arXiv subjects

Zhen-Min Zeng

Publications and source records attributed to Zhen-Min Zeng.

6 recordsLinked to original sources

arXiv2026

Loss of the scaling attractor in self-gravitating domain wall networks

Domain-wall(DW) networks are known to approach a relativistic scaling regime on fixed radiation- and matter-dominated backgrounds, forming the basis of the no-frustration conjecture. However, this picture assumes that the defect network remains gravitationally subdominant. We investigate the self-consistent evolution of DWs by coupling the velocity-dependent one-scale model to the Friedmann equation and radiation energy transfer. The resulting autonomous system allows the cosmic expansion history to evolve dynamically rather than being imposed externally. We demonstrate analytically that gravitational backreaction qualitatively changes the phase-space structure: the radiation-era scaling solution, which is a stable attractor on a fixed background, becomes a saddle once the expansion rate is promoted to a dynamical degree of freedom. Furthermore, we establish that no stable fixed point exists within the physical phase space. Consequently, the scaling regime survives only as a transient stage, and all trajectories are driven toward a wall dominated and kinematically frustrated state in which the walls freeze in comoving coordinates. Our results demonstrate that the scaling attractor is not preserved in self-gravitating DW networks and reveal the generic late-time frustration dynamics of wall domination.

gr-qc↗

arXiv2025

Critical behavior and ultraviolet scaling of induced gravitational waves from an early matter-dominated era

Critical behavior and ultraviolet scaling of induced gravitational waves (GWs) from an early matter-dominated (eMD) era are studied in the context of primordial black hole evaporation. The depth of the eMD is characterized by the minimum parameter of the equation of state $ω_{\min}$ that the Universe can attain during this phase. We identify a critical value $ω_{c}\sim 7.3\times10^{-3}$ that separates two regimes. For $ω_{\min}<ω_{c}$, the GW peak lies at the non-linear cut-off point and requires non-linear dynamics. For $ω_{\min}>ω_{c}$, the peak originates from modes that reenter near the matter-radiation equality, and the ultraviolet tail follows a distinct scaling $k^{-3/2}$. This critical behavior provides a clear definition of deep versus shallow eMD and a robust spectral signature for future GW observations.

gr-qc↗

arXiv2025

Bispectrum of induced gravitational waves in the poltergeist mechanism

In the poltergeist mechanism the enhancement of induced gravitational waves (GWs) occurs due to a sudden transition from an early matter-dominated era to the radiation-dominated era. In this work, we calculate the bispectrum of induced GWs from the poltergeist mechanism by adopting the sudden transition approximation. We find that the tensor bispectrum peaks either in the equilateral or squeezed configurations, depending on scales. Such a characteristic behavior enables us to distinguish it from that from other GW generation mechanisms.

gr-qc↗

arXiv2024

Phase transition catalyzed by primordial black holes

We investigate the first-order phase transition catalyzed by primordial black holes~(PBHs) in the early Universe. We find that super-horizon curvature perturbations generated in this scenario lead to the production of gravitational waves when the scalar modes re-enter the horizon. If PBHs with masses about $10^{-13}M_{\odot}$ constitute all dark matter, the first-order electroweak phase transition catalyzed by PBHs can explain the gravitational wave signal observed by pulsar timing array collaborations without the overproduction of PBHs.

astro-ph.CO↗

arXiv2023

Enhanced curvature perturbations from spherical domain walls nucleated during inflation

We investigate spherical domain walls~(DWs) nucleated via quantum tunneling in multifield inflationary models and curvature perturbations induced by the inhomogeneous distribution of those DWs. We consider the case that the Euclidean action $S_{E}$ of DWs changes with time during inflation so that most of DWs nucleate when $S_{E}$ reaches the minimum value and the radii of DWs are almost the same. When the Hubble horizon scale exceeds the DW radius after inflation, DWs begin to annihilate and release their energy into background radiation. Because of the random nature of the nucleation process, the statistics of DWs is of the Poisson type and the power spectrum of curvature perturbations has a characteristic slope ${\cal P}_{\cal R}(k)\propto k^{3}$. The amplitude of ${\cal P}_{\cal R}(k)$ depends on the tension and abundance of DWs at the annihilation time while the peak mode depends on the mean separation of DWs. We also numerically obtain the energy spectra of scalar-induced gravitational waves from predicted curvature perturbations which are expected to be observed in multiband gravitational-wave detectors.

astro-ph.CO↗

arXiv2022

Generation of gravitational waves in dynamical Chern-Simons gravity

We investigate gravitational waves (GWs) generated in a two-field inflationary model with a non-canonical kinetic term, in which the gravitational Chern-Simons term is coupled to a heavy dynamical field. In such a model, primordial GWs experience a period of resonant amplification for some modes. In addition, isocurvature perturbations suffer from a temporary tachyonic instability due to an effective negative mass, which source curvature perturbations, resulting in large induced GWs. These two stochastic gravitational wave backgrounds correspond to different frequency bands, which are expected to be detected by future GW detectors such as SKA, LISA and Taiji.

gr-qc↗