arXiv subjects
Publications and source records attributed to Yungui Gong.
We study the possibility to probe the spatial geometry of the Universe by supernova measurement of the cubic correction to the luminosity distance. We illustrate with an accelerating universe model with infinite-volume extra dimensions, for which the 1$σ$ level supernova results indicate that the Universe is closed.
The generalized Chaplygin gas model with parameter space $α>-1$ is studied in this paper. Some reasonable physical constraints are added to justify the use of the larger parameter space. The Type Ia supernova data and age data of some clusters are then used to fit the model. We find that the parameters have bimodal distributions. For the generalized Chaplygin gas model, we also find that less free parameters fit the data better. The best fit model is the spatially flat model with baryons. The best fit parameters are: $Ω_{m0}=0.044$, $w_{c0}=-0.881$ and $α=1.57$. The transition redshift is $z_T=0.395$.
The idea of relating the infrared and ultraviolet cutoffs is applied to Brans-Dicke theory of gravitation. We find that extended holographic dark energy from the Hubble scale or the particle horizon as the infrared cutoff will not give accelerating expansion. The dynamical cosmological constant with the event horizon as the infrared cutoff is a viable dark energy model.
In this paper, we use the type Ia supernova data to constrain the model of holographic dark energy. For $d=1$, the best fit result is $Ω^0_m = 0.25$, the equation of the state of the holographic dark energy $w^0_\d = -0.91$ and the transition between the decelerating expansion and the accelerating expansion happened when the cosmological red-shift was $z_T=0.72$. If we set $d$ as a free parameter, the best fit results are $d=0.21$, $Ω_m^0=0.46$, $w_\d^0=-2.67$, which sounds like a phantom today, and the transition redshift is $z_T=0.28$.
The recent observations of type Ia supernovae strongly support that the universe is accelerating now and decelerated in the recent past. This may be the evidence of the breakdown of the standard Friedmann equation. Instead of a linear function of the matter density, we consider a general function of the matter density to modify the Freidmann equation. We propose a new model which explains the recent acceleration and the past deceleration. Furthermore, the new model also gives a decelerated universe in the future. The new model gives $Ω_{m0}=0.46$ and $z_T=0.44$.
The recent observations of type Ia supernovae suggest that the universe is accelerating now and decelerated in the recent past. This may be the evidence of the breakdown of the standard Friedmann equation. The Friedmann equation $H^2\sim ρ$ is modified to be a general form $H^2=g(ρ)$. Three models with particular form of $g(ρ)$ are considered in detail. The supernova data published by \citet{tonry}, \citet{ddsg} and \citet{raknop03} are used to analyze the models. After the best fit parameters are obtained, we then find out the transition redshift $z_{\rm T}$ when the universe switched from the deceleration phase to the acceleration phase.
The recent observations of type Ia supernovae strongly support that the universe is accelerating now and decelerated in the recent past. By assuming a general relation between the quintessence potential and the quintessence kinetic energy, a general relation is found between the quintessence energy density and the scale factor. The potential includes both the hyperbolic and the double exponential potentials. A detailed analysis of the transition from the deceleration phase to the acceleration phase is then performed. We show that the current constraints on the transition time, the equation of state and the energy density of the quintessence field are satisfied in the model.
Brane cosmology takes the unconventional form $H\sim \sqrtρ$. To recover the standard cosmology, we have to assume that the matter density is much less than the brane tension. We show that the assumption can be justified even near the end of inflation if we fine-tune the coupling constant of the inflaton potential. As a consequence, the standard cosmology is recovered after inflation.
We discuss the physical equivalence between the Einstein and Jordan frames in Brans-Dicke theory. The inequivalence of conformal transformed theories is clarified with the help of an old equivalence theorem of Chrisholm's.
We find that the mixture of Ramond-Ramond fields and Neveu-Schwarz two form are transformed as Majorana spinors under the T-duality group $O(d,d)$. The Ramond-Ramond field transformation under the group $O(d,d)$ is realized in a simple form by using the spinor representation. The Ramond-Ramond field transformation rule obtained by Bergshoeff et al. is shown as a specific simple example. We also give some explicit examples of the spinor representation.
The holographic bound in Brans-Dicke $k=1$ matter dominated Cosmology is discussed. In this talk, both the apparent horizon and the particle horizon are taken for the holographic bound. The covariant entropy conjecture proposed by Bousso is also discussed.
We apply the holographic principle to the Brans-Dicke cosmology. We analyze the holographic bound in both the Jordan and Einstein frames. The holographic bound is satisfied for both the k=0 and k=-1 universe, but it is violated for the k=1 matter dominated universe.
We studied the Einstein-Brans-Dicke cosmology in detail. The difference of the evolution of the universe is significant between Einstein-Brans-Dicke cosmology and standard big-bang model during the radiation-dominated era. The power-law evolution of the scale factor is fast enough to solve the cosmological puzzles and slow enough to avoid the graceful exit problem. However, the constraints from the satisfactory bubble distribution (beta^2>0.25) and the solar system observations (beta^2<0.002) are mutually exclusive. This suggests that this kind of inflationary model is ruled out. We also clarify the distinction between Einstein frame and Jordan frame in Brans-Dicke theory.
The density perturbation during inflation seeds the large scale structure. We consider both new inflation-type and chaotic inflation-type potentials in the framework of Einstein-Brans-Dicke gravity. The density perturbation gives strong constraints on the parameters in these potentials. For both potentials, the constraints are not much different from those obtained in the original inflationary models by using of Einstein gravity.