Viscous effects in the dark sector of the Universe

2020 ◽  
Vol 35 (02n03) ◽  
pp. 2040041
Author(s):  
J. C. Fabris ◽  
T. R. P. Caramês ◽  
A. Wojnar ◽  
H. E. S. Velten
Keyword(s):  
Short Review ◽  
Point Of View ◽  
Accelerated Expansion ◽  
Viscous Effects ◽  
Dark Sector ◽  
Expansion Phase ◽  
Observational Point ◽  
Small Scales ◽  
Viscous Models ◽  
The Universe

Viscous properties are attributed to the dark sector of the Universe. They contribute to the accelerated expansion phase of the Universe and can alleviate existing tensions in the [Formula: see text]CDM model at small scales. We provide a short review of recent efforts on this topic. Different viscous models for the dark sector are analysed both from theoretical and observational point of view.

Polytropic Carnot heat engine

2019 ◽  
Vol 34 (24) ◽  
pp. 1950197 ◽  
Author(s):  
M. Askin ◽  
M. Salti ◽  
O. Aydogdu
Keyword(s):  
Equation Of State ◽  
Heat Engine ◽  
Point Of View ◽  
Carnot Cycle ◽  
Interesting Point ◽  
Thermal Equation ◽  
Expansion Phase ◽  
Polytropic Gas ◽  
Free Parameters ◽  
The Universe

Recent astrophysical datasets have implied that the universe has entered a speedy expansion phase. The Polytropic gas model, which describes a unified formulation of dark contents (matter plus energy), is one of the most reasonable definitions of this mysterious phenomenon. This interesting formulation allows to simulate the dark contents in the cosmic form of the perfect fluid and gives an interesting point of view in the discussion of fundamental theories of physics. In the first step of our investigation, we discuss the thermal equation-of-state (EoS henceforth) and obtain the EoS and deceleration parameters as explicit functions of temperature. Subsequently, we obtain a relation for the thermal efficiency of the Carnot heat engine which depends on free parameters given in the cosmological Polytropic gas description and the limits of maximal and minimal temperatures imposed on the Carnot cycle.

scholarly journals DISCRETE SYMMETRIES AND GENERAL RELATIVITY, THE DARK SIDE OF GRAVITY

2005 ◽  
Vol 20 (11) ◽  
pp. 2341-2345 ◽  
Author(s):  
FREDERIC HENRY-COUANNIER
Keyword(s):  
General Relativity ◽  
Discrete Symmetries ◽  
Blue Shift ◽  
Accelerated Expansion ◽  
Dark Side ◽  
Galactic Rotation ◽  
Expansion Phase ◽  
Coordinate Singularity ◽  
General Relativistic ◽  
The Universe

The parity and time reversal invariant actions, equations and their conjugated metric solutions are obtained in the context of a general relativistic model modified in order to suitably take into account discrete symmetries. The equations are not covariant however the predictions of the model, in particular its Schwarzschild metric solution in vacuum, only start to differ from those of General Relativity at the Post-Post-Newtonian order. No coordinate singularity (black hole) arises in the privileged coordinate system where the energy of gravity is found to vanish. Vacuum energies have no gravitational effects. A flat universe accelerated expansion phase is obtained without resorting to inflation nor a cosmological constant. The context may be promising to help us elucidate several outstanding enigmas such as the Pioneer anomalous blue-shift, flat galactic rotation curves or the universe voids.

scholarly journals Energy Definition and Dark Energy: A Thermodynamic Analysis

2018 ◽  
Vol 2018 ◽  
pp. 1-8 ◽  
Author(s):  
H. Moradpour ◽  
J. P. Morais Graça ◽  
I. P. Lobo ◽  
I. G. Salako
Keyword(s):  
Conservation Law ◽  
Momentum Conservation ◽  
Momentum Tensor ◽  
Accelerated Expansion ◽  
Expansion Phase ◽  
Definition Of ◽  
Relaxed Energy ◽  
Thermodynamic Pressure ◽  
The Universe ◽  
Energy Momentum Conservation

