Browsing by Author "Kao, WF"

Now showing 1 - 20 of 20

A new look at the pair-production width in a strong magnetic field
(2000-12-07)
Kao, WF
;
Lin, GL
;
Tseng, JJ
;
物理研究所
;
Institute of Physics
We reexamine the process gamma --> e(+) + e(-) in a background magnetic field comparable to B-c equivalent to m(e)(2)/e male. This process is known to be non-perturbative in the magnetic-field strength. However, it can be shown that the moments of the above pair production width are proportional to the derivatives of photon polarization function at the zero energy, which is perturbative in B. Hence, the pair-production width can be easily obtained from the latter by the inverse Mellin transform. The implications of our approach are discussed. (C) 2000 Published by Elsevier Science B.V.
Bianchi type-I space and the stability of the inflationary Friedmann-Robertson-Walker solution
(2001-11-15)
Kao, WF
;
物理研究所
;
Institute of Physics
A stability analysis of the Bianchi type-I universe in pure gravity theory is studied in detail, We first derive the nonredundant field equation of the system by introducing the generalized Bianchi type-I metric. This nonredundant equation reduces to the Friedmann equation in the isotropic limit. It is shown further that any unstable mode of the isotropic perturbation with respect to a de Sitter background is also unstable with respect to anisotropic perturbations. The implications of the choice of physical theories are discussed in detail in this paper.
Friedmann equation and stability of inflationary higher derivative gravity
(2001-06-15)
Kao, WF
;
Pen, UL
;
Zhang, PJ
;
物理研究所
;
Institute of Physics
A stability analysis of the de Sitter universe in pure gravity theory is known to be useful in many aspects. We first show how to complete the proof of an earlier argument based on a redundant field equation. It is shown further that the stability condition applies to k not equal 0 Friedmann-Robertson-Walker spaces based on the nonredundant Friedmann equation derived from a simple effective Lagrangian. We show how to derive this expression for the Friedmann equation of pure gravity theory. This expression is also generalized to include scalar field interactions.
Higher derivative Weyl gravity
(2000-02-15)
Kao, WF
;
物理研究所
;
Institute of Physics
A higher derivative Weyl model is analyzed carefully. We show that there is a nontrivial constraint if a symmetry-breaking potential is added to the system. A rigorous proof for the constraint is presented. One, hence, studies the applications of this conformal theory in the inflationary universe.
Induced Einstein-Kalb-Ramond theory and the black hole
(1996-02-15)
Kao, WF
;
Dai, WB
;
Wang, SY
;
Chyi, TK
;
Lin, SY
A four-dimensional induced Einstein-Kalb-Ramond theory with a conformally coupled Kalb-Ramond term is discussed. It is argued that the Kalb-Ramond field is difficult to interpret as an axion field from a dynamical point of view, and hence tends to be unstable. One can, however, apply Routh's method to extract an effective action for the axion field. It is found that the Kalb-Ramond hair will turn the event horizon into a naked singularity. Such an exact solution is found and analyzed carefully.
Inflationary universe in higher derivative induced gravity
(2000-10-15)
Kao, WF
;
物理研究所
;
Institute of Physics
In an induced-gravity model, the stability condition of an inflationary slow-rollover solution is shown to be phi (0)partial derivative (phi0) V(phi (0)) = 4 V(phi (0)). The presence of higher derivative terms will, however, act against the stability of this expanding solution unless further constraints on the field parameters are imposed. We find that these models will acquire a nonvanishing cosmological constant at the end of inflation. Some models are analyzed for their implication to the early universe.
Kaluza-Klein induced gravity inflation
(2000-10-15)
Kao, WF
;
物理研究所
;
Institute of Physics
A D-dimensional induced gravity theory is studied carefully in a 4 + (D-4) dimensional Friedmann-Robertson-Walker space-time. We try to extract information about the symmetry-breaking potential in search of an inflationary solution with a nonexpanding internal space. We find that the induced gravity model imposes strong constraints on the form of the symmetry-breaking potential in order to generate an acceptable inflationary universe. These constraints are analyzed carefully in this paper.
