Searcharxiv⌕ Search

arXiv subjects

Yungui Gong

Publications and source records attributed to Yungui Gong.

At least 145 records · Page 8Linked to original sources
arXiv2013

Testing the effects from dark radiation

In this letter, the effects of dark radiation (DR) are tested. Theoretically, the phase-space analysis method is applied to check whether the model is consist with the history of our universe which shows positive results. Observationally, by using the observational data ($SNLS$ (SuperNovae Legacy Survey), $WMAP9$(Wilkinson Microwave Anisotropy Probe 9 Years Result), $PLANCK$ (Planck First Data Release), $BAO$ (Baryon Acoustic Oscillations), $H(z)$ (Hubble Parameter Data) and $BBN$ (Big Bang Nucleosynthesis)), the dark radiation is found to have the effect of wiping out the tension between the $SNLS$ data and the other data in flat $ΛCDM$ model. The effects of dark radiation also make the best fit value of $N_{eff}$ slightly larger than 3.04.

astro-ph.CO↗
arXiv2013

Constraints on thawing scalar field models from fundamental constants

We consider a dark energy model with a relation between the equation of state parameter $w$ and the energy density parameter $Ω_ϕ$ derived from thawing scalar field models. Assuming the variation of the fine structure constant is caused by dark energy, we use the observational data of the variation of the fine structure constant to constrain the current value of $w_0$ and $Ω_{ϕ0}$ for the dark energy model. At the $1σ$ level, the observational data excluded some areas around $w_0=-1$, which explains the positive detection of the variation of the fine structure constant at the $1σ$ level, but $Λ$CDM model is consistent with the data at the $2σ$ level.

astro-ph.CO↗
arXiv2013

Cosmology in massive gravity

We argue that more cosmological solutions in massive gravity can be obtained if the metric tensor and the tensor $Σ_{μν}$ defined by Stückelberg fields take the homogeneous and isotropic form. The standard cosmology with matter and radiation dominations in the past can be recovered and $Λ$CDM model is easily obtained. The dynamical evolution of the universe is modified at very early times.

gr-qc↗
arXiv2013

The effect of different observational data on the constraints of cosmological parameters

The constraints on the $Λ$CDM model from type Ia supernova (SNe Ia) data alone and BAO data alone are similar, so it is worthwhile to compare their constraints on the property of dark energy. We apply the SNLS3 compilation of 472 SNe Ia data, the baryon acoustic oscillation measurement of distance, the cosmic microwave background radiation data from the WMAP7, and the Hubble parameter data to study the effect of their different combinations on the fittings of cosmological parameters in the modified holographic dark energy model and the Chevallier-Polarski-Linder model. Neither BAO nor WMAP7 data alone give good constraint on the equation of state parameter of dark energy, but both WMAP7 data and BAO data help SNe Ia data break the degeneracies among the model parameters, hence tighten the constraint on the variation of equation of state parameter $w_a$, and WMAP7 data do the job a little better. Although BAO and WMAP7 data provide reasonably good constraints on $Ω_m$ and $Ω_k$, they are not able to constrain the dynamics of dark energy. On the other hand, SNe Ia data do not provide good constraints on $Ω_m$ and $Ω_k$, but they provide good constraint on the dynamics of dark energy, especially the variation of the equation of state parameter of dark energy, so we need to combine SNe Ia with BAO and WMAP7 data to probe the property of dark energy. The addition of $H(z)$ data helps better constrain the geometry of the Universe $Ω_k$ and the property of dark energy. For the SNLS SNe Ia data, the nuisance parameters $α$ and $β$ are consistent for all different combinations of the above data, and their impacts on the fittings of cosmological parameters are minimal. By fitting the data to different models, $Λ$CDM model is still consistent with all the observational data.

