Research Rundown

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Science Journal, Fall 1995 Vol 13, No. 1

Astronomers have discovered direct evidence that most quasars came into existence during the same era, when the universe was still in its infancy. This discovery will help scientists use quasars, the most luminous objects in the sky, as tools for studying the universe back to a time when it was less than a billion years old.

"This survey allows scientists to investigate for the first time the era of quasar formation," says Maarten Schmidt, a Caltech astronomer and a coauthor of the study.

Using data from the recently completed quasar search known as the Palomar Transit Grism Survey, Schmidt, Donald P. Schneider of Penn State, and James Gunn of Princeton University published their discovery in a recent issue of the Astronomical Journal. (A grism is a transmission grating mounted on a clear, wedge-shaped piece of glass. The term is a combination of the words "grating" and "prism.")

The survey shows that the space density of quasarsóthe number of quasars in a given volume of spaceóreaches a maximum for those with redshifts between 1.7 and 2.7 and declines steeply for quasars with higher redshifts.

"This maximum means there was a peak in the rate of quasar formation between 1.9 and 3.0 billion years after the Big Bang," Gunn said, "and a much lower rate earlier in the history of the universe."

A typical quasar emits 100 times more energy than our home galaxy, the Milky Way, which makes them the most luminous and also some of the most distant known objects in the universe. Because light from quasars takes billions of years to reach Earth, scientists see quasars as they were billions of years ago. Quasars are important to astronomers as one of the best probes available for studying the conditions present in the early universe.

Astronomers first identified quasars in 1960 as starlike counterparts to strong sources of radio waves, but were initially unable to determine the nature of the objects. In February 1963, Maarten Schmidt made a breakthrough.

"I recognized that the pattern of spectral lines in one particularly bright quasar was due to hydrogen, but that the location of the lines was redshifted," Schmidt says. "This indicated that the object was moving away from the earth at a very high velocity."

Redshifting is an effect seen in rapidly receding sources of light, where the spectral lines of such sources move toward longer wavelengths, or toward the red end of the visible spectrum. The larger the redshift, the more the light is shifted toward red, and the greater the distance to the source.

The small size of quasars is as astonishing as their luminosity. Studies of the variability of quasars have shown that their brightness can change on time scales of days, or sometimes just a few hours, which implies that their physical size is not much larger than our solar system. Because of quasars' extraordinary brightness and small size, astronomers suspect that they are probably powered by matter spiraling into a supermassive black hole. But just how quasars form and whether black holes really power them remains a puzzle, one which studies such as the one reported here will help scientists solve.

The Palomar Transit Grism Survey was undertaken with the goal of finding a large number of high-redshift quasars so that scientists could study the evolution of these objects back to a time when the universe was less than a billion years old. The survey began in 1985 using a special electronic camera designed by James Gunn that was mounted on the 200-inch Hale Telescope at Palomar Observatory.

Finding a large number of quasars was like looking for needles in a haystack and required special software to separate the quasars from superficially similar foreground objects. "For every high-redshift quasar that we found, we recorded and sorted through thousands of nearby objects," Schneider said.

The Palomar Transit Grism Survey succeeded in identifying 90 quasars with redshifts between 2.75 and 4.75, with a typical luminosity more than a trillion times that of our sun. Analysis of the survey data has revealed that between redshifts of 2.7 and 4.7, the space density of luminous quasars declines by a factor of seven. That is, for quasars with redshifts greater than 2.7, the higher the redshift, the fewer quasars there are in a given volume of space.

Previous studies by other groups have shown that the space density of quasars increases dramaticallyóby a factor of 100 or moreóin the range of redshifts between 0 and 2.0. These results, combined with other studies of quasars with intermediate redshifts, show that the space density of quasars exhibits a sharp peak at a redshift between 1.7 and 2.7, indicating that the bulk of quasar formation must have occurred around 2.5 billion years after the Big Bang. This result will help astronomers refine their theories by placing important constraints both on models of galaxy and quasar formation and on ideas about the mechanism that supplies quasars with their tremendous energy.

Jay Aller (CalTech) and Barbara K. Kennedy

The first experimentally testable prediction about the quantum structure of the geometry of space has been produced by physicists developing a theory of quantum gravity.

