Washington: In a significant find, astronomers using NASA's Hubble Space Telescope have discovered that the universe is expanding five percent to nine percent faster than expected. There are a few possible explanations for the universe's excessive speed. One possibility is that dark energy, already known to be accelerating the universe, may be shoving galaxies away from each other with even greater -- or growing -- strength. "This surprising finding may be an important clue to understanding those mysterious parts of the universe that make up 95 per cent of everything and don't emit light, such as dark energy, dark matter and dark radiation," explained study leader and Nobel Laureate Adam Riess from the Space Telescope Science Institute and Johns Hopkins University. Riess' team made the discovery by refining the universe's current expansion rate to unprecedented accuracy, reducing the uncertainty to only 2.4 per cent. For the results, the team looked for galaxies containing both Cepheid stars and Type Ia supernova. Cepheid stars pulsate at rates that correspond to their true brightness, which can be compared with their apparent brightness as seen from Earth to accurately determine their distance. Type Ia supernovae are exploding stars that flare with the same brightness and are brilliant enough to be seen from relatively longer distances. By measuring about 2,400 Cepheid stars in 19 galaxies and comparing the observed brightness of both types of stars, they accurately measured their true brightness and calculated distances to roughly 300 Type Ia supernovae in far-flung galaxies. The team compared those distances with the expansion of space as measured by the stretching of light from receding galaxies. They used these two values to calculate how fast the universe expands with time, or the Hubble constant. The improved Hubble constant value 45.5 miles per second per megaparsec. (A megaparsec equals 3.26 million light-years.) The new value means the distance between cosmic objects will double in another 9.8 billion years. Measurements of the afterglow from the Big Bang by NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency's Planck satellite mission yield predictions which are 5 percent and 9 percent smaller for the Hubble constant, respectively. "Comparing the universe's expansion rate with WMAP, Planck, and Hubble is like building a bridge," Riess added. Another possibility of this expansion is that the cosmos contained a new subatomic particle in its early history that travelled close to the speed of light. Such speedy particles are collectively referred to as "dark radiation" and include previously known particles like neutrinos. More energy from additional dark radiation could be throwing off the best efforts to predict today's expansion rate from its post-Big Bang trajectory. The speedier universe may be telling astronomers that Albert Einstein's theory of gravity is incomplete. "We know so little about the dark parts of the universe, it's important to measure how they push and pull on space over cosmic history," said Lucas Macri from Texas A&M University in College Station. The results are forthcoming in The Astrophysical Journal.