astro-ph.CO↗
arXiv2012

Testing the interaction model with cosmological data and gamma-ray bursts

We use the new gamma-ray bursts (GRBs) data, combined with the baryon acoustic oscillation(BAO) observation from the spectroscopic Sloan Digital Sky Survey (SDSS) data release, the newly obtained $A$ parameter at $z=0.6$ from the WiggleZ Dark Energy Survey, the cosmic microwave background (CMB) observations from the 7-Year Wilkinson Microwave Anisotropy Probe (WMAP7) results, and the type Ia supernovae (SNeIa) from Union2 set, to constrain a phenomenological model describing possible interactions between dark energy and dark matter, which was proposed to alleviate the coincidence problem of the standard $Λ$CDM model. By using the Markov Chain Monte Carlo (MCMC) method, we obtain the marginalized $1σ$ constraints $Ω_{m}=0.2886\pm{0.0135}$, $r_m=-0.0047\pm{0.0046}$, and $w_X=-1.0658\pm{0.0564}$. We also consider other combinations of these data for comparison. These results show that: (1) the energy of dark matter is slightly transferring to that of dark energy; (2) even though the GRBs+BAO+CMB data present less stringent constraints than SNe+BAO+CMB data do, the GRBs can help eliminate the degeneracies among parameters.

astro-ph.CO↗
arXiv2011

The Unified First law in "Cosmic Triad" Vector Field Scenario

In this letter, we try to apply the unified first law to the "cosmic triad" vector field scenario both in the minimal coupling case and in the non-minimalcoupling case. After transferring the non-minimally coupling action in Jordan frame to Einstein frame, the correct dynamical equation (Friedmann equation) is gotten in a thermal equilibrium process by using the already existing entropy while the entropy in the non-minimal coupled "cosmic triad" scenario has not been derived. And after transferring the variables back to Jordan frame, the corresponding Friedmann equation is demonstrated to be correct. For complete arguments, we also calculate the related Misner-Sharp energy in Jordan and Einstein frames.

hep-th↗
arXiv2011

Notes on $f(T)$ Theories

The cosmological models based on teleparallel gravity with nonzero torsion are considered. To investigate the evolution of this theory, we consider the phase-space analysis of the $f(T)$ theory. It shows when the tension scalar can be written as an inverse function of $x$ where $x=ρ_{e}/(3m_{pl}^{2}H^{2})$ and $T=g(x)$, the system is an autonomous one. Furthermore,the $ω_{e}-ω'_{e}$ phase analysis is given out. We perform the dynamical analysis for the models $f(T)=βT\ln(T/T_{0})$ and $f(T)=αm_{pl}^{2}(-T/m_{pl}^{2})^{n}$ particularly. We find that the universe will settle into de-Sitter phase for both models. And we have examined the evolution behavior of the power law form in the $ω_{ep}-ω'_{ep}$ plane.

astro-ph.CO↗
arXiv2011

Current cosmological constraints on the curvature, dark energy and modified gravity

We apply the Union2 compilation of 557 supernova Ia data, the baryon acoustic oscillation measurement of distance, the cosmic microwave background radiation data from the seven year Wilkinson Microwave Anisotropy Probe, and the Hubble parameter data to study the geometry of the Universe and the property of dark energy by using models and parametrizations with different high redshift behaviours of $w(z)$. We find that $Λ$CDM model is consistent with current data, that the Dvali-Gabadadze-Porrati model is excluded by the data at more than $3σ$ level, that the Universe is almost flat, and that the current data is unable to distinguish models with different behaviours of $w(z)$ at high redshift. We also add the growth factor data to constrain the growth index of Dvali-Gabadadze-Porrati model and find that it is more than $1σ$ away from its theoretical value.

astro-ph.CO↗
arXiv2011

Interactions in Dark Energy Models

We perform a full dynamical analysis by considering the interactions between dark energy and radiation, and dark energy and dark matter. We find that the interaction helps alleviate the coincidence problem for the quintessence model.

astro-ph.CO↗
arXiv2011

Equipartition of energy and the first law of thermodynamics at the apparent horizon

