arXiv subjects

Hao Wei

Publications and source records attributed to Hao Wei.

At least 91 records · Page 5Linked to original sources

arXiv2018

Godel Metrics with Chronology Protection in Horndeski Gravities

Gödel universe, one of the most interesting exact solutions predicted by General Relativity, describes a homogeneous rotating universe containing naked closed time-like curves (CTCs). It was shown that such CTCs are the consequence of the null energy condition in General Relativity. In this paper, we show that the Gödel-type metrics with chronology protection can emerge in Einstein-Horndeski gravity. We construct such exact solutions also in Einstein-Horndeski-Maxwell and Einstein-Horndeski-Proca theories.

gr-qc↗

arXiv2017

Model-Independent Constraints on Lorentz Invariance Violation via the Cosmographic Approach

Since Lorentz invariance plays an important role in modern physics, it is of interest to test the possible Lorentz invariance violation (LIV). The time-lag (the arrival time delay between light curves in different energy bands) of Gamma-ray bursts (GRBs) has been extensively used to this end. However, to our best knowledge, one or more particular cosmological models were assumed {\it a priori} in (almost) all of the relevant works in the literature. So, this makes the results on LIV in those works model-dependent and hence not so robust in fact. In the present work, we try to avoid this problem by using a model-independent approach. We calculate the time delay induced by LIV with the cosmic expansion history given in terms of cosmography, without assuming any particular cosmological model. Then, we constrain the possible LIV with the observational data, and find weak hints for LIV.

gr-qc↗

arXiv2017

Observational Constraints on Varying Alpha in $Λ(α)$CDM Cosmology

In this work, we consider the so-called $Λ(α)$CDM cosmology with $Λ\proptoα^{-6}$ while the fine-structure "constant" $α$ is varying. In this scenario, the accelerated expansion of the universe is driven by the cosmological "constant" $Λ$ (equivalently the vacuum energy), and the varying $α$ is driven by a subdominant scalar field $ϕ$ coupling with the electromagnetic field. The observational constraints on the varying $α$ and $Λ\proptoα^{-6}$ models with various couplings $B_F(ϕ)$ between the subdominant scalar field $ϕ$ and the electromagnetic field are considered.

gr-qc↗

arXiv2017

Stability of the Einstein static universe in Eddington-inspired Born-Infeld theory

By considering the realization of the emergent universe scenario in Eddington-inspired Born-Infeld (EiBI) theory, we study the stability of the Einstein static universe filled with perfect fluid in EiBI theory against both the homogeneous and inhomogeneous scalar perturbations in this work. We find that in both the spatially flat and closed cases, the emergent universe scenario is no longer viable, since the Einstein static universe cannot be stable against both the homogeneous and inhomogeneous scalar perturbations simultaneously. However, the emergent universe scenario survives in the spatially open case, while the Einstein static universe can be stable under some conditions.

gr-qc↗

arXiv2017

Cosmological Constant, Fine Structure Constant and Beyond

In the present work, we consider the cosmological constant model $Λ\proptoα^{-6}$, which is well motivated from three independent approaches. As is well known, the hint of varying fine structure constant $α$ was found in 1998. If $Λ\proptoα^{-6}$ is right, it means that the cosmological constant $Λ$ should also be varying. Here, we try to develop a suitable framework to model this varying cosmological constant $Λ\proptoα^{-6}$, in which we view it from an interacting vacuum energy perspective. Then, we consider the observational constraints on these models by using the 293 $Δα/α$ data from the absorption systems in the spectra of distant quasars. We find that the model parameters can be tightly constrained to the very narrow ranges of ${\cal O}(10^{-5})$ typically. On the other hand, we can also view the varying cosmological constant model $Λ\proptoα^{-6}$ from another perspective, namely it can be equivalent to a model containing "dark energy" and "warm dark matter", but there is no interaction between them. We find that this is also fully consistent with the observational constraints on warm dark matter.

