arXiv subjects

Shuo Cao

Publications and source records attributed to Shuo Cao.

At least 127 records · Page 7Linked to original sources

arXiv2018

Cosmological investigation of multi-frequency VLBI observations of ultra-compact structure in $z\sim 3$ radio quasars

In this paper, we use multi-frequency angular size measurements of 58 intermediate-luminosity quasars reaching the redshifts $z\sim 3$ and demonstrate that they can be used as standard rulers for cosmological inference. Our results indicate that, for the majority of radio-sources in our sample their angular sizes are inversely proportional to the observing frequency. From the physical point of view it means that opacity of the jet is governed by pure synchrotron self-absorption, i.e. external absorption does not play any significant role in the observed angular sizes at least up to 43 GHz. Therefore, we use the value of the intrinsic metric size of compact milliarcsecond radio quasars derived in a cosmology independent manner from survey conducted at 2 GHz and rescale it properly according to predictions of the conical jet model. This approach turns out to work well and produce quite stringent constraints on the matter density parameter $Ω_m$ in the flat $Λ$CDM model and Dvali-Gabadadze-Porrati braneworld model. The results presented in this paper pave the way for the follow up engaging multi-frequency VLBI observations of more compact radio quasars with higher sensitivity and angular resolution.

astro-ph.CO↗

arXiv2018

A new test of $f(R)$ gravity with the cosmological standard rulers in radio quasars

As an important candidate gravity theory alternative to dark energy, a class of $f(R)$ modified gravity, which introduces a perturbation of the Ricci scalar $R$ in the Einstein-Hilbert action, has been extensively applied to cosmology to explain the acceleration of the universe. In this paper, we focus on the recently-released VLBI observations of the compact structure in intermediate-luminosity quasars combined with the angular-diameter-distance measurements from galaxy clusters, which consists of 145 data points performing as individual cosmological standard rulers in the redshift range $0.023\le z\le 2.80$, to investigate observational constraints on two viable models in $f(R)$ theories within the Palatini formalism: $f_1(R)=R-\frac{a}{R^b}$ and $f_2(R)=R-\frac{aR}{R+ab}$. We also combine the individual standard ruler data with the observations of CMB and BAO, which provides stringent constraints. Furthermore, two model diagnostics, $Om(z)$ and statefinder, are also applied to distinguish the two $f(R)$ models and $Λ$CDM model. Our results show that (1) The quasars sample performs very well to place constraints on the two $f(R)$ cosmologies, which indicates its potential to act as a powerful complementary probe to other cosmological standard rulers. (2) The $Λ$CDM model, which corresponds to $b=0$ in the two $f(R)$ cosmologies is still included within $1σ$ range. However, there still exists some possibility that $Λ$CDM may not the best cosmological model preferred by the current high-redshift observations. (3) The information criteria indicate that the cosmological constant model is still the best one, while the $f_1(R)$ model gets the smallest observational support. (4) The $f_2(R)$ model, which evolves quite different from $f_1(R)$ model at early times, still significantly deviates from both $f_1(R)$ and $Λ$CDM model at the present time.

astro-ph.CO↗

arXiv2018

Statistical analysis with cosmic-expansion-rate measurements and two-point diagnostics

Direct measurements of Hubble parameters $H(z)$ are very useful for cosmological model parameters inference. Based on them, Sahni, Shafieloo and Starobinski introduced a two-point diagnostic $Omh^2(z_i, z_j)$ as an interesting tool for testing the validity of the $Λ$CDM model. Applying this test they found a tension between observations and predictions of the $Λ$CDM model. We use the most comprehensive compilation $H(z)$ data from baryon acoustic oscillations (BAO) and differential ages (DA) of passively evolving galaxies to study cosmological models using the Hubble parameters itself and to distinguish whether $Λ$CDM model is consistent with the observational data with statistical analysis of the corresponding $Omh^2(z_i, z_j)$ two-point diagnostics. Our results show that presently available $H(z)$ data significantly improve the constraints on cosmological parameters. The corresponding statistical $Omh^2(z_i, z_j)$ two-point diagnostics seems to prefer the quintessence with $w>-1$ over the $Λ$CDM model. Better and more accurate prior knowledge of the Hubble constant, will considerably improve the performance of the statistical $Omh^2(z_i, z_j)$ method.

astro-ph.CO↗

arXiv2018

What is parameterized $Om(z)$ diagnostics telling us in light of recent observations?

