Dao-Jun Liu

Quintessence with O(N) symmetry

We study a new class of quintessence models, in which the scalar field possesses an O(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 do not satisfy the above condition, and therefore, the O(N) quintessence with these potentials will not lead to Q ball formation. It is worth noting that the O(N) quintessence with a cosine-type potential is especially interesting in that the angular contribution is not negligible.

Dynamics of quintessence with thermal interactions

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.

Modified holographic dark energy in DGP brane world

Abstract In this Letter, the cosmological dynamics of a modified holographic dark energy which is derived from the UV/IR duality by considering the black hole mass in higher dimensions as UV cutoff, is investigated in Dvali–Gabadadze–Porrati (DGP) brane world model. We choose Hubble horizon and future event horizon as IR cutoff respectively. And the two branches of the DGP model are both taken into account. When Hubble horizon is considered as IR cutoff, the modified holographic dark energy (HDE) behaves like an effect dark energy that modification of gravity in pure DGP brane world model acts and it can drive the expansion of the universe speed up at late time in ϵ = − 1 branch which in pure DGP model cannot undergo an accelerating phase. When future event horizon acts as IR cutoff, the equation of state parameter of the modified HDE can cross the phantom divide.

Revisiting the parametrization of equation of state of dark energy via SNIa data

In this paper, we revisit the parametrizations of the equation of state of dark energy and point out that comparing merely the χ 2 of different fittings may not be optimal for choosing the ‘best’ parametrization. Another figure of merit for evaluating different parametrizations based on the area of the w(z) − z band is proposed. In light of the analysis of some two-parameter parametrizations and models based on available SNIa data, the area of w(z) − z band seems to be a good figure of merit, especially in the situation that the value of χ 2 for different parametrizations are very close. Therefore, we argue that both the area of the w(z) − z band and χ 2 should be synthetically considered for choosing a better parametrization of dark

Exponential Cardassian universe

Abstract 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.

Quasinormal modes of stringy black holes

Abstract 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 perturbation is dominated by certain QNMs.

Non-commutative power-law inflation: mode equation and spectra index

Abstract Following an elegant approach that merges 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 ≃ 2.0 × 10 − 29 cm .

Quasi-normal modes for asymptotically safe black holes

Under the hypothesis of asymptotic safety of gravity, the static, spherically symmetric black hole solutions in the infrared limit are corrected by nonperturbative effects. Specifically, the metric is modified by the running of gravitational couplings. In this work, we investigate the effects of this correction to the quasi-normal modes of a test scalar field propagating in this kind of black hole background analytically and numerically. It is found that although the quasi-period frequencies and the damping of oscillations are respectively enhanced and weakened by the quantum correction term, the stability of the black hole remains.

Scalar clouds around Kerr?Sen black holes

In this paper, the behaviour of a charged massive scalar test field in the background of a Kerr–Sen black hole is investigated. A type of stationary solutions, dubbed scalar clouds, are obtained numerically and expressed by the existence lines in the parameter space. We show that for fixed background and a given set of harmonic indices, the mass and charge of the scalar clouds are limited in a finite region in the parameter space of the scalar field. Particularly, the maximum values of the mass and charge of the clouds around extremal Kerr–Sen black holes are independent of the angular velocity of the black hole, whereas those in the extremal Kerr–Newman background depend on the angular velocity. In addition, it is demonstrated that, as the static limit of the Kerr–Sen black hole, the Gibbons–Maeda–Garfinkle–Horowitz–Strominger black hole cannot support scalar cloud.

A MODIFIED MODEL OF NONLOCALLY CORRECTED GRAVITY: COSMOLOGY AND THE NEWTONIAN LIMIT

A modified form of nonlocally corrected theory of gravity is investigated in the context of cosmology and the Newtonian limit. This form of nonlocal correction to classic Einstein–Hilbert action can be locally represented by a triple-scalar–tensor theory in which one of the scalar degrees of freedom is phantom-like and the other two are quintessence-like. We show that there exists a stable de Sitter solution for the cosmological dynamics if a suitable form of potential function V(ϕ) (or equivalently, f(R)) is selected. However, no matter what a potential function is selected, there is always an early time repeller solution corresponding to a radiation dominated universe. Besides, the equations for linear scalar perturbations are presented and it is shown that the form of potential function V(ϕ) is stringently constrained by the solar system test, although the post-Newtonian parameter γ is not directly affected by this function.