Carlo Rovelli, professor of physics at the University of Pittsburgh, and Lee Smolin, professor of physics at Penn State and visiting member of the Institute for Advanced Study, describe their discovery in a paper published in a recent issue of the journal Nuclear Physics B. They discovered that the theory of quantum gravity requires that space, like ordinary matter, is not continuous but is made of a network of discrete elements. Rovelli and Smolin say the size of these elements is 10-33 centimetersó20 orders of magnitude smaller than the nucleus of an atom.

The idea that space and time may not be continuous but are built, like matter, from very tiny "atomic building blocks" is not newóit was proposed previously by Roger Penrose, of Oxford University and Penn State, and other physicists and mathematicians. The work of Rovelli and Smolin is, however, the first to show that these discrete structures are required by the laws of quantum mechanics and relativity theory.

"Just as an atom can have only a certain discrete set of energy levels, the results of geometrical measurements must come in discrete units," Smolin explains. In other words, measurements of the area or volume of any region in space cannot be just any number but must lie in certain sets of discrete numbers, which the calculations of Rovelli and Smolin are able to predict. "One way to describe these predictions is that the geometry of space itself is made out of discrete quanta analogous to the photons of light or the electron shells of the atom," Smolin explains.

Although no instrument exists today that actually can measure small enough areas or volumes to see the discrete structures they predict, Smolin says "We are confident about these predictions and believe they could be tested sometime in the future." Rovelli adds, "That is what one wants in scienceóto make definite predictions that could lead to confirmation of whether a theory is right or wrong."

Scientists have been looking for the right theory of quantum gravity to complete the revolutionary picture of nature begun early in this century with the invention of quantum mechanics, the modern theory of matter, and Einstein's theory of general relativity, the modern concept of space and time. "The unification of these two theories remains one of the key unsolved scientific problems," Rovelli explains.

Rovelli and Smolin's discovery of the atomic structure of the geometry of space is the result of a seven-year collaboration, which they say is built on a breakthrough discovery in 1986 by Abhay Ashtekar, Holder of the Eberly Family Chair in Physics and Director of the Center for Gravitational Physics and Geometry at Penn State. Ashtekar discovered that the equations of Einstein's general theory of relativity could be translated into a much simpler form, making it easier to combine relativity with quantum mechanics. "Ashtekar's work was the breakthrough that has enabled us and others to finally make progress on this problem," Smolin says. The following year, Smolin and Ted Jacobson, professor of physics at the University of Maryland, discovered that Ashtekar's form of the theory could be used to solve the equations of quantum gravity for the first time.

These solutions showed that the gravitational field could be seen in a new perspective. "Instead of thinking of the quantum unit of gravity as a particle or a wave, we describe the geometry of space as patterns of closed loopsóthe lines of force of the gravitational field," Smolin says. "The gravitational field has lines of force just like the lines of magnetic force around a bar magnet," Rovelli explains.

These gravitational lines of force are the basis for the "loop representation" picture of quantum gravity that Rovelli and Smolin invented in 1988, which is the basis of their predictions about the atomic nature of space. "The way the lines of force form loops that knot and link and meet each other is the basis for the quantum description of the geometry of space we have discovered," Smolin explains.

Rovelli and Smolin explain they use the word "loops" because the lines of gravitational force always form loops when there is no matter around. According to the physicists, Einstein's theory of relativity allows space to have a geometry even where there is no matter. "Where there is matter, the gravitational lines of force end on the matter," Rovelli explains, just as magnetic lines of force end on the poles of a magnet. He says lines of magnetic force in empty space form loops, as well.

Rovelli and Smolin's calculations also reveal that these quantum loops of space must be linked together in networks called spin networks, which look like an electric-circuit diagramóa pattern of lines joined to each other at various points, or nodes. The lines and nodes in spin networks are labeled by geometric quantities, such as units of area and volume, rather than by voltages and resistances, as in circuit diagrams. "Spin networks are the quantum states of gravity just as electron shells are the quantum states of the atom," Rovelli explains.

"Roger Penrose dreamed up spin networks 30 years ago as a beautiful picture of the quantum-mechanical geometry of space," Smolin says. He describes Penrose, the Francis R. Pentz and Helen M. Pentz Distinguished Visiting Professor of Physics and Mathematics at Penn State, as perhaps the most influential and creative relativity theorist living today. "Now our calculations have rediscovered these same wonderful structures mathematically, vindicating Roger's intuition that spin networks describe configurations of the basic building blocks of the geometry of space," Smolin says.

