Observations of helium give new clues as to how matter evolved in the
early universe.
The primordial helium gas left over from the Big Bang is spread
thinly through space between galaxies. NASA’s Far Ultraviolet
Spectroscopic Explorer (FUSE), which was launched in June 1999,
observed the light from a distant quasar and measured the absorption of
helium over 20 days. Simultaneous observations using the Hubble Space
Telescope measured the light at longer ultraviolet wavelengths, where the
spectrum is unaffected by helium.
Measuring how the absorption varies across the spectrum of the
quasar reveals how the gas evolved as the universe got older.
This technique has been used many times before to study the
intergalactic medium using the Lyman-aThe new observations trace the architecture of the universe back to
very early times. The helium structure arose from small gravitational
instabilities that were seeded in the chaos just after the Big Bang. The
excellent resolution of the results shows up the variations that existed in
the young universe, when galaxies were forming and when quasars were at
their most active.
The nuclei of helium atoms were formed in the first few minutes
after 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 the radiation from the first energetic objects in the universe.
