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Anzhong Wang

Publications and source records attributed to Anzhong Wang.

At least 127 records · Page 7Linked to original sources
arXiv2019

Preinflationary perturbations from the closed algebra approach in loop quantum cosmology

In this paper, the scalar and tensor perturbations in the closed algebra approach of loop quantum cosmology are studied. Instead of the distant past in the contracting phase, we choose the moment at which the initial conditions are imposed to be the silent point, which circumvents the problem due to the signature change in the super-inflationary phase and results in a well-defined Cauchy problem. For the ultraviolet and infrared modes, different approaches are applied in order to obtain analytical solutions with high accuracy. While previous numerical simulations reveal an exponentially divergent power spectrum in the ultraviolet regime, when the initial conditions are imposed in the remote contracting phase, we find a special set of initial conditions at the silent point, which can reproduce results that are consistent with current observations.

gr-qc↗
arXiv2019

Thermodynamical study on universal horizons in higher $D$-dimensional spacetime and aether waves

We investigate thermodynamic behaviors of the $D$-dimensional gravity coupled to a dynamical unit timelike vector, the aether, present two kinds of exact charged solutions and study the linearized wave spectrum of this theory. There is an universal horizon behind the Killing horizon in these aether black holes. It is interesting that there exist $D$-dimensional Schwarzschild, Reissner-Nordström and charged Schwarzschild black holes but now with an universal horizon inside. We find that in the uncharged case for both kinds, or charged case but the charge $\bar Q\ll \bar r_0/2$ for the first kind, one can construct a Smarr formula and the (slightly modified) first law of black hole mechanics at the universal horizons for the aether black holes. An entropy can be associated with the universal horizon and a temperature can be defined there. For the second kind aether black hole and for the first kind one in the extremal higher dimensions, the work term involving charge $V_{UH}δQ$ disappeared. For aether wave, our results show that the spin-1 and spin-2 modes are the same as those in 4-dimensional spacetime, and only the spin-0 one is different and dependant on the dimension number $n$.

gr-qc↗
arXiv2019

Gravitational wave forms, polarizations, response functions and energy losses of triple systems in Einstein-Aether theory

Gravitationally bound hierarchies containing three or more components are very common in our Universe. In this paper we study {\em periodic} gravitational wave (GW) form, their polarizations, response function, its Fourier transform, and energy loss rate of a triple system through three different channels of radiation, the scalar, vector and tensor modes, in Einstein-aether theory of gravity. In the weak-field approximations and with the recently obtained constraints of the theory, we first analyze the energy loss rate of a binary system, and find that the dipole contributions from the scalar and vector modes could be of the order of ${\cal{O}}\left(c_{14}\right){\cal{O}}\left(G_Nm/d\right)^2$, where $c_{14} \; (\equiv c_{1} + c_{4})$ is constrained to $c_{14} \lesssim {\cal{O}}\left(10^{-5}\right)$ by current observations, where $c_i$'s are the four coupling constants of the theory. On the other hand, the "strong-field" effects for a binary system of neutron stars are about six orders lower than that of GR. So, in this paper we ignore these "strong-field" effects and first develop the general formulas to the lowest post-Newtonian order, by taking the coupling of the aether field with matter into account. Within this approximation, we find that the scalar breather mode and the scalar longitudinal mode are all suppressed by a factor of ${\cal{O}}\left(c_{14}\right)$ with respect to the transverse-traceless modes ($h_{+}$ and $h_{\times}$), while the vectorial modes $(h_{X}$ and $h_{Y}$) are suppressed by a factor of $c_{13} \lesssim {\cal{O}}\left(10^{-15}\right)$. Applying the general formulas to a triple system with periodic orbits, we find that the corresponding GW form, response function, and its Fourier transform depend sensitively on both the configuration of the triple system and their orientations with respect to the detectors.

astro-ph.GA↗
arXiv2018

Non-adiabatic Evolution of Primordial Perturbations and non-Gaussinity in Hybrid Approach of Loop Quantum Cosmology

