Axionic domain walls at Pulsar Timing Arrays: QCD bias and particle friction

Understand

The recent results from the Pulsar Timing Array (PTA) collaborations show the first evidence for the detection of a stochastic background of gravitational waves at the nHz frequencies.

• This discovery has profound implications for the physics of both the late and the early Universe.
• In fact, together with the possible interpretation in terms of super massive black hole binaries, many sources in the early Universe can provide viable explanations as well.
• In this paper, we study the gravitational wave background sourced by a network of axion-like-particle (ALP) domain walls at temperatures around the QCD crossover, where the QCD-induced potential provides the necessary bias to annihilate the network.