Johns Hopkins Gazette: May 5, 1997

Much Of "Missing
Matter" Found

Emil Venere
News and Information
Two Hopkins astrophysicists may have solved a problem that has puzzled scientists for years.

Astronomers have been unable to account for all the matter they believe would have been needed for the creation of the universe in the Big Bang. But the Hopkins astrophysicists may have found much of the "missing matter" by using a new method to study the early cosmos.

Arthur Davidsen said the recent findings represent his best work in 20 years, since he embarked on a daunting challenge: to detect the original helium that was part of a primordial "intergalactic medium" of gas created at the birth of the universe.

In 1995 Davidsen and Hopkins astrophysicists Gerard Kriss and Wei Zheng used the Hopkins Ultraviolet Telescope, operated from within the cargo bay of the space shuttle Endeavour, to detect helium in the early universe. By showing that significant amounts of gas existed in the space between galaxies, the discovery reaffirmed the theory that hydrogen and helium were formed in the first three minutes after the Big Bang, a violent cosmic explosion that created the universe about 15 billion years ago, hurling matter in all directions.

Now Davidsen and Hopkins postdoctoral fellow HongGuang Bi have used a new analytical method they invented to show that some of the missing matter is the primordial medium of hydrogen and helium.

Their new analytical method was detailed in a scientific paper published on April 20 in the Astrophysical Journal. The astronomers have given several presentations about their work, and it has been well received by other scientists in their field, said Davidsen, who is interim dean of the faculty of the Krieger School of Arts and Sciences.

"I have been very excited about this recent work," said David Schramm, a University of Chicago astrophysicist involved in similar research. "A long-standing problem in cosmology is, Where is all the normal matter? Stars and galaxies do not add up to as much normal matter as we feel must be there from our analyses of nuclear processes that took place in the early universe. Davidsen and Bi appear to have found the normal matter out between the galaxies. Furthermore, the amount they find is completely consistent with the amount we expected to be there from our nuclear physics arguments, so the whole picture holds together remarkably well."

The dark-matter problem can be summarized like this: The universe is made of visible matter and so-called dark matter. Visible matter is seen in the form of stars and galaxies, which emit light and other forms of radiation. Dark matter has not been seen directly, but it is inferred to exist from the gravitational effects it appears to exert on the visible matter.

Dark matter itself apparently comes in at least two varieties. One component is made of ordinary "baryonic" matter-- subatomic particles such as protons and neutrons--the same stuff that makes up all the visible matter in the universe. But astronomers have been faced with a dilemma: the visible stars and galaxies contain only a small fraction of all the baryonic matter that is thought to exist. Where is the rest of it?

The other component of the dark matter is widely believed to be some unknown exotic particle that does not emit or absorb light.

Davidsen and Bi suggest that the missing baryonic matter has been found. It was spread throughout intergalactic space in the form of a diffuse gas of hydrogen and helium atoms. The newly found matter amounts to about 10 times as much as all the matter previously known to exist in stars and galaxies, Davidsen said.

The findings don't address the nature and amount of the exotic type of dark matter, which scientists believe makes up a majority of all matter in the universe.

"The exotic dark matter could still total an additional 10 to 20 times as much as the normal baryonic matter we have now found," Davidsen said.

Nevertheless, finding the missing baryonic matter represents a major milestone. "The missing baryons used to be one of the so-called 'dark matter problems,' but this matter is no longer dark, thanks to the work of Davidsen and Bi," Schramm said.

Astronomers believe that the simplest elements, hydrogen, helium and deuterium, were created in the Big Bang. Those simple elements formed stars, in which the more complex elements were manufactured. Exploding stars later released the more complex elements, such as carbon, nitrogen and oxygen.

But how did the hydrogen and helium come together to form stars in the first place? Astronomers believe that concentrations of the exotic type of dark matter formed gravity "wells" that attracted the gases, beginning the process of star and galaxy formation. The Johns Hopkins astronomers have calculated the effect of the dark matter on the ordinary baryonic matter, which started out as a uniform medium but eventually developed a "lumpy nature," with regions of higher and lower concentrations, Davidsen said.

He and other scientists have studied the nature of the medium by using spectrographs to analyze light emitted by extremely distant objects called quasars. Astronomers find places in the sky where there are no galaxies, to get a clear line of sight to a quasar. As the light from the quasar shines through space, it also shines through the gas, like a headlight through fog. The quasar is so far away that the light now reaching Earth is from a time when the universe was roughly one-quarter its present age, about 10 billion years ago. So, in effect, astronomers are looking back in time.

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