Articles de revues : « Einstein static universe »

1

Atazadeh, K., et F. Darabi. « Einstein static universe from GUP ». Physics of the Dark Universe 16 (juin 2017) : 87–93. http://dx.doi.org/10.1016/j.dark.2017.04.008.

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Atazadeh, K., Y. Heydarzade et F. Darabi. « Einstein static universe in braneworld scenario ». Physics Letters B 732 (mai 2014) : 223–27. http://dx.doi.org/10.1016/j.physletb.2014.03.009.

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Darabi, F., Y. Heydarzade et F. Hajkarim. « Stability of Einstein static universe over Lyra geometry ». Canadian Journal of Physics 93, n^o 12 (décembre 2015) : 1566–70. http://dx.doi.org/10.1139/cjp-2015-0312.

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The existence and stability conditions of Einstein static universe against homogeneous scalar perturbations in the context of Lyra geometry is investigated. The stability condition is obtained in terms of the constant equation of state parameter ω = p/ρ depending on energy density ρ0 and scale factor a0 of the initial Einstein static universe. Also, the stability against vector and tensor perturbations is studied. It is shown that a stable Einstein static universe can be found in the context of Lyra geometry against scalar, vector, and tensor perturbations for suitable range and values of physical parameters.

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Sharif, M., et Sana Saleem. « Stability of anisotropic perturbed Einstein universe in f(R) gravity ». Modern Physics Letters A 35, n^o 18 (13 mai 2020) : 2050152. http://dx.doi.org/10.1142/s0217732320501527.

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The aim of this paper is to investigate the existence of stable modes of the Einstein static universe in the background of [Formula: see text] theory. For this purpose, we take homogeneous anisotropic perturbations in scale factors as well as matter contents. We construct static and perturbed field equations that are further parameterized by linear equation of state parameter. We obtain the Einstein static solutions for two specific [Formula: see text] models and graphically analyze their stable regions. It is concluded that contrary to general relativity, there exists stable Einstein static universe with anisotropic perturbations.

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Atazadeh, K. « Stability of the Einstein static universe in Einstein-Cartan theory ». Journal of Cosmology and Astroparticle Physics 2014, n^o 06 (10 juin 2014) : 020. http://dx.doi.org/10.1088/1475-7516/2014/06/020.

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Heydarzade, Y., et F. Darabi. « Induced matter brane gravity and Einstein static universe ». Journal of Cosmology and Astroparticle Physics 2015, n^o 04 (20 avril 2015) : 028. http://dx.doi.org/10.1088/1475-7516/2015/04/028.

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Parisi, Luca, Marco Bruni, Roy Maartens et Kevin Vandersloot. « The Einstein static universe in loop quantum cosmology ». Classical and Quantum Gravity 24, n^o 24 (27 novembre 2007) : 6243–53. http://dx.doi.org/10.1088/0264-9381/24/24/007.

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Gergely, László Á., et Roy Maartens. « Brane-world generalizations of the Einstein static universe ». Classical and Quantum Gravity 19, n^o 2 (2 janvier 2002) : 213–21. http://dx.doi.org/10.1088/0264-9381/19/2/303.

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Barrow, John D., George F. R. Ellis, Roy Maartens et Christos G. Tsagas. « On the stability of the Einstein static universe ». Classical and Quantum Gravity 20, n^o 11 (2 mai 2003) : L155—L164. http://dx.doi.org/10.1088/0264-9381/20/11/102.

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Sharif, M., et Sana Saleem. « Stability of anisotropic perturbed Einstein universe in f(R,T) theory ». Modern Physics Letters A 35, n^o 27 (15 juillet 2020) : 2050222. http://dx.doi.org/10.1142/s0217732320502223.

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The aim of this paper is to investigate the stability of Einstein static cosmos using anisotropic homogeneous perturbations in the background of [Formula: see text] theory in which [Formula: see text] and [Formula: see text] express the Ricci scalar and trace of the stress–energy tensor, respectively. To accomplish this work, we consider perfect fluid distribution and adopt small anisotropic perturbations in the scale factors and matter contents. We develop static and perturbed field equations that are simplified by using equation of state parameter. For the specific models of [Formula: see text] theory with conserved and non-conserved stress–energy tensor, the Einstein solutions are explored and their stability regions are analyzed graphically. We conclude that the static Einstein stable universe with anisotropic perturbations exists in this framework contrary to general relativity.

