scholarly journals Limits on the Validity of the Semiclassical Theory — A Minisuperspace Example

1997 â—˝  
Vol 06 (03) â—˝  
pp. 363-375
Author(s):  
L. Sriramkumar
Keyword(s):  
Scalar Field â—˝  
Energy Momentum Tensor â—˝  
Momentum Tensor â—˝  
Matter Field â—˝  
Massless Scalar â—˝  
Back Reaction â—˝  
Massless Scalar Field â—˝  
Semiclassical Theory â—˝  
Dimensionless Quantity â—˝  
Complete Field

For want of a more natural proposal, it is generally assumed that the back-reaction of a quantized matter field on a classical metric is given by the expectation value of its energy–momentum tensor, evaluated in a specified state. This semiclassical theory can be reliable only when the fluctuations in the energy–momentum densities of the quantum field are negligible. Based on this condition, Kuo and Ford have constructed a dimensionless quantity, whose magnitude reflects the amount of fluctuation in the back-reaction term and hence on the validity of the semiclassical theory. In this paper, we evaluate this quantity for the minisuperspace model of a quantized massless scalar field in a Friedmann universe. We conclude that the semiclassical theory for the model we consider here can be relied upon only if the scalar field is in states like coherent states. The implications of our investigation on the complete field theory are also discussed.

scholarly journals Cosmological reconstruction of f(T, đť’Ż) gravity

2014 â—˝  
Vol 11 (08) â—˝  
pp. 1450077 â—˝  
Author(s):  
Davood Momeni â—˝  
Ratbay Myrzakulov
Keyword(s):  
Scalar Field â—˝  
Modified Gravity â—˝  
Energy Momentum Tensor â—˝  
Momentum Tensor â—˝  
Chaplygin Gas â—˝  
Massless Scalar â—˝  
Massless Scalar Field â—˝  
Two Fluids â—˝  
Consistent Model â—˝  
The Universe

Motivated by the newly proposal for gravity as the effect of the torsion scalar T and trace of the energy momentum tensor 𝒯, we investigate the cosmological reconstruction of different models of the Universe. Our aim here is to show that how this modified gravity model, f(T, 𝒯) is able to reproduce different epochs of the cosmological history. We explicitly show that f(T, 𝒯) can be reconstructed for ΛCDM as the most popular and consistent model. Also we study the mathematical reconstruction of f(T, 𝒯) for a flat cosmological background filled by two fluids mixture. Such model describes phantom–non-phantom era as well as the purely phantom cosmology. We extend our investigation to more cosmological models like perfect fluid, Chaplygin gas and massless scalar field. In each case we obtain some specific forms of f(T, 𝒯). These families of f(T, 𝒯) contain arbitrary function of torsion and trace of the energy momentum.

On the origin of black hole evaporation radiation

1976 â—˝  
Vol 351 (1664) â—˝  
pp. 129-139 â—˝  
Keyword(s):  
Black Hole â—˝  
Energy Momentum Tensor â—˝  
Momentum Tensor â—˝  
Massless Scalar â—˝  
Massless Scalar Field â—˝  
Static Vacuum â—˝  
Black Hole Evaporation â—˝  
A Value â—˝  
Special Case â—˝  
Collapse Process

The physical basis underlying the black hole evaporation process is clarified by a calculation of the expectation value of the energy-momentum tensor for a massless scalar field in a completely general two dimensional collapse scenario. It is found that radiation is produced inside the collapsing matter which propagates both inwards and outwards. The ingoing com­ponent eventually emerges from the star after travelling through the centre. The outgoing energy flux appears at infinity as the evaporation radiation discovered by Hawking. At late times, outside the star, the former component fades out exponentially, and the latter component approaches a value which is independent of the details of the collapse process. In the special case of a collapsing hollow, thin shell of matter, all the radiation is produced at the shell. These results are independent of regularization ambiguities, which enter only the static vacuum polariza­tion terms in the energy-momentum tensor. The significance of an earlier remark about black hole explosions is discussed in the light of these results.

scholarly journals QUANTUM NOETHER'S THEOREM AND CONFORMAL FIELD THEORY: A STUDY OF SOME MODELS

1999 â—˝  
Vol 11 (05) â—˝  
pp. 519-532 â—˝  
Author(s):  
SEBASTIANO CARPI
Keyword(s):  
Field Theory â—˝  
Quantum Field Theory â—˝  
Scalar Field â—˝  
Scaling Limit â—˝  
Momentum Tensor â—˝  
Massless Scalar â—˝  
Massless Scalar Field â—˝  
Quantum Field â—˝  
Time Dimension â—˝  
Complex Scalar

