arXiv subjects

Shuang Wang

Publications and source records attributed to Shuang Wang.

At least 199 records · Page 11Linked to original sources

arXiv2012

Generalized Holographic Dark Energy and its Observational Constraints

In the original holographic dark energy (HDE) model, the dark energy density is proposed to be $ρ_{de} = 3c^2M^2_{pl}L^{-2}$, with $c$ is a dimensionless constant characterizing the properties of the HDE. In this work, we propose the generalized holographic dark energy (GHDE) model by considering the parameter $c$ as a redshift-dependent function $c(z)$. We derive all the physical quantities of the GHDE model analytically, and fit the $c(z)$ by trying four kinds of parametrizations. The cosmological constraints of the $c(z)$ are obtained from the joint analysis of the present SNLS3+BAO+CMB+$H_0$ data. We find that, compared with the original HDE model, the GHDE models can provide a better fit to the data. For example, the GHDE model with JBP-type $c(z)$ can reduce the $χ^2_{min}$ of the HDE model by 2.16. We also find that, unlike the original HDE model with a phantom-like behavior in the future, the GHDE models can present many more different possibilities, i.e., it allows the GHDE in the future to be either quintessence like, cosmological constant like, or phantom like, depending on the forms of $c(z)$.

astro-ph.CO↗

arXiv2012

Testing modified gravity models with recent cosmological observations

We explore the cosmological implications of five modified gravity (MG) models by using the recent cosmological observational data, including the recently released SNLS3 type Ia supernovae sample, the cosmic microwave background anisotropy data from the Wilkinson Microwave Anisotropy Probe 7-yr observations, the baryon acoustic oscillation results from the Sloan Digital Sky Survey data release 7, and the latest Hubble constant measurement utilizing the Wide Field Camera 3 on the Hubble Space Telescope. The MG models considered include the Dvali-Gabadadze-Porrati(DGP) model, two $f(R)$ models, and two $f(T)$ models. We find that compared with the $Λ$CDM model, MG models can not lead to a appreciable reduction of the $χ^2_{min}$. The analysis of AIC and BIC shows that the simplest cosmological constant model($Λ$CDM) is still most preferred by the current data, and the DGP model is strongly disfavored. In addition, from the observational constraints, we also reconstruct the evolutions of the growth factor in these models. We find that the current available growth factor data are not enough to distinguish these MG models from the $Λ$CDM model.

astro-ph.CO↗

arXiv2012

Dark Energy and Fate of the Universe

We explore the ultimate fate of the Universe by using a divergence-free parametrization for dark energy $w(z)=w_0+w_a({\ln (2+z)\over 1+z}-\ln2)$. Unlike the CPL parametrization, this parametrization has well behaved, bounded behavior for both high redshifts and negative redshifts, and thus can genuinely cover many theoretical dark energy models. After constraining the parameter space of this parametrization by using the current cosmological observations, we find that, at the 95.4% confidence level, our Universe can still exist at least 16.7 Gyr before it ends in a big rip. Moreover, for the phantom energy dominated Universe, we find that a gravitationally bound system will be destroyed at a time $t \simeq P\sqrt{2|1+3w(-1)|}/[6π|1+w(-1)|]$, where $P$ is the period of a circular orbit around this system, before the big rip.

astro-ph.CO↗

arXiv2012

Field test of the wavelength-saving quantum key distribution network

We propose a wavelength-saving topology of quantum key distribution(QKD) network based on passive optical elements, and report the field test of this network on the commercial telecom optical fiber. In this network, 5 nodes are supported with 2 wavelengths, and every two nodes can share secure keys directly at the same time. All QKD links in the network operate at the frequency of 20 MHz. We also characterized the insertion loss and crosstalk effects on the point-to-point QKD system after introducing this QKD network.

quant-ph↗

arXiv2012

2-GHz clock quantum key distribution over 260 km of standard telecom fiber

We report a demonstration of quantum key distribution (QKD) over a standard telecom fiber exceeding 50 dB in loss and 250 km in length. The differential phase shift QKD protocol was chosen and implemented with 2 GHz system clock rate. By careful optimization of the 1-bit delayed Faraday-Michelson interferometer and the use of the super-conducting single photon detector (SSPD), we achieved a quantum bit error rate below 2% when the fiber length was no more than 205 km, and of 3.45% for the 260 km length fiber with 52.9 dB loss. We also improved the quantum efficiency of SSPD to obtain high key rate for 50 km length.

