Young universe was slow to expand, survey finds

Astronomers from the Lawrence Berkeley National Laboratory have combined a new analytical technique with a massive data-set to deduce how fast the universe was expanding in its earlier years.

Led by teams from the Berkeley Lab and the Centre de Saclay (France), they used the Baryon Oscillation Spectroscopic Survey (BOSS), a spectrograph used to observe how light of different frequencies is being absorbed and emitted by various objects in the universe. Commenting on the results, “at 3 billion years old, space-time itself was getting bigger by 1 per cent every 44 million years,” said David Schlegel, an astronomer with the Berkeley Lab, in an email to this Correspondent. He is the principal investigator on BOSS. The result was announced at a high accuracy of 2.2 per cent.

The conventional method has been to study how the frequency of light emitted by extremely bright objects called quasars is lowered as a natural consequence of the universe’s expansion. However, directly measuring it becomes difficult for those more than six billion light-years away. So, the astronomers picked over 140,000 quasars from the BOSS data and studied how they illuminated intervening gas clouds. The hydrogen in them absorbs some of the light, casting a shadow that reveals its density and distance from the quasar.

The Berkeley team, led by postdoctoral fellow Andreu Font-Ribera, compared the distribution of gas clouds with quasars, usually found at the centres of massive galaxies, to arrive at a cosmic map of distances. The team led by Timothée Delubac from Saclay focused on patterns in the hydrogen gas to measure the mass distribution in the young universe.

Putting them together, the Berkeley Lab astronomers' results correspond to a 10.5-billion year old universe expanding at 68 km/s for every million light-years away from the observer — which is lower than expected. Today, the rate is 67.15 km/s for every million parsec away from the observer (one parsec is 3.26 light-years), a value established in 2013 using the Planck space telescope.

In some theories of cosmology, the driver of the universe's accelerating expansion is a mysterious entity called dark energy. It accords the vacuum of space with some energy that resists the universe's implosion due to the gravitational pull of billions of galaxies. So, finding a young universe that expanded slowly puts constraints on the origins of dark energy.

“By studying the expansion we learn about how much matter and dark energy was present as a function of time. The higher the accuracy, the better the constraints,” Mr. Font-Ribera wrote in an email.