Magnetic monopole in induced Einstein-Yang-Mills models
(2000-02-15)
Kao, WF
;
物理研究所
;
Institute of Physics
We show that a regular SO(3) non-Abelian monopole cannot exist in induced gravity models with a variety of symmetry breaking potentials. The no-hair theorem is also shown to be true in these models. We also analyze the global behavior of the gauge field in the presence of a black hole with a nonvanishing cosmological constant. It is shown that the nontrivial non-Abelian monopole solution exists only if the radius of the event horizon is smaller than the characteristic radius of the classical monopole.
Magnetically charged black holes in induced gravity
(1998-10-01)
Dai, WB
;
Kao, WF
;
Wang, SY
;
物理研究所
;
Institute of Physics
It is shown that the regular SO3 non-abelian spherically symmetric monopole solution does not exist in an induced Einstein-Yang-Mills-Higgs model. It is also shown that Higgs scalar hair cannot exist in the presence of a black hole event horizon in this model. The corresponding global behavior of the gauge field is analyzed in a black hole background. In particular, it is shown that a non-trivial non-abelian monopole solution exists only if the radius of the event horizon is smaller than the characteristic radius of the classical monopole.
Neutrino-photon scattering and its crossed processes in a background magnetic field
(1999-11-04)
Chyi, TK
;
Hwang, CW
;
Kao, WF
;
Lin, GL
;
Ng, KW
We study the neutrino-photon processes such as gamma gamma --> , v gamma --> v gamma, and --> gamma gamma in a background magnetic field smaller than the critical magnetic field Bc + m(e)(2)/e. Using Schwinger's formalism, we extract leading magnetic-field contributions to the above processes. Our result is valid throughout the kinematic regime where both neutrino and photon energies are significantly smaller than m(w). We briefly discuss the astrophysical implications of our result. (C) 1999 Published by Elsevier Science B.V. All rights reserved.
On analytic properties of the photon polarization function in a background magnetic field
(2001-12-13)
Kao, WF
;
Lin, GL
;
Tseng, JJ
;
物理研究所
;
Institute of Physics
We examine the analytic properties of the photon polarization function in a background magnetic field, using the technique of inverse Mellin transform. The photon polarization function is first expressed as a power series of the photon energy omega with omega < 2m(e). Based upon this energy expansion and the branch cut of the photon polarization function in the complex omega plane. we compute the absorptive part of the polarization function with the inverse Mellin transform. Our results are valid for arbitrary photon energies and magnetic-fieid strengths. The applications of our approach are briefly discussed. (C) 2001 Published by Elsevier Science B.V.
On analytic properties of the photon polarization function in a background magnetic field (vol B 522, pg 257, 2001)
(2002-08-15)
Kao, WF
;
Lin, GL
;
Tseng, HJ
;
物理研究所
;
Institute of Physics
On metric dependence of Vilkovisky-DeWitt effective potential
(1998-08-30)
Kao, WF
;
Lin, SW
;
電子物理學系
;
物理研究所
;
Department of Electrophysics
It is known that the Vilkovisky-DeWitt effective potential for Einstein gravity depends on the choice of the metric of configuration space. We will show that this metric dependence occurs not only in gauge field theories, but also in scalar fields as well as other field theories with "unnatural" initial choices of metric. This means that the problem of metric dependence relates to initial field parametrization but not gauge symmetry.
Rotational perturbations of Friedmann-Robertson-Walker type brane-world cosmological models
(2002-08-05)
Chen, CM
;
Harko, T
;
Kao, WF
;
Mak, MK
;
物理研究所
First order rotational perturbations of the Friedmann-Robertson-Walker metric are Considered in the framework of the brane-world cosmological models. A rotation equation, relating the perturbations of the metric tensor to the angular velocity of the matter on the brane is derived under the assumption of slow rotation. The mathematical structure of the rotation equation imposes strong restrictions on the temporal and spatial dependence of the brane matter angular velocity. The study of the integrable cases of the rotation equation leads to three distinct models, which are considered in detail, As a general result we find that, similarly to the general relativistic case, the rotational perturbations decay due to the expansion of the matter on the brane. One of the obtained consistency conditions leads to a particular, purely inflationary brane-world cosmological model, with the cosmological fluid obeying a non-linear barotropic equation of state. (C) 2002 Elsevier Science B.V. All rights reserved.
