Best measurement yet of Universe's expansion

The team from the Sloan Digital Sky Survey (SDSS) combined two different methods of using quasars and intergalactic hydrogen gas to measure the rate of expansion of the universe. They looked at 140,000 distant quasars, luminous regions in the center of massive galaxies, when the universe was only one-quarter of its present age.

The position of the gas clouds are mapped in three dimensions and at different distances the gas blocks different coloured light from the luminous quasars. The astronomers then measure how much the universe has expanded since the light passed through each patch of hydrogen. This pioneering technique uses the distribution of hydrogen on the largest possible scale.

"A little over a year ago we tried this for the first time and demonstrated that it really works," said Dr Matthew Pieri, from the Institute of Cosmology and Gravitation at the University of Portsmouth. "Now we're back with twice as much data and with remarkable precision of 2 per cent. We are measuring the expansion of the universe with exquisite detail. Like the rings of a tree trunk that tell is its age, each quasar spectrum becomes an archive of the universe's history."

In the last five billion years the universe has started to rapidly expand, due to a mysterious repulsive force called dark energy. Scientists are investigating how and why it is expanding in order to understand the nature of dark energy.

"We are measuring the expansion rate better than at any point since the afterglow of the Big Bang, known as the Cosmic Microwave Background (CMB), and that precision is giving us a hint that maybe we aren't getting what we expected and so maybe the universe isn't quite as we had thought."

Dr Pieri and his colleague Professor Bob Nichol are members of the international Baryon Oscillation Spectroscopic Survey (BOSS) team, which undertook the research as part of SDSS.

Professor Nichol said: "Measuring the expansion rate of the universe over its entire history is key in determining the nature of the dark energy that has dominated the universe during the past six billion years. We measure these enormous structures using both the distribution of gas and the locations of the quasars themselves. This allows us to squeeze everything from the data."

The astronomers believe that by probing the universe when it was only a quarter of its present age, places an important anchor with which to compare more recent expansion measurements as dark energy has taken hold.

Dr Pieri said: "There is an intriguing hint of tension between our measurements and what you'd expect from observations of the CMB. It's odd, but nothing you'd want to hang your hat on just yet -- it's going to be fun finding out where the truth lies."

The results are being announced on Monday 7 April at a conference of the American Physical Society in Savannah, Georgia.