arXiv subjects
Publications and source records attributed to Dao-jun Liu.
A gravitating global k-monopole produces a tiny gravitational field outside the core in addition to a solid angular deficit in the k-field theory. As a new feature, the gravitational field can be attractive or repulsive depending on the non-canonical kinetic term.
Using the elegant method employed recently by Erickcek, Smith and Kamionkowski, on the premise that the space-time of Solar System is described by a metric with constant-curvature background added by a static perturbation, we show that many $f(R)$ gravities are ruled out by Solar System tests.
The expectation of explaining cosmological observations without requiring new energy sources is forsooth worthy of investigation. In this letter, a new kind of Cardassian models, called exponential Cardassian models, for the late-time universe are investigated in the context of the spatially flat FRW universe scenario. We fit the exponential Cardassian models to current type Ia supernovae data and find they are consistent with the observations. Furthermore, we point out that the equation-of-state parameter for the effective dark fluid component in exponential Cardassian models can naturally cross the cosmological constant divide $w=-1$ that observations favor mildly without introducing exotic material that destroy the weak energy condition.
The property and gravitational field of global monopole of tachyon are investigated in a four dimensional static space-time. We give an exact solution of the tachyon field in the flat space-time background. Using the linearized approximation of gravity, we get the approximate solution of the metric. We also solve analytically the coupled Einstein and tachyon field equations which is beyond the linearized approximation to determine the gravitational properties of the monopole solution. We find that the metric of tachyon monopole represents an asymptotically AdS space-time with a small effective mass at the origin. We show that this relatively tiny mass is actually negative, as it is in the case of ordinary scalar field.
The cosmological dynamics of minimally coupled scalar field that couple to the background matter with thermal interactions is investigated in exponential potential. The conditions for the existence and stability of various critical points as well as their cosmological implications are obtained. Although we show that the effects of thermal interaction such as depressing the equation-of-state parameter of quintessence, is only important at the early time, the evolution of equation-of-state parameter of quintessence is manifested. The upper bound is required on the coupling between quintessence and relativistic relic particles such as photons and neutrinos.
In this paper, a modified Chaplygin gas model of unifying dark energy and dark matter with exotic equation of state $p=Bρ-\frac{A}{ρ^α}$ which can also explain the recent accelerated expansion of the universe is investigated by the means of constraining the location of the peak of the CMBR spectrum. We find that the result of CMBR measurements does not exclude the nonzero value of parameter $B$, but allows it in the range $-0.35\lesssim B\lesssim0.025$.
Following an elegant approach that merge the effects of the stringy spacetime uncertainty relation into primordial perturbations suggested by Brandenberger and Ho, we show the mode equation up to the first order of non-commutative parameter. A new approximation is provided to calculate the mode functions analytically in the non-commutative power-law inflation models. It turns out that non-commutativity of spacetime can provide small corrections to the power spectrum of primordial fluctuations as the first-year results of WMAP indicate. Moreover, using the WMAP data, we obtain the value of expansion parameter, non-commutative parameter and find the approximation is viable. In addition, we determined the string scale $l_s \simeq 2.0\times 10^{-29}{cm}$.
The motivation for studying the rolling tachyon and non-commutative inflation comes from string theory. In the tachyon inflation scenario, metric perturbations are created by tachyon field fluctuations during inflation. We drive the exact mode equation for scalar perturbation of the metric and investigate the cosmological perturbations in the commutative and non-commutative inflationary spacetime driven by the tachyon field which have a Born-Infeld Lagrangian.
The property and gravitational field of global string of tachyon matter are investigated in a four dimentional approximately cylindrically-symmetric spacetime with a deficit angle. Especially, we give an exact solution of the tachyon field in the flat spacetime background and we also find the solution of the metric in the linearized approximation of gravity.
We study the perturbations on both "radial" and "angular" components of the quintessence with an internal abelian and non-abelian symmetry. The properties of the perturbation on the "radial" component depend on the specific potential of the model and is similiar for both abelian and non-abelian case. We show that the consine-type potential is very interesting for the O(\textit{N}) quintessence model and also give a critical condition of instability for the potential. While the properties of perturbations on "angular" components depend on whether the internal symmetry is abelian or non-abelian, which we have discussed respectively. In the non-abelian case, the fluctuation of the "angular" component will increase rapidly with time while in the abelian case it will not.
We study the evolution of Born-Infeld-type phantom in the second Randall-Sundrum brane scenario, and find that there exists attractor solution for the potential with a maximum, which implies a cosmological constant at the late time. Especially, we discuss the BI model of constant potential without and with dust matter. In the weak tension limit of the brane, we obtain an exact solution for the BI phantom and scale factor and show that there is no big rip during the evolution of the brane.
We study a new class of quintessence models, in which the scalar field possesses a O(\textit{N}) internal symmetry. We give a critical condition of instability for the potential against Q ball formation. We find that the most widely used potentials of quintessence don't satisfy the above condition, and therefore the O(\textit{N}) quintessence with these potential will not lead to the Q ball formation. It is worth noting that O(\textit{N}) quintessence with cosine-type potential is especially interesting in that the angular contribution is not negligible.
Although the formulas of tachyon inflation correspond to those of the inflation driven by the ordinary scalar field, there is obvious difference between them, which can not be neglected. We calculate the scalar and tensor perturbation of the string theory inspired tachyon inflation, which has been widely studied recently. We also show, through the Hamilton-Jacobi approach, that the the rolling tachyon can essentially produce enough inflation. An exact solution with the inverse squared potential of tachyon field has been proposed and its power spectra has been analyzed.
In light of the recent work by Sen and Gibbons, we present a phase-plane analysis on the cosmology containing a rolling tachyon field in a potential resulted from string theory. We show that there is no stable point on the phase-plane, which indicated that there is a coincidence problem if one consider tachyon as a candidate of quintessence. Furthermore, we also analyze the phase-plane of the cosmology containing a rolling tachyon field for an exactly solvable toy potential in which the critical point is stable. Therefore, it is possible for rolling tachyon to be quintessence if one give up the strict constraint on the potential or find a more appropriate effective potential for the tachyon from M/string theory.
String theory is a promising candidate for a fundamental quantum theory of all interactions including Einstein gravity. Some solutions in string theory can be interpreted as black holes. Using the semi-analytic method and WKB method,the quasinormal modes(QNMs)of 1+1 dimensional black hole in string theory are studied.The QNMs of 1+3 dimensional black hole in string theory are also calculated through numerical approach.The numerical investigation has shown that the late time gravitational oscillation of the black hole under an external perturbaton is dominated by certain QNMs.
Q ball solutions are considered within the theory of a complex scalar field with a gauged U(1) symmetry and a parabolic-type potential. In the thin-walled limit, we show explicitly that there is a maximum size for these objects because of the repulsive Coulomb force. The size of Q ball will increase with the decrease of local minimum of the potential. And when the two minima degenerate, the energy stored within the surface of the Q ball becomes significant. Furthermore, we find an analytic expression for gauged Q ball, which is beyond the conventional thin-walled limit.