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The European Pulsar Timing Array (EPTA) and Indian Pulsar Timing Array(InPTA) collaborations have measured a low-frequency common signal in thecombination of their second and first data releases respectively, with thecorrelation properties of a gravitational wave background (GWB). Such signalmay have its origin in a number of physical processes including a cosmicpopulation of inspiralling supermassive black hole binaries (SMBHBs);inflation, phase transitions, cosmic strings and tensor mode generation bynon-linear evolution of scalar perturbations in the early Universe;oscillations of the Galactic potential in the presence of ultra-light darkmatter (ULDM). At the current stage of emerging evidence, it is impossible todiscriminate among the different origins. Therefore, in this paper, we considereach process separately, and investigate the implications of the signal underthe hypothesis that it is generated by that specific process. We find that thesignal is consistent with a cosmic population of inspiralling SMBHBs, and itsrelatively high amplitude can be used to place constraints on binary mergertimescales and the SMBH-host galaxy scaling relations. If this origin isconfirmed, this is the first direct evidence that SMBHBs merge in nature,adding an important observational piece to the puzzle of structure formationand galaxy evolution. As for early Universe processes, the measurement wouldplace tight constraints on the cosmic string tension and on the level ofturbulence developed by first-order phase transitions. Other processes wouldrequire non-standard scenarios, such as a blue-tilted inflationary spectrum oran excess in the primordial spectrum of scalar perturbations at largewavenumbers. Finally, a ULDM origin of the detected signal is disfavoured,which leads to direct constraints on the abundance of ULDM in our Galaxy.

The second data release from the European Pulsar Timing Array: V. Implications for massive black holes, dark matter and the early Universe