Spitzer measures rate of universe's expansion

Posted by Kate Taylor

Astronomers have come up with their most accurate measurement yet of the Hubble constant, the rate at which the universe is expanding.

The team used NASA's Spitzer Space Telescope to take advantage of long-wavelength infrared light for the new measurement - making it three times more accurate than previous Hubble measurements and bringing the uncertainty down to just three percent.

The new value for the Hubble constant is 74.3 plus or minus 2.1 kilometers per second per megaparsec.

To get the measurement, the team examined variable stars called cepheids.

"These pulsating stars are vital rungs in what astronomers call the cosmic distance ladder: a set of objects with known distances that, when combined with the speeds at which the objects are moving away from us, reveal the expansion rate of the universe," says Spitzer program scientist Glenn Wahlgren.

Spitzer observed 10 cepheids in our own Milky Way galaxy and 80 in thenearby Large Magellanic Cloud. Making the observations in long-wavelength infrared light allowed the team to see right through the cosmic dust which obscures Hubble's view, achieving a much more accurate result.

"Just over a decade ago, using the words 'precision' and 'cosmology' in the same sentence was not possible, and the size and age of the universe was not known to better than a factor of two," says Wendy Freedman of the Observatories of the Carnegie Institution for Science in Pasadena.

"Now we are talking about accuracies of a few percent. It is quite extraordinary."