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Aygün, S., I. Tarhan et H. Baysal. « On the Energy-Momentum Problem in Static Einstein Universe ». Chinese Physics Letters 24, n^o 2 (18 janvier 2007) : 355–58. http://dx.doi.org/10.1088/0256-307x/24/2/015.

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Gruppuso, A., E. Roessl et M. Shaposhnikov. « Einstein static universe as a brane in extra dimensions ». Journal of High Energy Physics 2004, n^o 08 (5 août 2004) : 011. http://dx.doi.org/10.1088/1126-6708/2004/08/011.

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Atazadeh, K., et F. Darabi. « Einstein static Universe in non-minimal kinetic coupled gravity ». Physics Letters B 744 (mai 2015) : 363–68. http://dx.doi.org/10.1016/j.physletb.2015.04.022.

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Chakrabarti, A. « Fermions and gauge fields in the Einstein static universe ». Physics Letters B 212, n^o 2 (septembre 1988) : 145–46. http://dx.doi.org/10.1016/0370-2693(88)90514-x.

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ROY, BARNANA. « A NON-PERTURBATIVE APPROACH TO SYMMETRY BEHAVIOUR IN THE OPEN EINSTEIN UNIVERSE ». International Journal of Modern Physics A 06, n^o 09 (10 avril 1991) : 1525–32. http://dx.doi.org/10.1142/s0217751x91000800.

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Ebert, D., K. G. Klimenko, A. V. Tyukov et V. C. Zhukovsky. « Pion condensation of quark matter in a static Einstein universe ». European Physical Journal C 58, n^o 1 (24 septembre 2008) : 57–68. http://dx.doi.org/10.1140/epjc/s10052-008-0667-6.

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Herdeiro, Carlos A. R., Raquel H. Ribeiro et Marco Sampaio. « Scalar Casimir effect on a D -dimensional Einstein static universe ». Classical and Quantum Gravity 25, n^o 16 (5 août 2008) : 165010. http://dx.doi.org/10.1088/0264-9381/25/16/165010.

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Zhang, Kaituo, Puxun Wu et Hongwei Yu. « The stability of Einstein static universe in the DGP braneworld ». Physics Letters B 690, n^o 3 (juin 2010) : 229–32. http://dx.doi.org/10.1016/j.physletb.2010.05.040.

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Khokhlov, D. L. « The Einstein static model of the universe as a whole ». Astrophysics and Space Science 333, n^o 1 (16 décembre 2010) : 209–12. http://dx.doi.org/10.1007/s10509-010-0565-x.

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Berman, M. S. « A static universe with magnetic field in Einstein-Cartan’s theory ». Il Nuovo Cimento B Series 11 105, n^o 12 (décembre 1990) : 1373–75. http://dx.doi.org/10.1007/bf02742691.

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Sharif, M., et Ayesha Ikram. « Stability analysis of Einstein universe in f(𝒢,T) gravity ». International Journal of Modern Physics D 26, n^o 08 (22 février 2017) : 1750084. http://dx.doi.org/10.1142/s0218271817500845.

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This paper explores the stability of the Einstein universe against linear homogeneous perturbations in the background of [Formula: see text] gravity. We construct static as well as perturbed field equations and investigate stability regions for the specific forms of generic function [Formula: see text] corresponding to conserved as well as nonconserved energy-momentum tensor. We use the equation-of-state parameter to parameterize the stability regions. The graphical analysis shows that the suitable choice of parameters lead to stable regions of the Einstein universe.

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Darabi, F., et K. Atazadeh. « Einstein static universe, GUP, and natural IR and UV cut-offs ». International Journal of Geometric Methods in Modern Physics 15, n^o 05 (2 avril 2018) : 1850083. http://dx.doi.org/10.1142/s0219887818500834.