While loop quantum cosmology (LQC) predicts a robust quantum bounce of the background evolution of a Friedmann-Robertson-Walker (FRW) spacetime prior to the standard slow-roll inflation, whereby the big bang singularity is resolved, there are several different quantization procedures to cosmological perturbations, for instance, {\em the deformed algebra, dressed metric, and hybrid quantizations}. This paper devotes to study the quantum bounce effects of primordial perturbations in the hybrid approach. The main discrepancy of this approach is the effective positive mass at the quantum bounce for the evolution of the background that is dominated by the kinetic energy of the inflaton field at the bounce, while this mass is always nonpositive in the dressed metric approach. It is this positivity of the effective mass that violates the adiabatic evolution of primordial perturbations at the initial moments of the quantum bounce. With the assumption that the evolution of the background is dominated by the kinetic energy of the inflaton at the bounce, we find that the effective potentials for both scalar and tensor perturbations can be well approximately described by a Pöschl-Teller (PT) potential, which allows us to find analytical solutions of perturbations, and from these analytical expressions we are able to study the non-adiabatic evolution of primordial perturbations in details. In particular, we derive their quantum bounce effects and investigate their observational constraints. In addition, the impacts of quantum bounce effects on the non-Gaussinity and their implication on the explanations of observed power asymmetry in CMB have also been explored.

gr-qc↗
arXiv2018

Magnetic-oscillation mechanism for understanding periodic or quasi-periodic modulation of the timing residuals from pulsars

Highly precise pulsar timing is very important for understanding the nature of neutron stars and the implied physics, even being used to detect directly gravitational waves. Unfortunately, the accuracy of the pulsar timing is seriously affected by the spin-down irregularities, such as spin fluctuations with a manifestation of low frequency structures (the so-called red noise processes), and various activities of magnetospheres. Except from the random timing noise, significant periodic or quasi-periodic components could also be found in some timing residuals of pulsars, indicating the presence of unmodelled deterministic effects. The physical origins of these effects still remain unexplained. In this paper, we suggest a new mechanism involving the de Haas-van Alphen magnetic oscillation, which could trigger the observed low frequency structures. We find that the proposed magnetic oscillation period is about 1-$10^{2}$ yr, which is about $10^{-4}$ times as long as the classical characteristic time scale of interior magnetic field evolution for a normal neutron star. Due to the magnetic oscillation, we estimate both the braking index range between $10^{-5}$ and $10^{5}$ and the residuals range between 4 to 906 ms for some specific samples of quasi-periodic pulsars. Those are consistent with the pulsar timing observations. Nonlinear phenomena of de Haas-van Alphen effect and the Condon domain structure are considered. Avalanche-like magnetized process of domain wall motion (rapidly magnetic energy release) could associated with some special emitting manifestations of neuron star radiation.

astro-ph.HE↗
arXiv2018

Angular momentum loss for eccentric compact binary in screened modified gravity

Gravitational wave (GW) observations provide insight into the gravity regime. These observations relies on the comparison of the data to the GWs model, which depends on the orbital evolution of the binary systems. In this paper, we study the orbital evolution of eccentric compact binaries within screened modified gravity, which is a kind of scalar-tensor theory with screening mechanisms. According to the Noether's theorem, from the theory's GW action, we compute the angular momentum flux both as a function of the fields in the theory and as a function of the multipole moments of a N-body system. We specialize to the orbital parameters of eccentric compact binaries to express its orbital energy and angular momentum decay, and then derive the decay rates of its orbital eccentricity and semimajor axis. We find that these quantities decay faster than in General Relativity due to the presence of dipole radiation.

gr-qc↗
arXiv2018

Gravitational plane waves in Einstein-aether theory

In this paper, we systematically study spacetimes of gravitational plane waves in Einstein-aether theory. Due to the presence of the timelike aether vector field, now the problem in general becomes overdetermined. In particular, for the linearly polarized plane waves, there are five independent vacuum Einstein-aether field equations for three unknown functions. Therefore, solutions exist only for particular choices of the four free parameters $c_{i}$'s of the theory. We find that there exist eight cases, in two of which any form of gravitational plane waves can exist, similar to that in general relativity, while in the other six cases, gravitational plane waves exist only in particular forms. Beyond these eight cases, solutions either do not exist or are trivial (simply representing a Minkowski spacetime with a constant or dynamical aether field.).

gr-qc↗
arXiv2018

Preinflationary dynamics in loop quantum cosmology: Monodromy Potential

In this article we explore the pre-inflationary background dynamics of an FLRW universe sourced by a scalar field with monodromy potential in LQC framework. In particular we calculate the number of e-folds, $N_{inf}$, produced during the slowly rolling phase of the inflation and find out the critical value of the ratio of the kinetic to potential energy, $r_w^c$, at the quantum bounce that is required to produce $N_{inf}\simeq 60.$ Two different monodromy potentials, namely, linear and quadratic with a modulation term are investigated to this effect. The effects on the value of $N_{inf}$ due to parameters associated with the strength, decay constant and the phase factor of the modulation term are calculated. In addition to this we present the qualitative picture of the background dynamics by carrying out a dynamical system analysis. We produce the phase portraits and carry out a detailed linear stability analysis of the finite fixed points, if any, for each of the potentials.

