Restricted gravity

Based on the Abelian decomposition of Einstein's theory proposed recently we show that one can construct a restricted theory of gravitation made of the restricted connection which has the full general invariance but has no valence part of the connection. Moreover we show that classically the restricted gravity and Einstein's gravity have identical topology. In particular they have identical vacuum topology, which can be classi ed by 3(S3) ' 3(S2) when the 3-dimensional space allows a global chart.

Oppenheimer&Snyder-type gravitational collapse of the dust star to a black hole in the Brans-Dicke gravity theory

In the present wotk, we would like to explore the gravitational collapse of the spherically-symmetric, pressureless star to a black hole, namely, the Oppenheimer-Snyder solution but in the context of the Brans-Dicke(BD) theory of gravity. BD theory can be thought of as a minimal extension of general theory of relativity designed to properly accommodate both the Mach’s principle and the Dirac’s large number hypothesis by allowing the Newton’s constant G to vary with space and time and can be written in terms of a scalar(the “BD scalar”)field. To this end, we have to match the exterior and the interior metric of the spherically-symmetric collapsing star across its surface. Therefore, we need the spherically-symmetric vacuum solution, namely, the Schwarzschild solution for outside of the star whereas the homogeneous, isotropic Friedman-Robertson-Walker(FRW) metric solution for its inside filled with a perfect fluid(but without the pressure). Indeed, the author of this work published some time ago the BD gravity version of the Schwarzschild solution (Phys.Rev.D, 60,024001(1999)), the exterior metric and the BD version of the FRW solution by a perfect fluid (Mon. Not. R. Astron. Soc. 364,813(2005)), the interior metric that allow us to perform the matching and associated calculations quantitatively! To summarize the results of the present study; Firstly, the total mass of the dust star consists of contributions both from the dust matter and from the BD scalar field(which can be treated as a perfect fluid as well) since the BD gravity theory involves the scalar degree of freedom in addition to the tensor degree. Secondly, and more importantly, like in the context of general theory of relativity, for the case at hand, namely, in the context of the BD gravity theory, to a comoving observer sitting on the surface of the collapsing star, the whole collapse occurs in finite proper time which is of order the Planck time and this means that the collapse occurs almost instantaneously. To an observer staying outside the star and particularly to the one sitting at the Schwarzschild radius, however, it takes infinite time till the termination of the gravitational collapse and this is why the collapsar is a candidate model for the long-duration gamma-ray bursts. 

Analytic approach to density perturbations from an initially anisotropic universe

We present the analytic forms for the spectra of the cosmological perturbations from an initially anisotropic universe for the high momentum modes in the context of WKB approximations, as the continuation of the work [29]. We consider the Einstein gravity coupled to a light scalar field. We then assume that the scalar field has the zero velocity initially and then slowly rolls down on the potential toward the origin. In the slow-roll approximations, the Kasner-de Sitter universe with a planar symmetry is a good approximation as the background evolution. Quantization of the perturbations in the adiabatic vacuum, which we call the anisotropic vacuum, is carried out. For non-planar high momentum modes whose comoving momentum component orthogonal to the plane is bigger than the Hubble parameter at the inflationary phase, the WKB approximation is valid for the whole stage of the isotropization. On the other hand, the planar modes whose comoving momentum component orthogonal to the plane is comparable to the Hubble parameter, is amplified during the process of the anisotropic expansion. In the final gravitational wave spectra, we find that there is an asymmetry between the two polarizations of the gravitational wave because the initial mode mixing does not vanish.

Nonlinear electrodynamic effect of light bending by charged compact objects

A nonuniform electric or magnetic field can induce a continually varying index of refraction. Extremely strong electromagnetic fieldcan change the vacuum index of refraction by nonlinear electrodynamic effect. We calculate the bending angle of light under the strong electric and magnetic fields by charged black hole and neutron star according to the nonlinear electrodynamics of Euler-Heisenberg interaction. The bending by an electrically charged compact object is negligibly small compared to the gravitational bending. We found a general formula for the bending angle by a magnetic dipole of any orientation.

Thin shell wormhole accretion onto a black hole

Thin shell wormhole solutions are useful for treating the physical problems such as stability of wormhole. In this study the thin shell wormhole matter accretion onto a black hole is examined in a spherically symmetric way. The accretion result shows the behavior of accretion and the destiny of the black hole.  

On the holographic dual geometries for the strongly interacting systems

In the spirit of the AdS/CFT holographic principle, we try to find the relavant 5 dimensional gravity background geometry corresponding to the finite temperature QCD in the dense matter. We first propose the simple holographic dual geometry to the QCD with the finite density. We describe the the effect of the Hawking-Page transition. We then study an influence of matters on the deconfinement temperature and the meson masses. We now study alternative model based on the D7 brane embedding in black D3/D-instanton geometry. This corresponds to the Yang-Mills theory with background gluon condensation. We first consider the zero temperature and zero density limit. Then we extend to the finite temperature and finite density case. In this setup, we study the phase transitions among the chiral symmetry broken phase, chiral symmetry unbroken phase and the baryon phase.

