August 9, 2001: NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite has given astronomers their best glimpse yet at the ghostly cobweb of helium gas left over from the big bang, which underlies the universe's structure. The helium is not found in galaxies or stars but spread thinly through the vastness of space. The helium traces the architecture of the universe back to very early times. This structure arose from small gravitational instabilities seeded in the chaos just after the big bang. These FUSE observations help confirm theoretical models of how matter in the expanding universe condensed into a web-like structure pervading all of the space between galaxies.See the rest:
The FUSE observations were accomplished by collecting the light from a distant quasar, a brilliant, active nucleus of a galaxy that resides 10 billion light- years away from Earth. FUSE looked at this quasar for a total of twenty days, in August and October 2000. Along the trajectory to Earth intervening clouds containing hot helium gas modified the quasar's light. As light passes through intergalactic clouds, helium atoms in the gas absorb specific colors of the light in the far-ultraviolet range of the spectrum. 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.
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.
By studying the spectrum of this quasar, the FUSE scientists are able 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.
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 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). 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.