Lost and found: Hubble finds universe’s missing hydrogen

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 detected vast filaments of invisible hydrogen by using the light of a distant quasar as it passed through the dark space between the galaxies. The Hubble Space Telescope Imaging Spectrograph found the spectral “fingerprints” of highly ionized intervening oxygen (which is a tracer of the hydrogen) superimposed on the quasar’s light. Illustration: John Godfrey (STScI)

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. 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.

The results of three collaborators — Todd Tripp and Edward Jenkins of Princeton University, Princeton, N.J., and Blair Savage from the University of Wisconsin–Madison — were 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 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.

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.

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,” says 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.