Neutron Star Masses and Supercritical Accretion in Neutron Star Binaries

Recently, massive neutron star with 1.97 solar mass has been observed in neutron star-white dwarf binaries using Shapiro delay. Since this measurement was based on the Shapiro delay, the lower limit of the maximum neutron star mass is expected to be about 2 solar mass. On the other hand, all the well-measured neutron star masses in double neutron star binaries are still below 1.5 solar mass. In this talk, I will discuss the possibilities of supercritical accretion onto neutron stars in neutron star binaries. and its implication on the neutron star masses in different types of neutron star binaries.

Dense Stellar Matter with Strange Quark Matter Driven by Kaon Condensation

The core of neutron-star matter is supposed to be at a much higher density than the normal nuclear matter density for which various possibilities have been suggested such as, for example, meson or hyperon condensation and/or deconfined quark or color-superconducting matter. In this work, we explore the implication on hadron physics of a dense compact object that has three ``phases"", nuclear matter at the outer layer, kaon condensed nuclear matter in the middle and strange quark matter at the core. Using a drastically simplified but not unreasonable model, we develop the scenario where the different phases are smoothly connected with the kaon condensed matter playing a role of ``doorway"" to a quark core, the equation of state (EoS) of which by fine-tuning parameters within the range allowed by nature could be made compatible with the mass vs. radius constraint given by the 2-solar mass object PSR J1614-2230 recently observed.

Cosmological evolution for degenerate neutrinos

In this talk I will discuss the cosmological evolution for degenerate neutrinos. Since neutrino has different features from photons including the fact that it is fermion insread boson and it could be degenerate, it affect differently to the cosmological evolution.  

Classification of vacuum bubbles and oscillating instanton solutions

We study the possible types of the nucleation of vacuum bubbles and oscillating instanton solutions. We classify false vacuum bubbles of a self-gravitating scalar field within a finite size background. We show that there exist false vacuum bubbles and oscillating instanton solutions as a interpolation solution. The concept of the terminal vacuum or sink in the string theory landscape scenario can be modified due to the nucleation of false vacuum bubbles. The nontrivial solution corresponding to tunneling is possible only if gravity is taken into account.

Ultra-Fast Flash Observatory (UFFO) for early photon measurements from Gamma Ray Bursts

We propose the Ultra-Fast Flash Observatory (UFFO) to measure early optical photons from GRBs down to sub-second timescales for the first time, in order to gain a deeper understanding of GRB mechanisms, and potentially opening up the z>10 universe to point source emission probes. The UFFO will probe the early time regime and open a completely new frontier in GRB and transient studies using a fast-response MEMS (Micro-Electro-Mechanical Systems) Mirror Array (MMA)-based optical system, the Slewing Mirror Telescope (SMT), the only GRB system which can point and measure on these time scales. This new data will provide unique probes of the burst mechanism, shock breakouts in core-collapse supernovae, tidal disruptions around black holes, test Lorentz violation, and be the electromagnetic counterpart to neutrino and gravitational wave signatures of the violent universe.

General Relativistic Radiation Hydrodynamics: Moment Formalism

Radiation interacts with matter largely via exchange of energy and momentum. When matter is moving with relativistic velocity or when the background spacetime is strongly curved, rigorous relativistic treatment of hydrodynamics and radiative transfer is required. Here, I show how to derive fully general relativistic radiation hydrodynamic equations from covariant tensor formalism. The equations are easier to understand compared to previous comoving frame-based equations, and can be applied to any dimensional problems in arbitrary spacetime. Specific examples for various geometries will be explained. The limit and future prospect for the application to the astrophysical problems will be also discussed.

Dark matter and ultra high energy cosmic ray research in Korea

From the observations of supernova, cosmic micro wave background, light element abundances, and galactic dynamics revealed that the universe consists of 23% of dark matter, whose identity still needs to be clarified. ATIC, Antarctic balloon born experiment, and PAMELA, Russian satellite experiment, showed that some massive super symmetric particles of a few hundred GeV such as neutralino could be the dark matter. PAMELA and Fermi, satellite experiments, however, showed that there is no excess of antiproton which does not agree with the model of neutralino dark matter in the Galaxy. AMS, international space station high energy experiment, will be launched in April 2011 and the preliminary cosmic ray and dark matter results will be presented. Ultra high energy cosmic ray observations with ground array particle shower detectors, Telescope Array in Utah, and high energy cosmic ray Antarctic balloon experiment, CREAM, will also be presented. 

Nucleation and evolution of false vacuum bubbles in scalar-tensor gravity

In this talk, we discuss nucleation and subsequent evolution of false vacuum bubbles in scalar-tensor gravity. First, we can transform scalar-tensor type theory of gravity to the standard Brans-Dicke type theory. Second, we can transform the Brans-Dicke type theory from the Jordan frame to the Einstein frame. For a certain potential, a true vacuum bubble in the Einstein frame can be transformed to a false vacuum bubble in the Jordan frame by a conformal transformation. Therefore, in the Jordan frame, a nucleation of a false vacuum bubble can be possible and its subsequent evolution can be described by the thin shell approximation, as in the Einstein frame. False vacuum bubbles have physical importance: a set of false vacuum bubbles may generate a negative energy bath and it will have theoretical implications.