We explore the cosmological implications of five modified gravity(MG) models by using the recent cosmological observational data,including the recently released SNLS3 type Ia supernovae sample,the cosmic microwave background anisotropy data from the Wilkinson Microwave Anisotropy Probe 7-yr observations,the baryon acoustic oscillation results from the Sloan Digital Sky Survey data release 7,and the latest Hubble constant measurement utilizing the Wide Field Camera 3 on the Hubble Space Telescope.The MG models considered include the Dvali-Gabadadze-Porrati(DGP) model,two f(R) models,and two f(T) models.We find that compared with the ΛCDM model,MG models can not lead to an appreciable reduction of the χ 2 min.The analysis of AIC and BIC shows that the simplest cosmological constant model(ΛCDM) is still the most preferred by the current data,and the DGP model is strongly disfavored.In addition,from the observational constraints,we also reconstruct the evolutions of the growth factor in these models.We find that the current available growth factor data are not enough to distinguish these MG models from the ΛCDM model.
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.
The problem of dark energy is briefly reviewed in both theoretical and observational aspects. In the theoretical aspect, dark energy scenarios are classified into symmetry, anthropie principle, tuning mechanism, modified gravity, quantum cosmology, holographic principle, back-reaction and phenomenologieal types. In the observational aspect, we introduce cosmic probes, dark energy related projects, observational constraints on theoretical models and model independent reconstructions.
