Cosmic neutrinos could yield key insights, but detection devices would need to be the size of a star or galaxy

In "Too Hard for Science?" I interview scientists about ideas they would love to explore that they don't think could be investigated. For instance, they might involve machines beyond the realm of possibility, such as particle accelerators as big as the sun, or they might be completely unethical, such as lethal experiments involving people. This feature aims to look at the impossible dreams, the seemingly intractable problems in science. However, the question mark at the end of "Too Hard for Science?" suggests that nothing might be impossible.

The scientist: Lawrence Krauss, director of the Origins Project and co-director of the Cosmology Initiative at Arizona State University.

The idea: The most famous relics left over from soon after the Big Bang are microwaves that permeate the entire cosmos. However, this cosmic microwave background radiation — the first light to escape matter after the Big Bang — appeared nearly 400,000 after the birth of the universe, and much of what happened beforehand remains mysterious.

On the other hand, neutrinos were created just a second or so after the Big Bang, and an average of more than 150 should still fill each cubic centimeter of the universe. "The cosmic neutrino background holds direct signals of what the universe was like at its earliest moments, Krauss says. "These neutrinos could tell us a lot about what happened back then if we can directly detect them."

The problem: Normal neutrinos are extraordinarily difficult to detect, as they rarely collide with atoms. These oldest neutrinos are even harder to spot, having cooled down to just 1.95 degrees above absolute zero in the 13.7 billion years since the Big Bang, and such low-energy particles are even more loathe to interact with matter.

"One of these neutrinos can probably travel on average through something like 1 million or 1 billion light years of lead before interacting with it," Krauss says. "Their interaction rates are many orders of magnitude smaller than our most sensitive instruments can currently detect, so unless one can think of a tricky way to capture them, you might need a detector about the size of a star or maybe even our galaxy."

The solution? Although these relics of the Big Bang might be too hard to detect directly, cosmologist Roberto Trotta and his colleagues have found that collectively, the gravitational pull of these neutrinos influenced the development of the universe, with the effects of ripples in this primordial sea of neutrinos visible in the cosmic microwave background.

Scientists are also dreaming up other ways to detect these ancient particles. Theoretical physicist Andreas Ringwald of DESY, Germany's particle physics center near Hamburg, suggests experiments involving radioactive elements such as tritium or detectors focusing on the interactions of these old neutrinos with extremely high-energy cosmic neutrinos could provide the most direct evidence for these relics. "A crazy idea like this needs equally radical solutions," Krauss says.

Image of Lawrence Krauss from his Web page


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About the Author: Charles Q. Choi is a frequent contributor to Scientific American. His work has also appeared in The New York Times, Science, Nature, Wired, and LiveScience, among others. In his spare time he has traveled to all seven continents. Follow him on Twitter @cqchoi.

The views expressed are those of the author and are not necessarily those of Scientific American.