06/15/93
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STANFORD -- In the cosmic playground of the astrophysicist, where time and distance become incomprehensible, and the things that go bump in the night are indescribable, it is very difficult to know directly if the theories are right. No one can do a controlled cosmological experiment - we can't crash galaxies together or make stars collapse.
Scientists using NASA's Compton Gamma Ray Observatory (CGRO), part of which was designed and built at Stanford, may not have all the answers, but they have produced a series of mind-bending questions that have sent theoreticians back to their computer models.
They have found, among other things, that a well-known curiosity, once thought easily explained, is phenomenologically incomprehensible.
"It's just wonderful," said Stanford Physics Professor Peter Michelson. "I don't mean to be hard on theoreticians, but sometimes you have to wonder about them."
Michelson is the Stanford co-investigator for the Energetic Gamma Ray Experiment Telescope (EGRET), one of four devices on the CGRO, an earth-orbiting unmanned observatory designed to study gamma ray sources in space. EGRET was the brainchild of the late Nobel Laureate Robert Hofstadter of Stanford, who died less than a year before the orbiting observatory was launched in April 1991.
EGRET was built in collaboration with NASA's Goddard Space Flight Center, the Max Planck Institute in Germany, and Grumman Aerospace.
Hofstadter designed EGRET to detect hard gamma radiation - electromagnetic radiation ranging in energy from 30 MeV to 30 GeV - in deep space.
One of the targets of the instruments has been to study brief but unimaginably powerful bursts of high-energy gamma radiation coming from among the stars.
The bursts were first discovered by accident in the 1970s by satellites sent up by the Defense Department to monitor nuclear explosions on earth. When the Compton observatory was launched, it quickly confirmed the evidence of these cosmic explosions.
Then things got complicated.
The observatory's instruments found that each gamma ray event would last anywhere from a few milliseconds to a few hundred seconds, Michelson said. They came from random directions in the sky and were irregular in nature, with very complex energy fluctuations in time. The observatory has seen about two a day.
This disputed the theorists' model, which placed the source of the outbursts in the Milky Way galaxy.
"Before CGRO went up, most theorists thought that [the gamma ray events] had to do with a galactic population of neutron stars and that eventually, with the observatory, they would confirm this," Michelson said. "They expected to see a distribution of them in the sky that would reflect a population associated with the plane of our galaxy.
"That's not what the data show."
The observatory found completely random bursts, uniformly distributed over the sky. The incidents were, in the scientific term, "isotropic," meaning spread out all over and independent of direction.
This is hard to explain if they are in our galaxy. The only possible way out would be if the neutron stars producing the gamma rays were in the galactic halo, a theoretical ring of dark matter - space stuff that doesn't show on anyone's telescope - that encircles the Milky Way galaxy.
"That model also has problems," Michelson said, with some glee.
One problem is that there is no way to measure the distance to these events. If the events appear isotropic, that can mean only one of two things: The sources are very close (they appear isotropic because we are too close to discern patterns) or very far away (the whole universe is isotropic).
Now most theoreticians have gone back to their computers and chosen the latter solution. The favored explanation now is that the sources are cosmological, not galactic; they are at the edges of the universe.
"The only question that model does not answer is, what are the sources?" Michelson said. "Where does all that energy come from? How do you generate that much energy? This is one of the big puzzles to come out of the CGRO."
Another problem is that the events are detectable only in gamma rays. They can't be seen visually, for instance. Why?
Further, they do not repeat; a burst is only seen once from a particular direction in the sky. Why?
Also, since they are being seen in large numbers from a little planet in a backwater of a moderately sized galaxy located in no special place in the universe (Earth), these events must be very common. How is that possible?
Questions. Questions. Questions.
One possible solution - and scientists may, of course, never know if it is correct - is that these explosions came from very distant binary sources: two black holes, two neutron stars, or a black hole and a neutron star. When two of these dense objects whirl into each other at the end of their lifetimes, they might collapse into each other. That event might produce a considerable outburst of energy.
That raises another complication: No one knows how gamma rays specifically would be produced in this scenario.
Some bursts, by sheer luck, were detected by EGRET while other detectors on the Compton observatory were seeing them. EGRET found for the first time that very high-energy gamma rays are coming from the bursts - much higher than had been detected with other instruments.
"That is a big surprise," Michelson said. "I don't think anybody expected that."
The earlier models had led researchers to believe that there would be no high-energy gamma rays because those photons would have had trouble getting out of the source, due to interference from other radiation.
"As you go higher in energy, the spectrum should fall rapidly," Michelson said.
EGRET showed that sometimes the bursts persisted out to 10 GeV.
The fact that these are quick flashes in the sky means they must come from a very compact region; they are not things the size of galaxies," Michelson said. "They are coming from objects the size of small stars or planets.
"There is one out, one way you can do it," Michelson said. "If nature somehow could make a source where instead of gamma rays going isotropically in all directions from the source . . . if somehow the source beams the gamma rays in one direction so that they don't bump into each other, then the high-energy gamma rays can get out of the source."
If the radiation is coming from the edges of the universe, it must be that the sources are beamed strongly, like some unthinkably powerful headlight hurling off gamma-ray photons from billions of miles away. Quasars are the only other known astronomical objects that can beam huge amounts of high energy in this way (another discovery made by EGRET).
The objects in question here, however, are not quasars.
And if the explosions are beamed, then there are a lot more of them than we are seeing. Most beams would be aimed elsewhere than at Earth and would be missed.
"The problem with making the source cosmological is that it makes the problem worse," he said. "The further you put them away, the more energy would have to be involved."
"We don't even know the energy scale," he said. "If they are in our galaxy, say 10 kiloparsecs (30,000 light years) away, then the energy in the explosion is about 1041 ergs. [An erg is a unit of work or energy measured as one centimeter per gram per second.] If you make them cosmological, it's something like nine or 10 orders of magnitude larger - a hundred billion times larger."
That is 1017 times more than the sun produces. In other words, these objects, in 10 seconds, put out more energy than our sun has produced in tens of thousands of years.
"And that makes the problem even worse," Michelson said.
The cosmological solution may solve the isotropy problem, but now physicists are faced with the energy problem: How can that much energy be produced by a solitary event?
The Compton Gamma Ray Observatory has so changed the field that the astrophysicists who advocated the cosmological explanation before the satellite was launched were considered outside the mainstream. Since the observatory's readings, they have move into mainstream and now represent the current conventional wisdom - "all based on getting more data."
"It's amazing what a little data will do for a field," Michelson said with a smile.
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