arXiv subjects

Ligong Bian

Publications and source records attributed to Ligong Bian.

At least 19 recordsLinked to original sources

arXiv2026

Vacuum Decay and Baryogenesis Associated with Primordial Black Holes at Finite Temperature

Within finite-temperature effective potential framework, we study the effects of PBHs on vacuum decay, the associated gravitational wave spectrum, and the baryon asymmetry. For the PBHs considered, the Hawking temperatures are below the ambient temperature, and their dominant effect is taken to be the gravitational distortion of the Higgs field configuration. Relative to flat spacetime, the PBH background reduces the vacuum decay exponent, enhances the gravitational wave peak amplitude, and shifts the peak frequency downward. For PBH masses of roughly $10^{11}~{\rm g}-10^{13}~{\rm g}$, the sphaleron energy is reduced, while the small PBH abundance renders the associated washout negligible. Additionally, over roughly the same mass range, PBH-emitted BSM particle decays generate a baryon asymmetry too small to spoil the agreement between the baryon abundances inferred from BBN and CMB.

astro-ph.CO↗

arXiv2026

Probing High-Quality Axions with Gravitational Waves

We present a systematic study of gravitational wave (GW) signals from phase transitions and topological defects in a unified high-quality axion framework. The gauged $U(1)_g$ symmetry forbids any bias term that could lift the vacuum degeneracy, restricting the theory to the phenomenologically viable case $N_{\rm DW}=1$. Requiring the axion to account for the observed dark matter (DM) abundance and satisfy the high-quality condition constrains the gauge symmetry-breaking scale to $f_g \in [1.6\times10^{11},\,10^{16}]\,\mathrm{GeV}$ for the QCD axion, leading to a well-defined band of GW signals, part of which is consistent with current pulsar timing array observations. Two-step first-order phase transitions are common in this framework, with the lower-scale transition generating GWs with $f^{\rm peak} \gtrsim \mathcal{O}(10^7)\,\mathrm{Hz}$. For axion-like realizations, generic post-inflation models predict GW spectra that are nearly degenerate with the QCD axion case. We conclude that GWs alone cannot distinguish between these scenarios, highlighting the need for complementary probes.

hep-ph↗

arXiv2026

The phenomenon of the axion kinetic misalignment with a generic PQ-breaking operator

We investigate the phenomenology induced by generic PQ-breaking operators within the axion kinetic misalignment framework. We analyze their impact on the relic density of axion dark matter (DM), the PQ quality problem, axion-mediated fifth-force, as well as Big Bang Nucleosynthesis (BBN) and Cosmic Microwave Background (CMB) constraints. A nonzero initial axion velocity gives rise to brief periods of early matter domination and axion kinetic domination, leading to a nonstandard cosmological evolution. We compute the resulting gravitational wave (GW) signal from global cosmic strings and find that, because these nonstandard epochs are extremely short, the signal is highly suppressed and beyond the reach of existing experiments. Finally, we perform a parameter space scan, identify the regions and benchmark point that are consistent with all experimental constraints.

hep-ph↗

arXiv2026

Simulating first-order phase transition during inflation

Ending the inflation by vacuum decay is considered infeasible due to the graceful exit problem. Even if considering an alternative field other than the inflaton to realize a first-order phase transition (FoPT) during inflation, it is usually challenging for concrete model building, as bubble nucleations might not be fast and dense enough to successfully end the inflation. In this work, we propose a FoPT at the grand-unification-theory (GUT) scale within the Starobinsky inflation. The key construction is an exponentially evolving potential barrier dynamically controlled by the rolling inflaton, so that almost no bubble is nucleated during the early inflationary era, but with massive bubble nucleations near the end of inflation. With lattice numerical simulations, we have successfully tested this GUT-FoPT during Starobinsky inflation, and the resulting gravitational-wave energy density spectrum reproduces previous analytical estimation with a distinctive oscillation feature at high frequencies.

hep-ph↗

arXiv2026

Gravitational Wave Signature of Aspherical Bubbles Driven by Thermal Fluctuation

Cosmological first-order phase transitions are a well-motivated source of stochastic gravitational waves (GWs), but most predictions are made based on the highly idealized model of perfectly spherical vacuum bubbles, neglecting thermal fluctuations. In this work we use $(3+1)$-dimensional lattice simulations of a scalar model with thermal initial conditions to quantify how thermal fluctuations distort bubble profiles and modify the resulting GW spectrum. We find that thermal fluctuations can strongly break spherical symmetry at early times, allowing even an isolated bubble to emit GWs. In multi-bubble simulations, thermal fluctuations systematically reshape the spectrum, suppressing the infrared part while enhancing and broadening the high-$k$ tail. We further provide an analytical estimate for the ultraviolet regime of the GW spectrum, which is in good agreement with our lattice results and suggests that this regime is dominated by thermal fluctuations. These effects could leave observable imprints in future GW searches.