Accepting the Komar mass definition of a source with energy-momentum tensor Tμν and using the thermodynamic pressure definition, we find a relaxed energy-momentum conservation law. Thereinafter, we study some cosmological consequences of the obtained energy-momentum conservation law. It has been found out that the dark sectors of cosmos are unifiable into one cosmic fluid in our setup. While this cosmic fluid impels the universe to enter an accelerated expansion phase, it may even show a baryonic behavior by itself during the cosmos evolution. Indeed, in this manner, while Tμν behaves baryonically, a part of it, namely, Tμν(e) which is satisfying the ordinary energy-momentum conservation law, is responsible for the current accelerated expansion.

scholarly journals δ Gravity: Dark Sector, Post-Newtonian Limit and Schwarzschild Solution

Universe ◽  
2019 ◽  
Vol 5 (5) ◽  
pp. 96
Author(s):  
Jorge Alfaro ◽  
Pablo González
Keyword(s):  
Rotation Velocity ◽  
Accelerated Expansion ◽  
Symmetric Tensors ◽  
Dark Sector ◽  
Schwarzschild Solution ◽  
Perihelion Precession ◽  
Cosmological Inflation ◽  
Expansion Of The Universe ◽  
Matter Effect ◽  
The Universe

We present a new kind of model, which we call δ Theories, where standard theories are modified including new fields, motivated by an additional symmetry ( δ symmetry). In previous works, we proved that δ Theories just live at one loop, so the model in a quantum level can be interesting. In the gravitational case, we have δ Gravity, based on two symmetric tensors, g μ ν and g ˜ μ ν , where quantum corrections can be controlled. In this paper, a review of the classical limit of δ Gravity in a Cosmological level will be developed, where we explain the accelerated expansion of the universe without Dark Energy and the rotation velocity of galaxies by the Dark Matter effect. Additionally, we will introduce other phenomenon with δ Gravity like the deflection of the light produced by the sun, the perihelion precession, Black Holes and the Cosmological Inflation.

Quantum gravity at Hubble scales determines the cosmological constant and the amplitude of primordial perturbations

2017 ◽  
Vol 26 (12) ◽  
pp. 1743002
Author(s):  
T. Padmanabhan ◽  
Hamsa Padmanabhan
Keyword(s):  
Cosmological Constant ◽  
Energy Scale ◽  
Accelerated Expansion ◽  
Late Time ◽  
Quantum Structure ◽  
Scale Invariant ◽  
Expansion Phase ◽  
Perturbation Spectrum ◽  
The Universe

Gravity controls the amount of information that is accessible to any specific observer. We quantify the notion of cosmic information (‘CosmIn’) for the case of an eternal observer in the universe. Demanding the finiteness of CosmIn requires the universe to have a late-time accelerated expansion phase. Combined with some generic features of the quantum structure of spacetime, this leads to the determination of (i) the numerical value of the cosmological constant, as well as (ii) the amplitude of the primordial, scale invariant perturbation spectrum in terms of a single free parameter, which specifies the energy scale at which the universe makes a transition from a pre-geometric phase to the classical phase. This formalism also shows that the quantum gravitational information content of spacetime can be tested by using precision cosmology.

scholarly journals The Thermodynamics of Cosmic Gases in Expanding Universes Based on VlasowTheoretical Grounds

2021 ◽  
Vol 4 (2) ◽  
Keyword(s):  
Cosmic Time ◽  
Black Body ◽  
Distribution Functions ◽  
Accelerated Expansion ◽  
Physical Processes ◽  
Expansion Phase ◽  
Hubble Expansion ◽  
Expansion Of The Universe ◽  
Velocity Moments ◽  
The Universe

Contemporary cosmology is taking for granted that before the phase of matter recombination matter and radiation were in perfect thermodynamic equilibrium. This would mean that protons in this phase of the universe are described by Maxwell distributions and photons are described by a Planckian black body law. When looking, however, a bit deeper into the kinetic theory of the physical processes close to and just after the recombination phase of electrons and protons, it becomes evident that in a homologously expanding universe proton distribution functions will not maintain their Maxwellian profile and connected with it, that their most relevant velocity moments, i.e. their density and their temperature, vary in an unexpected nonclassical, non-adiabatic manner. As consequence of that, in contrast to the classical view, the entropy of free atoms does change with cosmic time contrary to the standard thermodynamically expectation. We shall also demonstrate here that the realistic behaviour of cosmic gases in this phase and later depends on the specific form of the Hubble expansion of the universe, especially an accelerated expansion phase as is discussed nowadays will strongly influence the thermodynamics of the cosmic gas.

