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Collapsing Domain Wall Networks: Impact on Pulsar Timing Arrays and Primordial Black Holes
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abstract
Unstable domain wall (DW) networks in the early universe are cosmologically viable and can emit a large amount of gravitational waves (GW) before annihilating. As such, they provide an interpretation for the recent signal reported by Pulsar Timing Array (PTA) collaborations. A related important question is whether such a scenario also leads to significant production of Primordial Black Holes (PBH). We investigate both GW and PBH production using 3D numerical simulations in an expanding background, with box sizes up to $N=3240$, including the annihilation phase. We find that: i) the network decays exponentially, i.e. the false vacuum volume drops as $\sim \exp(-\eta^3)$, with $\eta$ the conformal time; ii) the GW spectrum is larger than traditional estimates by more than one order of magnitude, due to a delay between DW annihilation and the sourcing of GWs. We then present a novel semi-analytical method to estimate the PBH abundances: rare false vacuum pockets of super-Hubble size collapse to PBHs if their energy density becomes comparable to the background when they cross the Hubble scale. Smaller (but more abundant) pockets will instead collapse only if they are close to spherical. This introduces very large uncertainties in the final PBH abundance. The first phenomenological implication is that the DW interpretation of the PTA signal is compatible with observational constraints on PBHs, within the uncertainties. Second, in a different parameter region, the dark matter can be entirely in the form of asteroid-mass PBHs from the DW collapse. Remarkably, this would also lead to a GW background in the observable range of LIGO-Virgo-KAGRA and future interferometers, such as LISA and Einstein Telescope.
Forward citations
Cited by 7 Pith papers
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Domain walls through different cosmologies
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Biased Domain Wall Networks and their Gravitational Waves
Population-biased domain wall networks annihilate at T_ann ~ T_s B_s^0.8 and emit a single-broken-power-law gravitational-wave spectrum peaking near twice the Hubble scale.
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Tensor perturbations from FOPT and domain-wall sources are claimed to induce second-order scalar perturbations large enough to form primordial black holes, potentially all of the dark matter.
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The Bearable Inhomogeneity of the Baryon Asymmetry
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Direct Detection of Cosmic Walls with Paleo Detectors
Ancient minerals could preserve parallel damage tracks left by a passing cosmic wall, enabling a direct search for these rare objects with paleo detectors.
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Primordial Black Hole Formation via Inverted Bubble Collapse
Isolated bubbles from an incomplete phase transition, inverted into false-vacuum regions by a later bulk transition, collapse into nearly monochromatic primordial black holes up to about 10^-5 solar masses.
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Domain Walls from $\Sigma(36 \times 3)$, $\Delta(54)$ and $\Delta(27)$ potentials
Degenerate vacua of Δ(27)/Δ(54)/Σ(36×3) scalar potentials give rise to one or two distinct domain-wall types depending on which orbits merge under CP or enlarged symmetries.
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