arXiv subjects

Zong-Hong Zhu

Publications and source records attributed to Zong-Hong Zhu.

At least 217 records · Page 12Linked to original sources

arXiv2008

Hybrid dark energy

Extending previous results [Phys. Rev. Lett. 97, 081301 (2006)], we explore the cosmological implications of a new quintessence scenario driven by a slow rolling homogeneous scalar field whose equation of state behaved as freezing over the entire cosmic evolution, is approaching -1 today, but will become thawing in the near future, thereby driving the Universe to an eternal deceleration. We argue that such a mixed behavior, named \emph{hybrid}, may reconcile the slight preference of current observational data for freezing potentials with the impossibility of defining observables in the String/M-theory context due to the existence of a cosmological event horizon in asymptotically de Sitter universes as, e.g., pure freezing scenarios.

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arXiv2008

Natural Phantom Dark Energy, Wiggling Hubble Parameter $H(z)$ and Direct $H(z)$ Data

Recent direct $H(z)$ data indicate that the parameter $H(z)$ may wiggle with respect to $z$. On the other hand the luminosity distance data of supernovae flatten the wiggles of $H(z)$ because of integration effect. It is expected that the fitting results can be very different in a model permitting a wiggling $H(z)$ because the data of supernovae is highly degenerated to such a model. As an example the natural phantom dark energy is investigated in this paper. The dynamical property of this model is studied. The model is fitted by the direct $H(z)$ data set and the SNLS data set, respectively. And the results are quite different, as expected. The quantum stability of this model is also shortly discussed. We find it is a viable model if we treat it as an effective theory truncated by an upperbound.

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arXiv2008

The optimal approach of detecting stochastic gravitational wave from string cosmology using multiple detectors

String cosmology models predict a relic background of gravitational wave produced during the dilaton-driven inflation. It's spectrum is most likely to be detected by ground gravitational wave laser interferometers (IFOs), like LIGO, Virgo, GEO, as the energy density grows rapidly with frequency. We show the certain ranges of the parameters that underlying string cosmology model using two approaches, associated with 5% false alarm and 95% detection rate. The result presents that the approach of combining multiple pairs of IFOs is better than the approach of directly combining the outputs of multiple IFOs for LIGOH, LIGOL, Virgo and GEO.

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arXiv2008

Testing the DGP model with gravitational lensing statistics

The self-accelerating braneworld model (DGP) seems to provide a simple alternative to the the standard $Λ$CDM cosmology to explain the current cosmic acceleration, which is strongly indicated by measurements of Type Ia supernovae, as well as other concordant observations. In this work, we investigate observational constraints on this scenario from gravitational lensing statistics using the Cosmic Lens All-Sky Survey (CLASS) lensing sample. We show that a large parameter space of the DGP model is in good agreement with this radio source gravitational lensing sample. In the flat case, $Ω_\mathrm{K}=0$, the likelihood is maximized, ${\cal L}={\cal L_\mathrm{max}}$, for $\1 = 0.30_{-0.11}^{+0.19}$. If we relax the prior on $Ω_\mathrm{K}$, the likelihood peaks at $\{\1,\2 \} \simeq \{0.29, 0.12\}$, just slightly in the region of open models. However the confidence contours are pretty elongated so that we can not discard either close or flat or open models

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arXiv2007

Reconstructing f(R) theory according to holographic dark energy

In this paper a connection between the holographic dark energy model and the $f(R)$ theory is established. We treat the $f(R)$ theory as an effective description for the holographic dark energy and reconstruct the function $f(R)$ with the parameter $c>1$, $c=1$ and $c<1$, respectively. We show the distinctive behavior of each cases realized in $f(R)$ theory, especially for the future evolution.

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arXiv2007

Testing power-law cosmology with galaxy clusters

Power-law cosmologies, in which the cosmological scale factor evolves as a power law in the age, $a \propto t^α$ with $α\ga 1$, regardless of the matter content or cosmological epoch, is comfortably concordant with a host of cosmological observations.} {In this article, we use recent measurements of the X-ray gas mass fractions in clusters of galaxies to constrain the $α$ parameter with curvature $k = \pm1, 0$. We find that the best fit happens for an open scenario with the power index $α= 1.14 \pm 0.05$, though the flat and closed model can not be rule out at very high confidence level.} {Our results are in agreement with other recent analyses and show that the X-ray gas mass fraction measurements in clusters of galaxies provide a complementary test to the power law cosmology.

