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August 9, 2001
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Space Telescope Science Institute, Baltimore
Johns Hopkins University News Office, Baltimore
NEW VIEW OF PRIMORDIAL HELIUM TRACES STRUCTURE OF EARLY UNIVERSE
NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite has given astronomers their best glimpse yet at the ghostly cobweb of helium gas left over from the Big Bang, which underlies the universe's structure. The helium is not found in galaxies or stars but spread thinly through the vastness of space.
The observations, published in the August 10 issue of the journal Science, help confirm theoretical models of how matter in the expanding universe condensed into a web-like structure pervading all the space between galaxies. The helium traces the architecture of the universe back to very early times. This structure arose from small gravitational instabilities seeded in the chaos just after the Big Bang.
"Visible galaxies are only the peaks in the structure of the early universe. The FUSE observations of ionized helium show us the details of the hills and valleys between the mountain tops," said Gerard Kriss, leader of the FUSE observing team and astronomer at the Space Telescope Science Institute in Baltimore.
The FUSE observations also bolster evidence that the early universe was re-energized by torrents of radiation from black holes in active galaxies, and a firestorm of star birth.
"The observed absorption by intergalactic helium agrees extremely well with theoretical predictions made at the University of Colorado of an intergalactic medium ionized by both quasars and starburst galaxies," said U.C. professor and FUSE team member Michael Shull.
The observations used the distant light from a quasar (a brilliant, active nucleus of a galaxy) to allow FUSE to peer across 10 billion light-years of seemingly empty space to make new and precise measurements of the universe's hidden structure.
The FUSE observations collected the light of a distant quasar for a total of twenty days during two observing campaigns in August and October 2000. Along the trajectory to Earth, intervening clouds containing hot helium gas modified the quasar's light. As light passes through intergalactic clouds, helium atoms in the gas absorb specific colors of the light in the far-ultraviolet region of the spectrum. The spectrum allows Kriss and co-investigators to trace how helium, opaque to radiation in the early universe, grew more transparent as the early universe expanded and was "re-ionized" by a flurry of quasar and galaxy formation, much as an early-morning fog is burned off by the rising sun.
The helium nuclei were forged in the first few minutes of the Big Bang. As the universe expanded the nuclei captured electrons to form a cool gas of neutral atoms. This gas was then reheated and ionized by a fireworks show in reverse as torrents of radiation poured into space from the powerful black holes at the centers of some newly formed galaxies and from the firestorm of star birth in other galaxies.
Astronomers have pondered exactly what energized the early universe. By comparing the absorption caused by intergalactic hydrogen -- visible in spectra from ground-based telescopes -- to the helium absorption seen with FUSE, astronomers can get a better understanding of the energy source. The FUSE comparison of helium to hydrogen absorption favors an energy source that is a mix of quasars powered by supermassive black holes and the light from newly formed stars. Quasars, historically, have been at the top of the list of probable power sources to heat the early universe.
"This is a very exciting discovery. The search for the spectral signatures of a forest of ionized helium gas in the early universe was one of the major objectives of the FUSE mission, and it has been fulfilled spectacularly," said Dr. George Sonneborn, FUSE Project Scientist at NASA's Goddard Space Flight Center in Greenbelt, MD. The team next plans to use FUSE to look at other quasars to trace the universe's structure.
FUSE is a NASA Origins mission developed and operated by The Johns Hopkins University in collaboration with Goddard; the Centre National d'Etudes Spatiales, France; the Canadian Space Agency; the University of Colorado; and the University of California, Berkeley. FUSE was launched June 24, 1999, on a three-year mission to obtain high-resolution spectra of faint galactic and extragalactic objects in the far ultraviolet wavelength region. More information on FUSE can be found at:
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