The Hubble constant: a mystery that keeps getting bigger

Scientists have found a discrepancy in estimates for the rate of expansion of the universe. Why is this and what does it mean?

Astronomers have reached a fundamental impasse in their understanding of the universe: they cannot agree how fast it is flying apart. And unless a reasonable explanation can be found for their differing estimates, they may be forced to completely rethink their ideas about time and space. Only new physics can now account for the cosmic conundrum they have uncovered, many believe.

"Five years ago, no one in cosmology was really worried about the question of how fast the universe was expanding. We took it for granted," says astrophysicist Daniel Mortlock of Imperial College London. "Now we are having to do a great deal of head scratching - and a lot of research."