arXiv subjects

W. F. Kao

Publications and source records attributed to W. F. Kao.

42 records · Page 3Linked to original sources

arXiv2000

The Effect of the (2+1)-Dimensional Gravity in the Spiral Galaxy

The slowly decreasing rotational velocity of the Milky Way suggests the existence of the dark matter. One finds that the effect of the dark matter in a galaxy can be described by a $(2+1)$-dimensional fluid model. The stability analysis for the corresponding fluid model is discussed in details in this paper. It is found that the pressure term plays an important role in the stability of the spiral arms and counteracts the increasing mass of the observed galaxy as the radial distance increases. When the fluid region ends, the $ nearly $ constant rotational velocity of the distant stars require no additional mass if the spatial dimension in the remote region is close to two. This indicates that the galaxies, we are interested, could be explained by a $(2+1)$-dimensional model. Possible implications are also discussed in this paper.

astro-ph↗

arXiv2000

Inflationary Universe in Higher Derivative Induced Gravity

In an induced-gravity model, the stability condition of an inflationary slow-rollover solution is shown to be $ϕ_0 \partial_{ϕ_0}V(ϕ_0)=4V(ϕ_0)$. 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 non-vanishing cosmological constant at the end of inflation. Some models are analyzed for their implication to the early universe.

hep-th↗

arXiv2000

Kaluza-Kelin Higher Derivative Gravity and Friedmann-Robertson-Walker Cosmology

Kaluza-Klein approach in an N(=1+3+D)-dimensional Friedmann-Robertson-Walker type space is often adopted in the literature. We derive a compact expression for the Friedmann equation in a (1+3+D)-dimensional space. The redundancy of the associated field equations due to the Bianchi identity is analyzed. We also study the dilaton gravity theory with higher-derivative gravitational couplings. It turns out that higher-order terms will not affect the Friedmann equation in a constant flat internal space. This is true only for the flat-De Sitter external space. The inflationary solution in an induced-gravity model is also discussed as an application.

hep-th↗

arXiv2000

The weak-field expansion for processes in a homogeneous background magnetic field

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 $γ\to ν\barν$ and $ν\to νγ$ decays, which involve charged fermions in the internal loop. The leading and subleading magnetic-field effects to the above processes are computed.

hep-th↗

arXiv2000

Kaluza-Klein Induced Gravity Inflation

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 of the symmetry breaking potential in search of an inflationary solution with non-expanding internal-space. We find that the induced gravity model imposes strong constraints on the form of symmetry breaking potential in order to generate an acceptable inflationary universe. These constraints are analyzed carefully in this paper.

hep-th↗

arXiv1999

Neutrino-photon scattering and its crossed processes in a background magnetic field

We study the neutrino-photon processes such as $γγ\to ν\barν$ and $νγ\to νγ$ in a background magnetic field smaller than the critical magnetic field $B_c\equiv m_e^2/e$. Using Schwinger's proper-time method, 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.

hep-ph↗