Scientists working at BOSS, the Baryon Oscillation Spectroscopic Survey, that is the largest program of the third Sloan Digital Sky Survey (SDSS-III), are trying to find out the role of dark energy in the evolution of the universe by investigating it over 10 billion years in the past using quasars. In March of this year, first results of the galaxy survey including about 300,000 galaxies were announced.
Quasars are very huge astronomical bodies with extreme remoteness in universe (showing redshift). They release a large amount of energy that sometimes reaches to the energy output of an entire galaxy.
“No technique for dark energy research has been able to probe this ancient era before, a time when matter was still dense enough for gravity to slow the expansion of the universe, and the influence of dark energy hadn’t yet been felt,” BOSS principal investigator David Schlegel, an astrophysicist in the Physics Division of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), said in a statement. “In our own time, expansion is accelerating because the universe is dominated by dark energy. How dark energy effected the transition from deceleration to acceleration is one of the most challenging questions in cosmology.”
BOSS studies dark energy by mapping baryon acoustic oscillations (BAO) with international collaboration. BAO represents regular rhythmic fluctuations in the density of the visible baryonic matter of the universe resulting from acoustic waves that existed in the early universe (Baryon is a composite subatomic particle). These rhythmic fluctuations of the matter density resulted in regular spacing of peaks, the remnants of which are visible in the cosmic microwave background radiation, that help to determine the rate of expansion.
Although, studying quasars to determine BAO directly is not easy but when the light of quasars pass through clouds of intergalactic gas it develops a huge amount of hydrogen absorption lines known as a Lyman-alpha forest. The absorption line represents the path of passage of quasar’s light through an intervening gas cloud and the individual absorption line’s prominence and redshifts could help to study the difference in the gas density. However, these Lyman-alpha absorption lines occur in the ultraviolet spectrum, which could be absorbed by the Earth’s atmosphere, so those quasars whose spectra are redshifted at the right amount could only be helpful in this study.
“We had one thing in our favor,” Berkeley Lab’s Nicholas Ross, who led the target selection team, said. “It didn’t matter what type of quasar it was, as long as it was high redshift. To identify them we could use ultraviolet data, near-infrared data – any method, any trick – because we were only going to use them as backlights to probe the intervening clouds of gas. Ultimately we found that all our selection algorithms worked well.”
Schlegel says, “There is no other credible way we could have measured BAO at redshifts of two or more. Five years ago it was chancy, but it was the only proposal on the table. We could have failed in any number of ways, but nature was good to us.”
Martin White, a professor of physics and astronomy at the University of California at Berkeley and the chair of the BOSS science survey teams, said, “We are seeing back to the matter-dominated universe, when expansion was decelerating and dark energy was hard to see. The transition from decelerating expansion to accelerating expansion was a sharp one, and now we live in a universe dominated by dark energy. The biggest puzzle in cosmology is, why now?”
Source: Berkeley Lab
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