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Catching Some Cosmic Rays

Vasiliki Pavlidou

Particle astrophysicist Vasiliki Pavlidou says that studying the mysterious gamma rays scattered across the universe is like being a doctor who has to make a diagnosis without ever doing a physical exam. "You don't get to ask them questions, you don't get to do a blood test; you can only look and try and figure out what's going on with them," she says. "I don't know of anybody in this field that does not get at least partially disappointed because our research has this unique property of where nothing works at ground state."

But Pavlidou, a second year postdoc at the University of Chicago's Kavli Institute of Cosmological Physics, likes working on very difficult problems, finding satisfaction in even the smallest forward steps. And one of her favored ways forward is "to use our understanding of exotic high-energy physics [to] see if the gamma rays would speak to us." Ideas developed to explain terrestrial experiments at particle accelerators, Pavlidou says, "not only provide hope for a better understanding of particle astrophysics observations--they are an essential everyday tool in any theoretical model which tries to interpret our observations."

The big picture and the long term

"It can get pretty frustrating and tiring at times, but staying motivated in this field is very important," Pavlidou says. "I am doing what I am doing because I really like it and not because it's necessarily an amazing career path."

Brian Fields, who was Pavlidou's Ph.D. adviser at the University of Illinois, Urbana-Champaign, echoes the thought. "You can get pretty depressed wondering how you can make progress," he says. "What's brilliant about Vasaliki's work is that she is finding ways, even despite this ignorance, to make progress by understanding that there are still some basic trends that you have to expect. She has the ability to step back and look at the larger cosmological picture and see how to place things in that picture."

Compton Gamma Ray Observatory. (Credit: NASA)

Before it fell to Earth in 2000, NASA's Compton Gamma Ray Observatory found 271 individual spots across the sky that produce gamma rays. These mysterious point sources, which produce gamma rays in the GeV range, are "nature's own particle accelerators," Pavlidou says. Unlike the more familiar gamma ray bursts, which are flashes measured in seconds, the objects Pavlidou studies are stable emitters. "You can see them, and you can go back and study them for very long time periods," she says. Some of these sources were determined to be "blazars"--supermassive black holes at the center of galaxies--pulsars, and other known objects. But 170 of those sources "remain unidentified, and we have not been able to associate them with any kind of known physical object," Pavlidou says. "We have absolutely no idea what's making these gamma rays."

Whereas the initial cosmological problem is figuring out what is powering these gigantic accelerators, scientists also want to know--for example--how these particles interact with their environment to produce photons. "These questions are particle physics questions, and models that are built to address them have particle physics built-in in every step," Pavlidou says. Thanks in part to particle physics, "we are finding that we are beginning to constrain some of our understandings and exclude some possibilities."

Dark matter

Pavlidou and her colleagues speculate that some of the unknown sources may be associated with clumps of dark matter. If they're right, this is one area where her work could result in a breakthrough. Nobody knows what dark matter is, and it can't be produced in terrestrial labs. So the best hope for understanding it may be to observe dark matter in its own environment. Up to now this was impossible because dark matter is, well, dark; it isn't directly observable by any technique yet devised. But, Pavlidou says, "Dark matter particles and antiparticles may be annihilating at the centers of very dense dark-matter clumps located in the halo of our Milky Way or in other galaxies. These annihilations can produce gamma rays with a very unique energy spectrum which, if observed, will be the 'smoking gun' of dark-matter particle interactions and will teach us a lot about the nature and properties of the dark-matter particle."

"When we observe an unknown gamma ray source, we look at its energy spectrum, and we use ideas from exotic particle physics to see if it looks like what we would expect from annihilating dark matter. In this sense, particle physics is guiding us in interpreting particle astrophysics observations. We may not have learned yet what the dark-matter particle is like, but we have increased our knowledge about what the dark-matter particle is not like. And in this sense, particle astrophysics is teaching us a lot about particle physics."

Compton map of gamma ray sources. (Credit: NASA)

A binary support system

Working with--and planning a career in--celestial phenomena is inherently uncertain. "From the science point of view, you just don't know enough to plan your career that far ahead. It might turn out in 2 years that some of what you are studying is irrelevant, so you have to remain flexible."

One key to that flexibility is her husband, who, as a postdoc in cosmology, knows just where to bend to absorb the stress the work applies. The two met while doing undergraduate physics degrees back in Greece. They moved to Urbana where they both did astronomy Ph.D.s. In 2005, both managed to snag postdoc fellowships at nearby University of Chicago, where today they continue their work. Despite the advantages of pairing up with another celestial scientist, "It can get kind of crazy, and sometimes you really have to force yourself to have some distractions because no matter how exciting this is, it can drive you crazy."

One such distraction is writing children's stories. One of Pavlidou's first stories was published in Greece and turned into a play and then a radio series. The stories are not about gamma rays or particle astrophysics but stars and galaxies manage to weave their way in. "I can't help it. I guess it's the science geek in me."

Andrew Fazekas is a correspondent at Next Wave and may be reached at

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DOI: 10.1126/science.caredit.a0700003

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