After an
extensive search, astronomers say they have definitely found half of the
universe's missing normal matter in the spaces between galaxies.
Astronomers
have long known that the amount
of matter we can see doesn't match up with what's actually there. Normal
matter (which includes galaxies, stars and us) makes up only about 4 percent of
the universe. This type of matter is also called "baryonic" because
it is made of baryons (protons, neutrons and other subatomic particles).
The missing
part of baryonic matter has largely escaped detection because it is too hot to
be seen in visible light but too cool to be seen in X-rays. Dubbed the
"intergalactic medium," or IGM, it extends essentially throughout all
of space like a cosmic spider web.
(This
missing matter is not to be confused with dark
matter, an exotic form of matter that can only be detected by its gravitational
pull.)
A team of
astronomers from the University of Colorado in Boulder used the light from
distant quasars (the bright cores of galaxies with active black holes) to probe
the almost-invisible web-like structure, like shining a flashlight through a
fog. Their results are detailed in the May 20 issue of the Astrophysical
Journal.
Using
Hubble's Space Telescope Imaging Spectrograph (STIS) and NASA's Far Ultraviolet
Spectroscopic Explorer (FUSE), the astronomers found the spectral
"fingerprints" of highly ionized hydrogen and oxygen, thought to form
the IGM.
"We
think we are seeing the strands of a web-like structure that forms the backbone
of the universe," said study team member Mike Shull. "What we are
confirming in detail is that intergalactic space, which intuitively might seem
to be empty, is in fact the reservoir for most of the normal, baryonic matter
in the universe."
Another
group of astronomers recently
found another filament of the missing baryonic matter connecting two
distant galaxies.
The Cosmic
Origins Spectrograph, to be installed on Hubble by astronauts later this year,
will help search for weaker signals of this missing matter.
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