Rovelli and Smolin suspect that, if the loop representation approach to quantum gravity turns out to be correct, it will have implications for other key unsolved problems. "In many theories in physics there are situations in which you try to make a physical prediction but you get an infinite quantity," Smolin explains. These infinite quantities are very troubling to scientists studying such things as the early universe, black holes, and other areas of physics. "If the loop representation is right about this prediction, then the infinities simply are not there because nothing can be infinitely small," Rovelli explains. "The networks of loops define space, they are not in it, so nothing can be smaller than them," says Smolin.

The two theorists are investigating the relationship of their results to other approaches to quantum gravityóparticularly string theory. "String theory is the only other approach to quantum gravity to have yielded definite physical predictions," Smolin says. "The loops in our theory are not strings, but there are deep connections between the two approaches that are most intriguing."

This research was supported partially by the National Science Foundation and by Penn State.

Barbara K. Kennedy

Note: This past May, an error was found in one of the calculations of Rovelli and Smolin that changed the values of some of the "units" of quantized volume, without changing the basic result that the volumes and areas are quantized. The error was detected by Renata Loll, a young German physicist who had been a postdoctoral fellow in the Penn State Center for Gravitational Physics and Geometry and is now a postdoctoral fellow in Florence, Italy. She did the calculation using a different method from the one used by Rovelli and Smolin. Looking back at their calculations, Rovelli and Smolin found they had made an error in the sign of a term in the expressions for the units of volume.

In another recent development, the prediction that areas should be quantized has been used by another group of physicists as the basis of calculations in which they make new predictions about radiation emitted from black holes.

Fossils discovered in East Africa represent a new species of human ancestor that lived about four million years ago, according to findings published in a recent issue of Nature magazine. The discovery sheds new light on the question of when and where upright posture first evolved.

"These fossils place the emergence of bipedalism further back in time by a half million years," said NSF-supported paleoanthropologist Alan Walker of Penn State, a member of the research team. "This gets close to the hypothesized time of splitting of the ape and human lineages, and fills in a bit more of the gap in our knowledge of human evolution."

The search for the earliest hominid Ñ the animals on the human family tree Ñ took place in two locations in Kenya's Turkana Basin area. Scientists knew that hominids and African apes share a common ancestor, but lacked the clues to indicate whether that ancestor was quadrupedal or bipedal. In search of this clue, Walker led a research team at Allia Bay while paleontologist Meave Leakey, with funding by the National Geographic Society, investigated a site called Kanapoi.

Fossils discovered at the Kanapoi site Ñ including jaws, teeth and a lower leg bone Ñ were dated at between 3.9 and 4.2 million years old, and showed clearly that their owner walked upright. Leakey and Walker placed the fossils in a new species named Australopithecus anamensis (A. anamensis).

Other research efforts have turned up clues to the emergence of uprightness. In 1994, paleoanthropologist Tim White of the University of California, Berkeley Ñ also supported by the National Science Foundation Ñ announced the discovery in Ethiopia of even older fossils, which he identified as Ardipithecus ramidus (A. ramidus), a new genus and species of hominid.

The researchers in Kenya theorize that the Ethiopian fossils discovered by White belong to a sister species to all later hominids, and represent a different branch of the hominid tree. They further theorize that the Kenyan fossils represent the surviving branch Ñ the actual forerunner to our species, Homo sapiens.

The discoveries at Turkana Basin help to paint a picture of the being that bridges the split between apes and humans but leave unanswered questions, pointed out Walker. "What is the relationship of bipedalism to this split? Did bipedalism cause the split? As with so many scientific discoveries, this one also provokes more fascinating questions," he said.

The Kenya findings are also discussed in an article by Leakey, published in the September 1995 issue of National Geographic magazine. Other members of the Kenyan research team were Craig Feibel of Rutgers University and Ian McDougall of Australian National University.

Mary Hanson (National Science Foundation)

A new method of "absolute genetic dating" announced recently by scientists at Penn State promises to rejuvenate molecular studies of the evolution of humans and other animals. While it has not yet resolved disputes over humanity's origin, the technical advance has undoubtedly shifted the terms of the debate.

David B. Goldstein, a postdoctoral fellow working in the laboratory of Andrew G. Clark, professor of biology, along with their colleagues, devised a way to measure genetic variation between populations at certain sites in nuclear DNA. This enabled them to calculate that an initial split in human evolutionary development probably occurred between Africans and non-Africans about 156,000 years ago.

