When it comes to unlocking the secrets of the universe, sometimes the devil is in the static. In the 1960s, scientists trying to find a source of radio interference discovered the microwave background radiation—extremely faint embers left over from the heat of the big bang that pervade the entire night sky. Now two physicists attempting to overcome some unexpected fuzziness in images of distant, supermassive black holes say they have found yet another potential big bang vestige: an extremely weak magnetic field that stretches across the universe. If scientists confirm the finding, it could help reveal the origins of magnetism in the cosmos.
The researchers who made the discovery found an important clue in images of the supermassive black holes that occupy the centers of most if not all galaxies. With the mass of up to a billion suns, these monsters can create havoc in their neighborhoods by gobbling up any stars, clouds of dust and gas, and even other black holes unfortunate enough to drift too close to their immense gravitational wells. As they consume matter, supermassive black holes expel gigantic amounts of energy in the form of huge jets of particles that extend well beyond their galactic confines and travel at nearly the speed of light.
In the new study, physicists Shin'ichiro Ando of the California Institute of Technology in Pasadena and Alexander Kusenko of the University of California, Los Angeles, examined images of supermassive black holes collected by the Fermi Gamma-ray Space Telescope looking for something that had been suspected for a long time but never observed: the possibility of a primordial, intergalactic magnetic field.
If it exists, such a magnetic field could scatter high-energy photons emitted from the jets of a supermassive black hole and blur images collected by detectors aboard the Fermi spacecraft just as a mist can blur an image on a photograph on Earth. The effect is so minuscule, however, that it can't be seen with current technology on a single image of jets from a supermassive black hole. So Ando and Kusenko took Fermi data from 170 different black holes and combined them into a single, composite image. Then they compared the composite with the product of a mathematical model, showing what the image should have looked like if all of the high-energy photons from the supermassive black holes had hit Fermi's detectors at the expected energy levels. But the real and simulated images didn't match (insets).
Further analysis yielded the result the two researchers were seeking: Somewhere in the vastness of space, the millions or billions of light-years between the black hole jets and Fermi's instruments, something was indeed scattering the photons—and very, very subtly. Calculations by the researchers, published online 17 September in The Astrophysical Journal Letters, suggest that a magnetic field equivalent to about one-quadrillionth the strength of Earth's magnetism had interacted with the photons. This phenomenon had distorted their energies just enough to create a "halo" effect in the gamma ray images of the supermassive black holes.
"The early universe had several mechanisms for generating magnetic fields," Ando says, explaining that those "primordial seed fields" were probably amplified within galaxies by the convective motions of hot gas. The findings could be "a real game changer," Kusenko adds.
"It's a very interesting paper that is really pushing us toward areas we've never observed before," says astrophysicist Christopher Reynolds of the University of Maryland, College Park. He says the researchers' conclusions are very sensible, but he cautions that there might be another potential source for the magnetic field, what he calls "intergalactic pollution" caused by those very same high-energy, magnetically charged particle jets emanating from supermassive black holes. On the other hand, he adds, now that Ando and Kusenko have provided a measurement of the intergalactic field's strength, scientists can try to model it to see whether it's primordial or a result of high-energy jets.