter ◽  
Effective Equation ◽  
De Sitter Universe ◽  
Friedmann Cosmological Models ◽  
Interacting Dark Energy Models

Abstract In this paper, applying the Hartman–Grobman theorem we carry out a qualitative late-time analysis of some unified dark energy-matter Friedmann cosmological models, where the two interact through linear energy exchanges, and the dark energy fluid obeys to the dynamical equation of state of Redlich–Kwong, Modified Berthelot, and Dieterici respectively. The identification of appropriate late-time attractors allows to restrict the range of validity of the free parameters of the models under investigation. In particular, we prove that the late-time attractors which support a negative deceleration parameter correspond to a de Sitter universe. We show that the strength of deviation from an ideal fluid for the dark energy does not influence the stability of the late-time attractors, as well as the values of all the cosmological parameters at equilibrium, but for the Hubble function (which represents the age of the universe). Our analysis also shows that a singularity in the effective equation of state parameter for the dark energy fluid is not possible within this class of models.

scholarly journals POLYTROPIC AND CHAPLYGIN f(R)-GRAVITY MODELS

2012 ◽  
Vol 21 (12) ◽  
pp. 1250083 ◽  
Author(s):  
K. KARAMI ◽  
M. S. KHALEDIAN
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
De Sitter Space ◽  
Chaplygin Gas ◽  
Gravity Models ◽  
Accelerated Expansion ◽  
De Sitter ◽  
Expansion Of The Universe ◽  
The Universe

We reconstruct different f(R)-gravity models corresponding to the polytropic, standard Chaplygin, generalized Chaplygin, modified Chaplygin and modified variable Chaplygin gas dark energy (DE) models. We also obtain the equation of state (EoS) parameters of the corresponding f(R)-gravity models which describe the accelerated expansion of the universe. We conclude that although the EoS parameters of the obtained f(R)-gravities can behave like phantom or quintessence DE models, they cannot justify the transition from the quintessence state to the phantom regime. Furthermore, the polytropic and Chaplygin f(R)-gravity models in de Sitter space can satisfy the inflation condition.

scholarly journals HYBRID CHAPLYGIN GAS

2009 ◽  
Vol 24 (07) ◽  
pp. 541-555 ◽  
Author(s):  
HONGSHENG ZHANG ◽  
ZONG-HONG ZHU ◽  
LIHUA YANG
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Transient Behavior ◽  
Chaplygin Gas ◽  
De Sitter ◽  
Coincidence Problem ◽  
Effective Equation ◽  
Phantom Divide ◽  
Perturbation Growth

Hybrid Chaplygin gas model is put forward, in which the gases play the role of dark energy. For this model the coincidence problem is greatly alleviated. The effective equation of state of the dark energy may cross the phantom divide w = -1. Furthermore, the crossing behavior is decoupled from any gravity theories. In the present model, w < -1 is only a transient behavior. There is a de Sitter attractor in the future infinity. Hence, the big rip singularity, which often afflicts the models with matter whose effective equation of state less than -1, naturally disappears. There exist stable scaling solutions, both at the early universe and the late universe. We discuss the perturbation growth of this model. We find that the index is consistent with observations.

Cosmological constant

2018 ◽  
Author(s):  
Michael Kachelriess
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Accelerated Expansion ◽  
Vacuum Fluctuations ◽  
Present Epoch ◽  
Modify Gravity ◽  
Expansion Of The Universe ◽  
The Universe

The contribution of vacuum fluctuations to the cosmological constant is reconsidered studying the dependence on the used regularisation scheme. Then alternative explanations for the observed accelerated expansion of the universe in the present epoch are introduced which either modify gravity or add a new component of matter, dubbed dark energy. The chapter closes with some comments on attempts to quantise gravity.

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