text: We're not alone -- but the universe may be less crowded than we think | EurekAlert! Science News
  "Most critically, we show that the ultraviolet luminosity function of our simulated galaxies is consistent with observations of redshift galaxy populations at the bright end of the luminosity function, but at lower luminosities is essentially flat rather than rising steeply," wrote researchers in their paper, called 'Probing the Ultraviolet Luminosity Function of the Earliest Galaxies with the Renaissance Simulations.'
  "Our work suggests that there are far fewer faint galaxies than one could previously infer," said principal investigator and lead author Brian W. O'Shea, an associate professor at Michigan State University with a joint appointment in the Department of Computational Mathematics, Science and Engineering; the Department of Physics and Astronomy; and the National Superconducting Cyclotron Laboratory. "Observations of high redshift galaxies provide poor constraints on the low-luminosity end of the galaxy luminosity function, and thus makes it challenging to accurately account for the full budget of ionizing photons during that epoch."
  "The Hubble Space Telescope can only see the what we might call the tip of the iceberg when it comes to taking inventory of the most distant galaxies," said SDSC Director Michael Norman, who was part of the research team for this study. "A key question is how many galaxies are too faint to see. By analyzing these new ultra-detailed simulations, we find that there are 10 to 100 times fewer galaxies than a simple extrapolation would predict."
  Because these simulations are so costly to generate, the team moved the entire output of the Renaissance Simulations to SDSC Cloud - some 100 terabytes of data, or the equivalent of about 150,000 audio compact discs. "A data access portal is being set up so that others can investigate their properties in more detail," added Norman, who also is a distinguished professor of physics at UC San Diego and a faculty member with the Center for Astrophysics & Space Sciences at the university.
  "The flattening at lower luminosities is a key finding in the study and significant to researchers' understanding of the reionization of the universe, when the gas in the universe changed from being mostly neutral to mostly ionized," said John H. Wise, Dunn Family Assistant Professor with the School of Physics at the Georgia Institute of Technology and a member of the research team.
  The term 'reionized' is used because the universe was ionized immediately after the fiery Big Bang. During that time, ordinary matter consisted mostly of hydrogen atoms with positively charged protons stripped of their negatively charged electrons. Eventually, the universe cooled enough for electrons and protons to combine and form neutral hydrogen. They didn't give off any optical or UV light and without that light, astrophysicists aren't able to see traces of how the cosmos evolved during these Dark Ages using conventional telescopes. The light returned when reionization began.
  In an earlier paper, simulations conducted by two researchers who were part of this new study concluded that about 300 million years after the 'Big Bang', the universe was 20 percent ionized, 50 percent ionized at 550 million years, and fully ionized at 860 million years after its creation.
  These simulations were done on the National Science Foundation-funded Blue Waters supercomputer, which is one of the largest academic supercomputers in the world. "These simulations are physically complex and very large - we simulate thousands of galaxies at a time, including their interactions through gravity and radiation, and that poses a tremendous computational challenge," said O'Shea. "Without the large memory, fast inter-node network, and powerful file system of Blue Waters, this calculation would not have been possible."
  In addition to O'Shea, Wise, and Norman, the research team also included Hao Xu, a postdoctoral research associate with the Center for Astrophysics & Space Sciences, at the University of California, San Diego. The research was funded by the National Science Foundation and NASA.
  http://ucsdnews.ucsd.edu/pressrelease/were_not_alone_but_the_universe_may_be_less_crowded_than_we_think