arXiv subjects

Jian-gang Hao

Publications and source records attributed to Jian-gang Hao.

18 recordsLinked to original sources

arXiv2004

Phantom-like GCG and the constraints of its parameters via cosmological dynamics

We extend the equation of state of GCG (Generalized Chaplygin Gas) to $w<-1$ regime and show, from the point of view of dynamics, that the parameters of GCG should be in the range of $0<α<1$. Also, dynamical analysis indicate that the phase $w_{g}=-1$ is a dynamical attractor and the equation of state of GCG approaches it from either $w_{g}>-1$ or $w_{g}<-1$ depending on the choice of its initial cosmic density parameter and the ratio of pressure to critical energy density.

astro-ph↗

arXiv2004

Cosmological dynamics of scalar fields with O(N) symmetry

In this paper, we study the cosmological dynamics of scalar fields with O(N) symmetry in general potentials. We compare the phase space of the dynamical systems of the quintessence and phantom and give the conditions for the existence of various attractors as well as their cosmological implications. We also show that the existence of tracking attractor in O(N) phantom models require the potential with $Γ<1/2$, which makes the models with exponential potential possess no tracking attractor.

astro-ph↗

arXiv2004

Phantom Cosmic Dynamics: Tracking Attractor and Cosmic Doomsday

We study the dynamics of phantom model and give the conditions for potentials to admit tracking attractor, de Sitter attractor and big rip attractor. Especially, we show that phantom models with exponential and inverse power law potentials do not admit the tracking attractor solution. The "tachyonic" stability of the system at/near the attractors as well as the quantum stabiltiy have been studied.

astro-ph↗

arXiv2004

Phantom Field with O(N) Symmetry in Exponential Potential

In this paper, we study the phase space of phantom model with O(\emph{N}) symmetry in exponential potential. Different from the model without O(\emph{N}) symmetry, the introduction of the symmetry leads to a lower bound $w>-3$ on the equation of state for the existence of stable phantom dominated attractor phase. The reconstruction relation between the potential of O(\textit{N}) phantom system and red shift has been derived.

hep-th↗

arXiv2003

Attractor Solution of Phantom Field

In light of recent study on the dark energy models that manifest an equation of state $w<-1$, we investigate the cosmological evolution of phantom field in a specific potential, exponential potential in this paper. The phase plane analysis show that the there is a late time attractor solution in this model, which address the similar issues as that of fine tuning problems in conventional quintessence models. The equation of state $w$ is determined by the attractor solution which is dependent on the $λ$ parameter in the potential. We also show that this model is stable for our present observable universe.

gr-qc↗

arXiv2003

Kantowski-Sachs universe cannot be closed

In this paper, by analyzing the instability against spatially homogeneous and anisotropic perturbations of the Kantowski-Sachs-type during different cosmological epoch, we show that it is a theoretical consequence of the general relativity that the KS universe must be open or flat if it underwent the matter dominated and/or radiation dominated era in its past evolution, which theoretically confirms the flatness of our observable universe.

astro-ph↗

arXiv2003

Constructing dark energy models with late time de Sitter attractor

In this paper, we describe a way to construct a class of dark energy models that admit late time de Sitter attractor solution. In the canonical scalar and Born-Infeld scalar dark energy models, we show mathematically that a simple sufficient condition for the existence of a late time de Sitter like attractor solution is that the potentials of the scalar field have non-vanishing minimum while this condition becomes that the potentials have non-vanishing maximum for the phantom models. These attractor solutions correspond to an equation of state $w=-1$ and a cosmic density parameter $Ω_ϕ=1$, which are important features for a dark energy model that can meet the current observations.

hep-th↗

arXiv2003

On the Quintessence with Abelian and Non-abelian Symmetry

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.

astro-ph↗

arXiv2003

Phantom with Born-Infield type Lagrangian

Recent analysis of the observation data indicates that the equation of state of the dark energy might be smaller than -1, which leads to the introduction of phantom models featured by its negative kinetic energy to account for the regime of equation of state $w<-1$. In this paper, we generalize the idea to the Born-Infield type Lagrangian with negative kinetic energy term and give the condition for the potential, under which the late time attractor solution exists and also analyze a viable cosmological model in such a scheme.

hep-th↗

arXiv2003

Cosmological Anisotropy and the Cycling of Universe

In this paper, we analyze the anisotropy of the scale factor in the Kantowski-Sachs spacetime. We show that the anisotropy will not increase when the expansion rate is greater than certain values while it will increase when the expansion rate is less than that value or the Universe is contracting. It is manifested that the matter dominated and radiation dominated era favor the flat spacetime if the anisotropy does not develop significantly. The relation between the cosmological anisotropy and the red-shift of the supernovae, which could be used to verify the anisotropy through the observation, has been derived.

hep-th↗

arXiv2003

Features of Motion Around Global Monopole in Asymptotically dS/AdS Spacetime

In this paper, we study the motion of test particle and light around the Global Monopole in asymptotically dS/AdS spacetime. The motion of a test particle and light in the exterior region of the global monopole in dS/AdS spacetime has been investigated. Although the test particle's motion is quite different from the case in asymptotically flat spacetime, the behaviors of light(null geodesic) remain unchanged except a energy(frequency) shift. Through a phase-plane analysis, we prove analytically that the existence of a periodic solution to the equation of motion for a test particle will not be altered by the presence of cosmological constant and the deficit angle, whose presence only affects the position and type of the critical point on the phase plane. We also show that the apparent capture section of the global monopole in dS/AdS spacetime is quite different from that in flat spacetime.

astro-ph↗

arXiv2002

O(N) quintessence

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.

astro-ph↗

arXiv2002

Global Monopole in Asymptotically dS/AdS Spacetime

In this paper, we investigate the global monopole in asymptotically dS/Ads spacetime and find that the mass of the monopole in the asymptotically dS spacetime could be positive if the cosmological constant is greater than a critical value. This shows that the gravitational field of the global monopole could be attractive or repulsive depending on the value of the cosmological constant.

hep-th↗

arXiv2002

Reconstructing the Equation of State of Tachyon

Recent progress in theoretical physics suggests that the dark energy in the universe might be resulted from the rolling tachyon field of string theory. Measurements to SNe Ia can be helpful to reconstruct the equation of state of the rolling tachyon which is a possible candidate of dark energy. We present a numerical analysis for the evolution of the equation of state of the rolling tachyon and derive the reconstruction equations for the equation of state as well as the potential.

hep-th↗

arXiv2002

On the tachyon inflation

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.

hep-th↗

arXiv2002

Can Quintessence Be The Rolling Tachyon?

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.

hep-th↗

arXiv2002

Quasinormal modes of stringy black holes

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.

gr-qc↗

arXiv2002

Gauged Q ball in a piecewise parabolic potential

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.

math-ph↗