"Our genetic data suggest that modern humans originated in Africa and spread from there to the rest of the world sometime during the last 150,000 years or so, lending strong support to the out-of-Africa theory of modern human origins," Goldstein and his coworkers conclude in a recent issue of the Proceedings of the National Academy of Sciences.

Most prior studies of genetic evolution used mitochondrial DNA, which is found outside the cell nucleus. Although this approach often supported the out-of-Africa model, statistical flaws undermined it.

Supporters of multiregional evolution argue instead that Homo sapiens evolved simultaneously in different parts of the world, beginning 1 million or more years ago.

Goldstein's group studied nuclear DNA segments called microsatellites. Nuclear DNA consists of 23 pairs of strand-like chromosomes built up from structural units called nucleotides. At microsatellite sites, chromosome pairs carry repeated nucleotide sequences; these sites often contain between two and five nucleotide repeats, but the number can reach 40 or more.

Several thousand microsatellites have been identified over the past decade. No one understands their functions fully. Because nucleotide repeat sequences often get added or deleted as a unit, the researchers theorize that the extent of population differences at microsatellite sites marks the passage of time since groups halted consistent interbreeding.

They applied a new statistical method for measuring the extent of multiple microsatellite differences to 30 microsatellite sites in 14 native populations from around the world. The entire sample consisted of 148 individuals.

An evolutionary tree reconstructed from the microsatellite evidence shows an initial split between Africans and non-Africans, the researchers contend. If the time span between ancient human generations averaged 27 years, the observed microsatellite differences indicate that the split occurred about 156,000 years ago, they maintain. That number may range from 75,000 to 287,000 years ago, depending on the accuracy of their estimate for microsatellite mutation rates.

It also remains unclear whether small, isolated groups of prehistoric people evolved independent microsatellite mutations that further complicate attempts to date human genetic origins, writes Penn State's Masatoshi Nei, distinguished professor of biology and director of the Institute of Molecular Evolutionary Genetics, in an accompanying comment. Still, Nei suspects that the genetic split of Africans and non-Africans dates to around 115,000 years ago if, as he suggests, an average of only 20 years separated succeeding prehistoric generations.

Goldstein's new measure of microsatellite differences represents a significant advance, says Alan R. Templeton of Washington University in St. Louis. But Goldstein uses the measure to help date an assumed split between Africans and non-Africans, which other genetic data indicate never occurred, Templeton holds. His earlier mitochondrial DNA analysis indicates that human populations grew separately within continents for nearly 1 million years, with occasional interbreeding across continents.

Bruce Bower

(reprinted with permission from Science News, the weekly newsmagazine of science, copyright 1995 by Science Service, Inc.)

A team of astronomers has caught one of the brightest radio galaxies in the act of producing X-rays from its "radio lobes"óa feat that has eluded astronomers since 1965 when the phenomenon, known as inverse Compton X-ray scattering, was first predicted. Using a powerful orbiting X-ray observatory, the astronomers have confirmed the prediction that fast-moving particles in radio lobes produce X-rays when they collide with low-energy photons in the cosmic microwave background.

The research is considered significant because it establishes, for the first time, the strength of the magnetic field in a radio galaxy's lobes and the energy output of its nucleus.

Using the German/U.S./British satellite, ROSAT, launched in 1990, the astronomers detected diffuse X-rays from the galaxy Fornax A (NGC 1316) coinciding with its two regions of intense radio emissions known as radio lobes. This galaxy, the brightest radio source in the constellation Fornax and the fourth-brightest radio source outside our galaxy, is 50 million light years from Earth.

Results of the research by Penn State professor of astronomy and astrophysics Eric D. Feigelson, Penn State graduate student Sally Laurent-Muehleisen, Penn State postdoctoral fellow Ronald I. Kollgaard, and National Radio Astronomy Observatory astronomer Edward Fomalont were presented in June 1995 at the American Astronomical Society meeting in Pittsburgh, PA.

Radio galaxies like Fornax A are thought to contain a central massive black hole that shoots out two jets of electrons and other particles at enormous energies. Spiraling rapidly outward in intense magnetic fields, the "relativistic" electrons in these jets emit powerful radio waves as they spread out into huge pools on opposite sides of the galaxy hundreds of thousands of light years from its center. In Fornax A and many other radio galaxies, these lobes, which are invisible to ordinary telescopes, are much larger than the region containing the galaxy's stars.

