Constraints on Flows in Horava-Lifshitz Gravity by Classical Solutions

We find exact static stringy solutions of Horava-Lifshitz gravity with the projectability condition but imposing the detailed balance condition near the UV fixed point, and propose a method on constraining the possible pattern of flows in Horava-Lifshitz gravity by using the obtained classical solutions. In the obtained vacuum solutions, the parameters related to the speed of the graviton and the coefficients of quartic spatial derivative terms lead to intriguing effects: the change of graviton speed yields a surplus angle and the quartic derivatives make the square of effective electric charge negative. The result of a few tests based on the geometries of a cone, an excess cone, a black string, and a charged (black) string seems suggestive. For example, the flow of constant graviton speed and variable Newton's coupling can be favored in the vicinity of IR fixed point, but the conclusion is indistinct and far from definite yet. Together with the numerous classical solutions, static or time-dependent, which have already been found, the accumulated data from various future tests will give some hints in constraining the flow patterns more deterministic.Comment: 27 pages, 3 figures, version to appear in PR

Surplus Solid Angle as an Imprint of Horava-Lifshitz Gravity

We consider the electrostatic field of a point charge coupled to Horava-Lifshitz gravity and find an exact solution describing the space with a surplus (or deficit) solid angle. Although, theoretically in general relativity, a surplus angle is hardly to be obtained in the presence of ordinary matter with positive energy distribution, it seems natural in Horava-Lifshitz gravity. We present the sudden disappearance and reappearance of a star image as an astrophysical effect of a surplus angle. We also consider matter configurations of all possible power law behaviors coupled to Horava-Lifshitz gravity and obtain a series of exact solutions.Comment: 23 pages, 1 figure; minor changes, published versio

Cosmic D- and DF-strings from D3Dbar3: Black Strings and BPS Limit

We study D- and DF-strings in a D3${\bar {\rm D}}3$ system by using Dirac-Born-Infeld type action. In the presence of an electric flux from the transverse direction, we discuss gravitating thick D-string solutions of a spatial manifold, ${\rm S}^{2}\times {\rm R}^{1}$, in which straight D-strings stretched along the R${}^{1}$ direction are attached to the south and north poles of the two-sphere. There is a horizon along its equator, which means the structure of black strings is formed. We also discuss the BPS limit for thin parallel D- and DF-strings in both flat and curved spacetime. We obtain the BPS sum rule for an arbitrarily-separated multi-string configuration with a Gaussian type tachyon potential. At the site of each thin BPS D(F)-string, the pressure takes a finite value. We find that there exists a maximum deficit angle $\pi$ in the conical geometry induced by thin BPS D- and DF-strings.Comment: 24 pages, 5 figure

Multi-BPS D-vortices

We investigate the BPS configuration of the multi D-vortices produced from the D2${\bar {\rm D}}$2 system. Based on the DBI-type action with a Gaussian-type runaway potential for a complex tachyon field, the BPS limit is achieved when the tachyon profile is thin. The solution states randomly-distributed $n$ static D-vortices with zero interaction. With the obtained BPS configuration, we derive the relativistic Lagrangian which describes the dynamics of free massive D-vortices. We also discuss the 90${}^{\circ}$ and 180${}^{\circ}$ scattering of two identical D-vortices, and present its implications on the reconnection in the dynamics of cosmic superstrings.Comment: 11 pages, 4 figures. To appear in the proceedings of APCTP Winter School on Black Hole Astrophysics 2008, Daejeon and Pohang, Korea, 24-29 January, 2008. Submitted to J. Korean Phys. So

Chinese-Style Of Reward-Based Crowd-Funding: Financial Approaches Or Marketing Tools?

Crowd-funding is an emerging industry in China. However, Chinese crowd-funding market has some different characteristics to its US-based or Europe-based predecessors. Some of Chinese founders regard reward-based crowd-funding as a funding approach, while others highlight its marketing functions. Thus, which play more important role at present? This paper investigates the reason for the special performance of reward-based crowd-funding in China from multiple perspectives. The results indicate that the special institutional environments of China---lack of perfect social credit systems and legal systems, not granting legal definition and status for crowd-funding, leads to an inevitable result that Chinese founders make heavy use of the marketing functions of crowd-funding but make light use of the funding function at the present stage. With the perfecting of the institutional and legal environment, many small-and-medium-enterprises and individual entrepreneurs are expected to pay more attention on the funding function of crowd-funding than today