Recently there's been a hint of discovery of stochastic gravitational wave (GWs) background by Pulsar Timing array at nHz frequencies. Supermassive black holes (SMBH) are the most natural source of such background. The data nonetheless says otherwise: they do not match the expectation. Although SMBHs are not ruled out yet, several cosmological sources fit the data better than SMBHs. This might be a drumroll for high-energy physics operating at scales beyond the reach of collider physics. I will give an overview of the status of the cosmological sources, e.g., gravitational waves from inflation, topological defects such as cosmic strings and domain walls, and first-order phase transition. I shall try to explain why 'Pulsars are the new collider' not literally, but in terms of physics potential.