quant-ph↗

arXiv2011

Attacking practical quantum key distribution system with wavelength dependent beam splitter and multi-wavelength sources

Unconditional security of quantum key distribution protocol can be guaranteed by the basic property of quantum mechanics. Unfortunately, the practical quantum key distribution system always have some imperfections, and the practical system may be attacked if the imperfection can be controlled by the eavesdropper Eve. Applying the fatal security loophole introduced by the imperfect beam splitter's wavelength dependent optical property, we propose wavelength-dependent attacking model, which can be applied to almost all practical quantum key distribution systems with the passive state modulation and photon state detection after the practical beam splitter. Utilizing our attacking model, we experimentally demonstrate the attacking system based on practical polarization encoding quantum key distribution system with almost 100% success probability. Our result demonstrate that all practical devices require tightened security inspection for avoiding side channel attacks in practical quantum key distribution experimental realizations.

quant-ph↗

arXiv2011

Dark Energy

We review the problem of dark energy, including a survey of theoretical models and some aspects of numerical studies.

astro-ph.CO↗

arXiv2011

Semi-device independent random number expansion without entanglement

By testing the classical correlation violation between two systems, the random number can be expanded and certified without applying classical statistical method. In this work, we propose a new random number expansion protocol without entanglement, and the randomness can be guaranteed only by the 2-dimension quantum witness violation. Furthermore, we only assume that the dimensionality of the system used in the protocol has a tight bound, and the whole protocol can be regarded as a semi-device independent black-box scenario. Comparing with the device independent random number expansion protocol based on entanglement, our protocol is much easier to implement and test.

quant-ph↗

arXiv2011

Probing Cosmic Acceleration by Using the SNLS3 SNIa Dataset

We probe the cosmic acceleration by using the recently released SNLS3 sample of 472 type Ia supernovae. Combining this type Ia supernovae dataset with the cosmic microwave background anisotropy data from the Wilkinson Microwave Anisotropy Probe 7-yr observations, the baryon acoustic oscillation results from the Sloan Digital Sky Survey data release 7, and the Hubble constant measurement from the Wide Field Camera 3 on the Hubble Space Telescope, we measure the dark energy equation of state $w$ and the deceleration parameter $q$ as functions of redshift by using the Chevallier-Polarski-Linder parametrization. Our result is consistent with a cosmological constant at 1$σ$ confidence level, without evidence for the recent slowing down of the cosmic acceleration. Furthermore, we consider three binned parametrizations ($w$ is piecewise constant in redshift $z$) based on different binning methods. The similar results are obtained, i.e., the $Λ$CDM model is still nicely compatible with current observations.

astro-ph.CO↗

arXiv2011

Exploring the Latest Union2 SNIa Dataset by Using Model-Independent Parametrization Methods

We explore the cosmological consequences of the recently released Union2 sample of 557 Type Ia supernovae (SNIa). Combining this latest SNIa dataset with the Cosmic microwave background (CMB) anisotropy data from the Wilkinson Microwave Anisotropy Probe 7 year (WMAP7) observations and the baryon acoustic oscillation (BAO) results from the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7), we measure the dark energy density function $f(z)\equiv ρ_{de}(z)/ρ_{de}(0)$ as a free function of redshift. Two model-independent parametrization methods (the binned parametrization and the polynomial interpolation parametrization) are used in this paper. By using the $χ^2$ statistic and the Bayesian information criterion, we find that the current observational data are still too limited to distinguish which parametrization method is better, and a simple model has advantage in fitting observational data than a complicated model. Moreover, it is found that all these parametrizations demonstrate that the Union2 dataset is still consistent with a cosmological constant at 1$σ$ confidence level. Therefore, the Union2 dataset is different from the Constitution SNIa dataset, which more favors a dynamical dark energy.

