Past exam of the mathematics course of the University of Cambridge / 2006 / iii / Paper 72 / 2 / ii / b / Solution

A pure positive cosmological constant in a flat universe gives constant $H=H_0$, the expanding de Sitter model. ThusThe redshift drift is a straight line through the origin with positive slope. Here the source and observer expansion rates are equal; the factor $1+z$ in the observer contribution makes every positive-redshift comoving source drift toward still larger redshift.