Newtonian Forms for Dilaton Spacetimes in String Theory

We show that the Newtonian forms for the motion of particles in mechanics and for light in geometrical optics can be extend to the Gibbons-Meada and the GarfinMe-Horne dilaton spacetimes in string theory. As an example,we study the bending of the light rays, the perihelion advance of planet, and the radar echo delay in the dilaton spacetimes. The results show that the gravitational effects arising from the dilaton can be observed provided that the dilaton is large enough.     

Observational Constraints on Purely Kinetic k-Essence Dark Energy Models

We consider constraints on purely kinetic k-essence models from the latest observational data （including 182 gold SNIa data, the shift parameter, and the acoustic scale）. We find the best-fit values of the parameters are Ωm = 0.37 ± 0.01 and ko = 0.064 ± 0.013 at 68.3% confidence level The phase transition from deceleration to acceleration is found to occur at redshift zq=o - 0.48-0.52 in 68.3% confidence level limits. By applying modelcomparison statistics （F-test, AICc, and BIC）, we find that the purely kinetic k-essence scenario is favored over the ACDM model by the combined data. These results are aJso confirmed by combined data 307 SNIa＋R ＋ la.     

Theoretical Constraint on Purely Kinetic κ-Essence

Purely kinetic k-essence models in which the Lagrangian contains only a kinetic factor and does not depend explicitly on the field itself are considered, and a theoretical constraint is obtained： Fx -= F0a^-3. Under this theoretical constraint, we discuss a kind of purely κ-essence with form F（X） = -（1 ＋ 2X^n）^1/2n, which can be considered as the generalized tachyon field, and find that this kind of κ-essence is not likely a candidate of dark energy to describe the present accelerated expansion of the Universe. This is contrary to a previous suggestion that κ-essence with such a form may be used to describe phantom cosmologies.     

Newtonian forms for the motion of particles and photons in dilaton spacetime

Using Newtonian forms for equations of motion, we consider the motion of particles and photons in the dilaton spacetime. Some classical gravitational effects, such as the bending of light rays, the perihelion advance of a planet, the delay of radar echo, and the gravitational redshifts, have been investigated. The results show that the gravitational effects arising from the dilaton can be observed provided that the dilaton is large enough.     

Effects of Lorentz Breaking on the Accretion onto a Schwarzschild-like Black Hole

We formulate and solve the problem of spherically symmetric, steady state, adiabatic accretion onto a Schwarzschild-like black hole obtained recently. We derive the general analytic expressions for the critical points, the critical velocity, the critical speed of sound, and subsequently the mass accretion rate. The case for polytropic gas is discussed in detail. We find the parameter characterizing the breaking of Lorentz symmetry will slow down the mass accretion rate, while has no effect on the gas compression and the temperature profile below the critical radius and at the event horizon.