arXiv subjects

Ligong Bian

Publications and source records attributed to Ligong Bian.

At least 37 records · Page 2Linked to original sources

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

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↗

arXiv2025

Theoretical Uncertainties in First-Order Electroweak Phase Transitions

We systematically investigate theoretical uncertainties in perturbative analyses of first-order electroweak phase transitions (EWPT). Utilizing the Standard Model Effective Field Theory (SMEFT) framework, we quantify the gauge dependence, renormalization scheme, and scale dependences in predicting phase-transition parameters across both zero and finite chemical potential regimes. Our key findings reveal that: 1) the gauge-parameter dependence and the chemical potential are subdominants, and 2) Baryon number preservation criteria and phase transition observables exhibit pronounced sensitivity to the renormalization scale in the ${\overline{\rm MS}}$ scheme. Comparative analyses with the on-shell scheme demonstrate that within strongly first-order EWPT parameter spaces, the ${\overline{\rm MS}}$ scheme predicts enhanced phase transition strengths and prolonged transition durations.

hep-ph↗

arXiv2025

Phase Transitions in the Early Universe: Impacts on BBN and CMB Observables

We explore the effects of low-scale cosmological first-order phase transitions on Big Bang Nucleosynthesis (BBN) and Cosmic Microwave Background (CMB) anisotropy and distortion. We examine two scenarios: the distribution of the phase-transition energy in the cosmic background and the phase transition as an additional source of energy injection. Our analysis reveals that the CMB spectrum from the Planck 2018 dataset imposes stringent constraints on sub-MeV-scale phase transitions, while keV-scale phase transitions can induce significant CMB distortions. Consequently, BBN and CMB observables serve as complementary tools to constrain phase-transition parameters, alongside gravitational wave detections.

hep-ph↗

arXiv2025

A quantitative analysis of Gravitational Wave spectrum sourced from First-Order Chiral Phase Transition of QCD

We investigate the cosmological first-order chiral phase transition of QCD, and for the first time calculate its parameters which can fully determine the gravitational wave spectrum. With the state-of-the-art calculation from the functional QCD method, we found that the large chemical potential of QCD phase transition results in very weak and fast first-order phase transitions at the temperature lower than $\mathcal{O}(10^2)$ MeV. These results further suggest that the GW signals of NANOGrav are very unlikely sourced from the chiral phase transition of QCD.

hep-ph↗

arXiv2025

Gravitational waves and dark matter with Witten effect

We investigate the breaking of dark $SU(2)_d$ symmetry at different temperature scales, occurring after Peccei-Quinn symmetry breaking or following QCD symmetry breaking. We focus on assessing the potential of the hidden monopoles generated during this process to serve as dark matter candidate. Additionally, we examine the impact of axion-monopole interactions on the axion mass. When the phase transition occurs at extremely high temperature ($\sim 10^8 \mathrm{GeV}$), the contribution of monopoles to the axion mass through witten effect becomes non-negligible, playing a crucial role in accurately determining the axion relic density. Moreover, the stochastic gravitational wave background generated by dark phase transition and axionic domain wall collapse may offer a potential explanation for the low-frequency gravitational wave signals observed in PTA experiments.

hep-ph↗

arXiv2024

Gravitational wave and dark matter from Axion-Higgs string

Axions have long been considered plausible candidates for dark matter. The axion dark matter emitted from cosmic strings after the Peccei-Quinn (PQ) symmetry breaking in the early Universe was extensively simulated. In this work, we study dark matter and gravitational waves through the lattice simulation of the Axion-Higgs string. We gave the dark matter overproduction and the Big Bang nucleosynthesis bounds on the axion decay constant $f_a$ and the axion mass $m_a$ for axion-like particles, and found that the predicted gravitational wave spectra cannot be probed by the dataset of the current pulsar timing array experiments.

hep-ph↗

arXiv2024

Cosmic Simulations of Axion String-Wall Networks: Probing Dark Matter and Gravitational Waves for Discovery

We simultaneously study gravitational waves (GWs) and free axions emitted from axionic string-wall networks in the early universe using advanced 3D lattice simulations. Our simulations start before the Peccei-Quinn phase transition and end with the destruction of string-wall networks below the QCD scale. The axion dark matter (DM) relic abundance radiated from string-wall networks are updated and refined for the scenarios of $N_{\rm DW}>1$. In this scenario, we observe that the GW spectrum is almost independent of the bias term and $N_{\rm DW}$, and $Ω_{\rm GW}h^2\propto f^{1.29}(f^{-0.43})$ in the IR and middle-frequency regions. After considering the constraints from DM relic abundance, we found that the QCD axion model predicts undetectable GW emissions, and the axion-like particles model allows for a detectable GW signal in the nano-Hertz to the milli-Hertz frequency range corresponding to axion masses range from KeV to TeV. For $N_{\rm DW}=1$, the GW energy density appears undetectable for QCD axions and axion-like particles.

astro-ph.CO↗

arXiv2024

Domain Wall Network: A Dual Solution for Gravitational Waves and Hubble Tension?