Rotational perturbations of high density matter in the brane cosmology
(2003-11-01)
Chen, CM
;
Harko, T
;
Kao, WF
;
Mak, MK
;
物理研究所
We consider the evolution of small rotational perturbations, with azimuthal symmetry, of the brane-world cosmological models. The equations describing the temporal, radial and angular dependence of the perturbations are derived by taking into account the effects of both scalar and tensor parts of the dark energy term on the brane. The time decay of the initial rotation is investigated for several types of equation of state of the ultra-high density cosmological matter. For an expanding Universe, rotation always decays in the case of the perfect dragging, for which the angular velocity of the matter on the brane equals the rotation metric tensor. For non-perfect dragging, the behaviour of the rotation is strongly equation of state dependent. For some classes of dense matter, like the stiff causal or the Chaplygin gas, the angular velocity of the matter on the brane is an increasing function of time. For other types of the ultra-dense matter, such as the Hagedorn fluid, rotation is smoothed out by the expansion of the Universe. Therefore the study of dynamics of rotational perturbations of brane-world models, as well as in general relativity, could provide some insights on the physical properties and equation of state of the cosmological fluid filling the very early Universe.
Semiclassical quantization for the spherically symmetric systems under an Aharonov-Bohm magnetic flux
(2002-05-01)
Kao, WF
;
Luan, PG
;
Lin, DH
;
物理研究所
;
Institute of Physics
The semiclassical quantization rule is derived for a system with a spherically symmetric potential V(r)similar tor(nu)(-2>1. The rule is even derived in the large principal quantum number limit n>>1. We also discuss the power parameter nu dependence of the energy spectra pattern in this paper.
Stability analysis of anisotropic inflationary cosmology
(2001-12-15)
Chen, CM
;
Kao, WF
;
物理研究所
;
Institute of Physics
The stability analysis of an anisotropic inflationary universe of the four-dimensional Neveu-Schwarz-Neveu-Schwarz string model with a nonvanishing cosmological constant is discussed in this paper. The accelerating expansion solution found earlier is shown to be stable against the perturbations with respect to the dilaton and axion fields once the dilaton field falls close to the local minimum of the symmetry-breaking potential. This indicates that the Bianchi type-I space tends to evolve to an isotropic flat Friedmann-Robertson-Walker space. This expanding solution is also shown to be stable against the perturbation with respect to anisotropic spatial directions.
Stability of the anisotropic brane cosmology
(2004-02-01)
Chen, CM
;
Kao, WF
;
物理研究所
;
Institute of Physics
The stability of the Bianchi type I anisotropic brane cosmology is analyzed in this paper. We also study the effects of the brane solution by comparing the models on the 3-brane and the models in the conventional Einstein space. The analysis is presented for two different models: one with a perfect fluid and the other with a dilaton field. It is shown that the anisotropic expansion is smeared out dynamically for both theories in the large time limit, independent of the models of different types of matter. The initial states are, however, dramatically different. A primordial anisotropic expansion will grow for the conventional Einstein's theory. On the other hand, it is shown that the initial state is highly isotropic for the brane universe, except for a very particular case. Moreover, it is also shown that the Bianchi type I anisotropic cosmology is stable against any anisotropic perturbation for both theories in the large time limit.
Weak-field expansion for processes in a homogeneous background magnetic field
(2000-11-15)
Chyi, TK
;
Hwang, CW
;
Kao, WF
;
Lin, GL
;
Ng, KW
The weak-field expansion of the charged fermion propagator under a uniform magnetic field is studied. Starting from Schwinger's proper-time representation, we express the charged fermion propagator as an infinite series corresponding to different Landau levels. This infinite series is then reorganized according to the powers of the external field strength B. For illustration, we apply this expansion to gamma --> nu<()over bar> and nu --> nu gamma decays, which involve charged fermions in the internal loop. The leading and subleading magnetic-field effects to the above processes are computed.
Weyl invariant black hole
(1996-02-15)
Kao, WF
;
Lin, SY
;
Chyi, TK
;
電子物理學系
;
物理研究所
The properties of the Weyl invariant black hole and its implications are studied. The calculation shows that there is no hair, up to a gauge choice, for a Weyl gauge field outside the event horizon of a black hole in the Weyl invariant limit. We also show that a scalar held will remain constant once the scale symmetry is broken spontaneously by the well-known Higgs potential. As a result, classical hair for the Weyl vector meson and scalar measuring field vanishes strictly in the presence of a spontaneously symmetry-breaking potential. Hence the no-hair theorem holds both in the Weyl invariant limit and the symmetry-breaking phase.