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.
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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 at Goddard. 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.
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