Johns Hopkins Gazette: June 12, 1995

Artful Discovery

     Once upon a time, Hopkins astrophysicist Arthur Davidsen
dreamed of proving that the Big Bang really happened. This week
in Pittsburgh, Davidsen and his scientific team make that dream a

Emil Venere
Homewood News and Information

     It has been likened to the cosmic soup from which the stars,
and ultimately everything else in the universe, formed--the
"primordial intergalactic medium" of hydrogen and helium produced
in the first three minutes after the birth of the cosmos.

     Finding it would provide a solid piece of evidence for the
standard Big Bang theory, which proposes that the universe was
born billions of years ago, exploding outward in a giant cosmic
explosion of nearly infinite density and temperature.

     Using a telescope sensitive to ultraviolet light, a team of
Hopkins astronomers led by Professor Arthur Davidsen has looked
back in time, making the first definitive detection of the
primordial helium. From that data, they were able to prove that
the intergalactic medium exists.

     Also involved in the work were Hopkins astrophysicists
Gerard A. Kriss and Wei Zheng. The discovery was announced June
12, during a meeting of the American Astronomical Society in

     The physicists used the Hopkins Ultraviolet Telescope on
NASA's Astro-2 observatory, which flew aboard the space shuttle
Endeavour in March.

     The findings confirm a critical prediction of the Big Bang
theory--that helium should have been widespread in the early
universe, long before the gas condensed to form the first stars.
The heavier elements, such as carbon, nitrogen, oxygen, silicon
and iron, came from nuclear reactions in the centers of stars,
and thus didn't form until some time after the Big Bang.

     In addition to finding the helium, Hopkins astronomers have
used HUT's data to estimate the total density of hydrogen and
helium in intergalactic space; the calculations confirm
predictions made by the Big Bang theory as to how much ordinary
matter was produced at the beginning of the universe.

     The data also allowed astronomers to account for a portion
of the invisible "dark matter" called baryonic matter, in the
early cosmos, a discovery that might help to identify some of the
"missing mass" in today's universe.

     The findings are the culmination of Davidsen's goal,
conceived in the late 1970s, to find the hypothetical
intergalactic medium. He reasoned that astronomers should be able
to detect the helium portion of the gas mixture by using a
spectrograph in space to measure a range of light called the far
ultraviolet spectrum.

     "It's a very rewarding feeling to find that we actually have
achieved what we set out to do at the beginning of the project 17
years ago," said Davidsen, a professor in the Department of
Physics and Astronomy.

     But detecting the primordial helium was never a certainty.

     The observations would depend on the existence of a quasar
just the right distance from Earth. Light from the distant object
would shine through the ancient helium, like a headlight through
fog, enabling astronomers to detect its existence with a

     "When we started the project, we didn't even have a good
quasar to look at to do this, but we thought we might find one,"
Davidsen said. "That was a leap of faith back in the late '70s,
that there would be enough quasars bright enough."

     The gamble paid off. German astronomers in the late 1980s
discovered a quasar just the right distance. It was named
HS1700+64, and it's about 10 billion light-years away in the
general direction of the constellation Draco.

     Quasars, the most luminous objects in the universe, are
extremely distant objects that look like stars when viewed
through a conventional telescope but actually emit more energy
than an entire galaxy. They were common long ago, when the
universe was young, and some scientists believe they played a
role in the formation of galaxies.

     Astronomers had been searching for the intergalactic medium
for 30 years, ever since astrophysicists James P. Gunn and Bruce
Peterson first postulated that scientists should be able to
detect the hydrogen originally created in the Big Bang by
analyzing the light from quasars. 

     But scientists, using a variety of telescopes and
instruments, were not able to detect the primordial hydrogen and
concluded that it may have been completely ionized by intense
radiation. To detect the primordial medium, Davidsen decided to
focus on the helium instead.

     The Big Bang theory proposes that a large amount of helium
should have been present in the early universe, said Kriss, an
associate research professor in the Department of Physics and
Astronomy who has been with the HUT project for more than a

     "Everyone was wondering, well, where is it?" he said. "And
by golly, we finally see it."

     A major obstacle in confirming the intergalactic medium's
existence has been the technical difficulty involved in detecting
the helium. It requires a telescope sensitive to wavelengths of
radiation falling within 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 far away that
their light is heavily "contaminated" by clouds of gas and
galaxies in the foreground, yet they are distant enough that
their light is stretched into the proper redshift to be observed
from within our galaxy. 

     Hydrogen gas between the stars of our own galaxy makes the
Milky Way opaque to ultraviolet light below a certain redshift.
The more distant an object is in space, the faster it is moving,
and the more its light has been stretched, or shifted, to longer

     Hopkins astronomers were able to detect the helium by
analyzing ultraviolet light from HS1700+64. By observing such a
remote object, astronomers were essentially looking back to a
time when the universe was less than a quarter of its present
age--about 10 billion years ago--a time when most of the original
hydrogen and helium gas produced by the Big Bang apparently had
not yet condensed into stars and galaxies.

     Intense radiation from quasars apparently has completely
ionized the hydrogen, stripping hydrogen atoms of their single
electrons, which makes them invisible to detection by
spectroscopy because they cannot absorb any of the quasar's
light. But helium atoms in their natural state have two
electrons; some of them have retained an electron, despite the
ionizing radiation, and HUT was able to detect the small portion
of helium atoms that were not fully ionized.

     From the data collected, scientists are able to calculate
how much total intergalactic helium and hydrogen may exist. The
degree of helium absorption detected by the spectrograph suggests
that a massive amount of gas was present in the intergalactic
medium about 10 billion years ago.

     "We are only seeing the tail of the dog," Davidsen said.
"It's enough of a tail to know that it's a very big dog." 

     Ultraviolet measurements made with the Faint Object Camera
on NASA's Hubble Space Telescope in 1994 uncovered a suspected
spectral signature of primordial helium. Astronomers had to wait,
however, for HUT's far-UV sensitivity and higher resolution to
make definitive measurements.

     The degree of the helium's ionization also enabled the
Hopkins astronomers to determine another important detail: while
scientists have debated whether quasars or hot stars in young
galaxies were more likely to have generated the ionizing
radiation, the new HUT data show that quasar radiation is the
most likely source, Davidsen said.

     Meanwhile, the HUT data appear to have provided a partial
answer to the puzzle of dark matter. 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. 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.

     Calculations based on HUT's data now show that the
primordial hydrogen and helium are about equal to the amount of
baryonic dark matter scientists believe exists in today's
universe, Davidsen said.

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