**Defogging the Early Universe**

By Trudy E. Bell\
revision #1 by TP

Much has been written about the first three minutes after the birth of
the Universe in the Big Bang. But what do astronomers know about the
*next* few billion years of the Universe\'s childhood?

Astronomers agree that matter somehow gathered together during that
epoch to form stars and galaxies -- but how? Did early stars form one at
a time in intergalactic space, or all at one in swirling whirlpool
galaxies? And what was left behind in the void after they formed: empty
space or vast drifting clouds of now-invisible gas?

How our Universe began -- and how it will end -- depends on the answers.

To probe such questions, NASA recently turned the Far-Ultraviolet
Spectroscopic Explorer (FUSE) toward a distant quasar \-- a brilliant,
active nucleus of a galaxy some 10 billion light-years away, near the
edge of the known Universe. For 20 days in August and October, 2000,
FUSE collected the quasar\'s light in the far ultraviolet region of the
spectrum. Simultaneously, others monitored the quasar at longer
ultraviolet and visual wavelengths using the Hubble Space Telescope and
the gigantic ground-based Keck telescope in Hawaii.

Then the analyses \-- and discoveries \-- began.

Actually, the astronomers were not interested in the quasar itself
(known only by its star-catalogue designation HE2347-4342). They were
trying to see whatever might lie between Earth and the quasar -- in this
case, rarefied clouds of helium gas in intergalactic space.

Helium is not the most abundant element in the Universe -- hydrogen is.
But during those first few billion years after the Big Bang, much of the
hydrogen between galaxies was transparent. Radiation from quasars, which
were more common in the early Universe than they are today, split
intergalactic hydrogen atoms into protons and electrons. Such
"free-ranging" sub-atomic particles can be hard to detect.

Helium, however, is not so stealthy. It has two electrons that are
difficult to pry from the atom. Even as radiation from quasars split
hydrogen atoms in two, many helium atoms would have remained more or
less intact. (A helium atom might lose one electron, but rarely two.)
According to Philippe Crane, Origins Theme Scientist at NASA
headquarters, this is why helium fascinates cosmologists: it is a
wonderful probe of the early Universe.

Hot helium gas between us and HE2347-4342 revealed its presence to FUSE
by absorbing specific \"colors\" or wavelengths of the quasar\'s light
in the far-ultraviolet region of the spectrum. Meanwhile, Hubble and
Keck observers working at longer wavelengths looked for hydrogen along
the same line of sight.

Looking far out in space is equivalent to looking far back in time. So,
when astronomers saw helium gas near the quasar, they were seeing it as
it appeared some 10 billion years old \-- impressively antique, as the
Universe is thought to be 14 or 15 billion years old. At least, that\'s
what astronomers calculated from how much spectral lines (specific
wavelengths or colors) were red-shifted (moved toward longer
wavelengths) from their normal positions.

Thus, by studying different red-shifted wavelengths of the quasar\'s
spectrum, the FUSE investigators were able to trace *through time* how
the transparency (ionization) of helium clouds waxed and waned as the
Universe expanded and evolved -- and how.

Intergalactic helium is some of the most ancient matter in the Universe.
Indeed, helium nuclei were forged during the first few minutes of the
Big Bang itself. At first the nuclei were bare. But then, as the
Universe expanded, the nuclei captured electrons and became neutral
helium atoms. These gaseous atoms created a semi-opaque fog that dimmed
the light from distant quasars.

Some time later torrents of radiation poured into space from unknown
sources that reheated and re-ionized some of the helium gas (that is,
stripped the electrons away again). Because helium ions absorb fewer
photons of light, the gas became transparent again, once more allowing
light from distant quasars to shine through \-- rather like energy from
the rising sun burns off morning fog to reveal distant hills.

By comparing FUSE's observations with data collected by Keck and Hubble,
the astronomers realized that the transparent periods -- times when
helium was substantially ionized -- were caused by radiation both from
brilliant quasars *and* from a firestorm of star birth in more ordinary
galaxies. Before, the leading candidate was quasars alone.

That wasn't the biggest surprise, however. FUSE also discovered helium
gas where no hydrogen gas has yet been found. This means the mass of
helium gas in intergalactic space actually could be greater than the
mass of all the visible stars and galaxies in the known Universe!

More hydrogen is likely hiding out there as well. How much? No one
knows. It's another foggy question for astronomers!

\- 30 -

Author: Trudy E. Bell

Useful links:

FUSE science summary on the intergalactic medium:

<http://fuse.pha.jhu.edu/wpb/sci_h2347.html>

another good summary

[http://oposite.stsci.edu/pubinfo/pr/2001/27/pr.html](http://oposite.stsci.edu/pubinfo/pr/2001/27/pr/html)

preprint of technical paper in *Science*

<http://oposite.stsci.edu/pubinfo/pr/2001/27/content/he2347_pp_072401.pdf>

FUSE exhibit at Maryland Science Center

<http://fuse.pha.jhu.edu/educ/MSCpage.html#Spec>

A summary history of the universe (I have no way of judging its
accuracy, but it seems as though it is an effective thumbnail account)

<http://www.innerx.net/~tsmith/cosm.html>

creation of universe

<http://www.lifeinuniverse.org/Lightelements-01-01-05.html>