gr-qc↗
arXiv2018

Waveforms of compact binary inspiral gravitational radiation in screened modified gravity

Scalar-tensor gravity, with the screening mechanisms to avoid the severe constraints of the fifth force in the Solar System, can be described with a unified theoretical framework, the so-called screened modified gravity (SMG). Within this framework, in this paper we calculate the waveforms of gravitational-waves (GWs) emitted by inspiral compact binaries, which include four polarization modes, the plus $h_{+}$, cross $h_{\times}$, breathing $h_{b}$, and longitudinal $h_{L}$ modes. The scalar polarizations $h_b$ and $h_L$ are both caused by the scalar field of SMG, and satisfy a simple linear relation. With the stationary phase approximations, we obtain their Fourier transforms, and derive the correction terms in the amplitude, phase, and polarizations of GWs, relative to the corresponding results in general relativity. The corresponding parametrized post-Einsteinian parameters in the general SMG are also identified. Imposing the noise level of the ground-based Einstein Telescope, we find that GW detections from inspiral compact binaries composed of a neutron star and a black hole can place stringent constraints on the sensitivities of neutron stars, and the bound is applicable to any SMG theory. Finally, we apply these results to some specific theories of SMG, including chameleon, symmetron, dilaton and $f(R)$.

gr-qc↗
arXiv2018

Gravitational collapse and formation of universal horizons in Einstein-aether theory

We numerically study the gravitational collapse of a massless scalar field with spherical symmetry in Einstein-aether theory, and show that apparent, spin-0 and dynamical universal horizons (dUHs) can be all formed. The spacetime and the aether field are well-behaved and regular, including regions nearby these horizons (but away from the center of spherical symmetry). The spacetime outside the apparent and spin-0 horizons settles down to a static configuration, and some of such resulting static black holes were already found numerically in the literature. On the other hand, the proper distance of the outermost dUH from the apparent (or spin-0) horizon keeps increasing on $\mboxæ$ther-orthogonal time slices. This indicates that the outermost dUH is evolving into the causal boundary, even for excitations with large speeds of propagation.

gr-qc↗
arXiv2018

Qualitative dynamics and inflationary attractors in loop cosmology

Qualitative dynamics of three different loop quantizations of spatially flat isotropic and homogeneous models is studied using effective spacetime description of the underlying quantum geometry. These include the standard loop quantum cosmology (LQC), its recently revived modification (referred to as mLQC-I), and another related modification of LQC (mLQC-II) whose dynamics is studied in detail for the first time. Various features of LQC, including quantum bounce and pre-inflationary dynamics, are found to be shared with the mLQC-I and mLQC-II models. We study universal properties of dynamics for chaotic inflation, fractional monodromy inflation, Starobinsky potential, non-minimal Higgs inflation, and an exponential potential. We find various critical points and study their stability, which reveal various qualitative similarities in the post-bounce phase for all these models. The pre-bounce qualitative dynamics of LQC and mLQC-II turns out to be very similar, but is strikingly different from that of mLQC-I. In the dynamical analysis, some of the fixed points turn out to be degenerate for which center manifold theory is used. For all these potentials, non-perturbative quantum gravitational effects always result in a non-singular inflationary scenario with a phase of super-inflation succeeded by the conventional inflation. We show the existence of inflationary attractors, and obtain scaling solutions in the case of the exponential potential. Since all of the models agree with general relativity at late times, our results are also of use in classical theory where qualitative dynamics of some of the potentials has not been studied earlier.

gr-qc↗
arXiv2018

Preinflationary dynamics of $α-$attractor in loop quantum cosmology

We systematically study the preinflationary dynamics of the spatially flat Friedmann-Lemaitre-Robertson-Walker universe filled with a single scalar field that has the generalized $α-$attractor potentials, in the framework of loop quantum cosmology, in which the big bang singularity is replaced generically by a non-singular quantum bounce due to purely quantum geometric effects. The evolution can be divided into two different classes, one is dominated initially (at the quantum bounce) by the kinetic energy of the scalar field, and one is not. In both cases, we identify numerically the physically viable initial conditions that lead to not only a slow-roll inflationary phase, but also enough $e$-folds to be consistent with observations, and find that the output of such a viable slow-roll inflationary phase is generic. In addition, we also show that in the case when the evolution of the universe is dominated initially by the kinetic energy of the scalar field (except for a very small set in the phase space), the evolution before reheating is aways divided into three different phases: {\em bouncing, transition and slow-roll inflation}. This universal feature does not depend on the initial conditions of the system nor on the specific potentials of the scalar field, as long as it is dominated initially by the kinetic energy of the scalar field at the bounce. Moreover, we carry out phase space analyses for the models under consideration and compare our results with the power-law and Starobinsky potentials.