We study the problem of recovering Wightman conserved currents from the canonical local implementations of symmetries which can be constructed in the algebraic framework of quantum field theory, in the limit in which the region of localization shrinks to a point. We show that, in a class of models of conformal quantum field theory in space-time dimension 1+1, which includes the free massless scalar field and the SU(N) chiral current algebras, the energy-momentum tensor can be recovered. Moreover we show that the scaling limit of the canonical local implementation of SO(2) in the free complex scalar field is zero, a manifestation of the fact that, in this last case, the associated Wightman current does not exist.

scholarly journals WAVE EQUATION OF THE SCALAR FIELD AND SUPERFLUIDS

2009 â—˝  
Vol 24 (40) â—˝  
pp. 3249-3256 â—˝  
Author(s):  
A. NADDEO â—˝  
G. SCELZA
Keyword(s):  
General Relativity â—˝  
Wave Equation â—˝  
Scalar Field â—˝  
Dimensional Space â—˝  
Massless Scalar â—˝  
Back Reaction â—˝  
Massless Scalar Field â—˝  
Sound Waves â—˝  
Hydrodynamical Equation â—˝  
Common Features

The new formal analogy between superfluid systems and cosmology, which emerges by taking into account the back-reaction of the vacuum to the quanta of sound waves,1 enables us to put forward some common features between these two different areas of physics. We find the condition that allows us to justify a General Relativity (GR) derivation of the hydrodynamical equation for the superfluid in a four-dimensional space whose metric is the Unruh one.2 Furthermore, we show how, in the particular case taken into account, our hydrodynamical equation can be deduced within a four-dimensional space from the wave equation of a massless scalar field.

WORMHOLES OF K-ESSENCE IN ARBITRARY SPACE–TIME DIMENSIONS

2008 â—˝  
Vol 23 (20) â—˝  
pp. 3165-3175 â—˝  
Author(s):  
J. ESTEVEZ-DELGADO â—˝  
T. ZANNIAS
Keyword(s):  
Energy Momentum Tensor â—˝  
Opposite Sign â—˝  
Momentum Tensor â—˝  
Space Time â—˝  
Parameter Family â—˝  
Massless Scalar â—˝  
Massless Scalar Field â—˝  
Asymptotically Flat â—˝  
Arbitrary Space â—˝  
Two Parameter

We consider a K-essence involving a massless scalar field Φ minimally coupled to Einstein gravity in D ≥ 4 space–time dimensions. This theory admits a two-parameter family of spherical wormholes representing two asymptotically-flat universes connected via a (D-2)-dimensional spherical throat. The ADM masses of the two ends are unequal and of opposite sign except for a one-parameter family where both ends possess vanishing ADM masses. By cut and paste techniques, we construct a two-parameter family of wormholes where the ends possess equal and positive ADM masses but the throat is a (D-1)-dimensional thin-shell. The structure of the surface energy–momentum tensor is also analyzed.

Vacuum polarization in the background of conical singularity

2020 â—˝  
Vol 35 (02n03) â—˝  
pp. 2040030
Author(s):  
Yuri V. Grats â—˝  
Pavel Spirin
Keyword(s):  
Green’S Function â—˝  
Green's Function â—˝  
Energy Momentum Tensor â—˝  
Vacuum Expectation â—˝  
Vacuum Expectation Value â—˝  
Momentum Tensor â—˝  
Massless Scalar â—˝  
Massless Scalar Field â—˝  
Dimensional Spacetime â—˝  
Higher Dimensional

We consider the gravity-induced effects associated with a massless scalar field living in a higher-dimensional spacetime being the tensor product of Minkowski space and spherically-symmetric space with angle deficit. These spacetimes are considered as simple models for a multidimensional global monopole or cosmic string with flat extra dimensions, where the deficit of solid angle is proportional to Newton constant and tension. Thus, we refer to them as conical backgrounds. In terms of the angular deficit value, we derive the perturbative expression for the scalar Green’s function and compute it to the leading order. With the use of this Green’s function we compute the renormalized vacuum expectation value of the scalar-field’s energy-momentum tensor. We make some general note on the linear-on-curvature part of the trace of even coefficients of Schwinger-deWitt expansion.

scholarly journals Examining a covariant and renormalizable quantum field theory in de Sitter space by studying “black hole radiation”