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arXiv2007

Dynamics of holographic vacuum energy in the DGP model

We consider the evolution of the vacuum energy in the DGP model according to the holographic principle under the assumption that the relation linking the IR and UV cut-offs still holds in this scenario. The model is studied when the IR cut-off is chosen to be the Hubble scale $H^{-1}$, the particle horizon $R_{\rm ph}$ and the future event horizon $R_{\rm eh}$, respectively. And the two branches of the DGP model are also taken into account. Through numerical analysis, we find that in the cases of $H^{-1}$ in the (+) branch and $R_{\rm eh}$ in both branches, the vacuum energy can play the role of dark energy. Moreover, when considering the combination of the vacuum energy and the 5D gravity effect in both branches, the equation of state of the effective dark energy may cross -1, which may lead to the Big Rip singularity. Besides, we constrain the model with the Type Ia supernovae and baryon oscillation data and find that our model is consistent with current data within $1σ$, and that the observations prefer either a pure holographic dark energy or a pure DGP model

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arXiv2007

Constraining bouncing cosmology caused by Casimir effect

In our paper we constrain Friedman Robertson Walker (FRW) model with ``radiation-like'' contribution to the Friedmann equation against the astronomical data. We analyze the observational limitations on a $(1+z)^4$ term from SNIa data, FRIIb radio galaxy (RG) data, baryon oscillation peak and CMBR observations. We argue that it is not possible to determine the energy densities of individual components scaling like radiation from a kinematic astronomical test. The bounds for density parameter for total radiation-like term can be obtained. We find different interpretations of the presence of scaling like radiation term: FRW universe filled with a massless scalar field in a quantum regime (Casimir effect), Friedmann-Robertson-Walker (FRW) model in a semi-classical approximation of loop quantum gravity, FRW model in the Randall Sundrum scenario with dark radiation or cosmological model with global rotation. In this paper we mainly concentrate on Casimir effect arising from quantum effects of the scalar field. This contribution can describe decaying part of cosmological constant. We discuss the back reaction of gravity on Casimir-type force which is a manifestation of the vacuum fluctuations of the quantum scalar field at low temperature. It is shown that while the Casimir energy gives rise to accelerating Universe, the cosmological constant term is still required. We argue that a small negative contribution of a radiation-like term can reconcile the tension between the observed primordial ${}^4He$ and $D$ abundance. Moreover the presence of such contribution can also remove the disagreement between Hubble parameter $H_0$ values obtained from both SNIa and Wilkinson Microwave Anisotropy Probe (WMAP) satellite data.

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arXiv2007

Exploring holographic dark energy model with Sandage-Loeb test

Taking into account that Sandage-Loeb test is unique in its coverage of the redshift desert and available in the near future, we explore the cosmic time evolution behavior of the source redshift for holographic dark energy model, an important competing cosmological model. As a result, we find that Sandage-Loeb test can provide a extremely strong bound on $Ω^0_m$, while its constraint on another dimensionless parameter $λ$ is weak. In addition, it is proposed here for the first time that we can also constrain various cosmological model by measuring the value of $z_{max}$ at which the peak of redshift velocity occurs. Combining this new proposed method with the traditional Sandage-Loeb test, we should be able to provide a better constraint on $λ$, at least from the theoretical perspective.

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arXiv2007

Limits from Weak Gravity Conjecture on Dark Energy Models

The weak gravity conjecture has been proposed as a criterion to distinguish the landscape from the swampland in string theory. As an application in cosmology of this conjecture, we use it to impose theoretical constraint on parameters of two types of dark energy models. Our analysis indicates that the Chaplygin-gas-type models realized in quintessence field are in the swampland, whereas the $a$ power-low decay model of the variable cosmological constant can be viable but the parameters are tightly constrained by the conjecture.

