Effects of a Time-Varying Color-Luminosity Parameter β on the Cosmological Constraints of Modified Gravity Models

It has been found that, for the Supernova Legacy Survey three-year(SNLS3) data, there is strong evidence for the redshift-evolution of color-luminosity parameter β. In previous studies, only dark energy(DE) models are used to explore the effects of a time-varying β on parameter estimation. In this paper, we extend the discussions to the case of modified gravity(MG), by considering Dvali–Gabadadze–Porrati(DGP) model, power-law type f(T) model and exponential type f(T) model. In addition to the SNLS3 data, we also use the latest Planck distance priors data,the galaxy clustering(GC) data extracted from Sloan Digital Sky Survey(SDSS) data release 7(DR7) and Baryon Oscillation Spectroscopic Survey(BOSS), as well as the direct measurement of Hubble constant H0 from the Hubble Space Telescope(HST) observation. We find that, for both cases of using the supernova(SN) data alone and using the combination of all data, adding a parameter of β can reduce χ2by ～ 36 for all the MG models, showing that a constantβ is ruled out at 6σ confidence level(CL). Moreover, we find that a time-varying β always yields a larger fractional matter density Ωm0and a smaller reduced Hubble constant h; in addition, it significantly changes the shapes of 1σ and2σ confidence regions of various MG models, and thus corrects systematic bias for the parameter estimation. These conclusions are consistent with the results of DE models, showing that β's evolution is completely independent of the cosmological models in the background. Therefore, our work highlights the importance of considering the evolution of βin the cosmology-fits.

Probing the interaction between dark energy and dark matter with the parametrized post-Friedmann approach

In the current universe,the dominant energy components are dark energy and dark matter.There is a longstanding conjecture that there might be some direct coupling between dark energy and dark matter(for a recent review,see ref.[1]).The advantages for considering such a possibility include,for example,those that it can alleviate the'cosmic coincidence'puzzle,can avoid the'big rip'in a phantom scenario,and so on.We call the scenario in which dark energy directly interacts with dark matter the'interacting dark energy'(IDE)scenario.Actually,besides the above reasons of theoretical aspect,one should be more concerned with the observational issue:How can we detect this interaction(this is essentially a kind of'fifth force')or falsify this scenario by using the observations?This requires us to be able to calculate how it affects the cosmological evolution,including both aspects