MARINA DEL REY, Calif., Feb. 20 — A dark unseen energy is steadily pushing the universe apart, just as Einstein predicted, suggesting the universe may have a more peaceful end than recent theories envision, according to striking new measurements of distant exploding stars by the orbiting Hubble Space Telescope. The energy, whose source remains unknown, was named the cosmological constant by Einstein. In a prediction in 1917 that he later called "my greatest blunder," Einstein posited a kind of antigravity force that was pushing galaxies apart with a strength that did not change over billions of years of cosmic history....