We apply the holographic principle and the equipartition law of energy to the apparent horizon of a Friedmann-Robertson-Walker universe and derive the Friedmann equation describing the dynamics of the universe. We also show that the equipartition law of energy can be interpreted as the first law of thermodynamics at the apparent horizon.

gr-qc↗
arXiv2010

Brane cosmology in the Horava-Witten heterotic M-Theory on $S^{1}/Z_{2}$

We study the radion stability and radion mass in the framework of the Horava-Witten (HW) heterotic M-Theory on $S^{1}/Z_{2}$, and find that the radion is stable and its mass can be of the order of GeV. The gravity is localized on the visible brane, and the spectrum of the gravitational Kaluza-Klein (KK) modes is discrete and can have a mass gap of TeV. The corrections to the 4D Newtonian potential from the higher order gravitational KK modes are exponentially suppressed. Applying such a setup to cosmology, we find the generalized Friedmann-like equations on each of the two orbifold branes.

hep-th↗
arXiv2010

Improved cosmological constraints on the curvature and equation of state of dark energy

We apply the Constitution compilation of 397 supernova Ia, the baryon acoustic oscillation measurements including the $A$ parameter, the distance ratio and the radial data, the five-year Wilkinson microwave anisotropy probe and the Hubble parameter data to study the geometry of the universe and the property of dark energy by using the popular Chevallier-Polarski-Linder and Jassal-Bagla-Padmanabhan parameterizations. We compare the simple $χ^2$ method of joined contour estimation and the Monte Carlo Markov chain method, and find that it is necessary to make the marginalized analysis on the error estimation. The probabilities of $Ω_k$ and $w_a$ in the Chevallier-Polarski-Linder model are skew distributions, and the marginalized $1σ$ errors are $Ω_m=0.279^{+0.015}_{-0.008}$, $Ω_k=0.005^{+0.006}_{-0.011}$, $w_0=-1.05^{+0.23}_{-0.06}$, and $w_a=0.5^{+0.3}_{-1.5}$. For the Jassal-Bagla-Padmanabhan model, the marginalized $1σ$ errors are $Ω_m=0.281^{+0.015}_{-0.01}$, $Ω_k=0.000^{+0.007}_{-0.006}$, $w_0=-0.96^{+0.25}_{-0.18}$, and $w_a=-0.6^{+1.9}_{-1.6}$. The equation of state parameter $w(z)$ of dark energy is negative in the redshift range $0\le z\le 2$ at more than $3σ$ level. The flat $Λ$CDM model is consistent with the current observational data at the $1σ$ level.

astro-ph.CO↗
arXiv2010

The dynamical behavior of the Extended Holographic Dark Energy with Hubble Horizon

The extended holographic dark energy model with the Hubble horizon as the infrared cutoff avoids the problem of the circular reasoning of the holographic dark energy model. Unfortunately, it is hit with the no-go theorem. In this paper, we consider the extended holographic dark energy model with a potential, $V(ϕ)$, for the Brans-Dicke scalar field. With the addition of a potential for the Brans-Dicke scalar field, the extended holographic dark energy model using the Hubble horizon as the infrared cutoff is a viable dark energy model, and the model has the dark energy dominated attractor solution.

