Zong-Hong Zhu

Generalized Chaplygin gas as a unified scenario of dark matter/energy: Observational constraints

Although various cosmological observations congruously suggest that our universe is dominated by two dark com- ponents, the cold dark matter without pressure and the dark energy with negative pressure, the nature and origin of these components is yet unknow. The generalized Chaplygin gas (gCg), parametrized by an equation of state, p = −A/ρ α ,w as recently proposed to be a candidate of the unified dark matter/energy (UDME) scenarios. In this work, we investigate some observational constraints on it. We mainly focus our attention on the constraints from recent measurements of the X-ray gas mass fractions in clusters of galaxies published by Allen et al. (2002, MNRAS, 334, L11; 2003, 342, 257) and the dimensionless coordinate distances to type Ia supernovae and Fanaroff-Riley type IIb radio galaxies compiled by Daly & Djorgovski (2003, ApJ, 597, 9). We obtain the confidence region on the two parameters fully characterizing gCg, As ≡ A/ρ (1+α) gCg0 and α, from a combined analysis of these databases, where ρgCg0 is the energy density of gCg at present. It is found that As = 0.70 +0.16 −0.17 and α = −0.09 +0.54 −0.33 , at a 95% confidence level, which is consistent within the errors with the standard dark matter + dark energy model, i.e., the case of α = 0. Particularly, the standard Chaplygin gas (α = 1) is ruled out as a feasible UDME by the data at a 99% confidence level.

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

We use the newly released 182 type la 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 Omega(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 pi 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, \Omega(K)\ > 0.06 has been excluded by 2 sigma yet the inclusion of Omega(K) can affect the measurement of DE parameters owing to their correlation and the present systematic effects of SNIa measurements. (c) 2007 Elsevier B.V. All rights reserved.

Constraints on Cardassian Expansion from Distant Type Ia Supernovae

The distant Type Ia supernovae data compiled by Perlmutter et al. are used to analyze the Cardassian expansion scenario, which was recently proposed by Freese & Lewis as an alternative to a cosmological constant (or more generally a dark energy component) in explaining the currently accelerating universe. We show that the allowed intervals for n and zeq, the two parameters of the Cardassian model, will give rise to a universe with a very low matter density, which can hardly be reconciled with the current value derived from the measurements of the cosmic microwave background anisotropy and galaxy clusters (cluster baryon fraction). As a result, this Cardassian expansion proposal does not seem to survive the magnitude-redshift test for the present Type Ia supernovae data, unless the universe contains primarily baryonic matter.

MODIFIED GRAVITY EMERGING FROM THERMODYNAMICS AND HOLOGRAPHIC PRINCIPLE

A new conception is proposed in [E. P. Verlinde, arXiv:1001.0785 [hep-th] and [T. Padmanabhan, Mod. Phys. Lett. A25 (2010) 1129] that gravity is one kind of entropic force. In this letter, we try to discuss its applications to the modified gravities by using three different corrections to the area law of entropy which are derived from the quantum effects and extra dimensions. According to the assumption of holographic principle, the number of bits N which is related to the equipartition law of energy is modified. Then, the modified law of Newtons gravity and the modified Friedmann equations are obtained by using the new notion. By choosing suitable parameters, the modified area law of entropy leads to de Sitter solutions which can be used to explain the accelerating expansion of our universe. It suggests that the accelerating phase in our universe may be an emergent phenomenon based on holographic principle and thermodynamics.

