There is a further source of uncertainty in the age caused by the fact that the
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 Hois
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.
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