Donald Savage

Headquarters, Washington, DC

(Phone: 202/358-1547)

Nancy Neal

Goddard Space Flight Center, Greenbelt, MD

(Phone: 301/286-0039)

Ray Villard

Space Telescope Science Institute, Baltimore, MD

(Phone: 410/338-4514)

RELEASE: 00-72

For the past decade astronomers have looked for vast
quantities of hydrogen that were cooked-up in the Big
Bang but somehow managed to disappear into the empty
blackness of space.

Now, NASA’s Hubble Space Telescope has uncovered
this long-sought missing hydrogen. It accounts for nearly
half of the "normal" matter in the universe —
the rest is locked up in myriad galaxies.

Astronomers believe at least 90 percent of the
matter in the universe is hidden in exotic "dark"
form that has not yet been seen directly. But more
embarrassing is that, until now, they have not been able
to see most of the universe’s ordinary, or baryonic,
matter (normal protons, electrons and neutrons).

The confirmation of this missing hydrogen will shed
new light on the large-scale structure of the universe.
The detection also confirms fundamental models of how
much hydrogen was manufactured in the first few minutes
of the universe’s birth in the Big Bang.

"This is a successful, fundamental test of
cosmological models," said Todd Tripp of Princeton
University, Princeton, NJ. "This provides strong
evidence that the models are on the right track." The
results of Tripp and his collaborators, Edward Jenkins
from Princeton and Blair Savage from the University of
Wisconsin-Madison, are being published in the May 1 issue
of the Astrophysical Journal Letters.

Previous observations show that billions of years
ago this missing matter formed vast complexes of hydrogen
clouds — but since then has vanished. Even Hubble’s keen
eye didn’t see the hydrogen directly because it is too
hot and rarified. Instead, Hubble found a telltale
elemental tracer — highly ionized (energized) oxygen —
between galaxies, which the hydrogen heats to the
temperatures observed in intergalactic space. The
presence of highly ionized oxygen between the galaxies
implies there are huge quantities of hydrogen in the
universe, which is so hot it escapes detection by normal
observational techniques.

In recent years, supercomputer models of the
expanding, evolving universe have predicted an intricate
web of gas filaments where hydrogen is concentrated along
vast chain-like structures. Clusters of galaxies form
where the filaments intersect. The models predict that
vast hydrogen clouds flowing along the chains should
collide and heat up. This would squelch the formation of
more galaxies in the hottest regions, so star birth was
more abundant in the early universe when the hydrogen was
cool enough to coalesce.

The oxygen "tracer" was probably created
when exploding stars in galaxies spewed the oxygen
(created in their cores through nuclear fusion) back into
intergalactic space where it mixed with the hydrogen and
then was shocked and heated to temperatures over 360,000
degrees Fahrenheit (100,000 degrees Kelvin).

Astronomers detected the highly ionized oxygen by
using the light of a distant quasar to probe the
invisible space between the galaxies, like shining a
flashlight beam through a fog. Hubble’s Space Telescope
Imaging Spectrograph found the spectral
"fingerprints" of intervening oxygen superimposed
on the quasar’s light. Slicing across billions of
light-years of space, the quasar’s brilliant beam
penetrated at least four separate filaments of the
invisible hydrogen laced with the telltale oxygen.

Hubble’s ultraviolet sensitivity and high-resolution
spectroscopic capability allowed it to probe the nearby
universe, where spectral features of hot gas can be seen
at ultraviolet wavelengths and the problems faced by
X-ray astronomers are avoided. "This result
beautifully illustrates the power of spectroscopy for
revealing fundamental information about the presence and
nature of the gaseous matter in the universe,"
according to Hubble spectroscopist Blair Savage.

Still, the hot hydrogen could not be seen directly
because it is fully ionized and so the hydrogen atoms are
stripped of their electrons. Without electrons, no
spectral features were etched into the quasar’s
earth-bound light. The oxygen is highly ionized too, but
still retains a few electrons which absorb specific
colors from the quasar’s light.

– end –

NOTE TO EDITORS: A ground-based image and illustration
associated with this release are available on the
Internet at: