The Experts below are selected from a list of 315 Experts worldwide ranked by ideXlab platform
P H R S Moraes - One of the best experts on this subject based on the ideXlab platform.
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stable relativistic polytropic objects with Cosmological Constant
arXiv: General Relativity and Quantum Cosmology, 2020Co-Authors: Jose D V Arbanil, P H R S MoraesAbstract:The effects of the Cosmological Constant on the static equilibrium configurations and stability against small radial perturbations of relativistic polytropic spheres are investigated. This study numerically solves the hydrostatic equilibrium equation and the radial stability equation, both of which are modified from their standard form to introduce the Cosmological Constant. For the fluid, we consider a pressure $p$ and an energy density $\rho$, which are connected through the equation of state $p=\kappa\delta^{\Gamma}$ with $\delta=\rho-p/(\Gamma-1)$, where $\kappa$, $\Gamma$ and $\delta$ represent the polytropic Constant, adiabatic index and rest mass density of the fluid, respectively. The dependencies of the mass, radius and eigenfrequency of oscillations on both the Cosmological Constant and the adiabatic index are analyzed. For ranges of both the central rest mass density $\delta_c$ and the adiabatic index $\Gamma$, we show that the stars have a larger (lower) mass and radius and a diminished (enhanced) stability when the Cosmological Constant $\Lambda>0$ ($\Lambda 0$ and $dM/d\delta_c<0$, respectively.
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stable relativistic polytropic objects with Cosmological Constant
European Physical Journal Plus, 2020Co-Authors: Jose D V Arbanil, P H R S MoraesAbstract:The effects of the Cosmological Constant on the static equilibrium configurations and stability against small radial perturbations of relativistic polytropic spheres are investigated. This study numerically solves the hydrostatic equilibrium equation and the radial stability equation, both of which are modified from their standard form to introduce the Cosmological Constant. For the fluid, we consider a pressure p and an energy density , which are connected through the equation of state with , where , and represent the polytropic Constant, adiabatic index and rest mass density of the fluid, respectively. The dependencies of the mass, radius and eigenfrequency of oscillations on both the Cosmological Constant and the adiabatic index are analyzed. For ranges of both the central rest mass density and the adiabatic index , we show that the stars have a larger (lower) mass and radius and a diminished (enhanced) stability when the Cosmological Constant () is increased (decreased). In addition, in a sequence of compact objects with fixed and , the regions constructed by stable and unstable static equilibrium configurations are recognized by the conditions and , respectively.
Nikodem J Poplawski - One of the best experts on this subject based on the ideXlab platform.
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Cosmological Constant from quarks and torsion
Annalen der Physik, 2011Co-Authors: Nikodem J PoplawskiAbstract:We present a simple and natural way to derive the observed small, positive Cosmological Constant from the gravitational interaction of condensing fermions. In the Riemann-Cartan spacetime, torsion gives rise to the axial–axial vector four-fermion interaction term in the Dirac Lagrangian for spinor fields. We show that this nonlinear term acts like a Cosmological Constant if these fields have a nonzero vacuum expectation value. For quark fields in QCD, such a torsion-induced Cosmological Constant is positive and its energy scale is only about 8 times larger than the observed value. Adding leptons to this picture could lower this scale to the observed value.
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Cosmological Constant from quarks and torsion
arXiv: General Relativity and Quantum Cosmology, 2010Co-Authors: Nikodem J PoplawskiAbstract:We present a simple and natural way to derive the observed small, positive Cosmological Constant from the gravitational interaction of condensing fermions. In the Riemann-Cartan spacetime, torsion gives rise to the axial-axial four-fermion interaction term in the Dirac Lagrangian for spinor fields. We show that this nonlinear term acts like a Cosmological Constant if these fields have a nonzero vacuum expectation value. For quark fields in QCD, such a torsion-induced Cosmological Constant is positive and its energy scale is only about 8 times larger than the observed value. Adding leptons to this picture could lower this scale to the observed value.
Steven D Bass - One of the best experts on this subject based on the ideXlab platform.
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The Cosmological Constant puzzle
Journal of Physics G: Nuclear and Particle Physics, 2015Co-Authors: Steven D BassAbstract:The accelerating expansion of the Universe points to a small positive vacuum energy density and negative vacuum pressure. A strong candidate is the Cosmological Constant in Einstein's equations of General Relativity. Possible contributions are zero-point energies and the condensates associated with spontaneous symmetry breaking. The vacuum energy density extracted from astrophysics is 10 56 times smaller than the value expected from quantum fields and Standard Model particle physics. Is the vacuum energy density time dependent ? We give an introduction to the Cosmological Constant puzzle and ideas how to solve it.
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Vacuum energy and the Cosmological Constant
Modern Physics Letters A, 2015Co-Authors: Steven D BassAbstract:The accelerating expansion of the Universe points to a small positive value for the Cosmological Constant or vacuum energy density. We discuss recent ideas that the Cosmological Constant plus Large Hadron Collider (LHC) results might hint at critical phenomena near the Planck scale.
Surjeet Rajendran - One of the best experts on this subject based on the ideXlab platform.
