Xiawei Wang

Journey away from the city and electric lights and, with the help of a small telescope, you can see the astronomical objects that populate the universe: galaxies, nebulae, and star clusters. And between these objects is the seemingly empty intergalactic space.

“We used to think that there was nothing between these objects, but actually there is this diffusive radiation background that is spread throughout the universe,” explains Xiawei Wang, a PhD candidate in the Department of Astronomy. Cosmic background radiation was discovered by chance over 50 years ago, but the origin of this radiation continues to stump astronomers.

In the center of all galaxies is a black hole. Some black holes are quiescent, others active. “When a galaxy has an active black hole that is accumulating material from its surroundings, we call this type of galaxy a quasar,” says Wang. But these active black holes are not only sucking up matter. They also spew out gas and material at supersonic speeds. These quasar outflows, Wang argues, can explain the origin of the background radiation that permeates all of space.

“Quasar outflows travel at around a few thousand kilometers per second and this produces shockwaves that accelerate particles in the surrounding medium,” explains Wang. As the particles gain energy, they go through a number of fundamental interactions that lead to the production of three high-energy components of background radiation that Wang studies: gamma-rays, neutrinos, and ultra-high energy cosmic rays.

Wang’s model represents the first time quasar outflows have been proposed as the source of cosmic background radiation and she credits her model to a change in how we measure quasar outflows. “We’ve known about the existence of these outflows for a long time, but signals from individual outflows are faint, far below the sensitivity of our telescopes.” But when Wang looked at the emissions from all the quasars in the universe, suddenly she found not only a significant signal that telescopes can detect, but also something far more interesting. “The integrated emissions from the entire quasar population, can explain the three high energy components of the background radiation,” Wang says. “An individual quasar outflow cannot explain this phenomenon, but the aggregate of all quasars can.”