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arXiv2007

Observational Constraints on Agegraphic Dark Energy

In this paper, we use the Type Ia supernova data as well as the CMB and LSS data to constrain the agegraphic dark energy model recently proposed by Cai. Due to its peculiar nature, the parameter $n$ of this model cannot be well constrained by the SNIa data, while the other parameter $Ω_{m0}$ can be constrained to be $0.34\pm0.04$. When combined with CMB and LSS data, the range of $1σ$ confidence level for $n$ is greatly narrowed, albeit still very large. The best fit result is $Ω_{m0}=0.28\pm0.02$, which is consistent with most observations like WMAP and SDSS, and $n=3.4$, of which a meaningful range of confidence level can not be obtained due to the fact that the contours are not closed. Despite of this result, we conclude that for $n>1$ this model is consistent with SNIa, CMB and LSS observations. Furthermore, the fitting results indicate a generalized definition for the agegraphic dark energy.

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arXiv2007

Measuring Dark Energy with Gamma-Ray Bursts and Other Cosmological Probes

It has been widely shown that the cosmological parameters and dark energy can be constrained by using data from type-Ia supernovae (SNe Ia), the cosmic microwave background (CMB) anisotropy, the baryon acoustic oscillation (BAO) peak from Sloan Digital Sky Survey (SDSS), the X-ray gas mass fraction in clusters, and the linear growth rate of perturbations at z=0.15 as obtained from the 2dF Galaxy Redshift Survey. Recently, gamma-ray bursts (GRBs) have also been argued to be promising standard candles for cosmography. In this paper, we present constraints on the cosmological parameters and dark energy by combining a recent GRB sample including 69 events with the other cosmological probes. First, we find that for the LambdaCDM cosmology this combination makes the constraints stringent and the best fit is close to the flat universe. Second, we fit the flat Cardassian expansion model and find that this model is consistent with the LambdaCDM cosmology. Third, we present constraints on several two-parameter dark energy models and find that these models are also consistent with the LambdaCDM cosmology. Finally, we reconstruct the dark energy equation-of-state parameter w(z) and the deceleration parameter q(z). We see that the acceleration could have started at a redshift from z_T=0.40_{-0.08}^{+0.14} to z_T=0.65_{-0.05}^{+0.10}. This difference in the transition redshift is due to different dark energy models that we adopt. The most stringent constraint on w(z) lies in the redshift range z\sim 0.3-0.6.

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arXiv2006

Probing for dynamics of dark energy and curvature of universe with latest cosmological observations

We use the newly released 182 Type Ia supernova data combined with the third-year Wilkinson Microwave Anisotropic Probe data (WMAP3) and large scale structure (LSS) information including SDSS and 2dFGRS to constrain the dark energy equation of state (EoS) as well as the curvature of universe $Ω_K$. Using the full dataset of Cosmic Microwave Background (CMB) and LSS rather than the shift parameter and linear growth factor, we make a Markov Chain Monte Carlo (MCMC) global fit, while paying particular attention to the dark energy perturbation. Parameterizing the EoS as $w_{DE}(a) = w_{0} + w_{1}(1-a)$, we find the best fit of ($w_0,w_1$) is ($-1.053,0.944$) and for $w_{DE}(a) = w_{0} + w_{1}\sin({3/2}π\ln(a))$, the best fit for ($w_0,w_1$) is ($-1.614,-1.046$). We find that a flat universe is a good approximation, namely, $|Ω_K|>0.06$ has been excluded by 2$σ$ yet the inclusion of $Ω_K$ can affect the measurement of DE parameters owing to their correlation and the present systematic effects of SNIa measurements.

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arXiv2006

Curvaton reheating mechanism in inflation on warped Dvali-Gabadadze-Porrati brane

An impressed feature of inflation on warped Dvali-Gabadadze-Porrati (DGP) brane is that the inflationary phase exits spontaneously for a scalar inflaton field with exponential potential, which presents a graceful exit mechanism for the inflation. But its reheating mechanism leaves open. We investigate the curvaton reheating in inflation on warped DGP brane model. The reheating may occur in effctively 5 dimensional or 4 dimensional stage. We study the permitted parameter space of the curvaton field in detail. We demonstrate how the inflation model of the warped DGP brane is improved by the curvaton mechanism.

