 Low-redshift acceleration cosmography

A local derivation makes the acceleration sign transparent. Normalize $a(t_0)=1$ and put $\tau =t_0-t_e$. A Taylor series givesThe radial null geodesic in FLRW spacetime gives $D_C=c{\int }_{t_e}^{t_0}dt/a(t)=c[\tau +H_0{\tau }^2/2+O({\tau }^3)]$. Invert the redshift series, use $d_L=(1+z)D_C$ through second order, and obtain the displayed luminosity distance. Spatial curvature of an FLRW universe first changes this relation at third order. Lower $q_0$ therefore means larger $d_L$ at a fixed sufficiently small positive redshift and common $H_0$.

At $z=0.05$, the quadratic approximation gives $222.4353\mathrm{Mpc}$ for $q_0=-0.55$, within $0.07\%$ of the full flat-model value $222.2891\mathrm{Mpc}$. At $z=0.5$ it gives $2971.1574\mathrm{Mpc}$, about $4.9\%$ too large. A useful sign argument at low redshift is not a precision replacement for the full distance integral in a distant supernova analysis.