91av

ONLY about half the hydrogen gas in the early Universe ended up in galaxies
like our own—the rest went missing. Now astronomers claim they’ve found
it. “A substantial amount of it appears to be in superhot clouds hiding in
intergalactic space,” says Todd Tripp of Princeton University in New Jersey.

Astronomers suspected that a huge amount of hydrogen had gone missing because
deuterium—heavy hydrogen—was once abundant in interstellar space.
The deuterium is thought to have been forged in the first few minutes after the
big bang, but it is easily converted into helium if there is a lot of hydrogen
about. “The scarcity of deuterium [today] tells us that when it was born there
was about twice as much hydrogen around as we see today tied up in galaxies and
galaxy clusters,” says Tripp.

One explanation for the disappearance of the hydrogen was that it became so
hot its atoms lost their electrons. Since atoms can’t emit or absorb light
without electrons, the hydrogen effectively became invisible. According to
theorists, this could have happened if some hydrogen was “shock-heated” to over
100 000 °C as clouds of the gas collided at incredible speeds. “The stuff
that remained cool congealed into galaxies,” says Tripp. “The stuff that was hot
was simply too hot to be held together by gravity and got left out.”

Tripp and his colleagues, Blair Savage of the University of Wisconsin in
Madison and Edward Jenkins at Princeton, have now found evidence of the
“left-out stuff” by pointing the Hubble Space Telescope at a distant quasar.
They found slices had been taken out of the quasar’s spectrum by highly ionised
oxygen in gas clouds between the quasar and Earth. The oxygen atoms had lost
five electrons, which could be because they were being remorselessly bombarded
by a huge number of superhot particles. Tripp’s team has extrapolated these
results to the whole Universe. “Our calculations show there’s easily enough
[hydrogen] nuclei to substantially alleviate the missing hydrogen problem,”
Tripp says.

“Finding evidence for an amount of hydrogen gas equivalent to that in all the
stars and galaxies is clearly a major achievement,” says David Hough of Trinity
University in San Antonio, Texas. But he adds that all the ordinary matter in
the Universe accounts for only a few per cent of all the matter there is.

There is 10 times as much “dark matter” as ordinary matter, and its identity
remains elusive. What’s more, it has recently been discovered that empty space
contains twice as much “dark energy” as ordinary and dark matter combined.
“Astronomers have got a long way to go,” Hough says.

Confirmation that the Universe contains a prodigious amount of dark energy
has just been found by a team led by David Wittman of Bell Labs in New Jersey
(Nature, vol 405, p 143). They conclude from the way the matter of the
Universe bends light from distant objects—a phenomenon called
gravitational lensing—that there is not enough matter, including dark
matter, to make space “flat”. But recent measurements of the “afterglow” of the
big bang (91av, 6 May, p 22) confirm that the Universe is
indeed flat—so vast amounts of dark energy must make up the deficit.