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February 9, 1995
FOR IMMEDIATE RELEASE
CONTACT: Emil Venere
Astrophysicists Devise New Method for
Investigating Dark Matter
(Editors: This release on recent Johns Hopkins research on
the dark matter question also is relevant to coverage of the
Astro-2 space shuttle flight, an ultraviolet astronomy mission
now scheduled for a March 2, 1995, launch)
Technique will be used with data from March shuttle flight
Two Johns Hopkins astrophysicists have devised a novel method for
calculating how much of the original hydrogen and helium produced
in the Big Bang remains diffused in the vast space between
galaxies, knowledge that might help cosmologists identify a
portion of the hypothetical "dark matter."
The technique could lead to a greater understanding of how the
universe mysteriously evolved from its original state of smoothly
distributed matter to its present clumpy condition in which
clusters of galaxies are distributed unevenly. Although the
well-accepted Big Bang theory suggests that the primordial
"intergalactic medium" of hydrogen and helium should exist, it
has not actually been detected.
Proving its existence and calculating its mass would provide
vital clues in attempts to learn how the cosmos developed from
its explosive birth, said Hopkins astrophysicists Arthur F.
Davidsen and Wei Zheng.
"We're looking for much more detail about how the universe got
from there to here," said Dr. Davidsen, a professor in the
Hopkins Department of Physics and Astronomy. "Given that the Big
Bang did what we believe it did, how did the galaxies form? The
intergalactic medium is like a missing link in that chain of
events that connects the Big Bang to what we have today."
Their innovative technique is outlined in a scientific paper to
be published Feb. 20 in The Astrophysical Journal: Letters. The
technique adds a new dimension to a well-known concept conceived
30 years ago by astrophysicists James P. Gunn and Bruce Peterson.
The Gunn-Peterson effect postulated that scientists should be
able to detect the hydrogen originally created in the Big Bang by
analyzing the light from quasars, the most luminous objects in
the universe. As the light passes through the presumed medium,
the gas should absorb a portion of the ultraviolet spectrum,
producing a distinct spectrographic signature.
But scientists, using a variety of telescopes and instruments,
have not detected the primordial hydrogen. One theory is that the
hydrogen has been ionized -- intense radiation has stripped
hydrogen atoms of their single electrons -- rendering them unable
to absorb light. But helium has two electrons, making it possible
that some of the helium atoms have not been fully ionized. If a
portion of the helium atoms have retained an electron, they
should absorb some of the ultraviolet light from quasars, making
the gas detectable with the right kind of ultraviolet
The Hopkins Ultraviolet Telescope, sensitive to a portion of the
spectrum best suited for the research, is scheduled for a March 2
launch aboard the space shuttle Endeavour, on the Astro-2
mission. Dr. Davidsen and members of his science team plan to use
HUT to search for signs of the primordial helium in the
But even with data from HUT, without the new method devised by
Dr. Davidsen and Dr. Zheng, scientists would only be able to
confirm the presence of helium. They would not be able to
calculate how much total helium and hydrogen are present.
The Hopkins scientists have determined that the closer the helium
is to a quasar, the more ionized it should be. That means
researchers should be able to construct a "proximity profile" of
changing ultraviolet absorption in the space near quasars. By
analyzing the curving profile's shape, scientists plan to
calculate the total mass of the hypothetical helium. The
computation takes into account the theoretical ratio of hydrogen
to helium in the universe. So, once the mass of helium is known,
the total mass of gas can be determined.
Current theories propose that the intergalactic medium of gas is
diffused throughout the universe between the galaxies. At the
birth of the cosmos the two simplest elements, hydrogen and
helium, were produced and evenly dispersed. The primordial gas
subsequently condensed to form the stars and galaxies we see
today. But some, or perhaps much of that gas may still remain. A
major stumbling block in confirming its existence has been the
technical difficulty involved in detecting the helium. HUT is
sensitive to a range of ultraviolet light called the far
ultraviolet spectrum. That spectral range is best suited to the
search for the intergalactic medium because it enables
astronomers to study quasars that are just the right distance
from Earth: they are not so distant that their light is heavily
"contaminated" by clouds of gas and galaxies in the foreground,
yet their redshift is high enough to bring the helium absorption
into HUT's view.
However, because even "nearby" quasars are very distant from
Earth, astronomers will be looking back in time. HUT's target, a
quasar known as HS1700+64, is located at such a distance that its
light now reaching Earth would be about 75 percent of the age of
the universe -- roughly 10 billion years old if the universe was
born 12 billion to 15 billion years ago.
"We are looking for some evidence of this smooth intergalactic
medium that was the original product of the Big Bang, trying to
see how much was there, directly," Dr. Davidsen said. The
intergalactic medium could help to provide an answer to a major
riddle in cosmology: where is the "missing mass" in the universe.
The observable universe adds up to no more than 1 percent of the
mass required to produce the gravitational force that seems to be
present throughout the universe. The standard Big Bang theory
predicts that a portion of the remaining, unseen mass is in the
form of normal, or baryonic matter -- the stuff people and
planets are made of. Theories suggest that up to 10 percent of
the missing mass is baryonic, and the rest is possibly some form
of exotic matter -- perhaps a variety of unknown subatomic
particles that are difficult to detect. The intergalactic medium
is a popular candidate for at least a portion of the baryonic
matter making up the missing mass.
Calculating the intergalactic medium's mass also could give
scientists valuable clues about the radiation sources that
existed in the early universe.
"What is the cause of the ionization?" Dr. Zheng asked. "Most of
the diffuse matter in the early universe is highly ionized. The
electrons are removed from the atoms. That means in the early
universe there is very strong radiation that ionized everything,
and we would like to know how strong are these radiation fields
in the early universe, and what could have produced them."
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