hep-ph↗

arXiv2026

Vacuum Decay Rate in D-dimensional Electroweak theories

We present a systematic framework for computing the functional determinant contribution to vacuum decay rates in D-dimensional electroweak theories. It consistently incorporates quantum fluctuations from scalar, fermion, and gauge fields while ensuring rapid convergence even at high angular momenta. Demonstrated through applications to the $D=4$ SMEFT and its $D=3$ thermal counterpart, our method provides a general and efficient tool for analyzing vacuum stability and decay across dimensions in the presence of new physics beyond the Standard Model.

hep-ph↗

arXiv2026

Towards Accurate Gravitational Wave Predictions: Gauge-Invariant Nucleation in the Electroweak Phase Transition

The vacuum decay in the early Universe should be gauge-invariant. In this work, we study the gauge dependence of the vacuum decay occurring through a first-order phase transition and the associated gravitational wave production. We investigate the gauge dependence of the bubble nucleation and phase transition parameters within the framework of the Standard model effective field theory in three dimension. By considering the power-counting and utilizing the Nielsen identity at finite temperature, we show that, depending on the power-counting scheme favored by the new physics scale, the perturbative computation methodology allow we get the gauge-independent nucleation rates and phase transition, this enables more accurate predictions of gravitational wave signatures.

hep-ph↗

arXiv2025

Chiral magnetic effect amplified baryogenesis at first-order phase transitions

In this study, we show that, in the background of the primordial magnetic field, the chiral magnetic effect effect can significantly amplify the chiral chemical potential sourced by the CP violation near the bubble walls during the first-order electroweak phase transition. This effect can lift the generated baryon asymmetry by several orders, and make it possible to explain the baryon asymmetry of the Universe with a CPV in the fermion sector far beyond the limitation of the electron dipole moment.

hep-ph↗

arXiv2025

Gauge-Invariant Bubble Nucleation and Gravitational Waves from First-Order Electroweak Phase Transitions

A long-standing problem in electroweak phase transition studies is the spurious gauge dependence of bubble nucleation rate calculations, which introduces large systematic uncertainties in predictions of gravitational waves and related cosmological phenomena. We address this issue by presenting a systematic study of gauge-invariant bubble nucleation within a phenomenologically realistic effective field theory framework. Using the three-dimensional thermal effective field theory formalism with a consistent power-counting scheme, we rigorously establish the gauge invariance of the physical nucleation rate up to two-loop order. This proof eliminates the gauge-parameter dependence of key phase transition parameters, providing a robust theoretical foundation for precise calculations of gravitational waves, electroweak baryogenesis, and primordial magnetogenesis.

hep-ph↗

arXiv2025

Collapse of Axion Domain Wall Induced by Helical Primordial Magnetic Fields

Stable domain wall (DW) must decay to avoid overclose the Universe. A commonly used solution is to slightly break the PQ symmetry by introducing a bias term in the potential. In this work, we propose an alternative, symmetry-preserving mechanism: coupling the axion field to a helical primordial magnetic field (PMF) via the Chern-Simons term. Using three-dimensional lattice simulations, we evolve the DW network and demonstrate that it can successfully drive DW decay. Our quantitative results further show that the correlation length of the PMF plays a crucial role in determining the decay rate of the DW network and the resulting axion and gravitational wave radiation.

astro-ph.CO↗

arXiv2025

Bayesian analysis of the complex singlet model with phase transition gravitational waves

We explore the prospects of probing the complex singlet extension of the Standard Model (CxSM) with gravitational waves from the electroweak phase transition. The study establishes a connection of the scalar potential parameters, the thermodynamic properties of the phase transition, with the directly measured stochastic gravitational-wave background in the presence of astrophysical background and foreground. Considering the space-based gravitational-wave detector Taiji, we construct a frequency-domain likelihood that incorporates instrumental and astrophysical noises, and we perform both Fisher-matrix forecasts and Bayesian nested sampling analysis. The comparison of these two approaches demonstrates consistent parameter recovery and highlights the sensitivity of Taiji to millihertz gravitational-wave signals. We further propagate the inferred constraints on the gravitational-wave spectrum back to the underlying CxSM parameters, obtaining meaningful limits on the Higgs self-couplings. The results emphasize the complementarity between gravitational-wave observations and collider measurements, showing that future missions such as Taiji can serve as a powerful probe of electroweak-scale new physics and the dynamical origin of the Higgs sector.

hep-ph↗

arXiv2025

Gravitational wave cosmology

Gravitational waves (GWs) originating from cosmological sources offer direct insights into the physics of the primordial Universe, the fundamental nature of gravity, and the cosmic expansion of the Universe. In this review paper, we present a comprehensive overview of our recent advances in GW cosmology, supported by the national key research and development program of China, focusing on cosmological GW sources and their implications for fundamental physics and cosmology. We first discuss the generation mechanisms and characteristics of stochastic gravitational wave backgrounds generated by physical processes occurred in the early Universe, including those from inflation, phase transitions, and topological defects, and summarize current and possible future constraints from pulsar timing array and space-based detectors. Next, we explore the formation and observational prospects of primordial black holes as GW sources and their potential connection to dark matter. We then analyze how GWs are affected by large-scale structure, cosmological perturbations, and possible modifications of gravity on GW propagation, and how these effects can be used to test fundamental symmetry of gravity. Finally, we discuss the application of GW standard sirens in measuring the Hubble constant, the expansion history, and dark energy parameters, including their combination with electromagnetic observations. These topics together show how GW observations, especially with upcoming space-based detectors, such as LISA, Taiji, and Tianqin, can provide new information about the physics of the early Universe, cosmological evolution, and the nature of gravity.

