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Glimpsing the early universe


NASA's FUSE satellite is probing the intergalactic medium.

Astronomers have caught their best glimpse yet of a gas that traces the architecture of the very early universe.

The observations of primordial helium, from NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, shed light on the so-called 'tenuous intergalactic matter'.

This very low-density matter, found in the vast spaces between galaxies, can only be seen through the absorption lines produced in the spectra of bright, distant sources of light - usually quasars (brilliant active nuclei of galaxies).

"Visible galaxies are only the peaks in the structure of the early universe," says Gerard Kriss, leader of the FUSE observing team. "The FUSE observations show us the details of the hills and valleys between the mountain tops."

The new data, described in this week's Science, support the notion that radiation from black holes and star birth re-energised the early universe.

The light FUSE detected was modified as it passed through intergalactic clouds on its way to the satellite. Helium atoms in the gas absorbed specific colours of the light in the far-ultraviolet region of the spectrum. This pattern allowed the astronomers to trace how helium, opaque to radiation in the early universe, grew more transparent as the early universe expanded.

The team say it was "re-ionised by a flurry of quasar and galaxy formation", much as an early-morning fog is burned off by the rising sun.

Historically, quasars have been top of the list of probable power sources to heat the early universe. But comparisons of the absorption caused by intergalactic hydrogen (visible from ground-based telescopes) with the helium absorption seen from FUSE, is seeing new insights emerge.

The energy source may have been a mix of quasars powered by supermassive back holes and the light from newly-formed stars.