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We study the Einstein static universe (ESU) in the framework of Generalized Uncertainty Principle (GUP) constructed by the Snyder non-commutative space. It is shown that the deformation parameter can induce an effective energy density subject to GUP which obeys the holographic principle (HP) and plays the role of a cosmological constant. Using the holographic feature of this effective energy density, we introduce natural IR and UV cut-offs which depend on the GUP-based effective equation of state. Moreover, we propose a solution to the cosmological constant problem (CCP). This solution is based on the result that the Einstein equations just couple to the tiny holographic-based surface energy density (cosmological constant) induced by the deformation parameter, rather than the large quantum gravitational-based volume energy density (vacuum energy) having contributions of order [Formula: see text].

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GUENDELMAN, E. I. « NONSINGULAR ORIGIN OF THE UNIVERSE AND THE COSMOLOGICAL CONSTANT PROBLEM ». International Journal of Modern Physics D 20, n^o 14 (31 décembre 2011) : 2767–71. http://dx.doi.org/10.1142/s0218271811020718.

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We consider a nonsingular origin for the universe starting from an Einstein static universe in the framework of a theory which uses two volume elements [Formula: see text] and Φd4x, where Φ is a metric independent density, also curvature, curvature square terms, first order formalism and for scale invariance a dilaton field ϕ are considered in the action. In the Einstein frame we also add a cosmological term that parametrizes the zero point fluctuations. The resulting effective potential for the dilaton contains two flat regions, for ϕ → ∞ relevant for the nonsingular origin of the universe and ϕ → -∞, describing our present universe. Surprisingly, avoidance of singularities and stability as ϕ → ∞ imply a positive but small vacuum energy as ϕ → -∞. Zero vacuum energy density for the present universe is the "threshold" for universe creation.

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SHOJAI, ALI, FATIMAH SHOJAI et NARESH DADHICH. « STATIC EINSTEIN'S UNIVERSE AS A QUANTUM SOLUTION OF CAUSAL QUANTUM GRAVITY ». International Journal of Modern Physics A 20, n^o 13 (20 mai 2005) : 2773–80. http://dx.doi.org/10.1142/s0217751x05022913.

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We shall in the framework of Bohmian quantum gravity show that it is possible to find a pure quantum state which leads to the static Einstein universe whose classical counterpart is flat space–time. We obtain the solution in the long-wavelength approximation. At the end an exact solution is found.

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Böhmer, Christian G., et Francisco S. N. Lobo. « Stability of the Einstein static universe in IR modified Hořava gravity ». European Physical Journal C 70, n^o 4 (23 novembre 2010) : 1111–18. http://dx.doi.org/10.1140/epjc/s10052-010-1503-3.

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26

Heydarzade, Y., M. Khodadi et F. Darabi. « Deformed Hořava–Lifshitz cosmology and stability of the Einstein static universe ». Theoretical and Mathematical Physics 190, n^o 1 (janvier 2017) : 130–39. http://dx.doi.org/10.1134/s0040577917010111.

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27

Zhang, Kaituo, Puxun Wu, Hongwei Yu et Ling-Wei Luo. « Stability of Einstein static state universe in the spatially flat branemodels ». Physics Letters B 758 (juillet 2016) : 37–41. http://dx.doi.org/10.1016/j.physletb.2016.04.049.

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Sharif, M., et Arfa Waseem. « On the stability of Einstein universe in f(R, T, Rμν Tμν) gravity ». Modern Physics Letters A 33, n^o 36 (28 novembre 2018) : 1850216. http://dx.doi.org/10.1142/s0217732318502164.

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This paper investigates the existence and stability of Einstein universe in the context of f(R, T, Q) gravity, where Q = R[Formula: see text] T[Formula: see text]. Considering linear homogeneous perturbations around scale factor and energy density, we formulate static as well as perturbed field equations. We parametrize the stability regions corresponding to conserved as well as non-conserved energy–momentum tensor using linear equation of state parameter for particular models of this gravity. The graphical analysis concludes that for a suitable choice of parameters, stable regions of the Einstein universe are obtained which indicates that the big bang singularity can be avoided successfully by the emergent mechanism in non-minimal matter-curvature coupled gravity.

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HERDEIRO, CARLOS A. R., RAQUEL H. RIBEIRO et MARCO SAMPAIO. « REPULSIVE GRAVITY AND THE CASIMIR EFFECT ON SPHERICAL UNIVERSES ». International Journal of Modern Physics A 24, n^o 08n09 (10 avril 2009) : 1821–24. http://dx.doi.org/10.1142/s0217751x09045406.

