One of the greatest intellectual breakthroughs of the 20th century was the discovery that the universe is constantly expanding. It is difficult to fathom, however, why this revelation took so long to discover, since Isaac Newton and his colleagues should have known that gravity would cause an inert universe ultimately to shrink. Further study could have helped physicists determine the minimal rate at which the universe must be growing, in order to compensate for the gravitational forces holding it back. Instead, the theory of a static universe persisted centuries after Newton's death. The author of the passage mentions Isaac Newton in order to