In this paper, we propose a new parametrization of $Om(z)$ diagnostics and show how the most recent and significantly improved observations concerning the $H(z)$ and SN Ia measurements can be used to probe the consistency or tension between $Λ$CDM model and observations. Our results demonstrates that $H_0$ plays a very important role in the consistency test of $Λ$CDM with the $H(z)$ data. Adopting the Hubble constant priors from \textit{Planck} 2013 and Riess (2016), one finds a considerable tension between the current $H(z)$ data and $Λ$CDM model and confirms the conclusions obtained previously by the others. However, with the Hubble constant prior taken from WMAP9, the discrepancy between $H(z)$ data and $Λ$CDM disappears, i.e., the current $H(z)$ observations still support the cosmological constant scenario. This conclusion is also supported by the results derived from the JLA SNe Ia sample. The best-fit Hubble constant from the combination of $H(z)$+JLA ($H_0=68.81^{+1.50}_{-1.49}$ km/s/Mpc) is well consistent with the results derived both by Planck 2013 and WMAP9, which is significantly different from the recent local measurement by Riess (2016).

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arXiv2017

Ultra-compact structure in radio quasars as a cosmological probe: a revised study of the interaction between cosmic dark sectors

A new compilation of $120$ angular-size/redshift data for compact radio quasars from very-long-baseline interferometry (VLBI) surveys motivates us to revisit the interaction between dark energy and dark matter with these probes reaching high redshifts $z\sim 3.0$. In this paper, we investigate observational constraints on different phenomenological interacting dark energy (IDE) models with the intermediate-luminosity radio quasars acting as individual standard rulers, combined with the newest BAO and CMB observation from Planck results acting as statistical rulers. The results obtained from the MCMC method and other statistical methods including Figure of Merit and Information Criteria show that: (1) Compared with the current standard candle data and standard clock data, the intermediate-luminosity radio quasar standard rulers , probing much higher redshifts, could provide comparable constraints on different IDE scenarios. (2) The interaction between dark energy and dark matter seems to be vanishing or slightly smaller than zero. At the 68.3\% confidence level, the energy is seen transferred from dark matter to dark energy, which implies that those IDE models can not alleviate the coincidence problem or even more sever. However, the strong degeneracies between the interaction term and Hubble constant may contribute to alleviate the tension of $H_0$ between the recent Planck and HST measurements. (3) Concerning the ranking of competing dark energy models, IDE with more free parameters are substantially penalized by the BIC criterion, which agrees very well with the previous results derived from other cosmological probes.

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arXiv2017

Testing and selecting cosmological models with ultra-compact radio quasars

In this paper, we place constraints on four alternative cosmological models under the assumption of the spatial flatness of the Universe: CPL, EDE, GCG and MPC. A new compilation of 120 compact radio quasars observed by very-long-baseline interferometry, which represents a type of new cosmological standard rulers, are used to test these cosmological models. Our results show that the fits on CPL obtained from the quasar sample are well consistent with those obtained from BAO. For other cosmological models considered, quasars provide constraints in agreement with those derived with other standard probes at $1σ$ confidence level. Moreover, the results obtained from other statistical methods including Figure of Merit, $Om(z)$ and statefinder diagnostics indicate that: (1) Radio quasar standard ruler could provide better statistical constraints than BAO for all cosmological models considered, which suggests its potential to act as a powerful complementary probe to BAO and galaxy clusters. (2) Turning to $Om(z)$ diagnostics, CPL, GCG and EDE models can not be distinguished from each other at the present epoch. (3) In the framework of statefinder diagnostics, MPC and EDE will deviate from $\rmΛ$CDM model in the near future, while GCG model cannot be distinguished from $\rmΛ$CDM model unless much higher precision observations are available.