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We describe the Casimir effect of a free scalar field coupled to gravity on an n + 1-dimensional Einstein Static Universe (ESU), with arbitrary low energy effective operators (up to mass dimension n + 1). We discuss the variation of the effect from attractive to repulsive and some possible physical consequences.

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Eshkobilov, Olimjon, Emilio Musso et Lorenzo Nicolodi. « On the restricted conformal group of the (1+n)-Einstein static universe ». Journal of Geometry and Physics 146 (décembre 2019) : 103517. http://dx.doi.org/10.1016/j.geomphys.2019.103517.

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CHATTERJEE, S., Y. Z. ZHANG et D. PANIGRAHI. « EINSTEIN–STRAUS PROBLEM IN HIGHER DIMENSIONS ». International Journal of Modern Physics D 12, n^o 03 (mars 2003) : 395–405. http://dx.doi.org/10.1142/s0218271803003116.

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Years ago Einstein and Straus (ES) showed that it is possible to match a static Schwarzschild region to an external expanding Friedmann universe. This model is extended in this work to a spacetime of arbitrary dimensions. Frequency shift of radiation coming from the boundary of the two spacetimes is calculated. Depending on the relative magnitude of gravitational and doppler effects our model gives both blue shift and red shift. The dynamical behaviour of the boundary is investigated and it is found that like the ES case our model is also unstable against small perturbation.

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Hamil, B., M. Merad et T. Birkandan. « Particle creation in the context of the emergent universe ». Revista Mexicana de Física 67, n^o 2 Mar-Apr (15 juillet 2021) : 219–25. http://dx.doi.org/10.31349/revmexfis.67.219.

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We study the mechanism of particle creation in the context of the emergent universe (EU) scenario which is privileged by certain important characteristics such as the absence of time-like singularity. EU asymptotically coincides with an Einstein static model in the infinite past and it approaches to a de Sitter expansion phase at late times. By introducing the conformal time, we obtain the solution of the Klein-Gordon equation and by applying the "in" and "out" states method, the total number of produced particles and the total energy associated with them are determined.

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Mousavi, M., et F. Darabi. « On the stability of Einstein static universe at background level in massive bigravity ». Nuclear Physics B 919 (juin 2017) : 523–40. http://dx.doi.org/10.1016/j.nuclphysb.2017.04.002.

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Miao, Haitao, Puxun Wu et Hongwei Yu. « Stability of the Einstein static Universe in the scalar–tensor theory of gravity ». Classical and Quantum Gravity 33, n^o 21 (13 octobre 2016) : 215011. http://dx.doi.org/10.1088/0264-9381/33/21/215011.

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Cavaglià, Marco, et Vittorio de Alfaro. « Quantization of an Integrable Minisuperspace Model in Dilaton-Einstein Gravity ». International Journal of Modern Physics D 06, n^o 01 (février 1997) : 39–47. http://dx.doi.org/10.1142/s0218271897000030.

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We quantize the minisuperspace model of Einstein gravity plus dilaton that describeseither a static spherically symmetric configuration or a Kantowski–Sachs like universe. We develop the canonical formalism and identify canonical quantities that generate rigid symmetries of the Hamiltonian. Quantization is performed both by the Dirac and the reduced methods. Since the system is classically integrable we show that both approaches lead to the same positive definite Hilbert space.

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Riazi, N., H. Moradpour et A. Sheykhi. « Conformally Schwarzschild black holes in an accelerating universe ». International Journal of Modern Physics D 23, n^o 05 (30 avril 2014) : 1450048. http://dx.doi.org/10.1142/s0218271814500485.

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We investigate the physical properties of conformally Schwarzschild black holes (BHs) in a background of accelerating universe. We discuss the effect of the cosmic expansion on the principal quantities of the BH such as mass, temperature, entropy and horizon radius. In particular, we investigate the quasi-static thermodynamics of the BH by considering the fact that the cosmic expansion is very slow on local time scales. We show that, by imposing a condition on Ricci scalar, one can describe time-dependent, spherically symmetric solutions of Einstein gravity in an accelerating universe background in a unified way. A generalization of cosmological horizon, its temperature and other properties of conformally Schwarzschild BHs in a background of accelerating universe is addressed.