gr-qc↗

arXiv2016

Godel Universe from String Theory

Gödel universe is a direct product of a line and a three-dimensional spacetime we call G$_α$. In this paper, we show that the Gödel metrics can arise as exact solutions in Einstein-Maxwell-Axion, Einstein-Proca-Axion, or Freedman-Schwarz gauged supergravity theories. The last allows us to embed Gödel universe in string theory. The ten-dimensional spacetime is a direct product of a line and the nine-dimensional one of an $S^3\times S^3$ bundle over G$_α$, and it can be interpreted as some decoupling limit of the rotating D1/D5/D5 intersection. For some appropriate parameter choice, the nine-dimensional metric becomes an AdS$_3\times S^3$ bundle over squashed 3-sphere. We also study the properties of the Gödel black holes that are constructed from the double Wick rotations of the Gödel metrics.

hep-th↗

arXiv2016

Stability of Differentially Rotating Disks in $f(T)$ Theory

To explain the accelerated expansion of our universe, many dark energy models and modified gravity theories have been proposed so far. It is argued in the literature that they are difficult to be distinguished on the cosmological scales. Therefore, it is well motivated to consider the relevant astrophysical phenomena on (or below) the galactic scales. In this work, we study the stability of self-gravitating differentially rotating galactic disks in $f(T)$ theory, and obtain the local stability criteria in $f(T)$ theory, which are valid for all $f(T)$ theories satisfying $f(T=0)=0$ and $f_T (T=0)\not=0$, if the adiabatic approximation and the weak field limit are considered. The information of the function $f(T)$ is mainly encoded in the parameter $α\equiv 1/f_T(T=0)$. We find that the local stability criteria in $f(T)$ theory are quite different from the ones in Newtonian gravity, general relativity, and other modified gravity theories such as $f(R)$ theory. We consider that this might be a possible hint to distinguish $f(T)$ theory from general relativity and other modified gravity theories on (or below) the galactic scales.

gr-qc↗

arXiv2016

Dyonic (A)dS Black Holes in Einstein-Born-Infeld Theory in Diverse Dimensions

We study Einstein-Born-Infeld gravity and construct the dyonic (A)dS planar black holes in general even dimensions, that carry both the electric charge and magnetic fluxes along the planar space. In four dimensions, the solution can be constructed with also spherical and hyperbolic topologies. We study the black hole thermodynamics and obtain the first law. We also classify the singularity structure.

hep-th↗

arXiv2016

New Generalizations of Cosmography Inspired by the Pade Approximant

The current accelerated expansion of the universe has been one of the most important fields in physics and astronomy since 1998. Many cosmological models have been proposed in the literature to explain this mysterious phenomenon. Since the nature and cause of the cosmic acceleration are still unknown, model-independent approaches to study the evolution of the universe are welcome. One of the powerful model-independent approaches is the so-called cosmography. It only relies on the cosmological principle, without postulating any underlying theoretical model. However, there are several shortcomings in the usual cosmography. For instance, it is plagued with the problem of divergence (or an unacceptably large error), and it fails to predict the future evolution of the universe. In the present work, we try to overcome or at least alleviate these problems, and we propose two new generalizations of cosmography inspired by the Padé approximant. One is to directly parameterize the luminosity distance based on the Padé approximant, while the other is to generalize cosmography with respect to a so-called $y_β$-shift $y_β=z/(1+βz)$, which is also inspired by the Padé approximant. Then, we confront them with the observational data with the help of the Markov chain Monte Carlo (MCMC) code emcee, and find that they work fairly well.

gr-qc↗

arXiv2016

An Overview of Transmission Theory and Techniques of Large-scale Antenna Systems for 5G Wireless Communications

To meet the future demand for huge traffic volume of wireless data service, the research on the fifth generation (5G) mobile communication systems has been undertaken in recent years. It is expected that the spectral and energy efficiencies in 5G mobile communication systems should be ten-fold higher than the ones in the fourth generation (4G) mobile communication systems. Therefore, it is important to further exploit the potential of spatial multiplexing of multiple antennas. In the last twenty years, multiple-input multiple-output (MIMO) antenna techniques have been considered as the key techniques to increase the capacity of wireless communication systems. When a large-scale antenna array (which is also called massive MIMO) is equipped in a base-station, or a large number of distributed antennas (which is also called large-scale distributed MIMO) are deployed, the spectral and energy efficiencies can be further improved by using spatial domain multiple access. This paper provides an overview of massive MIMO and large-scale distributed MIMO systems, including spectral efficiency analysis, channel state information (CSI) acquisition, wireless transmission technology, and resource allocation.