2016 â—˝  
Vol 31 (11) â—˝  
pp. 1650052 â—˝  
Author(s):  
Hamed Pejhan â—˝  
Surena Rahbardehghan
Keyword(s):  
Field Theory â—˝  
Quantum Field Theory â—˝  
Black Hole â—˝  
Energy Momentum Tensor â—˝  
Momentum Tensor â—˝  
Massless Scalar â—˝  
Massless Scalar Field â—˝  
Quantum Field â—˝  
De Sitter â—˝  
Black Hole Radiation

Respecting that any consistent quantum field theory in curved space–time must include black hole radiation, in this paper, we examine the Krein–Gupta–Bleuler (KGB) formalism as an inevitable quantization scheme in order to follow the guideline of the covariance of minimally coupled massless scalar field and linear gravity on de Sitter (dS) background in the sense of Wightman–Gärding approach, by investigating thermodynamical aspects of black holes. The formalism is interestingly free of pathological large distance behavior. In this construction, also, no infinite term appears in the calculation of expectation values of the energy–momentum tensor (we have an automatic and covariant renormalization) which results in the vacuum energy of the free field to vanish. However, the existence of an effective potential barrier, intrinsically created by black holes gravitational field, gives a Casimir-type contribution to the vacuum expectation value of the energy–momentum tensor. On this basis, by evaluating the Casimir energy–momentum tensor for a conformally coupled massless scalar field in the vicinity of a nonrotating black hole event horizon through the KGB quantization, in this work, we explicitly prove that the hole produces black-body radiation which its temperature exactly coincides with the result obtained by Hawking for black hole radiation.

scholarly journals Scalar Casimir effect in a high-dimensional cosmic dispiration spacetime

2018 â—˝  
Vol 27 (12) â—˝  
pp. 1850107 â—˝  
Author(s):  
H. F. Mota â—˝  
E. R. Bezerra de Mello â—˝  
K. Bakke
Keyword(s):  
Cosmic String â—˝  
Energy Momentum Tensor â—˝  
Casimir Effect â—˝  
Topological Defects â—˝  
Vacuum Expectation â—˝  
Scalar Fields â—˝  
Momentum Tensor â—˝  
High Dimensional â—˝  
Massless Scalar â—˝  
Massless Scalar Field

In this paper we present a complete and detailed analysis of the calculation of both the Wightman function and the vacuum expectation value of the energy–momentum tensor that arise from quantum vacuum fluctuations of massive and massless scalar fields in the cosmic dispiration spacetime, which is formed by the combination of two topological defects: a cosmic string and a screw dislocation. This spacetime is obtained in the framework of the Einstein–Cartan theory of gravity and is considered to be a chiral spacelike cosmic string. For completeness we perform the calculation in a high-dimensional spacetime, with flat extra dimensions. We found closed expressions for the energy–momentum tensor and, in particular, in [Formula: see text]-dimensions, we compare our results with previous existing ones in the literature for the massless scalar field case.

scholarly journals Accelerated paths and Unruh effect. Part I. Scalars and fermions in Anti De Sitter spacetime

2021 â—˝  
Vol 2021 (9) â—˝  
Author(s):  
Shahnewaz Ahmed â—˝  
Mir Mehedi Faruk
Keyword(s):  
Scalar Field â—˝  
Response Function â—˝  
Matter Field â—˝  
Detector Response â—˝  
Dirac Field â—˝  
Massless Scalar â—˝  
Unruh Effect â—˝  
Massless Scalar Field â—˝  
De Sitter â—˝  
Ads Spacetime

Abstract We have investigated the Unruh effect in Anti de-Sitter (AdS) spacetime by examining the response function of an Unruh-DeWitt particle detector with uniform constant acceleration. An exact expression of the detector response function for the scalar field has been obtained with different levels of non-linearity in even dimensional AdS spacetime. We also showed how the response of the accelerated Unruh detector coupled quadratically to massless Dirac field in D dimensional (D ≥ 2) AdS spacetime is proportional to that of a detector linearly coupled to a massless scalar field in 2D dimensional AdS spacetime. Here, the fermionic and scalar matter field is coupled minimally and conformally to the background AdS metric, respectively. Finally, we discuss about the extension of the results for more general stationary motion.

Mass Generation by a Massless Scalar Field

2020 â—˝  
Vol 26 (4) â—˝  
pp. 358-372
Author(s):  
A. P. Lelyakov
Keyword(s):  
Scalar Field â—˝  
Massless Scalar â—˝  
Massless Scalar Field â—˝  
Mass Generation
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