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GUENDELMAN, E. I. « NONSINGULAR ORIGIN OF THE UNIVERSE AND ITS PRESENT VACUUM ENERGY DENSITY ». International Journal of Modern Physics A 26, n^o 17 (10 juillet 2011) : 2951–72. http://dx.doi.org/10.1142/s0217751x11053614.

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We consider a nonsingular origin for the universe starting from an Einstein static universe, the so-called "emergent universe" scenario, in the framework of a theory which uses two volume elements [Formula: see text] and Φd4x, where Φ is a metric independent density, used as an additional measure of integration. Also curvature, curvature square terms and for scale invariance a dilaton field ϕ are considered in the action. The first-order formalism is applied. The integration of the equations of motion associated with the new measure gives rise to the spontaneous symmetry breaking of scale invariance. After spontaneous symmetry breaking of scale invariance it is found that a nontrivial potential for the dilaton is generated. In the Einstein frame we also add a cosmological term that parametrizes the zero point fluctuations. The resulting effective potential for the dilaton contains two flat regions, for ϕ → ∞ relevant for the nonsingular origin of the universe, followed by an inflationary phase and ϕ → - ∞, describing our present universe. The dynamics of the scalar field becomes nonlinear and these nonlinearities are instrumental in the stability of some of the emergent universe solutions, which exists for a parameter range of values of the vacuum energy in ϕ → - ∞, which must be positive but not very big, avoiding the extreme fine tuning required to keep the vacuum energy density of the present universe small. Zero vacuum energy density for the present universe defines the threshold for the creation of the universe.

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Tung, Michael M. « Metamaterial Acoustics on the (2 + 1)D Einstein Cylinder ». Mathematics 9, n^o 17 (28 août 2021) : 2079. http://dx.doi.org/10.3390/math9172079.

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The Einstein cylinder is the first cosmological model for our universe in modern history. Its geometry not only describes a static universe—a universe being invariant under time reversal—but it is also the prototype for a maximally symmetric spacetime with constant positive curvature. As such, it is still of crucial importance in numerous areas of physics and engineering, offering a fruitful playground for simulations and new theories. Here, we focus on the implementation and simulation of acoustic wave propagation on the Einstein cylinder. Engineering such an extraordinary device is the territory of metamaterial science, and we will propose an appropriate tuning of the relevant acoustic parameters in such a way as to mimic the geometric properties of this spacetime in acoustic space. Moreover, for probing such a space, we derive the corresponding wave equation from a variational principle for the underlying curved spacetime manifold and examine some of its solutions. In particular, fully analytical results are obtained for concentric wave propagation. We present predictions for this case and thereby investigate the most significant features of this spacetime. Finally, we produce simulation results for a more sophisticated test model which can only be tackled numerically.

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KAN, NAHOMI, KOICHIRO KOBAYASHI et KIYOSHI SHIRAISHI. « GRAPH-THEORY INDUCED GRAVITY AND STRONGLY-DEGENERATE FERMIONS IN A SELF-CONSISTENT EINSTEIN UNIVERSE ». International Journal of Modern Physics A 27, n^o 23 (18 septembre 2012) : 1250131. http://dx.doi.org/10.1142/s0217751x1250131x.

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We study UV-finite theory of induced gravity. We use scalar fields, Dirac fields and vector fields as matter fields whose one-loop effects induce the gravitational action. To obtain the mass spectrum that satisfies the UV-finiteness condition, we use a graph-based construction of mass matrices. The existence of a self-consistent static solution for an Einstein universe is shown in the presence of degenerate fermions.

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Rebecca, Louise, C. Sivaram et Kenath Arun. « Dark Energy and Cosmological Constant ». Mapana - Journal of Sciences 17, n^o 1 (1 janvier 2018) : 25–32. http://dx.doi.org/10.12723/mjs.44.3.

