Gaze up at the sky on a clear night, far from the city lights, and you might see the moon and some stars, a few planets and maybe even the fuzzy glow of the Milky Way. “In between, there seems to be nothing but emptiness and darkness,” says Xiawei Wang, an astronomy graduate student at Harvard University. “At least that was our view of the cosmos until 50 or so years ago when astronomers realized that space was not as empty as we once thought.”
In 1964, astronomers were surprised to discover a uniform sea of radiation coming from all directions, all the time. Later dubbed the cosmic microwave background (CMB), this low-energy light turned out to be the residual radiation from the Big Bang itself, cooled during its journey over the past 13.8 billion years. Studying the CMB has led to tremendous insights about the structure and composition of our universe, its exact age and even its shape.
In the decades since, astronomers have learned there’s much more to the unseen universe, finding background radiation at practically every wavelength observed. The highest frequency and most energetic background signals they’ve found are made up of a form of light called gamma rays, plus exotic particles called neutrinos and cosmic rays. And, unlike the CMB, the source of these emissions is still enigmatic. No one knows where they come from.