astro-ph.CO↗

arXiv2017

Ultra-compact structure in intermediate-luminosity radio quasars: building a sample of standard cosmological rulers and improving the dark energy constraints up to $z\sim 3$

In this paper, we present a new compiled milliarcsecond compact radio data set of 120 intermediate-luminosity quasars in the redshift range $0.46< z <2.76$. These quasars show negligible dependence on redshifts and intrinsic luminosity, and thus represents, in the standard model of cosmology, a fixed comoving-length of standard ruler. We implement a new cosmology-independent technique to calibrate the linear size of of this standard ruler as $l_m= 11.03\pm0.25$ pc, which is the typical radius at which AGN jets become opaque at the observed frequency $ν\sim 2$ GHz. In the framework of flat $Λ$CDM model, we find a high value of the matter density parameter, $Ω_m=0.322^{+0.244}_{-0.141}$, and a low value of the Hubble constant, $H_0=67.6^{+7.8}_{-7.4}\; \rm{kms}^{-1}\rm{Mpc}^{-1}$, which is in excellent agreement with the CMB anisotropy measurements by \textit{Planck}. We obtain ${Ω_m}=0.309^{+0.215}_{-0.151}$, $w=-0.970^{+0.500}_{-1.730}$ at 68.3% CL for the constant $w$ of a dynamical dark-energy model, which demonstrates no significant deviation from the concordance $Λ$CDM model. Consistent fitting results are also obtained for other cosmological models explaining the cosmic acceleration, like Ricci dark energy (RDE) or Dvali-Gabadadze-Porrati (DGP) brane-world scenario. While no significant change in $w$ with redshift is detected, there is still considerable room for evolution in $w$ and the transition redshift at which $w$ departing from -1 is located at $z\sim 2.0$. Our results demonstrate that the method extensively investigated in our work on observational radio quasar data can be used to effectively derive cosmological information. Finally, we find the combination of high-redshift quasars and low-redshift clusters may provide an important source of angular diameter distances, considering the redshift coverage of these two astrophysical probes.

astro-ph.CO↗

arXiv2017

New observational constraints on $f(T)$ cosmology from radio quasars

Using a new recently compiled milliarcsecond compact radio data set of 120 intermediate-luminosity quasars in the redshift range $0.46< z <2.76$, whose statistical linear sizes show negligible dependence on redshifts and intrinsic luminosity and thus represent standard rulers in cosmology, we constrain three viable and most popular $f(T)$ gravity models, where $T$ is the torsion scalar in teleparallel gravity. Our analysis reveals that constraining power of the quasars data (N=120) is comparable to the Union2.1 SN Ia data (N=580) for all three $f(T)$ models. Together with other standard ruler probes such as Cosmic Microwave Background and Baryon Acoustic Oscillation distance measurements, the present value of the matter density parameter $Ω_m$ obtained by quasars is much lager than that derived from other observations. For two of the models considered ($f_1$CDM and $f_2$CDM) a small but noticeable deviation from $Λ$CDM cosmology is present, while in the framework of $f_3$CDM the effective equation of state may cross the phantom divide line at lower redshifts. These results indicate that intermediate-luminosity quasars could provide an effective observational probe comparable to SN Ia at much higher redsifts, and $f(T)$ gravity is a reasonable candidate for the modified gravity theory.

astro-ph.CO↗

arXiv2017

Thermodynamics of de Sitter black hole in massive gravity

In this paper, by taking de Sitter space-time as a thermodynamic system, we study the equivalent thermodynamic quantities of de Sitter black hole in massive gravity, and furthermore obtain the equivalent thermodynamic quantities of the space-time. Our results show that the entropy of this type of space-time takes the same form as that in Reissner-Nordstrom-de Sitter space-time, which lays a solid foundation for deeply understanding the universal thermodynamic characteristics of de Sitter space-time in the future. Moreover, our analysis indicates that the equivalent thermodynamic quantities and relevant parameters play a very important role, especially in the investigation of the stability and evolution of de Sitter space-time.