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relaxation of the Cosmological Constant
Physical Review D, 2019Co-Authors: Peter W Graham, David E Kaplan, Surjeet RajendranAbstract:We present a model that naturally tunes a large positive Cosmological Constant to a small Cosmological Constant. A slowly rolling scalar field decreases the Cosmological Constant to a small negative value, causing the universe to contract, thus reheating it. An expanding universe with a small positive Cosmological Constant can be obtained, respectively, by coupling this solution to any model of a Cosmological bounce and coupling the scalar field to a sector that undergoes a technically natural phase transition at the meV scale. A robust prediction of this model is a rolling scalar field today with some coupling to the standard model. This can potentially be experimentally probed in a variety of Cosmological and terrestrial experiments, such as probes of the equation of state of dark energy, birefringence in the cosmic microwave background and terrestrial tests of Lorentz violation.
T Padmanabhan - One of the best experts on this subject based on the ideXlab platform.
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Cosmological Constant—the weight of the vacuum
Physics Reports, 2020Co-Authors: T PadmanabhanAbstract:Recent Cosmological observations suggest the existence of a positive Cosmological Constant $\Lambda$ with the magnitude $\Lambda(G\hbar/c^3) \approx 10^{-123}$. This review discusses several aspects of the Cosmological Constant both from the Cosmological (sections 1-6) and field theoretical (sections 7-11) perspectives. The first section introduces the key issues related to Cosmological Constant and provides a brief historical overview. This is followed by a summary of the kinematics and dynamics of the standard Friedmann model of the universe paying special attention to features involving the Cosmological Constant. Section 3 reviews the observational evidence for Cosmological Constant, especially the supernova results, constraints from the age of the universe and a few others. Theoretical models (quintessence, tachyonic scalar field, ...) with evolving Cosmological `Constant' are described from different perspectives in the next section. Constraints on dark energy from structure formation and from CMBR anisotropies are discussed in the next two sections. The latter part of the review (sections 7-11) concentrates on more conceptual and fundamental aspects of the Cosmological Constant. Section 7 provides some alternative interpretations of the Cosmological Constant which could have a bearing on the possible solution to the problem. Several relaxation mechanisms have been suggested in the literature to reduce the Cosmological Constant to the currently observed value and some of these attempts are described in section 8. Next section gives a brief description of the geometrical structure of the de Sitter spacetime and the thermodynamics of the de Sitter universe is taken up in section 10. The last section deals with the role of string theory in the Cosmological Constant problem.Comment: Final version to appear as Physics Reports; references added, typos and some labelling errors in figures corrected; 114 pages; macros include
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Cosmological Constant the weight of the vacuum
Physics Reports, 2003Co-Authors: T PadmanabhanAbstract:Recent Cosmological observations suggest the existence ofa positive Cosmological Constantwith the magnitude � (G˝=c 3 ) ≈ 10 −123 . This review discusses several aspects ofthe Cosmological Constant both f rom the Cosmological (Sections 1- 6) and .eld theoretical (Sections 7-11) perspectives. After a brief introduction to the key issues related to Cosmological Constant and a historical overview, a summary ofthe kinematics and dynamics ofthe standard Friedmann model ofthe universe is provided. The observational evidence for Cosmological Constant, especially from the supernova results, and the constraints from the age of the universe, structure formation, Cosmic Microwave Background Radiation (CMBR) anisotropies and a few others are described in detail, followed by a discussion of the theoretical models (quintessence, tachyonic scalar .eld, :::) from di4erent perspectives. The latter part of the review (Sections 7-11) concentrates on more conceptual and f aspects ofthe Cosmological Constant like some alternative interpretations ofthe Cosmological Constant, relaxation mechanisms to reduce the Cosmological Constant to the currently observed value, the geometrical structure ofthe de Sitter spacetime, thermodynamics ofthe de Sitter universe and the role of string theory in the Cosmological Constant problem. c
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Cosmological Constant—the weight of the vacuum
Physics Reports, 2003Co-Authors: T PadmanabhanAbstract:Recent Cosmological observations suggest the existence ofa positive Cosmological Constantwith the magnitude � (G˝=c 3 ) ≈ 10 −123 . This review discusses several aspects ofthe Cosmological Constant both f rom the Cosmological (Sections 1- 6) and .eld theoretical (Sections 7-11) perspectives. After a brief introduction to the key issues related to Cosmological Constant and a historical overview, a summary ofthe kinematics and dynamics ofthe standard Friedmann model ofthe universe is provided. The observational evidence for Cosmological Constant, especially from the supernova results, and the constraints from the age of the universe, structure formation, Cosmic Microwave Background Radiation (CMBR) anisotropies and a few others are described in detail, followed by a discussion of the theoretical models (quintessence, tachyonic scalar .eld, :::) from di4erent perspectives. The latter part of the review (Sections 7-11) concentrates on more conceptual and f aspects ofthe Cosmological Constant like some alternative interpretations ofthe Cosmological Constant, relaxation mechanisms to reduce the Cosmological Constant to the currently observed value, the geometrical structure ofthe de Sitter spacetime, thermodynamics ofthe de Sitter universe and the role of string theory in the Cosmological Constant problem. c