astro-ph.CO↗

arXiv2010

Revisit of cosmic age problem

We investigate the cosmic age problem associated with 9 extremely old globular clusters in M31 galaxy and 1 very old high-$z$ quasar APM 08279 + 5255 at $z=3.91$. These 9 globular clusters have not been used to study the cosmic age problem in the previous literature. By evaluating the age of the universe in the $Λ$CDM model with the observational constraints from the SNIa, the BAO, the CMB, and the independent $H_0$ measurements, we find that the existence of 5 globular clusters and 1 high-$z$ quasar are in tension (over 2$σ$ confidence level) with the current cosmological observations. So if the age estimates of these objects are correct, the cosmic age puzzle still remains in the standard cosmology. Moreover, we extend our investigations to the cases of the interacting dark energy models. It is found that although the introduction of the interaction between dark sectors can give a larger cosmic age, the interacting dark energy models still have difficulty to pass the cosmic age test.

astro-ph.CO↗

arXiv2010

Cosmic age test in inhomogeneous cosmological models mimicking $Λ$CDM on the light cone

The possibility of reconstructing a spherically symmetric inhomogeneous Lemaitre-Tolman-Bondi (LTB) model with $Λ$CDM observations has drawn a lot of attention. Recently, a LTB model having the same luminosity-distance and light-cone mass density as the $Λ$CDM model was reconstructed. We calculate the cosmic age at our position in this model, and get the age $t_{LTB}<11.7$Gyr in 2$σ$ confidence level from the Seven-Year WMAP observations. This result is inconsistent with some Milky Way observations.

astro-ph.CO↗

arXiv2010

Revisit of Tension in Recent SNIa Datasets

Although today there are many observational methods, Type Ia supernovae (SNIa) is still one of the most powerful tools to probe the mysterious dark energy (DE). The most recent SNIa datasets are the 307 SNIa "Union" dataset \cite{kow08} and the 397 SNIa "Constitution" dataset \cite{hic09}. In a recent work \cite{wei10}, Wei pointed out that both Union and Constitution datasets are in tension with the observations of cosmic microwave background (CMB) and baryon acoustic oscillation (BAO), and suggested that two truncated versions of Union and Constitution datasets, namely "UnionT" and "ConstitutionT", should be used to constrain various DE models. But in \cite{wei10}, only the $Λ$CDM model is used to select the outliers from the Union and the Constitution dataset. In principle, since different DE models may select different outliers, the truncation procedure should be performed for each different DE model. In the present work, by performing the truncation procedure of \cite{wei10} for 10 different models, we demonstrate that the impact of different models is negligible, and the approach adopted in \cite{wei10} is valid. Moreover, by using the 4 SNIa datasets mentioned above, as well as the observations of CMB and BAO, we perform best-fit analysis on the 10 models. It is found that: (1) For each DE model, the truncated SNIa datasets not only greatly reduce $χ_{min}^{2}$ and $χ_{min}^{2}/dof$, but also remove the tension between SNIa data and other cosmological observations. (2) The CMB data is very helpful to break the degeneracy among different parameters, and plays a very important role in distinguishing different DE models. (3) The current observational data are still too limited to distinguish all DE models.

astro-ph.CO↗

arXiv2009

New Primordial-Magnetic-Field Limit from The Latest LIGO S5 data

Since the energy momentum tensor of a magnetic field always contains a spin-2 component in its anisotropic stress, stochastic primordial magnetic field (PMF) in the early universe must generate stochastic gravitational wave (GW) background. This process will greatly affect the relic gravitational wave (RGW), which is one of major scientific goals of the laser interferometer GW detections. Recently, the fifth science (S5) run of laser interferometer gravitational-wave observatory (LIGO) gave a latest upper limit $Ω_{GW}<6.9\times10^{-6}$ on the RGW background. Utilizing this upper limit, we derive new PMF Limits: for a scale of galactic cluster $λ=1$ Mpc, the amplitude of PMF, that produced by the electroweak phase transition (EPT), has to be weaker than $B_λ \leq 4\times 10^{-7}$ Gauss; for a scale of supercluster $λ=100$ Mpc, the amplitude of PMF has to be weaker than $B_λ \leq 9\times 10^{-11}$ Gauss. In this manner, GW observation has potential to make interesting contributions to the study of primordial magnetic field.