cs.IT↗

arXiv2015

Exact Cosmological Solutions of $f(R)$ Theories via Hojman Symmetry

Nowadays, $f(R)$ theory has been one of the leading modified gravity theories to explain the current accelerated expansion of the universe, without invoking dark energy. It is of interest to find the exact cosmological solutions of $f(R)$ theories. Besides other methods, symmetry has been proved as a powerful tool to find exact solutions. On the other hand, symmetry might hint the deep physical structure of a theory, and hence considering symmetry is also well motivated. As is well known, Noether symmetry has been extensively used in physics. Recently, the so-called Hojman symmetry was also considered in the literature. Hojman symmetry directly deals with the equations of motion, rather than Lagrangian or Hamiltonian, unlike Noether symmetry. In this work, we consider Hojman symmetry in $f(R)$ theories in both the metric and Palatini formalisms, and find the corresponding exact cosmological solutions of $f(R)$ theories via Hojman symmetry. There exist some new solutions significantly different from the ones obtained by using Noether symmetry in $f(R)$ theories. To our knowledge, they also have not been found previously in the literature. This work confirms that Hojman symmetry can bring new features to cosmology and gravity theories.

gr-qc↗

arXiv2015

Cosmological Models and Gamma-Ray Bursts Calibrated by Using Pade Method

Gamma-ray bursts (GRBs) are among the most powerful sources in the universe. In the recent years, GRBs have been proposed as a complementary probe to type Ia supernovae (SNIa). However, as is well known, there is a circularity problem in the use of GRBs to study cosmology. In this work, based on the Padé approximant, we propose a new cosmology-independent method to calibrate GRBs. We consider a sample consisting of 138 long Swift GRBs and obtain 79 calibrated long GRBs at high-redshift $z>1.4$ (named Mayflower sample) which can be used to constrain cosmological models without the circularity problem. Then, we consider the constraints on several cosmological models with these 79 calibrated GRBs and other observational data. We show that GRBs are competent to be a complementary probe to the other well-established cosmological observations.

astro-ph.CO↗

arXiv2015

Hojman Symmetry in $f(T)$ Theory

Today, $f(T)$ theory has been one of the popular modified gravity theories to explain the accelerated expansion of the universe without invoking dark energy. In this work, we consider the so-called Hojman symmetry in $f(T)$ theory. Unlike Noether conservation theorem, the symmetry vectors and the corresponding conserved quantities in Hojman conservation theorem can be obtained by using directly the equations of motion, rather than Lagrangian or Hamiltonian. We find that Hojman symmetry can exist in $f(T)$ theory, and the corresponding exact cosmological solutions are obtained. We find that the functional form of $f(T)$ is restricted to be the power-law or hypergeometric type, while the universe experiences a power-law or hyperbolic expansion. These results are different from the ones obtained by using Noether symmetry in $f(T)$ theory. Therefore, it is reasonable to find exact cosmological solutions via Hojman symmetry.

gr-qc↗

arXiv2015

$f(T)$ non-linear massive gravity and the cosmic acceleration

Inspired by the $f(R)$ non-linear massive gravity, we propose a new kind of modified gravity model, namely $f(T)$ non-linear massive gravity, by adding the dRGT mass term reformulated in the vierbein formalism, to the $f(T)$ theory. We then investigate the cosmological evolution of $f(T)$ massive gravity, and constrain it by using the latest observational data. We find that it slightly favors a crossing of the phantom divide line from the quintessence-like phase ($w_{de} > -1$) to the phantom-like one ($w_{de} < -1$) as redshift decreases.

