We explore the ultimate fate of the Universe by using a divergence-free parametrization for dark energy w(z)=w0+wa[ln(2+z)/(1+z)-ln2]. Unlike the Chevallier-Polarski-Linder parametrization, this parametrization has well behaved, bounded behavior for both high redshifts and negative redshifts, and thus can genuinely cover many theoretical dark energy models. After constraining the parameter space of this parametrization by using the current cosmological observations, we find that, at the 95.4% confidence level, our Universe can still exist at least 16.7 Gyr before it ends in a big rip. Moreover, for the phantom energy dominated Universe, we find that a gravitationally bound system will be destroyed at a time t≌P(2|1+3w(-1)|)～1/2/[6π|1+w(-1)|], where P is the period of a circular orbit around this system, before the big rip.
LI XiaoDongWANG ShuangHUANG QingGuoZHANG XinLI Miao
I show the formulation of de Sitter Special Relativity (dS-SR) based on Dirac-Lu-Zou-Guo's discussions. dS-SR quantum mechanics is formulated, and the dS-SR Dirac equation for hydrogen is suggested. The equation in the earth-QSO framework reference is solved by means of the adiabatic approach. It's found that the fine-structure 'constant' α in dS-SR varies with time. By means of the t-z relation of the ΛCDM model, α's time-dependency becomes redshift z-dependent. The dS-SR's predictions of △α/α agree with data of spectra of 143 quasar absorption systems, the dS-space-time symmetry is SO(3, 2) (i.e., anti-dS group) and the universal parameter R (de Sitter ratio) in dS-SR is estimated to be R ≈ 2.73×1012 ly. The effects of dS-SR become visible at the cosmic space-time scale (i.e., the distance 109 ly). At that scale, dS-SR is more reliable than Einstein SR. The α-variation with time is evidence of SR with de Sitter symmetry.
We compare some popular dark energy models under the assumption of a flat universe by using the latest observational data including the type Ia supernovae Constitution compilation,the baryon acoustic oscillation measurement from the Sloan Digital Sky Survey,the cosmic microwave background measurement given by the seven-year Wilkinson Microwave Anisotropy Probe observations and the determination of H0 from the Hubble Space Telescope.Model comparison statistics such as the Bayesian and Akaike information criteria are applied to assess the worth of the models.These statistics favor models that give a good fit with fewer parameters.Based on this analysis,we find that the simplest cosmological constant model that has only one free parameter is still preferred by the current data.For other dynamical dark energy models,we find that some of them,such as the αdark energy,constant w,generalized Chaplygin gas,Chevalliear-Polarski-Linder parametrization,and holographic dark energy models,can provide good fits to the current data,and three of them,namely,the Ricci dark energy,agegraphic dark energy,and Dvali-Gabadadze-Porrati models,are clearly disfavored by the data.
LI Miao1,2,LI XiaoDong2,3 & ZHANG Xin1,4 1Kavli Institute for Theoretical Physics China,Chinese Academy of Sciences,Beijing 100190,China
Extending the holographic program of our previous work,we derive f(R) gravity and the Maxwell equations from the holographic principle,using time-like holographic screens.We find that to derive the Einstein equations and f(R) gravity by a natural holographic approach,the quasi-static condition is necessary.We also find the surface stress tensor and the surface electric current,surface magnetic current on a holographic screen for f(R) gravity and Maxwell's theory,respectively.
We propose a new holographic program of gravity in which we introduce a surface stress tensor.Our proposal differs from Verlinde's in several aspects.First,we use an open or a closed screen.Second,the temperature is not necessary,but a surface energy density and pressure are introduced.The surface stress tensor is proportional to the extrinsic curvature.Third,the energy we use is Brown-York energy and the equipartition theorem is violated by a non-vanishing surface pressure.We discuss holographic thermodynamics of a gas of weak gravity and find a chemical potential,and then show that Verlinde's program does not lead to reasonable thermodynamics.The holographic entropy is similar to the Bekenstein entropy bound.
Taking into account the noise from intrinsic ellipticities of source galaxies,we study the efficiency and completeness of halo detections from weak lensing convergence maps.Particularly,with numerical simulations,we compare the Gaussian filter with the so called MRLens treatment based on the modification of the Maximum Entropy Method.For a pure noise field without lensing signals,a Gaussian smoothing results in a residual noise field that is approximately Gaussian in terms of statistics if a large enough number of galaxies are included in the smoothing window.On the other hand,the noise field after the MRLens treatment is significantly non-Gaussian,resulting in complications in characterizing the noise effects.Considering weak-lensing cluster detections,although the MRLens treatment effectively deletes false peaks arising from noise,it removes the real peaks heavily due to its inability to distinguish real signals with relatively low amplitudes from noise in its restoration process.The higher the noise level is,the larger the removal effects are for the real peaks.For a survey with a source density ng-30 arcmin-2,the number of peaks found in an area of 3×3 deg2 after MRLens filtering is only-50 for the detection threshold κ = 0.02,while the number of halos with M > 5 × 1013 M and with redshift z ≤ 2 in the same area is expected to be-530.For the Gaussian smoothing treatment,the number of detections is-260,much larger than that of the MRLens.The Gaussianity of the noise statistics in the Gaussian smoothing case adds further advantages for this method to circumvent the problem of the relatively low efficiency in weak-lensing cluster detections.Therefore,in studies aiming to construct large cluster samples from weak-lensing surveys,the Gaussian smoothing method performs significantly better than the MRLens treatment.
