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Axionic domain walls at Pulsar Timing Arrays: QCD bias and particle friction
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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. Remarkably, this implies a peak amplitude at frequencies around the sensitivity range of PTAs. We extend previous analysis by taking into account the unavoidable friction on the network stemming from the topological coupling of the ALP to QCD in terms of gluon and pion reflection off the domain walls at high and low temperatures, respectively. We identify the regions of parameter space where the network annihilates in the scaling regime ensuring compatibility with the PTA results, as well as those where friction can be important and a more detailed study around the QCD crossover is required.
Forward citations
Cited by 16 Pith papers
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Collapsing axionic domain walls can produce the baryon asymmetry via spontaneous baryogenesis, with a maximum yield set by the annihilation temperature, but minimal post-inflationary realisations suffer a suppression ...
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Biased Domain Wall Networks and their Gravitational Waves
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Collapsing domain walls with $\mathbb{Z}_2$-violating coupling to thermalized fermions and their impact on gravitational wave detections
Thermal corrections from a Z2-violating Yukawa coupling alter domain-wall annihilation temperatures and can change predicted gravitational wave spectra by orders of magnitude.
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Monodromic transparency of axion domain walls
Axion domain walls become transparent to low-energy photons at E/N=8/3 because of axion-pion cancellation, making thermal friction scale as T^8 rather than e^{-ma/T}.
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This paper derives, for the first time, the scalar-induced gravitational wave background sourced by domain wall perturbations, finding a resonant peak at the wall annihilation scale and a k^-16 high-frequency tail.
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Right-handed-neutrino couplings generate the energy bias that collapses Z2 domain walls, linking the type-I seesaw mass scale to observable gravitational-wave peaks and to resonant leptogenesis.
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With renormalisation-group running included, the one-loop high-temperature Daisy-resummed potential at µ=πT reproduces the phase-transition parameters of the two-loop dimensionally reduced EFT, while the no-running on...
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The power of SKA to Constrain cosmological gravitational-wave backgrounds below the astrophysical foreground noise
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NANOGrav 15-year gravitational-wave signals from binary supermassive black-holes seeded by primordial black holes, and implications for the origins of Little Red Dots
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Pre-Big-Bang Cosmology Cannot Explain NANOGrav 15-year Signal
Pre-Big Bang string cosmology cannot explain the NANOGrav 15-year signal; a power-law spectrum is preferred by a Bayes factor of about 468.
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