arXiv subjects

Yungui Gong

Publications and source records attributed to Yungui Gong.

At least 163 records · Page 9Linked to original sources

arXiv2009

A Modified Holographic Dark Energy Model with Infrared Infinite Extra Dimension(s)

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). As a concrete example, we consider the Dvali-Gabadadze-Porrati (DGP) model and its generalization. We find that the DGP model is dual to the MHDE model in five dimensions, and the $Λ$CDM model is dual to the MHDE model in six dimensions. Fitting the MHDE model to the observational data, we obtain that $Ω_{m0}=0.269^{+0.030}_{-0.027}$, $Ω_{k0}=0.003^{+0.011}_{-0.012}$, and the number of the spatial dimensions is $N=4.78^{+0.68}_{-0.44}$. The best fit value of $N$ implies that there might exist two IR infinite extra dimensions.

hep-th↗

arXiv2009

Growth factor parametrization in curved space

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 $Ω_{m}^γ$ and then suggest a new form, $f_a=Ω_m^γ+(γ-4/7)Ω_k$, as an approximation for the growth factor when the curvature $Ω_k$ is not negligible, and where the growth index $γ$ is usually model dependent. The expression recovers the standard results for the curved and flat $Λ$CDM and Dvali-Gabadadze-Porrati models. Using the best fit values of $Ω_{m0}$ and $Ω_{k0}$ to the expansion/distance measurements from Type Ia supernovae, baryon acoustic oscillation, WMAP5, and $H(z)$ data, we fit the growth index parameter to current growth factor data and obtain $γ_Λ(Ω_{k} \not= 0) = 0.65^{+0.17}_{-0.15}$ and $γ_{DGP}(Ω_{k} \not= 0) = 0.53^{+0.14}_{-0.12}$. For the $Λ$CDM model, the 1-$σ$ observational bounds are found consistent with theoretical value, unlike the case for the Dvali-Gabadadze-Porrati model. We also find that the current data we used is not enough to put significant constraints when the 3 parameters in $f_a$ are fit simultaneously. Importantly, we find that, in the presence of curvature, the analytical expression proposed for $f_a$ provides a better fit to the growth factor than other forms and should be useful for future high precision missions and studies.

astro-ph.CO↗

arXiv2009

A quantitative criteria for the coincidence problem

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. Quantitatively, we find that the coincidence index C for the interacting Chaplygin gas model is smaller than that for the interacting quintessence and phantom models by six orders of magnitude.

astro-ph.CO↗

arXiv2009

Fixed points in interacting dark energy models

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.

gr-qc↗

arXiv2009

Phase-space analysis of interacting phantom cosmology

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. It seems that interacting phantom cosmology cannot fulfill the basic requirement that led to its construction.

gr-qc↗

arXiv2008

Growth factor parametrization and modified gravity

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 $γ$. We discuss the dependence of $γ$ on the matter energy density $Ω$ and its current value $Ω_0$ for a more accurate approximation of the growth factor. The observational data, including the data of the growth rate, are used to fit different models. The data strongly disfavor the Dvali-Gabadadze-Porrati model. For the dark energy model with a constant equation of state, we find that $Ω_0=0.27\pm 0.02$ and $w=-0.97\pm 0.09$. For the $Λ$CDM model, we find that $γ=0.64^{+0.17}_{-0.15}$. For the Dvali-Gabadadze-Porrati model, we find that $γ=0.55^{+0.14}_{-0.13}$.

astro-ph↗

arXiv2008

Non-existence of 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. We show that the infrared cutoff of the extended holographic dark energy model cannot be the Hubble horizon provided that the Brans-Dicke parameter $ω$ satisfies the experimental constraint $ω> 10^4$, and this is proved as a no-go theorem. The no-go theorem also applies to the case in which the dark matter interacts with the dark energy.

gr-qc↗

arXiv2008

Constraints on Dark Energy Models from Weak Gravity Conjecture

We study the constraints on the dark energy model with constant equation of state parameter $w=p/ρ$ and the holographic dark energy model by using the weak gravity conjecture. The combination of weak gravity conjecture and the observational data gives $w<-0.7$ at the $3σ$ confidence level. The holographic dark energy model realized by a scalar field is in swampland.

gr-qc↗

arXiv2008

Dark energy and cosmic curvature: Monte-Carlo Markov Chain approach

We use the Monte-Carlo Markov Chain method to explore the dark energy property and the cosmic curvature by fitting two popular dark energy parameterizations to the observational data. The new 182 gold supernova Ia data and the ESSENCE data both give good constraint on the DE parameters and the cosmic curvature for the dark energy model $w_0+w_a z/(1+z)$. The cosmic curvature is found to be $|Ω_k|\la 0.03$. For the dark energy model $w_0+w_a z/(1+z)^2$, the ESSENCE data gives better constraint on the cosmic curvature and we get $|Ω_k|\leq 0.02$.

astro-ph↗

arXiv2008

Cosmological constant and late transient acceleration of the universe in the Horava-Witten Heterotic M-Theory on S^{1}/Z_{2}

Orbifold branes are studied in the framework of the 11-dimensional Horava-Witten heterotic M-Theory. It is found that the effective cosmological constant can be easily lowered to its current observational value by the mechanism of large extra dimensions. The domination of this constant over the evolution of the universe is only temporarily. Due to the interaction of the bulk and the branes, the universe will be in its decelerating expansion phase again in the future, whereby all problems connected with a far future de Sitter universe are resolved.