scholarly journals The Sharma–Mittal Model’s Implications on FRW Universe in Chern–Simons Gravity

Universe ◽  
2021 ◽  
Vol 7 (11) ◽  
pp. 428
Author(s):  
Sarfraz Ali ◽  
Muhammad Hummad Waheed ◽  
Muhammad Imran Asjad ◽  
Khuram Ali Khan ◽  
Thanin Sitthiwirattham ◽  
...  
Keyword(s):  
Observational Data ◽  
Holographic Dark Energy ◽  
Accelerated Expansion ◽  
De Sitter ◽  
Stiff Fluid ◽  
Frw Universe ◽  
Expansion Phase ◽  
Modified Gravity Theory ◽  
Chern Simons ◽  
The Universe

The Sharma–Mittal holographic dark energy model is investigated in this paper using the Chern–Simons modified gravity theory. We investigate several cosmic parameters, including the deceleration, equation of state, square of sound speed, and energy density. According to the deceleration parameter, the universe is in an decelerating and expanding phase known as de Sitter expansion. The Sharma–Mittal HDE model supports a deceleration to acceleration transition that is compatible with the observational data. The EoS depicts the universe’s dominance era through a number of components, such as ω=0, 13, 1, which indicate that the universe is influenced by dust, radiation, and stiff fluid, while −1<ω<13, ω=−1, and ω<−1 are conditions for quintessence DE, ΛCDM, and Phantom era dominance. Our findings indicate that the universe is in an accelerated expansion phase, and this is similar to the observational data.

scholarly journals Constraining effective equation of state in f(Q, T) gravity

2021 ◽  
Vol 81 (6) ◽  
Author(s):  
Simran Arora ◽  
Abhishek Parida ◽  
P. K. Sahoo
Keyword(s):  
Equation Of State ◽  
State Parameter ◽  
Momentum Tensor ◽  
Accelerated Expansion ◽  
Dark Sector ◽  
Effective Equation ◽  
Best Fitting ◽  
Expansion Of The Universe ◽  
Two Parameters ◽  
The Universe

AbstractNew high-precision observations are now possible to constrain different gravity theories. To examine the accelerated expansion of the Universe, we used the newly proposed f(Q, T) gravity, where Q is the non-metricity, and T is the trace of the energy–momentum tensor. The investigation is carried out using a parameterized effective equation of state with two parameters, m and n. We have also considered the linear form of $$f(Q,T)= Q+bT$$ f ( Q , T ) = Q + b T , where b is constant. By constraining the model with the recently published 1048 Pantheon sample, we were able to find the best fitting values for the parameters b, m, and n. The model appears to be in good agreement with the observations. Finally, we analyzed the behavior of the deceleration parameter and equation of state parameter. The results support the feasibility of f(Q, T) as a promising theory of gravity, illuminating a new direction towards explaining the Universe dark sector.

scholarly journals Toy Models of Universe with an Effective VaryingΛ-Term in Lyra Manifold

2015 ◽  
Vol 2015 ◽  
pp. 1-10 ◽  
Author(s):  
Martiros Khurshudyan
Keyword(s):  
Dark Energy ◽  
Scalar Field ◽  
Accelerated Expansion ◽  
Observational Constraints ◽  
Constant Deceleration Parameter ◽  
Expansion Phase ◽  
Lyra Manifold ◽  
New Form ◽  
Transit Universe ◽  
The Universe

Research on the accelerated expansion of our Universe captures a lot of attention. The dark energy (DE) is a way to explain it. In this paper we will consider scalar field quintessence DE withωDE>-1EoS, where the dynamics of the DE models related to the dynamics of the scalar field. We are interested in the study of the behavior of the Universe in the presence of interacting quintessence DE models in Lyra manifold with a varyingΛt. In a considered framework we also would like to propose a new form forΛt. We found that the models correspond to the transit Universe, which will enter the accelerated expansion phase and will remain there with a constant deceleration parameterq. We found also that theΛtis a decreasing function which takes a small positive value withΩm≠0andΩQ→constdominatingΩmin the old Universe. Observational constraints are applied and causality issue viaCS2is discussed as a possible way to either reject or accept the models.

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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