astro-ph↗

arXiv2009

Probing interaction and spatial curvature in the holographic dark energy model

In this paper we place observational constraints on the interaction and spatial curvature in the holographic dark energy model. We consider three kinds of phenomenological interactions between holographic dark energy and matter, i.e., the interaction term $Q$ is proportional to the energy densities of dark energy ($ρ_Λ$), matter ($ρ_{m}$), and matter plus dark energy ($ρ_m+ρ_Λ$). For probing the interaction and spatial curvature in the holographic dark energy model, we use the latest observational data including the type Ia supernovae (SNIa) Constitution data, the shift parameter of the cosmic microwave background (CMB) given by the five-year Wilkinson Microwave Anisotropy Probe (WMAP5) observations, and the baryon acoustic oscillation (BAO) measurement from the Sloan Digital Sky Survey (SDSS). Our results show that the interaction and spatial curvature in the holographic dark energy model are both rather small. Besides, it is interesting to find that there exists significant degeneracy between the phenomenological interaction and the spatial curvature in the holographic dark energy model.

astro-ph.CO↗

arXiv2009

Fitting the Constitution SNIa Data with Redshift Binned Parameterization Method

In this work, we explore the cosmological consequences of the recently released Constitution sample of 397 Type Ia supernovae (SNIa). By revisiting the Chevallier-Polarski-Linder (CPL) parameterization, we find that, for fitting the Constitution set alone, the behavior of dark energy (DE) significantly deviate from the cosmological constant $Λ$, where the equation of state (EOS) $w$ and the energy density $ρ_Λ$ of DE will rapidly decrease along with the increase of redshift $z$. Inspired by this clue, we separate the redshifts into different bins, and discuss the models of a constant $w$ or a constant $ρ_Λ$ in each bin, respectively. It is found that for fitting the Constitution set alone, $w$ and $ρ_Λ$ will also rapidly decrease along with the increase of $z$, which is consistent with the result of CPL model. Moreover, a step function model in which $ρ_Λ$ rapidly decreases at redshift $z\sim0.331$ presents a significant improvement ($Δχ^{2}=-4.361$) over the CPL parameterization, and performs better than other DE models. We also plot the error bars of DE density of this model, and find that this model deviates from the cosmological constant $Λ$ at 68.3% confidence level (CL); this may arise from some biasing systematic errors in the handling of SNIa data, or more interestingly from the nature of DE itself. In addition, for models with same number of redshift bins, a piecewise constant $ρ_Λ$ model always performs better than a piecewise constant $w$ model; this shows the advantage of using $ρ_Λ$, instead of $w$, to probe the variation of DE.

astro-ph.CO↗

arXiv2009

Field Experiment on a Robust Hierarchical Metropolitan Quantum Cryptography Network

A hierarchical metropolitan quantum cryptography network upon the inner-city commercial telecom fiber cables is reported in this paper. The seven-user network contains a four-node backbone net with one node acting as the subnet gateway, a two-user subnet and a single-fiber access link, which is realized by the Faraday-Michelson Interferometer set-ups. The techniques of the quantum router, optical switch and trusted relay are assembled here to guarantee the feasibility and expandability of the quantum cryptography network. Five nodes of the network are located in the government departments and the secure keys generated by the quantum key distribution network are utilized to encrypt the instant video, sound, text messages and confidential files transmitting between these bureaus. The whole implementation including the hierarchical quantum cryptographic communication network links and corresponding application software shows a big step toward the practical user-oriented network with high security level.

quant-ph↗

arXiv2009

Holographic dark energy models: A comparison from the latest observational data

The holographic principle of quantum gravity theory has been applied to the dark energy (DE) problem, and so far three holographic DE models have been proposed: the original holographic dark energy (HDE) model, the agegraphic dark energy (ADE) model, and the holographic Ricci dark energy (RDE) model. In this work, we perform the best-fit analysis on these three models, by using the latest observational data including the Union+CFA3 sample of 397 Type Ia supernovae (SNIa), the shift parameter of the cosmic microwave background (CMB) given by the five-year Wilkinson Microwave Anisotropy Probe (WMAP5) observations, and the baryon acoustic oscillation (BAO) measurement from the Sloan Digital Sky Survey (SDSS). The analysis shows that for HDE, $χ_{min}^{2}=465.912$; for RDE, $χ_{min}^{2}=483.130$; for ADE, $χ_{min}^{2}=481.694$. Among these models, HDE model can give the smallest $χ^2_{min}$. Besides, we also use the Bayesian evidence (BE) as a model selection criterion to make a comparison. It is found that for HDE, ADE, and RDE, $Δ\ln \mathrm{BE}= -0.86$, -5.17, and -8.14, respectively. So, it seems that the HDE model is more favored by the observational data.

astro-ph.CO↗