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

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

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

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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