Feigelson says the prediction that electrons in these radio lobes would collide with photons in the cosmic microwave backgroundóthe residual radiation from the Big Bang that permeates the entire universeówas made by the distinguished astrophysicist Fred Hoyle in 1965. This process, known as inverse Compton scattering, is "analogous to what happens in a game of pool when a fast-moving cue ball transfers some of its energy to a stationary billiard ball, which then goes shooting off," Feigelson explains. Similarly, when a low-energy photon gets hit with one of the fast-moving electrons, it gets "scattered up" to X-ray energies.

"The X-ray emission is much fainter than Hoyle predicted," Feigelson says, explaining that its faintness is one reason the X-rays have remained so elusive for three decades. "In order to detect the emission, we had to focus the best X-ray telescope in the world on the best radio source in the sky, and then persevere in making numerous detailed corrections to remove various sources of background noise." Feigelson says he selected Fornax A because, among the brightest radio galaxies, it is the least contaminated with extraneous sources of X-rays.

By measuring the intensity of the X-rays emerging from the radio lobes, the astronomers were able to calculate, for the first time, how much energy is in the relativistic electrons. Before this research, astronomers could measure only radio emissions from the lobes and were unable to distinguish the energy contribution of the relativistic electrons from that of the magnetic fields. Now, by also measuring the intensity of the X-rays produced by inverse Compton scattering in the lobes, they can directly measure the number of relativistic electrons.

"These observations are critical for establishing the total energy in the lobes and the energy being emitted by the powerful cauldron buried at the center of the galaxy," explains Kollgaard. "The idea that the energy contributions from the electrons and the fields should be about the same is appealing because it is nearly the same as the minimum energy needed to produce the radio emission," he says.

"Our observations roughly agree with the prediction of minimum energy," says Feigelson. But there is some uncertainty in the interpretation of the ROSAT data. "While the spatial agreement between the X-ray and radio structures is excellent, the X-ray spectral data are inconclusive," he says. "An independent research group working with the Japanese ASCA satellite confirms the ROSAT discovery, but suggests that X-ray-emitting hot gas, as well as inverse Compton scattering, may be associated with the Fornax A radio lobes."

Feigelson, who has been persistently looking for X-rays in radio galaxies since he failed to find them when he was working on his Ph.D. thesis fifteen years ago, says that "Fornax A provides the most convincing case to date for inverse Compton X-ray emission from scattering of the cosmic microwave background by relativistic electrons in radio lobes." He adds that he never could have done it without ROSAT. "When you have such a well-built and sensitive instrument you can make satisfying discoveries," he says.

This research was sponsored by the National Aeronautics and Space Administration. The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation.

Barbara K. Kennedy

The eighty-five-foot-diameter dome covering the William P. Hobby-Robert E. Eberly telescope, which was conceived by Lawrence W. Ramsey and Daniel W. Weedman, Penn State professors of astronomy and astrophysics, was lifted into place at the McDonald Observatory in Texas on July 7, 1995.

The Hobby-Eberly Telescope is a partnership involving Penn State, the University of Texas at Austin, Stanford University, and two German universities. According to Ramsey, the telescope gives Penn State and its partner institutions access to one of the largest telescopes in the world at a "bargain-basement" price less than one-fifth the cost of others in its class. It will be the largest and most powerful telescope in the world designed for spectroscopic astronomy, the measurement of individual wavelengths of light from objects in space.

"The installation of the dome puts in place the last large structural component of this unique telescope facility," Ramsey says. The telescope structure (visible in the picture on the cover) and the 11-meter mirror truss were installed in late spring and early summer, 1995. The 91 mirror segments that will make up the primary mirror will be installed during the next year. "We eagerly anticipate first lightówhen the telescope will first begin to realize its potential to carry out scientific researchóin mid 1996," Ramsey says.

Penn State scientists intend to use the Hobby-Eberly Telescope to study the most distant quasars, to understand the early history of the universe, and to probe for the existence of dark matter in and around galaxies, as well as to search for planets around other stars and to study the properties of newly born stars.

When completed in 1997, the Hobby-Eberly Telescope will be the largest telescope in the world that members of the public can view from a visitors' gallery.

Barbara K. Kennedy

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