PITTSBURGH -- Johns Hopkins University astronomers say they have detected and measured intergalactic helium believed to be left over from the birth of the universe.
From that, they have figured out, for the first time, how much hydrogen existed along with the helium 10 billion years ago. That helps to confirm the Big Bang theory of the origin of the universe, and adds to the growing inventory of what is out there.
There was so much of the two gases that the discovery multiplies the amount of matter now known to exist in the universe by a factor of 5 or 10, the astronomers said.
"It's a spectacular end to a very long and arduous journey," said Hopkins astrophysicist Dr. Arthur F. Davidsen, who began the quest for the intergalactic gas 17 years ago. He is the principal investigator on the Baltimore-built Hopkins Ultraviolet Telescope (HUT).
On a human scale, there wasn't much helium out there to be detected.
"The density amounts to one helium atom in 50 typical bedrooms," Dr. Davidsen said. But the mass adds up because of the enormous volume of space.
"We're only seeing the tail of the dog . . . but of course it's a big tail, so we're inferring there's a very big dog attached to it," Dr. Davidsen said.
HUT made the discovery in March during the 17-day Astro 2 mission aboard the space shuttle Endeavor. It was the instrument's primary objective and required 5 1/2 hours of observation during 12 separate orbits.
The findings were announced yesterday at the 186th semiannual meeting of the American Astronomical Society. The details will be published later this year in the scientific journal Nature, Dr. Davidsen said.
Hubble Space Telescope scientists announced last year the likely detection of intergalactic helium. But because of technical limitations, the Hubble results were tentative and did not measure the gas. The HUT findings are regarded as definitive.
The widely accepted Big Bang theory of the origin of the universe says that the simplest elements -- hydrogen and helium -- were created during the first three minutes after the universe began in a colossal explosion, perhaps 12 billion or 13 billion years ago.
The two elements filled the expanding universe. Heavier elements, like oxygen, carbon and iron, were formed later, after regions of the original hydrogen and helium had condensed to form stars and galaxies.
For more than 30 years, astronomers have predicted that, if the Big Bang theory is correct, much of the primordial hydrogen would probably remain detectable in the space between the early galaxies.
But efforts to find it failed. Astronomers suspected the intense radiation of the early universe had stripped the individual hydrogen atoms of their only electron, making them invisible to astronomers.
They proposed, instead, to look for the primordial helium. It might be easier to detect than hydrogen, they said, because helium atoms have two electrons, and more might have survived the radiation bombardment. But they lacked the technology to see it.
In 1978, Dr. Davidsen and his colleagues began working to build a spectrographic telescope capable of detecting the intergalactic helium in the far-ultraviolet wavelengths, and at the distances required.
To observe the universe as it existed 10 billion years ago, scientists must aim their telescopes at objects 10 billion light years away. That's because light just now arriving at Earth from those objects had to get started on that long journey 10 billion years ago.
In this case, HUT scientists picked a distant quasar designated HS1700+64. Quasars are extremely bright objects believed to be the violent cores of giant early galaxies. Their light is frequently used as a "searchlight" to illuminate material in intervening regions of space.
By studying the ultraviolet light from HS1700+64, Dr. Davidsen said his team found "strong evidence" that portions of the light had been absorbed by helium in the intergalactic space within a billion light years of the quasar.
The astronomers then measured the density of the helium, and used it to infer the amount of invisible hydrogen that also must exist in that region. Based on theory and observation, scientists say there are 12 invisible hydrogen atoms for every one of the helium atoms they detected. Taken together, the intergalactic helium and hydrogen revealed by HUT amount to 5 or 10 times the mass of all the visible objects in the universe at the time.
With the discovery, Dr. Davidsen said science has accounted for nearly all of the normal or "baryonic" matter in the universe -- things like planets, stars and people that are composed of protons, neutrons and electrons.
Investigators, however, are still looking for huge amounts of exotic sub-atomic particles believed to compose the other 90 percent of the universe -- mass still "missing" but necessary to explain the gravitational forces that appear to be at work.
HUT's planned launch in 1986 was delayed four years by the Challenger disaster. Its first mission in 1990 was hobbled by aiming difficulties, and the helium experiment failed.