hep-ph↗

arXiv2007

Current constraints on interacting holographic dark energy

Although there is mounting observational evidence that the cosmic expansion is undergoing a late-time acceleration, the physical mechanism behind such a phenomenon is yet unknown. In this paper, we investigate a holographic dark energy (HDE) model with interaction between the components of the dark sector in the light of current cosmological observations. We use both the new \emph{gold} sample of 182 type Ia supernovae (SNe Ia) and the 192 SNe Ia ESSENCE data, the baryon acoustic oscillation measurement from the Sloan Digital Sky Survey and the shift parameter from the three-year Wilkinson Microwave Anisotropy Probe data. In agreement with previous results, we show that these observations suggest a very weak coupling or even a noninteracting HDE. The phantom crossing behavior in the context of these scenarios is also briefly discussed.

astro-ph↗

arXiv2007

Friedmann Equations and Thermodynamics of Apparent Horizons

With the help of a masslike function which has dimension of energy and equals to the Misner-Sharp mass at the apparent horizon, we show that the first law of thermodynamics of the apparent horizon $dE=T_AdS_A$ can be derived from the Friedmann equation in various theories of gravity, including the Einstein, Lovelock, nonlinear, and scalar-tensor theories. This result strongly suggests that the relationship between the first law of thermodynamics of the apparent horizon and the Friedmann equation is not just a simple coincidence, but rather a more profound physical connection.

hep-th↗

arXiv2007

Dark energy and the future fate of the Universe

We consider the possibility of observing the onset of the late time inflation of our patch of the Universe. The Hubble size criterion and the event horizon criterion are applied to several dark energy models to discuss the problem of future inflation of the Universe. We find that the acceleration has not lasted long enough to confirm the onset of inflation by present observations for the dark energy model with constant equation of state, the holographic dark energy model and the generalized Chaplygin gas (GCG) model. For the flat $Λ$CDM model with $Ω_{m0}=0.3$, we find that if we use the Hubble size criterion, we need to wait until the $a_v$ which is the scale factor at the time when the onset of inflation is observed reaches 3.59 times of the scale factor $a_T$ when the Universe started acceleration, and we need to wait until $a_v=2.3 a_T$ to see the onset of inflation if we use the event horizon criterion. For the flat holographic dark energy model with $d=1$, we find that $a_v=3.46 a_T$ with the Hubble horizon and $a_v=2.34 a_T$ with the event horizon, respectively. For the flat GCG model with the best supernova fitting parameter $α=1.2$, we find that $a_v=5.50 a_T$ with the Hubble horizon and $a_v=2.08 a_T$ with the event horizon, respectively.

gr-qc↗

arXiv2007

Testing the viability of the interacting holographic dark energy model by using combined observational constraints

Using the data coming from the new 182 Gold type Ia supernova samples, the shift parameter of the Cosmic Microwave Background given by the three-year Wilkinson Microwave Anisotropy Probe observations, and the baryon acoustic oscillation measurement from the Sloan Digital Sky Survey, $H(z)$ and lookback time measurements, we have performed a statistical joint analysis of the interacting holographic dark energy model. Consistent parameter estimations show us that the interacting holographic dark energy model is a viable candidate to explain the observed acceleration of our universe.

astro-ph↗

arXiv2007

Energy conditions and current acceleration of the universe

The energy conditions provide a very promising model-independent study of the current acceleration of the universe. However, in order to connect these conditions with observations, one often needs first to integrate them, and then find the corresponding constraints on some observational variables, such as the distance modulus. Those integral forms can be misleading, and great caution is needed when one interprets them physically. A typical example is that the transition point of the deceleration parameter $q(z)$ is at about $z \simeq 0.76$ in the $Λ$CDM model. However, with the same model when we consider the dimensionless Hubble parameter $E(z)$, which involves the integration of $q(z)$, we find that $E(z)$ does not cross the line of $q(z) = 0$ before $z = 2$. Therefore, to get the correct result, we cannot use the latter to determine the transition point. With these in mind, we carefully study the constraints from the energy conditions, and find that, among other things, the current observational data indeed strongly indicate that our universe has ocne experienced an accelerating expansion phase between the epoch of galaxy formation and the present.

astro-ph↗

arXiv2007

Direct evidence of acceleration from distance modulus redshift graph

The energy conditions give upper bounds on the luminosity distance. We apply these upper bounds to the 192 essence supernova Ia data to show that the Universe had experienced accelerated expansion. This conclusion is drawn directly from the distance modulus-reshift graph. In addition to be a very simple method, this method is also totally independent of any cosmological model. From the degeneracy of the distance modulus at low redshift, we argue that the choice of $w_0$ for probing the property of dark energy is misleading. One explicit example is used to support this argument.

astro-ph↗

arXiv2007

The generalized second law of thermodynamics in the accelerating universe

We show that in the accelerating universe the generalized second law of thermodynamics holds only in the case where the enveloping surface is the apparent horizon, but not in the case of the event horizon. The present analysis relies on the most recent SNe Ia events, being model independent. Our study might suggest that event horizon is not a physical boundary from the point of view of thermodynamics.

gr-qc↗

arXiv2007

Thermodynamical properties of dark energy

We have investigated the thermodynamical properties of dark energy. Assuming that the dark energy temperature $T\sim a^{-n}$ and considering that the volume of the Universe enveloped by the apparent horizon relates to the temperature, we have derived the dark energy entropy. For dark energy with constant equation of state $w>-1$ and the generalized Chaplygin gas, the derived entropy can be positive and satisfy the entropy bound. The total entropy, including those of dark energy, the thermal radiation and the apparent horizon, satisfies the generalized second law of thermodynamics. However, for the phantom with constant equation of state, the positivity of entropy, the entropy bound, and the generalized second law cannot be satisfied simultaneously.

gr-qc↗