hep-th↗

arXiv2017

The Dynamics of Inhomogeneous Dark Energy

In this paper, by analyzing the dynamics of the inhomogeneous quintessence dark energy, we find that the gradient energy of dark energy will oscillate and gradually vanish, which indicates the gradient energy of the scalar field present in the early universe does not affect the current dynamics of the universe. Moreover, with the decaying of gradient energy, there exists a possible mutual transformation between kinetic energy and gradient energy. In the framework of interacting dark energy models, we argue that inhomogeneous dark energy may have a significant effect on the evolution of the cosmic background, the investigation of which still requires fully relativistic $N$-body numerical simulations in the future.

astro-ph.CO↗

arXiv2017

The thermodynamic relationship between the RN-AdS black holes and the RN black hole in canonical ensemble

In this paper, by analyzing the thermodynamic properties of charged AdS black hole and asymptotically flat space-time charged black hole in the vicinity of the critical point, we establish the correspondence between the thermodynamic parameters of asymptotically flat space-time and nonasymptotically flat space-time, based on the equality of black hole horizon area in the two different space-time. The relationship between the cavity radius (which is introduced in the study of asymptotically flat space-time charged black holes) and the cosmological constant (which is introduced in the study of nonasymptotically flat space-time) is determined. The establishment of the correspondence between the thermodynamics parameters in two different space-time is beneficial to the mutual promotion of different time-space black hole research, which is helpful to understand the thermodynamics and quantumproperties of black hole in space-time.

hep-th↗

arXiv2017

Measuring the speed of light with ultra-compact radio quasars

In this paper, based on a 2.29 GHz VLBI all-sky survey of 613 milliarcsecond ultra-compact radio sources with $0.0035<z<3.787$, we describe a method of identifying the sub-sample which can serve as individual standard rulers in cosmology. If the linear size of the compact structure is assumed to depend on source luminosity and redshift as $l_m=l L^β(1+z)^n$, only intermediate-luminosity quasars ($10^{27}$ W/Hz$<L<$ $10^{28}$ W/Hz) show negligible dependence ($|n|\simeq 10^{-3}$, $|β|\simeq 10^{-4}$), and thus represent a population of such rulers with fixed characteristic length $l=11.42$ pc. With a sample of 120 such sources covering the redshift range $0.46<z<2.80$, we confirm the existence of dark energy in the Universe with high significance under the assumption of a flat universe, and obtain stringent constraints on both the matter density $Ω_m=0.323^{+0.245}_{-0.145}$ and the Hubble constant $H_0=66.30^{+7.00}_{-8.50}$ km sec$^{-1}$ Mpc$^{-1}$. Finally, with the angular diameter distances $D_A$ measured for quasars extending to high redshifts ($z\sim 3.0$), we reconstruct the $D_A(z)$ function using the technique of Gaussian processes. This allows us to identify the redshift corresponding to the maximum of the $D_A(z)$ function: $z_m=1.70$ and the corresponding angular diameter distance $D_A(z_m)=1719.01\pm43.46$ Mpc. Similar reconstruction of the expansion rate function $H(z)$ based on the data from cosmic chronometers and BAO gives us $H(z_m)=176.77\pm6.11$ km sec$^{-1}$ Mpc$^{-1}$. These measurements are used to estimate the speed of light: $c=3.039(\pm0.180)\times 10^5$ km/s. This is the first measurement of the speed of light in a cosmological setting referring to the distant past.

astro-ph.CO↗

arXiv2017

Revisiting Studies of the Statistical Property of a Strong Gravitational Lens System and Model-independent Constraint on the Curvature of the Universe

