In 2023, NANOGrav, EPTA, PPTA, and IPTA announced that they found evidence of a universal gravitational wave background. North American Nanohertz Observatory for Gravitational Waves states, that they were created over cosmological time scales by supermassive black holes, identifying the distinctive Hellings-Downs curve in 15 years of radio observations of 25 pulsars. Similar results are published by European Pulsar Timing Array, who claimed a 3 σ σ -significance. They expect that a 5 σ σ -significance will be achieved by 2025 by combining the measurements of several collaborations.