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May 4, 2000

Lost and Found: Scientists Uncover Much of the Universe's Missing Hydrogen

PRINCETON, N.J. -- For the past decade astronomers have looked for vast quantities of hydrogen that was cooked-up in the big bang but somehow managed to disappear into the empty blackness of space.

Now, a team of scientists led by researchers at Princeton has used NASA's Hubble Space Telescope to uncover 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 that 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," says Todd Tripp, a postdoctoral fellow at Princeton. "This provides strong evidence that the models are on the right track." The results of Tripp and his collaborators, Princeton Professor of Astrophysics Edward Jenkins and Blair Savage from the University of Wisconsin-Madison, are being published in the May 1 issue of the Astrophysical Journal Letters.

Previous observations had shown that billions of years ago hydrogen left over from the Big Bang formed vast complexes of hydrogen clouds - but since then 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. The oxygen is a tracer because the hydrogen was needed to heat it to the temperatures observed in intergalactic space.

The presence of highly ionized oxygen between the galaxies implies that the universe contains huge quantities of hydrogen, which is so hot it escapes detection by normal observational techniques.

Tripp's observation of the oxygen tracer supports models of the expanding universe, developed in recent years by scientists using supercomputers. These models predict the existence of 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 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 100,000 degrees Kelvin (360,000 degrees Fahrenheit).

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, a new, more powerful spectrograph than the first-generation HST instruments, 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.

Observations with ground-based telescopes previously detected vast clouds of relatively cool intergalactic hydrogen in the early universe. Closer to home the question has been, Where did the hydrogen go? Was it all locked up in galaxies or lost in space?

Searches for the faint X-ray glow of the suspected hot gas have been ambiguous due to confusion from the large number of point sources such as quasars, which emit X-rays. In addition, X-ray emission and absorption from our own Milky Way further complicate the X-ray picture.

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 onto in 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.

The researchers next plan to conduct more observations with Hubble to reduce uncertainties in this initial result. They also are planning to use NASA's Far Ultraviolet Spectroscopic Explorer to carry out supporting observations such as searching for highly ionized neon as well as nearby highly ionized oxygen.

In the more distant future they hope to study how this highly ionized gas evolves with time by looking at increasingly farther systems until the gap between what's seen from the ground and what is seen from space is filled in.

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. for NASA, under contract with NASA's Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.

Note to Editors: Ground-based images and illustrations associated with this release are available on the Internet at: and via links in and