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One of the unresolved problems in cosmology is that the measured mass density of the universe has revealed a value that was about 30% of the critical density. Since the universe is very nearly spatially flat, as is indicated by measurements of the cosmic microwave background, about 70% of the energy density of the universe was left unaccounted for. Another observation seems to be connected to this mystery. Generally one would expect the rate of expansion to slow down once the universe started expanding. The measurements of Type Ia supernovae have revealed that the expansion of the universe is actually accelerating. This accelerated expansion is attributed to the so-called dark energy (DE).Here we give a brief overview on the observational basis for DE hypothesis and how cosmological constant, initially proposed by Einstein to obtain a static universe, can play the role of dark energy.

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Böhmer, C. G. « The Einstein static universe with torsion and the sign problem of the cosmological constant ». Classical and Quantum Gravity 21, n^o 4 (22 janvier 2004) : 1119–24. http://dx.doi.org/10.1088/0264-9381/21/4/025.

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GUENDELMAN, EDUARDO I., et PEDRO LABRAÑA. « CONNECTING THE NONSINGULAR ORIGIN OF THE UNIVERSE, THE VACUUM STRUCTURE AND THE COSMOLOGICAL CONSTANT PROBLEM ». International Journal of Modern Physics D 22, n^o 09 (26 juin 2013) : 1330018. http://dx.doi.org/10.1142/s0218271813300188.

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We consider a nonsingular origin for the universe starting from an Einstein static universe, the so-called "emergent universe" scenario, in the framework of a theory which uses two volume elements [Formula: see text] and Φd4x, where Φ is a metric independent density, used as an additional measure of integration. Also curvature, curvature square terms and for scale invariance a dilaton field ϕ are considered in the action. The first-order formalism is applied. The integration of the equations of motion associated with the new measure gives rise to the spontaneous symmetry breaking (SSB) of scale invariance (SI). After SSB of SI, it is found that a nontrivial potential for the dilaton is generated. In the Einstein frame we also add a cosmological term that parametrizes the zero point fluctuations. The resulting effective potential for the dilaton contains two flat regions, for ϕ → ∞ relevant for the nonsingular origin of the universe, followed by an inflationary phase and ϕ → -∞, describing our present universe. The dynamics of the scalar field becomes nonlinear and these nonlinearities produce a nontrivial vacuum structure for the theory and are responsible for the stability of some of the emergent universe solutions, which exists for a parameter range of values of the vacuum energy in ϕ → -∞, which must be positive but not very big, avoiding the extreme fine tuning required to keep the vacuum energy density of the present universe small. The nontrivial vacuum structure is crucial to ensure the smooth transition from the emerging phase, to an inflationary phase and finally to the slowly accelerated universe now. Zero vacuum energy density for the present universe defines the threshold for the creation of the universe.

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NUSSBAUMER, HARRY. « THE DISCOVERY OF THE EXPANDING UNIVERSE AND 80 YEARS OF BIG BANG ». International Journal of Modern Physics D 20, supp01 (31 juillet 2011) : 87–103. http://dx.doi.org/10.1142/s0218271811019402.

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Modern cosmology began in 1917 when Einstein published his model of a static Universe built on general relativity. A few months later de Sitter came forward with a competing, but also static model which contained no matter but had the intriguing quality that the spectrum of a test particle appeared redshifted to a distant observer. It was thought that de Sitter's model might explain the redshifted spectra observed by Slipher in spiral nebulae. However, in 1927 Lemaître showed that de Sitter's model violated the principle of homogeneity. He then formulated a dynamical cosmological model and combined it with the available observations, showing that our Universe is expanding. He theoretically derived the linear distance–velocity relationship which today is called the "Hubble-relation." Hubble confirmed the relation in 1929 on purely observational grounds. 80 years ago, in 1931 in a letter to Nature, Lemaître suggested that the Universe had a definite beginning in a rapid expansion out of a highly condensed state: the primeval atom. This event became later known as the Big Bang.

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Al-Ali, Usamah S., et Ashfaque H. Bokhari. « Exact vacuum solutions to modified Einstein field equations in f(R) gravity ». Modern Physics Letters A 35, n^o 25 (8 juin 2020) : 2050205. http://dx.doi.org/10.1142/s0217732320502053.

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Considering the plane symmetric non-static spacetimes in the context of [Formula: see text] gravity, we obtain exact solutions of the vacuum field equations by assuming constant scalar curvature. By suitable transformations, it is shown that the obtained solutions can be transformed to Bianchi type [Formula: see text], a type of Taub’s and the De Sitter solutions. Of particular interest is a solution that represents a model that has initial singularity and under an appropriate transformation can be converted to a Bianchi-type V model. This solution, like the Bianchi type V model, leads to predictions about evolution in the sense of an expanding universe starting from an initial singularity. In this context, we show that the expansion of the universe in [Formula: see text] gravity can be explained without invoking the cosmological constant [Formula: see text].

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Nian, Jun, et Yachao Qian. « Exact multi-instanton solutions to self-dual Yang–Mills equation on curved spaces ». International Journal of Modern Physics A 36, n^o 17 (15 juin 2021) : 2150132. http://dx.doi.org/10.1142/s0217751x21501323.

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We find exact multi-instanton solutions to the self-dual Yang–Mills equation on a large class of curved spaces with [Formula: see text] isometry, generalizing the results previously found on [Formula: see text]. The solutions are featured with explicit multi-centered expressions and topological properties. As examples, we demonstrate the approach on several different curved spaces, including the Einstein static universe and [Formula: see text], and show that the exact multi-instanton solutions exist on these curved backgrounds.

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Lerner, Eric J., Renato Falomo et Riccardo Scarpa. « UV surface brightness of galaxies from the local universe to z ~ 5 ». International Journal of Modern Physics D 23, n^o 06 (mai 2014) : 1450058. http://dx.doi.org/10.1142/s0218271814500588.

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The Tolman test for surface brightness (SB) dimming was originally proposed as a test for the expansion of the universe. The test, which is independent of the details of the assumed cosmology, is based on comparisons of the SB of identical objects at different cosmological distances. Claims have been made that the Tolman test provides compelling evidence against a static model for the universe. In this paper we reconsider this subject by adopting a static Euclidean universe (SEU) with a linear Hubble relation at all z (which is not the standard Einstein–de Sitter model), resulting in a relation between flux and luminosity that is virtually indistinguishable from the one used for ΛCDM models. Based on the analysis of the UV SB of luminous disk galaxies from HUDF and GALEX datasets, reaching from the local universe to z ~ 5, we show that the SB remains constant as expected in a static universe. A re-analysis of previously published data used for the Tolman test at lower redshift, when treated within the same framework, confirms the results of the present analysis by extending our claim to elliptical galaxies. We conclude that available observations of galactic SB are consistent with a SEU model. We do not claim that the consistency of the adopted model with SB data is sufficient by itself to confirm what would be a radical transformation in our understanding of the cosmos. However, we believe this result is more than sufficient reason to examine this combination of hypotheses further.

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Garcia de Andrade, L. C. « Generation of Primordial Magnetic Fields from QED and Higgs-like Domain Walls in Einstein–Cartan Gravity ». Universe 8, n^o 12 (14 décembre 2022) : 658. http://dx.doi.org/10.3390/universe8120658.

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Spacetime torsion is known to be highly suppressed at the end of inflation, which is called preheating. This result was recently shown in (EPJ C (2022)) in the frame of Einstein–Cartan–Brans–Dicke inflation. In this paper, it is shown that a torsionful magnetogenesis in QED effective Lagrangean drives a torsion damping in order to be subsequently amplified by the dynamo effect after the generation of these magnetic fields seeds. This damping on amplification would depend upon the so-called torsion chirality. Here, a cosmic factor gkK is present where K is the contortion vector and k is the wave vector which is connected to the inverse of magnetic coherence length. In a second example, we find Higgs inlationary fields in Einstein–Cartan gravity thick domain walls (DWs). Recently, a modified Einstein–Cartan gravity was given by Shaposhnikov et al. [PRL (2020)] to obtain Higgs-like inflatons as a portal to dark energy. In the case of thick DW, we assume that there is a torsion squared influence, since we are in the early universe where torsion is not so weak as in the late universe as shown by Paul and SenGupta [EPJ C (2019)] in a 5D brane-world. A static DW solution is obtained when the inflationary potential vanishes and Higgs potential is a helical function. Recently, in the absence of inflation, domain wall dynamos were obtained in Einstein–Cartan gravity (EC) where the spins of the nucleons were orthogonal to the wall.