In this paper we use a recently compiled data set, which comprises 118 galactic-scale strong gravitational lensing (SGL) systems to constrain the statistic property of SGL system, as well as the curvature of universe without assuming any fiducial cosmological model. Based on the singular isothermal ellipsoid (SIE) model of SGL system, we obtain that the constrained curvature parameter $Ω_{\rm k}$ is close to zero from the SGL data, which is consistent with the latest result of planck measurement. More interestingly, we find that the parameter $f$ in the SIE model is strongly correlated with the curvature $Ω_{\rm k}$. Neglecting this correlation in the analysis will significantly overestimate the constraining power of SGL data on the curvature. Furthermore, the obtained constraint on $f$ is different from previous results: $f=1.105\pm0.030$ ($68\%$ C.L.), which means that the standard singular isothermal sphere (SIS) model ($f=1$) is disfavored by the current SGL data at more than $3σ$ confidence level. We also divide the whole SGL data into two parts according to the centric stellar velocity dispersion $σ_{\rm c}$ and find that the larger value of $σ_{\rm c}$ the subsample has, the more favored the standard SIS model is. Finally, we extend the SIE model by assuming the power-law density profiles for the total mass density, $ρ=ρ_0(r/r_0)^{-α}$, and luminosity density, $ν=ν_0(r/r_0)^{-δ}$, and obtain the constraints on the power-law indexes: $α=1.95\pm0.04$ and $δ=2.40\pm0.13$ at 68\% confidence level. When assuming the power-law index $α=δ=γ$, this scenario is totally disfavored by the current SGL data, $χ^2_{\rm min,γ} - χ^2_{\rm min,SIE} \simeq 53$.

astro-ph.CO↗

arXiv2017

Test of parametrized post-Newtonian gravity with galaxy-scale strong lensing systems

Based on a mass-selected sample of galaxy-scale strong gravitational lenses from the SLACS, BELLS, LSD and SL2S surveys and using a well-motivated fiducial set of lens-galaxy parameters we tested the weak-field metric on kiloparsec scales and found a constraint on the post-Newtonian parameter $γ= 0.995^{+0.037}_{-0.047}$ under the assumption of a flat $Λ$CDM universe with parameters taken from \textit{Planck} observations. General relativity (GR) predicts exactly $γ=1$. Uncertainties concerning the total mass density profile, anisotropy of the velocity dispersion and the shape of the light-profile combine to systematic uncertainties of $\sim 25\%$. By applying a cosmological model independent method to the simulated future LSST data, we found a significant degeneracy between the PPN $γ$ parameter and spatial curvature of the Universe. Setting a prior on the cosmic curvature parameter $-0.007< Ω_k <0.006$, we obtained the following constraint on the PPN parameter: $γ=1.000^{+0.0023}_{-0.0025}$. We conclude that strong-lensing systems with measured stellar velocity dispersions may serve as another important probe to investigate validity of the GR, if the mass-dynamical structure of the lensing galaxies is accurately constrained in the future lens surveys.

astro-ph.CO↗

arXiv2016

$Q-Φ$ criticality in the extended phase space of $(n+1)$-dimensional RN-AdS black holes

In order to achieve a deeper understanding of gravity theories, it is important to further investigate the thermodynamic properties of black hole at the critical point, besides the phase transition and critical behaviors. In this paper, by using Maxwell's equal area law, we choose $T,Q,Φ$ as the state parameters and study the phase equilibrium problem of general $(n+1)$-dimensional RN-AdS black holes thermodynamic system. The boundary of the two-phase coexistence region and its isotherm and isopotential lines are presented, which may provide theoretical foundation for studying the phase transition and phase structure of black hole systems.

