# Yungui Gong - CQUPT

## Contact Details

NameYungui Gong |
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AffiliationCQUPT |
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Location |
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## Pubs By Year |
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## Pub CategoriesGeneral Relativity and Quantum Cosmology (41) Cosmology and Nongalactic Astrophysics (38) High Energy Physics - Theory (36) High Energy Physics - Phenomenology (3) Astrophysics (2) High Energy Astrophysical Phenomena (1) |

## Publications Authored By Yungui Gong

We derive a lower bound on the field excursion for the tachyon inflation, which is determined by the amplitude of the scalar perturbation and the number of $e$-folds before the end of inflation. Using the relation between the observables like $n_s$ and $r$ with the slow-roll parameters, we reconstruct three classes of tachyon potentials. The model parameters are determined from the observations before the potentials are reconstructed, and the observations prefer the concave potential. Read More

We analyze the polarization states of gravitational waves in Horndeski theory. In addition to the familiar plus and cross modes appearing in Einstein's general relativity, there is one more polarization state which is the mixture of the transverse breathing and longitudinal modes. The additional mode is excited by the massive scalar field. Read More

We give the procedure to reconstruct the extended inflationary potentials for general scalar-tensor theory of gravity and use the $\alpha$ attractor and the constant slow-roll model as examples to show how to reconstruct the class of extended inflationary potentials in the strong coupling limit. The class of extended inflationary potentials have the same attractor in the strong coupling limit, and the reconstructed extended inflationary potentials are consistent with the observational constraints. We also derive the condition on the coupling constant $\xi$ for satisfying the strong coupling. Read More

We investigate the cosmological perturbations of the topological model recently proposed. The model has an exact de Sitter background solution associated with a Becchi-Rouet-Stora(BRS) quartet terms which are regarded as a Lagrangian density of the topological field theory, and the de Sitter solution can be selected without spontaneously breaking the BRS symmetry. The BRS symmetry is preserved for the perturbations around the de Sitter background before we solve the constraints of general relativity. Read More

We point out that there are only three polarizations for gravitational waves in $f(R)$ gravity, and the polarization due to the massive scalar mode is a mix of the pure longitudinal and transverse breathing polarization. The classification of the six polarizations by the Newman-Penrose quantities is based on weak, plane and null gravitational waves, so it is not applicable to the massive mode. Read More

Compact objects with a light sphere such as black holes and wormholes can reflect light rays like a mirror. This gravitational lensing phenomenon is called retrolensing and it is an interesting tool to survey dark and compact objects with a light sphere near the solar system. In this paper, we calculate the deflection angle analytically in the strong deflection limit in the Reissner-Nordstrom spacetime without Taylor expanding it in the power of the electric charge. Read More

We show explicitly how the T model, E model, and Hilltop inflations are obtained from the general scalar-tensor theory of gravity with arbitrary conformal factors in the strong coupling limit. We argue that $\xi$ attractors can give any observables $n_s$ and $r$ by this method. The existence of attractors imposes a challenge to distinguish different models. Read More

With the usual definitions for the entropy and the temperature associated with the apparent horizon, we discuss the first law of the thermodynamics on the apparent in the general scalar-tensor theory of gravity with the kinetic term of the scalar field non-minimally coupling to Einstein tensor. We show the equivalence between the first law of thermodynamics on the apparent horizon and Friedmann equation for the general models, by using a mass-like function which is equal to the Misner-Sharp mass on the apparent horizon. The results further support the universal relationship between the first law of thermodynamics and Friedmann equation. Read More

We perform a phase space analysis of a generalized modified gravity theory with nonminimally coupling between geometry and matter. We apply the dynamical system approach to this generalized model and find that in the cosmological context, different choices of Lagrangian density will apparently result in different phases of the Universe. By carefully choosing the variables, we prove that there is an attractor solution to describe the late time accelerating universe when the modified gravity is chosen in a simple power-law form of the curvature scalar. Read More

The nonminimal coupling of the kinetic term to Einstein's tensor helps the implementation of inflationary models due to the gravitationally enhanced friction. We calculate the parametrized post-Newtonian (PPN) parameters for the scalar--tensor theory of gravity with nonminimally derivative coupling. We find that under experimental constraint from the orbits of millisecond pulsars in our galaxy, the theory deviates from Einstein's general relativity in the order of $10^{-20}$, and the effect of the nonminimal coupling is negligible if we take the scalar field as dynamical dark energy. Read More

The observational data on the anisotropy of the cosmic microwave background constraints the scalar spectral tilt $n_s$ and the tensor to scalar ratio $r$ which depend on the first and second derivatives of the inflaton potential. The information can be used to reconstruct the inflaton potential in the polynomial form up to some orders. However, for some classes of potentials, $n_s$ and $r$ behave as $n_s(N)$ and $r(N)$ universally in terms of the number of e-folds $N$. Read More

In quantum theory of gravity, we expect the Lorentz Invariance Violation (LIV) and the modification of the dispersion relation between energy and momentum for photons. The effect of the energy-dependent velocity due to the modified dispersion relation for photons was studied in the standard cosmological context by using a sample of Gamma Ray Bursts (GRBs). In this paper we mainly discuss the possible LIV effect by using different cosmological models for the accelerating universe. Read More

