April 11, 2014

Astronomers from the Sloan Digital Sky Survey have used 140,000 distant quasars to measure the expansion rate of the Universe when it was only one-quarter of its present age (now, the Universe is 13 billion years old). This is the best measurement yet of the expansion rate at any epoch in the last 13 billion years. Andreu Font Ribera, who holds a PhD from UB and researches at the Lawrence Berkeley National Laboratory (LBNL, United States), and Jordi Miralda, ICREA researcher from the Institute of Cosmos Sciences of UB (ICCUB), have participated in the study. Measuring the expansion rate of the Universe over its entire history is key to determine the nature of the dark energy that is responsible for causing this expansion rate to increase during the past six billion years.

The Baryon Oscillation Spectroscopic Survey (BOSS), the largest component of the third Sloan Digital Sky Survey (SDSS-III), pioneered the technique of measuring the structure of the young Universe by using quasars to map the distribution of intergalactic hydrogen gas. New BOSS observations of this structure were presented at the April 2014 meeting of the American Physical Society in Savannah (United States).

To know the evolution of the Universe with quasars

The light emitted by quasars, the brightest cosmic objects found billions of light-years away, passes through intervening hydrogen gas distributed throughout the Universe. The analysis of hydrogen absorption patterns intercepted by quasars’ light on their trip towards us is a new method to measure the structure of the Universe.

These latest results combine two different methods of using quasars and intergalactic gas to measure the rate of expansion of the Universe. The first analysis, by Andreu Font Ribera and collaborators, compares the distribution of quasars to the distribution of hydrogen gas to measure distances in the Universe. A second analysis team, led by Timothée Delubac, from the Centre de Saclay (France), is focused on the patterns in the hydrogen gas itself to measure the distribution of mass in the young Universe. Both BOSS analyses establish that, 10.8 billion years ago, the Universe was expanding by one per cent every 44 million years.

When galaxies were closer together

According to Professor Jordi Miralda, “the expansion of the Universe means that galaxies move away from one another, as if space was being stretched everywhere like a rubber band. When we observe distant galaxies or gas clouds, we are seeing the past of the Universe because of the time light takes to reach us”.

“If we look back to the Universe when galaxies were three times closer together than they are today, we'd see that a pair of galaxies separated by a million light-years would be drifting apart at a speed of 68 kilometres per second as the Universe expands”, says the expert Andreu Font Ribera.

Delubac explains: “We have measured the expansion rate in the young Universe with an unprecedented precision of 2%”. Measuring the expansion rate of the Universe over its entire history is key in determining the nature of the dark energy that is responsible for causing this expansion rate to increase during the past six billion years. “By probing the Universe when it was only a quarter of its present age, BOSS has placed a key anchor to compare to more recent expansion measurements as dark energy has taken hold”, says Delubac.

Acoustic oscillations in the early Universe

BOSS determines the expansion rate at a given time in the Universe by measuring the size of baryon acoustic oscillations (BAO), a signature imprinted in the way matter is distributed, resulting from sound waves in the early Universe. This imprint is visible in the distribution of galaxies, quasars, and intergalactic hydrogen throughout the cosmos. According to Jordi Miralda “acoustic oscillations were propagating through intergalactic matter; when the Universe was 400,000 years old, they left an excess of matter at a fixed and known distance from the sites where galaxies, quasars and gas clouds were formed later”. “It is as if around each object —he adds— there were a ring of known size where there is an excess of matter, and this is what has enabled us to make an accurate measure of the expansion rate of the Universe”.

David Schlegel, from the Lawarence Berkeley National Laboratory and principal investigator of BOOS, emphasizes that “three years ago, BOSS used 14,000 quasars to demonstrate we could make the biggest 3D maps of the Universe”. “Two years ago —he adds—, with 48,000 quasars, we first detected baryon acoustic oscillations in these maps. Now, with more than 140,000 quasars, we've made extremely precise measures of BAO”.