PINGU · Pulsar timing array Inference of the Nanohertz Gravitational wave Universe

We are on the verge of the next big breakthrough in gravitational wave (GW) astronomy: namely the detection of a nano-Hz GW signal with Pulsar Timing Arrays (PTAs). Within the next few years nano-Hz GWs will be established as a completely new window on our Universe, unlocking an unprecedented opportunity to unveil its secrets. The signal is anticipated to come from a cosmic population of supermassive black hole binaries (SMBHBs), which are a fundamental, yet observationally missing, piece in the process of structure formation and galaxy evolution. However, alternative Early Universe origins, including backgrounds arising from inflation or phase transitions, cannot be dismissed a priori.To exploit the scientific breakthrough potential of this new window we need an innovative, robust framework to build our way forward in uncharted territory. A framework that allows us to establish the nature of the nano-Hz GW signal and understand its implications for astrophysics and cosmology. PINGU is this framework; it is a concerted multimessenger project for connecting the GW and electromagnetic (EM) Universe in a novel way. On the one hand, it will leverage on the 15-year long expertise of the PI in PTA observations, data analysis and signal characterization to pin down the properties of the nano-Hz GW signal and characterize its features. On the other hand, it will exploit the most powerful all sky survey and state of the art galaxy formation models to construct a live nano-Hz GW map of our Universe and match it with the upcoming results of PTA observations. This will allow us to exploit the full potential of the nano-Hz GW sky, including: i) establishing the origin of the GW signal and probe its astrophysical nature, ii) gain unprecedented insights into the formation and evolution of SMBHBs and their role in galaxy formation, iii) identify SMBHBs and map their distribution in the Universe, iv) enable, for the first time, multimessenger astronomy in the nano-Hz GW band.