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Moradpour, H., et M. Valipour. « Generalized Misner–Sharp energy in generalized Rastall theory ». Canadian Journal of Physics 98, n^o 9 (septembre 2020) : 853–56. http://dx.doi.org/10.1139/cjp-2019-0492.

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Employing the unified first law of thermodynamics and the field equations of the generalized Rastall theory, we get the generalized Misner–Sharp mass of space–times for which gtt = –grr = –f(r). The obtained result differs from those of the Einstein and Rastall theories. Moreover, using the first law of thermodynamics, the obtained generalized Misner–Sharp mass, and the field equations, the entropy of static spherically symmetric horizons are also addressed in the framework of the generalized Rastall theory. In addition, by generalizing the study to a flat Friedmann–Robertson–Walker (FRW) universe, the apparent horizon entropy is also calculated. Considering the effects of applying the Newtonian limit to the field equations on the coupling coefficients of the generalized Rastall theory, our study indicates (i) the obtained entropy–area relation is the same as that of the Rastall theory, and (ii) the Bekenstein entropy is recovered when the generalized Rastall theory reduces to the Einstein theory. The validity of the second law of thermodynamics is also investigated in the flat FRW universe.

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López-Corredoira, M., F. Melia, E. Lusso et G. Risaliti. « Cosmological test with the QSO Hubble diagram ». International Journal of Modern Physics D 25, n^o 05 (avril 2016) : 1650060. http://dx.doi.org/10.1142/s0218271816500607.

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A Hubble diagram (HD) has recently been constructed in the redshift range [Formula: see text] using a nonlinear relation between the ultraviolet (UV) and X-ray luminosities of quasi stellar objects (QSOs). The Type Ia Supernovae (SN) HD has already provided a high-precision test of cosmological models, but the fact that the QSO distribution extends well beyond the supernova range ([Formula: see text]), in principle provides us with an important complementary diagnostic whose significantly greater leverage in [Formula: see text] can impose tighter constraints on the distance versus redshift relationship. In this paper, we therefore perform an independent test of nine different cosmological models, among which six are expanding, while three are static. Many of these are disfavored by other kinds of observations (including the aforementioned Type Ia SNe). We wish to examine whether the QSO HD confirms or rejects these earlier conclusions. We find that four of these models (Einstein–de Sitter, the Milne universe, the static universe with simple tired light and the static universe with plasma tired light) are excluded at the [Formula: see text] C.L. The quasi-steady state model is excluded at [Formula: see text] C.L. The remaining four models ([Formula: see text]CDM/[Formula: see text]CDM, the [Formula: see text] universe, the Friedmann open universe and a static universe with a linear Hubble law) all pass the test. However, only [Formula: see text]CDM/[Formula: see text]CDM and [Formula: see text] also pass the Alcock–Paczyński (AP) test. The optimized parameters in [Formula: see text]CDM/[Formula: see text]CDM are [Formula: see text] and [Formula: see text] (the dark energy equation-of-state). Combined with the AP test, these values become [Formula: see text] and [Formula: see text]. But whereas this optimization of parameters in [Formula: see text]CDM/[Formula: see text]CDM creates some tension with their concordance values, the [Formula: see text] universe has the advantage of fitting the QSO and AP data without any free parameters.

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LIU, BIN, YUN-CHUAN DAI, XIAN-RU HU et JIAN-BO DENG. « THE FRIEDMANN EQUATION IN MODIFIED ENTROPY-AREA RELATION FROM ENTROPY FORCE ». Modern Physics Letters A 26, n^o 07 (7 mars 2011) : 489–500. http://dx.doi.org/10.1142/s021773231103492x.

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According to the formal holographic principle, a modification to the assumption of holographic principle in Verlinder's investigation of entropy force is obtained. A more precise relation between entropy and area in the holographic system is proposed. With the entropy corrections to the area-relation, we derivate Newton's laws and Einstein equation with a static spherically symmetric holographic screen. Furthermore, we derived the correction terms to the modified Friedmann equation of the FRW universe starting from the holographic principle and the Debye model.

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