More Evidence for the Redshift Dependence of Color from the JLA Supernova Sample Using Redshift Tomography

In this work, by applying the redshift tomography method to Joint Light-curve Analysis (JLA) supernova sample, we explore the possible redshift-dependence of stretch-luminosity parameter $α$ and color-luminosity parameter $β$. The basic idea is to divide the JLA sample into different redshift bins, assuming that $α$ and $β$ are piecewise constants. Then, by constraining the $Λ$CDM model, we check the consistency of cosmology-fit results given by the SN sample of each redshift bin. We also adopt the same technique to explore the possible evolution of $β$ in various subsamples of JLA. Using the full JLA data, we find that $α$ is always consistent with a constant. In contrast, at high redshift $β$ has a significant trend of decreasing, at $\sim 3.5σ$ confidence level (CL). Moreover, we find that low-$z$ subsample favors a constant $β$; in contrast, SDSS and SNLS subsamples favor a decreasing $β$ at 2$σ$ and $3.3σ$ CL, respectively. Besides, by using a binned parameterization of $β$, we study the impacts of $β$'s evolution on parameter estimation. We find that compared with a constant $β$, a varying $β$ yields a larger best-fit value of fractional matter density $Ω_{m0}$, which slightly deviates from the best-fit result given by other cosmological observations. However, for both the varying $β$ and the constant $β$ cases, the $1σ$ regions of $Ω_{m0}$ are still consistent with the result given by other observations.

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