On the Possible Variations of the Hubble Constant with Distance

Abstract

Current measurements of the Hubble constant $H_0$ on scale less than $\sim100$ Mpc appear to be controversial, while the observations made at high redshift seem to provide a relatively low value. On the other hand, the Hubble expansion is driven by the matter content of the universe. The dynamical analysis on scale of a few $\sim10$ Mpc indicates that the matter density $Ω_0$ is only $\sim0.2$--$0.3$, which is significantly smaller than $Ω_0=1$ predicted in the standard inflation model. This might support the tendency of a decreasing Hubble constant towards distance. In this paper, we discuss the influence of a possible variant Hubble constant on two fundamental relations in astronomy: the magnitude-redshift ($m$--$z$) and the number-magnitude relations. Using a distant type Ia supernova at $z=0.458$, we show that the deceleration parameter $q_0$ or $Ω_0$ cannot be determined from the $m$--$z$ relation at moderate/high redshift unless the variation of the Hubble constant is {\it a priori} measured. It is further demonstrated that the number density of distant sources would be underestimated when their local calibration is employed, which may partially account for the number excess of the faint blue galaxies observed at moderate/high redshift.