We study the non-equilibrium condensation process in a holographic superconductor with nonlinear corrections to the U(1) gauge field. We start with an asymptotic Anti-de-Sitter(AdS) black hole against a complex scalar perturbation at the initial time, and solve the dynamics of the gravitational systems in the bulk. When the black hole temperature T is smaller than a critical value Tc, the scalar perturbation grows exponentially till saturation, the final state of spacetime approaches to a hairy black hole. Read More

We derive the general formulae for the the scalar and tensor spectral tilts to the second order for the inflationary models with non-minimally derivative coupling without taking the high friction limit. The non-minimally kinetic coupling to Einstein tensor brings the energy scale in the inflationary models down to be sub-Planckian. In the high friction limit, the Lyth bound is modified with an extra suppression factor, so that the field excursion of the inflaton is sub-Planckian. Read More

With the usual definitions for the entropy and the temperature associated with the apparent horizon, we show that the unified first law on the apparent horizon is equivalent to the Friedmann equation for the scalar--tensor theory with non-minimally derivative coupling. The second law of thermodynamics on the apparent horizon is also satisfied. The results support a deep and fundamental connection between gravitation, thermodynamics, and quantum theory. Read More

We perform a dynamical analysis for the exponential scalar field with non-minimally derivative coupling. For the quintessence case, the stable fixed points are the same with and without the non-minimally derivative coupling. For the phantom case, the attractor with dark energy domination exists for the minimal coupling only. Read More

To reconcile the BICEP2 measurement on the tensor-to-scalar ratio $r$ with Planck constraint, a large negative running of scalar spectral index $n_s$ is needed. So the inflationary observable such as $n_s$ should be expanded at least to the second-order slow-roll parameters for single-field inflationary models. The large value of $r$ and the Lyth bound indicate that it is impossible to obtain the sub-Planckian excursion for the inflaton. Read More

The BICEP2 experiment confirms the existence of primordial gravitational wave with the tensor-to-scalar ratio $r=0$ ruled out at $7\sigma$ level. The consistency of this large value of $r$ with the {\em Planck} data requires a large negative running $n'_s$ of the scalar spectral index. Herein we propose two types of the single field inflation models with simple potentials to study the possibility of the consistency of the models with the BICEP2 and {\em Planck} observations. Read More

The detection of B-mode power spectrum by the BICEP2 collaboration constrains the tensor-to-scalar ratio $r=0.20^{+0.07}_{-0. Read More

We discuss the general dynamical behaviors of quintessence field, in particular, the general conditions for tracking and thawing solutions are discussed. We explain what the tracking solutions mean and in what sense the results depend on the initial conditions. Based on the definition of tracking solution, we give a simple explanation on the existence of a general relation between $w_\phi$ and $\Omega_\phi$ which is independent of the initial conditions for the tracking solution. Read More

We get the same degeneracy relation between $w_0$ and $w_a$ for the tachyon fields as for quintessence and phantom fields. Our results show that the dynamics of scalar fields with different origins becomes indistinguishable when the equation of state parameter $w$ does not deviate too far away from -1, and the time variation $w'$ satisfies the same bound for the same class of models. For the tachyon fields, a limit on $w'$ exists due to the Hubble damping and we derived the generic bounds on $w'$ for different classes of models. Read More

Planck measurements of the cosmic microwave background power spectra find a lower value of the Hubble constant $H_0$ and a higher value of the fractional matter energy density $\Omega_{m0}$ for the concordance $\Lambda$CDM model, and these results are in tension with other measurements. The {\em Planck} group argued that the tension came either from some sources of unknown systematic errors in some astrophysical measurements or the wrong $\Lambda$CDM model applied in fitting the data. We studied the reason for the tension on $H_0$ from different measurements by considering two dynamical dark energy models. Read More

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 $\Lambda CDM$ model. Read More

The dynamics of scalar fields as dark energy is well approximated by some general relations between the equation of state parameter $w(z)$ and the fraction energy density $\Omega_\phi$. Based on the approximation, for slowly-rolling scalar fields, we derived the analytical expressions of $w(z)$ which reduce to the popular Chevallier-Polarski-Linder parametrization with explicit degeneracy relation between $w_0$ and $w_a$. The models approximate the dynamics of scalar fields well and help eliminate the degeneracies among $w_a$, $w_0$ and $\Omega_{m0}$. Read More

We consider a dark energy model with a relation between the equation of state parameter $w$ and the energy density parameter $\Omega_\phi$ 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 $\Omega_{\phi 0}$ for the dark energy model. At the $1\sigma$ 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 $1\sigma$ level, but $\Lambda$CDM model is consistent with the data at the $2\sigma$ level. Read More

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 $\Lambda$CDM model. By using the Markov Chain Monte Carlo (MCMC) method, we obtain the marginalized $1\sigma$ constraints $\Omega_{m}=0. Read More

