There is a further source of uncertainty in the age caused by the fact that the present speed of galaxies may not equal their average speed over all time; ordinary gravity retards the expansion and causes galaxies to slow down whereas a repulsive Λ makes them speed up. Hidden in every deduction of the age of the Universe from H_ois always some assumption about the past speeds of galaxies. This assumption adds an extra uncertainty of about +/- 20% to quoted ages but is not usually mentioned in popular accounts.