hep-th↗

arXiv2016

The Distance Duality Relation from Strong Gravitational Lensing

Under very general assumptions of metric theory of spacetime, photons traveling along null geodesics and photon number conservation, two observable concepts of cosmic distance, i.e. the angular diameter and the luminosity distances are related to each other by the so-called distance duality relation (DDR) $D^L=D^A(1+z)^2$. Observational validation of this relation is quite important because any evidence of its violation could be a signal of new physics. In this paper we introduce a new method to test DDR based on strong gravitational lensing systems and type Ia supernovae under a flat universe. The method itself is worth attention, because unlike previously proposed techniques, it does not depend on all other prior assumptions concerning the details of cosmological model. We tested it using a new compilation of strong lensing systems and JLA compilation of type Ia supernovae and found no evidence of DDR violation. For completeness, we also combined it with previous cluster data and showed its power on constraining DDR. It could become a promising new probe in the future in light of forthcoming massive strong lensing surveys and because of expected advances in galaxy cluster modlelling.

astro-ph.CO↗

arXiv2016

What are $Omh^2(z_1,z_2)$ and $Om(z_1,z_2)$ diagnostics telling us in light of $H(z)$ data?

Two-point diagnostics $Om(z_i,z_j)$ and $Omh^2(z_i,z_j)$ have been introduced as an interesting tool for testing the validity of the $Λ$CDM model. Quite recently, Sahni, Shafieloo $\&$ Starobinsky (2014) combined two independent measurements of $H(z)$ from BAO data with the value of the Hubble constant $H_0$, and used the second of these diagnostics to test the $Λ$CDM model. Their result indicated a considerable tension between observations and predictions of the $Λ$CDM model. Since reliable data concerning expansion rates of the Universe at different redshifts $H(z)$ are crucial for the successful application of this method, we investigate both two-point diagnostics on the most comprehensive set of $N=36$ measurements of $H(z)$ coming from the BAO and differential ages (DA) of passively evolving galaxies. We discuss the uncertainties of two-point diagnostics and find that they are strongly non-Gaussian and follow the patterns deeply rooted in their very construction. Therefore we propose that non-parametric median statistics is the most appropriate way of treating this problem. Our results support the claims that $Λ$CDM is in tension with $H(z)$ data according to the two-point diagnostics developed by Shafieloo, Sahni and Starobinsky. However, other alternatives to the $Λ$CDM, such as wCDM or CPL models perform even worse. We also notice that there are serious systematic differences between BAO and DA methods which ought to be better understood before $H(z)$ measurements can become competitive to the other probes.

astro-ph.CO↗

arXiv2016

Limits on the power-law mass and luminosity density profiles of elliptical galaxies from gravitational lensing systems

We use 118 strong gravitational lenses observed by the SLACS, BELLS, LSD and SL2S surveys to constrain the total mass profile and the profile of luminosity density of stars (light-tracers) in elliptical galaxies up to redshift $z \sim 1$. Assuming power-law density profiles for the total mass density, $ρ=ρ_0(r/r_0)^{-α}$, and luminosity density, $ν=ν_0(r/r_0)^{-δ}$, we investigate the power law index and its first derivative with respect to the redshift. Using Monte Carlo simulations of the posterior likelihood taking the Planck's best-fitted cosmology as a prior, we find $γ= 2.132\pm0.055$ with a mild trend $\partial γ/\partial z_l= -0.067\pm0.119$ when $α=δ=γ$, suggesting that the total density profile of massive galaxies could have become slightly steeper over cosmic time. Furthermore, similar analyses performed on sub-samples defined by different lens redshifts and velocity dispersions, indicate the need of treating low, intermediate and high-mass galaxies separately. Allowing $δ$ to be a free parameter, we obtain $α=2.070\pm0.031$, $\partial α/\partial z_l= -0.121\pm0.078$, and $δ= 2.710\pm0.143$. The model in which mass traces light is rejected at $>95\%$ confidence and our analysis robustly indicates the presence of dark matter in the form of a mass component that is differently spatially extended than the light. In this case, intermediate-mass elliptical galaxies ($200$ km/s $ < σ_{ap} \leq 300$ km/s) show the best consistency with the singular isothermal sphere as an effective model of galactic lenses.

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