The ghost free massive gravity modified Friedmann equations at cosmic scale and provided an explanation of cosmic acceleration without dark energy. We analyzed the cosmological solutions of the massive gravity in detail and confronted the cosmological model with current observational data. We found that the model parameters $\alpha_3$ and $\alpha_4$ which are the coefficients of the third and fourth order nonlinear interactions cannot be constrained by current data at the background level. Read More

We argue that more cosmological solutions in massive gravity can be obtained if the metric tensor and the tensor $\Sigma_{\mu\nu}$ defined by St\"{u}ckelberg fields take the homogeneous and isotropic form. The standard cosmology with matter and radiation dominations in the past can be recovered and $\Lambda$CDM model is easily obtained. The dynamical evolution of the universe is modified at very early times. Read More

We explore, in the context of AdS/CFT correspondence, the causality constraints on the Noncritical Einstein-Weyl (NEW) gravity model in five dimensions. The scalar and shear channels are considered as small metric perturbations around an AdS black brane background. Our results show that causality analysis on the propagation of these two channels imposes a new bound on the coupling of the Weyl-squared terms in the NEW gravity. Read More

The transient acceleration which the current cosmic acceleration is not eternal is possible by introducing the interaction between dark matter and dark energy. If the energy transfer is from dark energy to dark matter, then it is possible to realize the transient acceleration. We study the possibility of transient acceleration by considering two time-dependent phenomenological interaction forms so that the energy transfer increases as the universe evolves. Read More

The constraints on the $\Lambda$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. Read More

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. Read More

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. Read More

**Category:**Cosmology and Nongalactic Astrophysics

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 $x=\rho_{e}/(3m_{pl}^{2}H^{2})$ and $T=g(x)$, the system is an autonomous one. Read More

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 $\Lambda$CDM model is consistent with current data, that the Dvali-Gabadadze-Porrati model is excluded by the data at more than $3\sigma$ 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 $1\sigma$ away from its theoretical value. Read More

We examine the effectiveness of the weak gravity conjecture in constraining the dark energy by comparing with observations. For general dark energy models with plausible phenomenological interactions between dark sectors, we find that although the weak gravity conjecture can constrain the dark energy, the constraint is looser than that from the observations. Read More

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 $\chi^2$ 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 $\Omega_k$ and $w_a$ in the Chevallier-Polarski-Linder model are skew distributions, and the marginalized $1\sigma$ errors are $\Omega_m=0. Read More

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. Read More

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. Read More

By assuming a phenomenological form for the ratio of the DE and DM densities $\rho_X\propto \rho_{m} a^{\xi}$ (Dalal et al.2001), we discuss the cosmic coincidence problem in light of current observational data. $\xi$ is a key parameter to denote the severity of the coincidence problem. Read More

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. Read More

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, $V(\phi)$, for the Brans-Dicke scalar field. Read More

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 $\Omega_{m0}$. Read More

We propose a modified holographic dark energy (MHDE) model with the Hubble scale as the infrared (IR) cutoff. Introducing the infinite extra dimension(s) at very large distance scale, we consider the black hole mass in higher dimensions as the ultraviolet cutoff. Thus, we can probe the effects of the IR infinite extra dimension(s). Read More

The cosmic coincidence problem is a serious challenge to dark energy model. We suggest a quantitative criteria for judging the severity of the coincidence problem. Applying this criteria to three different interacting models, including the interacting quintessence, interacting phantom, and interacting Chaplygin gas models, we find that the interacting Chaplygin gas model has a better chance to solve the coincidence problem. Read More

The growth rate of matter perturbation and the expansion rate of the Universe can be used to distinguish modified gravity and dark energy models in explaining cosmic acceleration. We explore here the inclusion of spatial curvature into the growth factor. We expand previous results using the approximation $\Omega_{m}^\gamma$ and then suggest a new form, $f_a=\Omega_m^\gamma+(\gamma-4/7)\Omega_k$, as an approximation for the growth factor when the curvature $\Omega_k$ is not negligible, and where the growth index $\gamma$ is usually model dependent. Read More

We perform a detailed phase-space analysis of various phantom cosmological models, where the dark energy sector interacts with the dark matter one. We examine whether there exist late-time scaling attractors, corresponding to an accelerating universe and possessing dark energy and dark matter densities of the same order. We find that all the examined models, although accepting stable late-time accelerated solutions, cannot alleviate the coincidence problem, unless one imposes a form of fine-tuning in the model parameters. Read More

The dynamical behaviors of two interacting dark energy models are considered. In addition to the scaling attractors found in the non-interacting quintessence model with exponential potential, new accelerated scaling attractors are also found in the interacting dark energy models. The coincidence problem is reduced to the choice of parameters in the interacting dark energy models. Read More

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. Read More

The growth rate of matter perturbation and the expansion rate of the Universe can be used to distinguish modified gravity and dark energy models in explaining the cosmic acceleration. The growth rate is parametrized by the growth index $\gamma$. We discuss the dependence of $\gamma$ on the matter energy density $\Omega$ and its current value $\Omega_0$ for a more accurate approximation of the growth factor. Read More