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  • richardmitnick 8:43 am on July 28, 2015 Permalink | Reply
    Tags: , , BOSS,   

    From CAASTRO: “Impact of IGM temperature fluctuations on large-scale clustering” 

    CAASTRO bloc

    CAASTRO ARC Centre of Excellence for All Sky Astrophysics

    28 July 2015
    No Writer Credit


    In 2014, the measurements of the clustering properties from large-scale structures at high redshift by the Baryon Oscillation Spectroscopic Survey (BOSS) solidified our current understanding of the accelerated expansion of the Universe.


    The wealth and quality of BOSS data further allows for detailed studies of the intergalactic medium (IGM; i.e. diffuse gas between galaxies). By measuring the absorption of light by neutral hydrogen gas in the IGM in the spectra of over 100,000 bright background sources – known as quasars – it is possible to deduce the location and, through detailed modelling, the amount of neutral hydrogen along the line-of-sight. From this, global properties of the IGM, namely its temperature, as well as the intensity of ionising background radiation from star-forming galaxies and quasars, can be calculated. This is an important factor in the evolution of the Universe because helium reionisation at redshift z ~ 3 was driven by radiation from quasars. Due to the sparsity of these sources, along with the energy spectrum of ionising radiation, this process can produce spatial fluctuations in the IGM temperature of more than 10,000 K. These fluctuations impact on the individual absorption properties which could in turn affect the interpretation on these important clustering measurements.

    As part of his PhD project at the University of Melbourne, ex-CAASTRO student Dr Bradley Greig (now in Pisa, Italy) and his supervisors used detailed semi-analytical simulations developed to investigate the impact of these temperature fluctuations on the clustering properties. These large-volume simulations employ a simple, physically motivated model for helium reionisation from which the researchers could construct synthetic quasar absorption spectra drawn through the IGM, mimicking observational programmes such as BOSS. This approach had the advantage of efficiently achieving a larger dynamic range than in computationally expensive, fully numerical simulations.

    They found that these temperature fluctuations could cause a 20-30% increase in the amplitude of the clustering measurements. While this increase does not affect the precision cosmology inferred from clustering, it has important consequences for recovering information regarding the IGM properties. The team concluded that any attempt to infer information on the IGM temperature or strength of the ionising background from the BOSS data must include detailed forward modelling of the IGM temperature fluctuations from helium reionisation and the quasars responsible.

    Publication details:
    Bradley Greig, James S. Bolton and J. Stuart B. Wyithe in MNRAS (2015) The impact of temperature fluctuations on the large-scale clustering of the Ly-alpha forest

    See the full article here.

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Astronomy is entering a golden age, in which we seek to understand the complete evolution of the Universe and its constituents. But the key unsolved questions in astronomy demand entirely new approaches that require enormous data sets covering the entire sky.

    In the last few years, Australia has invested more than $400 million both in innovative wide-field telescopes and in the powerful computers needed to process the resulting torrents of data. Using these new tools, Australia now has the chance to establish itself at the vanguard of the upcoming information revolution centred on all-sky astrophysics.

    CAASTRO has assembled the world-class team who will now lead the flagship scientific experiments on these new wide-field facilities. We will deliver transformational new science by bringing together unique expertise in radio astronomy, optical astronomy, theoretical astrophysics and computation and by coupling all these capabilities to the powerful technology in which Australia has recently invested.


    The University of Sydney
    The University of Western Australia
    The University of Melbourne
    Swinburne University of Technology
    The Australian National University
    Curtin University
    University of Queensland

  • richardmitnick 12:25 pm on March 30, 2012 Permalink | Reply
    Tags: , , BOSS,   

    From Berkeley Lab: “Clocking an Accelerating Universe: First Results from BOSS” 

    Berkeley Lab

    Berkeley Lab scientists are the leaders of BOSS, the Baryon Oscillation Spectroscopic Survey. They and their colleagues in the third Sloan Digital Sky Survey have announced the most precise measurements ever made of the era when dark energy turned on.

    Julie Chao
    March 30, 2012

    “Some six billion light years ago, almost halfway from now back to the big bang, the universe was undergoing an elemental change. Held back until then by the mutual gravitational attraction of all the matter it contained, the universe had been expanding ever more slowly. Then, as matter spread out and its density decreased, dark energy took over and expansion began to accelerate.

    Today BOSS, the Baryon Oscillation Spectroscopic Survey, the largest component of the third Sloan Digital Sky Survey (SDSS-III), announced the most accurate measurement yet of the distance scale of the universe during the era when dark energy turned on.

    ‘We’ve made precision measurements of the large-scale structure of the universe five to seven billion years ago – the best measure yet of the size of anything outside the Milky Way,’ says David Schlegel of the Physics Division at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), BOSS’s principal investigator. “We’re pushing out to the distances when dark energy turned on, where we can start to do experiments to find out what’s causing accelerating expansion.”

    BOSS measures the three-dimensional clustering of galaxies at various redshifts, revealing their precise distance, the age of the universe at that redshift, and how fast the universe has expanded. The measurement uses a “standard ruler” based on the regular variations of the temperature of the cosmic microwave background (CMB), which reveal variations in the density of matter in the early universe that gave rise to the later clustering of galaxies and large-scale structure of the universe today. (Click on image for best resolution. Credit: Eric Huff, the SDSS-III team, and the South Pole Telescope team. Graphic by Zosia Rostomian)

    For each 15-minute exposure of the deep sky, astronomers plug a thousand optical fibers into a plate that fits at the focal plane of the Sloan Telescope. Each fiber feeds the light of an individual galaxy to BOSS’s advanced spectrograph. (Photo by Dan Long, Senior Operations Engineer, Apache Point Observatory)

    See the full article here.

    A US Department of Energy National Laboratory Operated by the University of California


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