Do quasars have cosmologically long lifetimes

An alternative explanation to gravitational lensing is examined, by which problems inherent in space density evolution are avoided without invoking gravitational effects. Apparent and unreal density evolution follows as an immediate consequence, if the quasar lifetimes that are the only free parameter in the model proposed are of the order of three billion years. If such lifetimes are the case, while quasars may occur less frequently than has been thought, the local density of quasars may have been grossly underestimated.