gr-qc↗
arXiv2010

Constraints on growth index parameters from current and future observations

We use current and future simulated data of the growth rate of large scale structure in combination with data from supernova, BAO, and CMB surface measurements, in order to put constraints on the growth index parameters. We use a recently proposed parameterization of the growth index that interpolates between a constant value at high redshifts and a form that accounts for redshift dependencies at small redshifts. We also suggest here another exponential parameterization with a similar behaviour. The redshift dependent parametrizations provide a sub-percent precision level to the numerical growth function, for the full redshift range. Using these redshift parameterizations or a constant growth index, we find that current available data from galaxy redshift distortions and Lyman-alpha forests is unable to put significant constraints on any of the growth parameters. For example both $Λ$CDM and flat DGP are allowed by current growth data. We use an MCMC analysis to study constraints from future growth data, and simulate pessimistic and moderate scenarios for the uncertainties. In both scenarios, the redshift parameterizations discussed are able to provide significant constraints and rule out models when incorrectly assumed in the analysis. The values taken by the constant part of the parameterizations as well as the redshift slopes are all found to significantly rule out an incorrect background. We also find that, for our pessimistic scenario, an assumed constant growth index over the full redshift range is unable to rule out incorrect models in all cases. This is due to the fact that the slope acts as a second discriminator at smaller redshifts and therefore provide a significant test to identify the underlying gravity theory.

astro-ph.CO↗
arXiv2010

Probing the cosmic acceleration from combinations of different data sets

We examine in some detail the influence of the systematics in different data sets including type Ia supernova sample, baryon acoustic oscillation data and the cosmic microwave background information on the fitting results of the Chevallier-Polarski-Linder parametrization. We find that the systematics in the data sets does influence the fitting results and leads to different evolutional behavior of dark energy. To check the versatility of Chevallier-Polarski-Linder parametrization, we also perform the analysis on the Wetterich parametrization of dark energy. The results show that both the parametrization of dark energy and the systematics in data sets influence the evolutional behavior of dark energy.

astro-ph.CO↗
arXiv2010

Using a phenomenological model to test the coincidence problem of dark energy

By assuming a phenomenological form for the ratio of the DE and DM densities $ρ_X\propto ρ_{m} a^ξ$ (Dalal et al.2001), we discuss the cosmic coincidence problem in light of current observational data. $ξ$ is a key parameter to denote the severity of the coincidence problem. In this scenario, $ξ=3$ and $ξ=0$ correspond to $Λ$CDM and the self-similar solution without coincidence problem, respectively. Hence, any solution with a scaling parameter $0<ξ<3$ makes the coincidence problem less severe. In addition, the standard cosmology without interaction between DE and DM is characterized by $ξ+3ω_X=0$, whereas the inequality $ξ+3ω_X\neq 0$ represents non-standard cosmology. We place observational constraints on the parameters $(Ω_{X,0},ω_{X},ξ)$ of this model, by using the Constitution Set, the CMB shift parameter from the 5-year WMAP and the SDSS baryon acoustic peak. Combining the three samples, we get $Ω_{X,0}=0.72\pm0.02$, $ω_{X}=-0.98\pm0.07$ and $ξ=3.06\pm0.35$ at 68.3% confidence level. The result shows that the $Λ$CDM model still remains a good fit to the recent observational data, as well as, the coincidence problem indeed exists and is quite severe, in the framework of this simple phenomenological model. We further constrain the model with the transition redshift (deceleration / acceleration). It shows that if the transition from deceleration to acceleration happens at the redshit $z > 0.73$, within the framework of this model, we can conclude that the interaction between DE and DM is necessary.

astro-ph.CO↗
arXiv2010

Observational constraint on dynamical evolution of dark energy

We use the Constitution supernova, the baryon acoustic oscillation, the cosmic microwave background, and the Hubble parameter data to analyze the evolution property of dark energy. We obtain different results when we fit different baryon acoustic oscillation data combined with the Constitution supernova data to the Chevallier-Polarski-Linder model. We find that the difference stems from the different values of $Ω_{m0}$. We also fit the observational data to the model independent piecewise constant parametrization. Four redshift bins with boundaries at $z=0.22$, 0.53, 0.85 and 1.8 were chosen for the piecewise constant parametrization of the equation of state parameter $w(z)$ of dark energy. We find no significant evidence for evolving $w(z)$. With the addition of the Hubble parameter, the constraint on the equation of state parameter at high redshift isimproved by 70%. The marginalization of the nuisance parameter connected to the supernova distance modulus is discussed.

astro-ph.CO↗