Constraints on cosmological models from strong gravitational lensing systems

Strong lensing has developed into an important astrophysical tool for probing both cosmology and galaxies (their structure, formation, and evolution). Using the gravitational lensing theory and cluster mass distribution model, we try to collect a relatively complete observational data concerning the Hubble constant independent ratio between two angular diameter distances Dds/Ds from various large systematic gravitational lens surveys and lensing by galaxy clusters combined with X-ray observations, and check the possibility to use it in the future as complementary to other cosmological probes. On one hand, strongly gravitationally lensed quasar-galaxy systems create such a new opportunity by combining stellar kinematics (central velocity dispersion measurements) with lensing geometry (Einstein radius determination from position of images). We apply such a method to a combined gravitational lens data set including 70 data points from Sloan Lens ACS (SLACS) and Lens Structure and Dynamics survey (LSD). On the other hand, a new sample of 10 lensing galaxy clusters with redshifts ranging from 0.1 to 0.6 carefully selected from strong gravitational lensing systems with both X-ray satellite observations and optical giant luminous arcs, is also used to constrain three dark energy models (ΛCDM, constant w and CPL) under a flat universe assumption. For the full sample (n = 80) and the restricted sample (n = 46) including 36 two-image lenses and 10 strong lensing arcs, we obtain relatively good fitting values of basic cosmological parameters, which generally agree with the results already known in the literature. This results encourages further development of this method and its use on larger samples obtained in the future.

Cardassian Expansion: Constraints from Compact Radio Source Angular Size versus Redshift Data

The Cardassian Expansion Scenario was recently proposed by Freese & Lewis as an alternative to a cosmological constant in explaining the current accelerating universe. In this paper we investigate observational constraints on this scenario from recent measurements of the angular size of high-z compact radio sources compiled by Gurvits and coworkers. We show that the allowed intervals for n and zeq, the two parameters of the Cardassian model, are heavily dependent on the value of the mean projected linear size l. However, the best fit to the current angular size data prefers the conventional flat ? cold dark matter model to this Cardassian expansion proposal, though the latter is cosmologically credible and compatible with the ?-z diagram for some values of l.

Reconstructing f(R) theory according to holographic dark energy

Abstract In this Letter 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.

Measurement of reaction cross section for proton-rich nuclei ( A< 30) at intermediate energies

Radioactive ion beams were produced through the projectile fragmentation induced by 69 MeV/nucleon Ar-36 primary beam on a Be-9\ target. Measurements of reaction cross sections (sigma(R)s) for 44 nuclei with A < 30 (mostly proton-rich), on carbon were performed on RIBLL (Radioactive Ion Beam Line in Lanzhou) of HIRFL (Heavy Ion Research Facility in Lanzhou) at intermediate energies around 30 MeV/nucleon by a transmission method. The experimental sigma(R) values for Al-23 and P-27 are abnormally large compared with their neighboring nuclei. Together with the previous experimental facts such as the binding energy and ground state data, it suggests anomalously large matter root-mean-square radii and proton halo structure in Al-23 and in P-27. There is an enhancement for the sigma(R) of F-17 + C-12 compared with the neighboring isotopes. Considering that the ground state of F-17 is 1d(5/2), this can indicate that there is a proton skin in F-17. The calculation of relativistic density-dependent Hartree model (RDDH) shows that the nuclei Al-23 and P-27 may have proton-halo structure and F-17 may have proton-skin structure. The significance of these measurements is discussed

Observational Constraints on Cosmology from the Modified Friedmann Equation

Recent measurements of Type Ia supernovae, as well as other concordant observations, suggest that the expansion of our universe is accelerating. A dark energy component has usually been invoked as the most feasible mechanism for the acceleration. However, effects arising from possible extra dimensions can mimic dark energy through a modified Friedmann equation. In this work, we investigate some observational constraints on a scenario in which this modification is given by H2 = (8πG/3)(ρ + Cρn). We mainly focus our attention on the constraints from recent measurements of the dimensionless coordinate distances to Type Ia supernovae and Fanaroff-Riley Type IIb radio galaxies compiled by Daly & Djorgovski and the X-ray gas mass fractions in clusters of galaxies published by Allen et al. We obtain the confidence region on the power index n and the density parameter Ωm of the universe from a combined analysis of these databases. We find that n = 0.06 and Ωm = 0.30, at the 95.4% confidence level, which is consistent within the errors with the standard ΛCDM model. These parameter ranges give a universe whose expansion switches from deceleration to acceleration at a redshift between 0.52 to 0.73.

Is there evidence for dark energy evolution

Recently, Sahni, Shafielo o & Starobinsky (2014) combined two independent measurements of