Abstract: When two galaxies merge the supermassive black holes in their centers come together, forming a binary pair that emits gravitational waves. The combined emission from many such black hole binaries is a primary source for pulsar timing arrays, which are sensitive to very low frequency gravitational waves. We will discuss how an ensemble black hole binaries can produce a stochastic background of gravitational waves, and how pulsar timing arrays can detect it. Finally, we will discuss the evidence for such a background observed by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) and the International Pulsar Timing Array (IPTA).

Before the talk (~3:45pm), tea and coffee will be served outside 116 Weniger.

After the talk, there will be a reception with food and drink in 247 Weniger.