Scientists from the Baryon Oscillation Spectroscopic Survey (BOSS) have used 140,000 quasars to measure the expansion rate of the Universe when it was about 3 billion years old.
An artist’s conception of how BOSS uses quasars to measure the distant Universe. Image credit: Zosia Rostomian, Lawrence Berkeley National Laboratory / Andreu Font-Ribera, BOSS Lyman-alpha team, Berkeley Lab.
The light emitted by quasars, the brightest cosmic objects found billions 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 Earth is a new method to measure the structure of the Universe.
The method was reported last year in the journal Astronomy & Astrophysics. Analysis using a tested approach, but with far more data than before, has just been published in the Journal of Cosmology and Astroparticle Physics (arXiv.org version).
“The expansion of the Universe means that galaxies go away one from another as if the space was stretched like a piece of elastic. When we observe distant galaxies, we are seeing the past of the Universe as light takes much time to get to us,” said Prof Jordi Miralda-Escude from the Institute of Cosmos Sciences of the University of Barcelona, a co-author on both papers.
The two analyses together establish the expansion rate at 68 km per second per million light years at redshift 2.34, with an unprecedented accuracy of 2.2 percent.
“This means if we look back to the Universe when it was less than a quarter of its present age, we’d see that a pair of galaxies separated by a million light years would be drifting apart at a velocity of 68 km a second as the Universe expands. The uncertainty is plus or minus only a kilometer and a half per second,” said Dr Andreu Font-Ribera from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, the lead author of the new paper and a co-author of the Astronomy & Astrophysics paper.
“We have measured the expansion rate in the young Universe with an unprecedented precision of 2 per cent. 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,” said co-author Dr Timothee Delubac from the Center de Saclay, France.
BOSS employs galaxies and distant quasars to measure baryon acoustic oscillations (BAO), a signature imprint in the way matter is distributed, resulting from conditions in the early Universe.
While also present in the distribution of invisible dark matter, the imprint is evident in the distribution of ordinary matter, including galaxies, quasars, and intergalactic hydrogen.
Co-author Dr David Schlegel from the Lawrence Berkeley National Laboratory, said: “three years ago BOSS used 14,000 quasars to demonstrate we could make the biggest 3-D maps of the Universe.”
“Two years ago, with 48,000 quasars, we first detected baryon acoustic oscillations in these maps.”
“Now, with more than 150,000 quasars, we’ve made extremely precise measures of BAO.”
The BAO imprint corresponds to an excess of about 5 percent in the clustering of matter at a separation known as the BAO scale.
Recent experiments including put the BAO scale, as measured in today’s Universe, at very close to 450 million light years – a ‘standard ruler’ for measuring expansion.
BAO directly descends from pressure waves moving through the early Universe, when particles of light and matter were inextricably entangled; 380,000 years after the Big Bang, the Universe had cooled enough for light to go free.
The cosmic microwave background radiation preserves a record of the early acoustic density peaks; these were the seeds of the subsequent BAO imprint on the distribution of matter.
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Font-Ribera A. et al. 2014. Quasar-Lyman α Forest Cross-Correlation from BOSS DR11: Baryon Acoustic Oscillations. Journal of Cosmology and Astroparticle Physics, submitted for publication; arXiv: 1311.1767
N.G. Busca et al. 2013. Baryon acoustic oscillations in the Lyα forest of BOSS quasars. A&A 552, A96; doi: 10.1051/0004-6361/201220724
