Helium gas left over from the Big Bang underlies the universe's structure and resembles a cobweb spread thinly through the vastness of space between galaxies, according to new findings.
Scientists have long known that the universe was comprised almost entirely of helium and hydrogen in the early years after the Big Bang.
Now, new data from NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite help confirm models of how matter in the expanding universe condensed into a web-like structure pervading intergalactic space.
According to the newly bolstered theory, density fluctuations in helium gas, which is not found in galaxies and stars, caused it to clump together gravitationally. The densest clumps then formed the first galaxies and quasars.
"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 FUSE science chief Gerard Kriss in a prepared statement. Kriss is astronomer at the Space Telescope Science Institute in Baltimore.
The FUSE observations also back up other findings that suggest 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 Michael Shull, a member of the FUSE team and professor at Colorado.
The results were published in the latest issue of the journal Science.
More quasar-based findings
The observations were made by collecting the light of a distant quasar (quasars are very young cosmic objects that burn with the energy of billions of Suns and are thought to be galactic nuclei) for 20 days, during August and October 2000.
Intervening clouds of hot helium modified the quasar's light en route to Earth. As the light passed through intergalactic clouds, helium atoms in the gas absorbed specific colors of the light in the far-ultraviolet range of the spectrum.
Complementary, simultaneous observations using NASA's Hubble Space Telescope showed the brightness of the quasar at longer ultraviolet wavelengths where the spectrum is unaffected by helium.
The spectrum allowed Kriss and his colleagues to trace how helium, which was 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, like 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.
Energizer mystery
Astronomers have pondered what exactly energized the early universe. By comparing the absorption caused by intergalactic hydrogen, which is visible in spectra from ground-based telescopes like the Keck Observatory, to the helium absorption seen with FUSE, astronomers are able to achieve a better understanding of the energy source.
Though more abundant, intergalactic hydrogen is less easily detected because it is so highly energized (ionized). Even when ionized, helium manages to retain an electron. This etches the light from the quasar with a "forest" of spectral absorption features. Because the universe is expanding, these absorption features are found at many different wavelengths depending on the distances of the intergalactic clouds from Earth.
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 the preferred power source to heat the early universe. The FUSE observations support other recent suggestions that star formation is also important.
"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 George Sonneborn, FUSE Project Scientist at NASA's Goddard Space Flight Center in Maryland.
FUSE was launched on June 24, 1999 on a three-year mission to obtain high-resolution spectra in the far ultraviolet wavelength region of faint galactic and extragalactic objects.
HUT hinted
Studying the intergalactic medium in ultraviolet light is one of the top tasks for FUSE.
In the 1990s astronomers probed the distant universe using the ultraviolet capabilities of the Hubble Space Telescope and the Hopkins Ultraviolet Telescope (HUT) on the ASTRO-2 Space Shuttle mission.
The HUT observations, led by the late Arthur Davidsen of the Johns Hopkins University, gave the first inkling that the intergalactic medium was not a smooth distribution of gas between the galaxies.
FUSE, built and operated by Johns Hopkins University, has a combination of far-ultraviolet sensitivity and spectral resolution that could observe structure in the intergalactic medium traced by ionized helium.
The team next plans to use FUSE to look at other quasars to trace the universe's structure.
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