Astronomers, Using New Method, Find Evidence For Missing Matter

April 21, 1997

For years, scientists have been unable to account for all of the material they believe would have been needed to form the cosmos billions of years ago. But now two Johns Hopkins astrophysicists may have found much of the "missing matter" by using a new method to study the early universe.

Their new analytical method is detailed in a scientific paper to be published on April 20 in the Astrophysical Journal. The paper was written by astrophysicists Arthur F. Davidsen and HongGuang Bi.

"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 appears to come in at least two varieties. One component is made of ordinary "baryonic" matter, the same stuff that makes up all the visible matter in the universe. Baryons are ordinary matter particles like protons and neutrons. But astronomers have not been able to account for all the baryonic matter that is thought to exist, based on their studies of the nuclear reactions that occurred during the Big Bang. The visible stars and galaxies contain only a small fraction of the total amount of such ordinary baryonic matter believed to exist. The other component of the dark matter is widely believed to be some sort of exotic particle that does not emit or absorb light.

The analysis reported in the Hopkins paper suggests that the missing baryonic matter has been found. It was spread throughout intergalactic space in the form of a very diffuse gas of hydrogen and helium atoms whose presence is detected through its effects on light passing through it. These 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.

Astronomers had thought that the primordial medium of gases that existed in the early universe was contained in individual "clouds," with nearly empty space in between. But the Johns Hopkins astronomers have found evidence that the gases were not arranged that way. Using their method, Davidsen and Bi propose that the early universe contained a "continuous medium" of hydrogen and helium gases, with regions of higher and lower density blending together smoothly.

Although other scientists are using powerful supercomputers to make similar calculations about the evolution of the universe, the Johns Hopkins scientist have devised a method that requires only "fairly simple analytical equations," Davidsen said. They used their analytical method to explain data from observations made by other astronomers over the past 20 years.

Astronomers have detected the primordial hydrogen gas by using spectrographs to analyze light emitted by very 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.

But intense radiation from quasars and early galaxies has ionized much of the gas, stripping away electrons from the atoms and making the gas largely invisible to detection by spectroscopy. So astronomers are only detecting a small portion of the gas.

"The gas is so highly ionized that we are seeing only the tail of the dog," Davidsen said. "It's a big dog but we are only seeing the tail. If we had a theory that told us exactly what dog it is, based on what the tail looks like, then we could say something. That's what we have now -- a theory that connects the tail to the dog. We now believe we can say how much intergalactic gas, baryonic material, there must have been."

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 those more complex elements.

But how did the hydrogen and helium come together to form stars in the first place? Astronomers believe that concentrations of the exotic form of dark matter formed gravity "wells" that attracted the gases, beginning the process of star and galaxy formation. The Johns Hopkins astronomers have used their method to see that process going on in the universe about 10 billion years ago, Davidsen said.

"Although a small fraction of baryons had by then managed to condense into stars, galaxies, and quasars, it now appears that most of them were still spread throughout intergalactic space, in the form of very diffuse hydrogen and helium gas that was ionized by the ultraviolet radiation of the quasars," Davidsen said.

The method was inspired by previous findings with the Hopkins Ultraviolet Telescope, which was operated from the cargo bay of a space shuttle in 1995. HUT observations of the primordial helium yielded data that contradicted the theory that the primordial gases were contained only in discrete clouds.

"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.
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Johns Hopkins University

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