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arXiv2006

Lookback time as a test for brane cosmology

The observed late-time acceleration of the Universe may be the result of unknown physical processes involving either modifications of gravitation theory or the existence of new fields in high energy physics. In the former case, such modifications are usually related to the possible existence of extra dimensions (which is also required by unification theories), giving rise to the so-called brane cosmology. In this paper we investigate the viability of this idea by considering a particular class of brane scenarios in which a large scale modification of gravity arises due to a gravitational \emph{leakage} into extra dimensions. To this end, differently from other recent analyses, we combine \emph{orthogonal} age and distance measurements at intermediary and high redshifts. We use observations of the lookback time to galaxy clusters, indirect estimates of the age of the Universe from the most recent Large-Scale Structure (LSS) and Cosmic Microwave Background (CMB) data, along with the recent detection of the baryon acoustic oscillations at $z = 0.35$. In agreement with other recent analyses we show that, although compatible with these age and distance measurements, a spatially closed scenario is largely favoured by the current observational data. By restricting our analysis to a spatially flat universe, we also find that the standard $Λ$CDM model is favoured over the particular braneworld scenario here investigated.

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arXiv2006

Constraints on the DGP Model from Recent Supernova Observations and Baryon Acoustic Oscillations

Although there is mounting observational evidence that the expansion of our universe is undergoing a late-time acceleration, the mechanism for this acceleration is yet unknown. In the so-called Dvali-Gabadadze-Porrati (DGP) model this phenomena is attributed to gravitational leakage into extra dimensions. In this work, we mainly focus our attention to the constraints on the model from the gold sample of type Ia supernovae (SNeIa), the first year data from the Supernova Legacy Survey (SNLS) and the baryon acoustic oscillation (BAO) peak found in the Sloan Digital Sky Survey (SDSS). At 99.73% confidence level, the combination of the three databases provides Omega_m=0.270^{+0.018}_{-0.017} and Omega_{r_c}=0.216^{+0.012}_{-0.013} (hence a spatially closed universe with Omega_k=-0.350^{+0.080}_{-0.083}), which seems to be in contradiction with the most recent WMAP results indicating a flat universe. Based on this result, we also estimated the transition redshift (at which the universe switches from deceleration to acceleration) to be 0.70 < z_{q=0} < 1.01, at 2 sigma confidence level.

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arXiv2006

Interacting Chaplygin gas

We investigate a kind of interacting Chaplygin gas model in which the Chaplygin gas plays the role of dark energy and interacts with cold dark matter particles. We find that there exists a stable scaling solution at late times with the Universe evolving into a phase of steady state. Furthermore, the effective equation of state of Chaplygin gas may cross the so-called phantom divide $w=-1$. The above results are derived from continuity equations, which means that they are independent of any gravity theories. Assuming standard general relativity and a spatially flat FRW metric, we also find the deceleration parameter is well consistent with current observations.

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arXiv2004

Complementary Constraints on Brane Cosmology

The acceleration of the expansion of the universe represents one of the major challenges to our current understanding of fundamental physics. In principle, to explain this phenomenon, at least two different routes may be followed: either adjusting the energy content of the Universe -- by introducing a negative-pressure dark energy -- or modifying gravity at very large scales -- by introducing new spatial dimensions, an idea also required by unification theories. In the cosmological context, the role of such extra dimensions as the source of the dark pressure responsable for the acceleration of our Universe is translated into the so-called brane world (BW) cosmologies. Here we study complementary constraints on a particular class of BW scenarios in which the modification of gravity arises due to a gravitational \emph{leakage} into extra dimensions. To this end, we use the most recent Chandra measurements of the X-ray gas mass fraction in galaxy clusters, the WMAP determinations of the baryon density parameter, measurements of the Hubble parameter from the \emph{HST}, and the current supernova data. In agreement with other recent results, it is shown that these models provide a good description for these complementary data, although a closed scenario is always favored in the joint analysis. We emphasize that observational tests of BW scenarios constitute a natural verification of the role of possible extra dimensions in both fundamental physics and cosmology.

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