astro-ph.CO↗
arXiv2018

Smarr Integral Formula of D-dimensional Stationary Spacetimes in Einstein-Æther-Maxwell Theroy

Using the Wald formalism, we investigate the thermodynamics of charged black holes in D-dimensional stationary spacetimes with or without rotations in Einstein-æther-Maxwell theory. In particular, assuming the existence of a scaling symmetry of the action, we obtain the Smarr integral formula, which can be applied to both Killing and universal horizons. When restricted to 4-dimensional spherically symmetric spacetimes, previous results obtained by a different method are re-derived.

gr-qc↗
arXiv2018

Prospect for cosmological parameter estimation using future Hubble parameter measurements

We constrain cosmological parameters using only Hubble parameter data and quantify the impact of future Hubble parameter measurements on parameter estimation for the most typical dark energy models. We first constrain cosmological parameters using 52 current Hubble parameter data including the Hubble constant measurement from the Hubble Space Telescope. Then we simulate the baryon acoustic oscillation signals from WFIRST (Wide-Field Infrared Survey Telescope) covering the redshift range of $z\in [0.5, 2]$ and the redshift drift data from E-ELT (European Extremely Large Telescope) in the redshift range of $z\in [2,5]$. It is shown that solely using the current Hubble parameter data could give fairly good constraints on cosmological parameters. Compared to the current Hubble parameter data, with the WFIRST observation the $H(z)$ constraints on dark energy would be improved slightly, while with the E-ELT observation the $H(z)$ constraints on dark energy is enormously improved.

astro-ph.CO↗
arXiv2018

An Alternative Channel For High-Mass Binary Black Holes-----Dark Matter Accretion Onto Black Hole

By a method of population synthesis we construct a model of dark matter (DM) accretion onto binary black holes (BHs), and investigate the merger rate of the binary BHs. We find that the merger rate can weakly increase (less than 10\%). However, the DM accretion can efficiently enhance the masses of binary BHs. In our model, the result for $Z = 0.01$ without the DM accretion can not explain the GW170104, GW170814, and GW150914, while with the DM accretion, it can cover all observations well. For the higher metallicity ($Z = 0.02$), our model can not explain the mergers of high-mass binary BHs like GW170104, GW170814, and GW150914. We estimate that the merger rate of binary BHs lies between $\rm55Gpc^{-3}yr^{-1}$ to $\rm197Gpc^{-3}yr^{-1}$.

astro-ph.HE↗
arXiv2018

Distinguishing rotating Kiselev black hole from naked singularity using spin precession of test gyroscope

We study the critical values of the quintessential and spin parameters, to distinguish a rotating Kiselev black hole (RKBH) from a naked singularity. For any value of the dimensionless quintessential parameter $ω_{q} \in (-1, -1/3)$, when increasing the value of quintessential parameter $α$, the size of the event horizon increases, whereas the size of the outer horizon decreases. We then study the spin precession of a test gyroscope attached to a stationary observer in this spacetime. Using the spin precessions we differentiate black holes from naked singularities. If the precession frequency becomes large, as approaching to the central object in the quintessential field along any direction, then the spacetime is a black hole. A spacetime will contain a naked singularity if the precession frequency remains finite everywhere except at the singularity itself. Finally, we study the Lense-Thirring precession frequency for rotating Kiseleb black hole and the geodetic precession for Kiselev black hole.

gr-qc↗
arXiv2018

Constraints on Einstein-aether theory after GW170817

In this paper, we carry out a systematic analysis of the theoretical and observational constraints on the dimensionless coupling constants $c_i$ ($i=1,2,3,4$) of the Einstein-aether theory, taking into account the events GW170817 and GRB 170817A. The combination of these events restricts the deviation of the speed $c_T$ of the spin-2 graviton to the range, $- 3\times 10^{-15} < c_T -1 < 7\times 10^{-16}$, which for the Einstein-aether theory implies $\left|c_{13}\right| \le 10^{-15}$ with $c_{ij} \equiv c_{i} + c_{j}$. The rest of the constraints are divided into two groups: those on the ($c_1, c_{14}$)-plane and those on the ($c_2, c_{14}$)-plane, except the strong-field constraints. The latter depend on the sensitivities $σ_æ$ of neutron stars, which are not known at present in the new ranges of the parameters found in this paper.

gr-qc↗