gr-qc↗

arXiv2025

Enhancing Phase Transition Calculations with Fitting and Neural Network

The computation of bounce action in a phase transition involves solving partial differential equations, inherently introducing non-negligible numerical uncertainty. Deriving characteristic temperatures and properties of this transition necessitates both differentiation and integration of the action, thereby exacerbating the uncertainty. In this work, we fit the action curve as a function of temperature to mitigate the uncertainties inherent in the calculation of the phase transition parameters. We find that, after extracting a factor, the sixth-order polynomial yields an excellent fit for the action in the high temperature approximated potential. In a realistic model, the singlet extension of the Standard Model, this method performs satisfactorily across most of the parameter space after trimming the fitting data. This approach not only enhances the accuracy of phase transition calculations but also systematically reduces computation time and facilitates error estimation, particularly in models involving multiple scalar fields. Furthermore, we discussed the possible of using multiple neural networks to predict the action curve from model parameters.

hep-ph↗

arXiv2025

Baryogenesis Induced by Magnetic Field Effects During the Electroweak Phase Transition

We numerically investigate the first-order electroweak phase transition in the background of a hypermagnetic field with three-dimensional lattice simulation. The generation of baryon asymmetry is observed, and we present the relationship between baryon number asymmetry and magnetic field strength and its helicity. We find the magnetic field strength required to achieve the correct matter-antimatter asymmetry is about $10^{-17}\sim10^{-14}$ Gauss at present, depending on the correlation length of the helical magnetic field. This study provides a mechanism for explaining the baryon number asymmetry with cosmic magnetic fields.

hep-ph↗

arXiv2025

Impact of Primordial Magnetic Fields on the First-Order Electroweak Phase Transition

We numerically study how the primordial magnetic field affects the first-order electroweak phase transition in the early Universe. We observe that: 1) the phase transition process would be slowed down by the magnetic field; 2) the phenomenon of vortex structure of the Higgs condensation appears when the homogenesis hypermagentic field $g'B_Y^{ex}/m_W^2\gtrsim3.63$; and, 3) the helical hypermagnetic field can dramatically enhance the sphaleron rate and validate the generation of the baryon asymmetry through the chiral anomaly.

hep-ph↗

arXiv2025

Testing Nambu-Goto approximation of cosmic string by lattice field simulations

The precise calculation of gravitational wave (GW) from cosmic string networks is of significant theoretical and experimental interest. The Nambu--Goto (NG) approximation has long been employed to calculate GW emission from such networks; however, its validity has never been systematically verified. We perform large-scale zero-temperature Abelian-Higgs lattice simulations under different gauge couplings, and compare them with NG predictions. We find excellent agreement in the power-law region for near-global strings but strong deviation for strongly coupled local strings with $m_v/m_s \sim 1$, quantitatively establishing the breakdown of the NG approximation. Additionally, we confirm that particle emission significantly dominates the energy loss of the string network, with the ratio of GW energy to particle energy approximately $10^{-3}$ to $10^{-2}$ for both near-global and local string scenarios.

astro-ph.CO↗

arXiv2025

False Vacuum Decay across the Quantum-to-Thermal Crossover: A Comparison of Real-Time Observables

We develop a real-time Wigner-functional lattice framework with positive Hartree-Gaussian initial sampling and introduce a connected-cluster survival criterion for extracting false-vacuum decay rates across the crossover from quantum fluctuations to thermal nucleation. At high temperatures, the connected-cluster rate agrees well with the Hartree-resummed thermal nucleation benchmark, while the commonly used global-survival criterion can give substantially smaller rates because of multi-seed dynamics and global averaging. At low temperatures, the connected-cluster and global-survival rates approach each other in the dilute-event regime, whereas the false-vacuum fraction observable can be contaminated by transient spatial conversion and kink-antikink reflection. Our results clarify how different real-time observables encode distinct aspects of metastable decay.

hep-th↗

arXiv2025

Numerical simulations on First-order phase transition through thermal fluctuation

In this Letter, we numerically present the possibility of the first-order phase transition occurring through the thermal fluctuation in the early universe. We find that when the temperature is slightly higher than the mass scale of the background field, the bubble-like field configurations appear proceeded by oscillons, which expand and collide to finish the phase transition. We provide the false vacuum decay rate and the accompanied gravitational waves. We also present the vacuum phase transition comparison of the quantum tunneling case and thermal fluctuation case.

hep-ph↗