gr-qc↗

arXiv2015

Post-Newtonian Approximation of Teleparallel Gravity Coupled with a Scalar Field

We use the parameterized post-Newtonian (PPN) formalism to explore the weak field approximation of teleparallel gravity non-minimally coupling to a scalar field $ϕ$, with arbitrary coupling function $ω(ϕ)$ and potential $V(ϕ)$. We find that all the PPN parameters are identical to general relativity (GR), which makes this class of theories compatible with the Solar System experiments. This feature also makes the theories quite different from the scalar-tensor theories, which might be subject to stringent constraints on the parameter space, or need some screening mechanisms to pass the Solar System experimental constraints.

gr-qc↗

arXiv2015

Exploring Hierarchies in Online Social Networks

Social hierarchy (i.e., pyramid structure of societies) is a fundamental concept in sociology and social network analysis. The importance of social hierarchy in a social network is that the topological structure of the social hierarchy is essential in both shaping the nature of social interactions between individuals and unfolding the structure of the social networks. The social hierarchy found in a social network can be utilized to improve the accuracy of link prediction, provide better query results, rank web pages, and study information flow and spread in complex networks. In this paper, we model a social network as a directed graph G, and consider the social hierarchy as DAG (directed acyclic graph) of G, denoted as GD. By DAG, all the vertices in G can be partitioned into different levels, the vertices at the same level represent a disjoint group in the social hierarchy, and all the edges in DAG follow one direction. The main issue we study in this paper is how to find DAG GD in G. The approach we take is to find GD by removing all possible cycles from G such that G = U(G) + GD where U(G) is a maximum Eulerian subgraph which contains all possible cycles. We give the reasons for doing so, investigate the properties of GD found, and discuss the applications. In addition, we develop a novel two-phase algorithm, called Greedy-&-Refine, which greedily computes an Eulerian subgraph and then refines this greedy solution to find the maximum Eulerian subgraph. We give a bound between the greedy solution and the optimal. The quality of our greedy approach is high. We conduct comprehensive experimental studies over 14 real-world datasets. The results show that our algorithms are at least two orders of magnitude faster than the baseline algorithm.

cs.SI↗

arXiv2015

Age Problem in Lemaitre-Tolman-Bondi Void Models

As is well known, one can explain the current cosmic acceleration by considering an inhomogeneous and/or anisotropic universe (which violates the cosmological principle), without invoking dark energy or modified gravity. The well-known one of this kind of models is the so-called Lema\^ıtre-Tolman-Bondi (LTB) void model, in which the universe is spherically symmetric and radially inhomogeneous, and we are living in a locally underdense void centered nearby our location. In the present work, we test various LTB void models with some old high redshift objects (OHROs). Obviously, the universe cannot be younger than its constituents. We find that an unusually large $r_0$ (characterizing the size of the void) is required to accommodate these OHROs in LTB void models. There is a serious tension between this unusually large $r_0$ and the much smaller $r_0$ inferred from other observations (e.g. SNIa, CMB and so on). However, if we instead consider the lowest limit 1.7\,Gyr for the quasar APM 08279+5255 at redshift $z=3.91$, this tension could be greatly alleviated.

gr-qc↗

arXiv2014

Cosmological Evolution of Einstein-Aether Models with Power-law-like Potential

The so-called Einstein-Aether theory is General Relativity coupled (at second derivative order) to a dynamical unit time-like vector field (the aether). It is a Lorentz-violating theory, and gained much attention in the recent years. In the present work, we study the cosmological evolution of Einstein-Aether models with power-law-like potential, by using the method of dynamical system. In the case without matter, there are two attractors which correspond to an inflationary universe in the early epoch, or a de Sitter universe in the late time. In the case with matter but there is no interaction between dark energy and matter, there are only two de Sitter attractors, and no scaling attractor exists. So, it is difficult to alleviate the cosmological coincidence problem. Therefore, we then allow the interaction between dark energy and matter. In this case, several scaling attractors can exist under some complicated conditions, and hence the cosmological coincidence problem could be alleviated.

gr-qc↗