We explore the possibility that domain wall networks generate the stochastic gravitational wave background (SGWB) observed as a strong common power-law process in the Data Release-2 of Parkes Pulsar Timing Array. We find that a broad range of parameters, specifically wall tension around $σ_{\textrm{DW}} \sim (29-414 , \textrm{TeV})^3$ and wall-decay temperature within $T_d \sim 20-257 , \textrm{MeV}$, can explain this phenomenon at a $68\%$ credible level. Meanwhile, the same parameters could ease the Hubble tension if particles from these domain wall networks decay into dark radiation. We establish a direct analytical relationship, $Ω_{\textrm{GW}}(f_p,T_0) h^2 \sim Ω_{\textrm{rad}} h^2 ( Ω_νΔN_{\textrm{eff}})^2$, to illustrate this coincidence, underlining its importance in the underlying physics and potential applicability to a wider range of models and data. Conversely, if the common power-law process is not attributed to domain wall networks, our findings impose tight limits on the wall tension and decay temperature.

hep-ph↗

arXiv2024

First-order Electroweak phase transition with Gauge-invariant approach

We study the electroweak phase transition dynamics with a three-dimensional standard model effective field theory under a gauge-invariant approach. We observe that, at the two-loop level, the phase transition parameters obtained with the gauge invariant approach can at most deviate from that of the dimensional reduction method around the percent level. We further found that the predicted gravitational wave signals at the new physics scale $Λ\gtrsim 590$ GeV are unreachable by the space-based interferometers, such as: LISA, TianQin, and Taiji.

hep-ph↗

arXiv2024

First-order Electroweak phase transition at finite density

We study the Electroweak phase transition with the Standard Model effective field theory at finite temperature and finite density. Utilizing the dimensional reduction approach, we construct the tree dimensional thermal effective field theory at finite density and investigate the phase transition dynamics. We evaluate how the results depend on the renormalization scale and the chemical potential. Our results show that, with the tree dimensional thermal effective potential at 2-loop level, we can effectively reduce the theoretical uncertainty in the calculations of the phase transition parameters due to the renormalization scale dependence, and the new physics scale is restricted to be $Λ\lesssim (770-800)$ GeV by the baryon number washout avoidance condition. Meanwhile, the presence of the chemical potential would affect the phase transition parameter and make the constraints from the baryon number washout avoidance condition more strict, especially for weaker first-order phase transition scenarios at higher new physics scales.

hep-ph↗

arXiv2023

Heavy QCD axion model in light of pulsar timing arrays

Recently, pulsar timing array experiments reported the observation of a stochastic gravitational wave background in the nanohertz range frequency band. We show that such a signal can be originated from a cosmological first-order phase transition (PT) within a well-motivated heavy (visible) QCD axion model. Considering the Peccei-Quinn symmetry breaking at the TeV scale in the scenario, we find a supercooled PT, in the parameter space of the model, prolonging the PT with the reheating temperature at the GeV scale.

hep-ph↗

arXiv2023

Constraints on new physics around the MeV scale with cosmological observations

We investigate the joint effect of the cosmological phase transitions, thermal light dark matter, and the lepton asymmetry on the big bang nucleosynthesis and cosmic microwave background. We find that all of them can modify the predictions of the effective number of neutrino species and primordial nucleosynthesis. In turn, we observe that: 1) the cosmological observations can exclude slow and strong phase transitions with strength even smaller than $\mathcal{O}(10^{-3}-10^{-2})$; 2) a much larger portion of dark matter mass region is excluded when the phase transition temperature is closer to 1 MeV; and 3) the magnitude of the non-vanishing neutrino lepton asymmetry is limited to be around $\mathcal{O}(10^{-2}-10^{-1})$ depending on the phase transition strength. These phase transitions can produce stochastic gravitational wave background to be probed by pulsar timing array experiments.

hep-ph↗

arXiv2023

Gravitational wave sources for Pulsar Timing Arrays

Very recently, several pulsar timing array collaborations, including CPTA, EPTA, and NANOGrav, reported their results from searches for an isotropic stochastic gravitational wave background (SGWB), with each finding positive evidence for SGWB. In this work, we assessed the credibility of interpreting the Hellings-Downs correlated free-spectrum process of EPTA, PPTA, and NANOGrav as either the result of supermassive black hole binary mergers or various stochastic SGWB sources that originated in the early Universe, including first-order phase transitions, cosmic strings, domain walls, and large-amplitude curvature perturbations. Our observations show that the current new datasets do not display a strong preference for any specific SGWB source based on Bayesian analysis.

astro-ph.HE↗

arXiv2023

Solving the domain wall problem with first-order phase transition

Domain wall networks are two-dimensional topological defects generally predicted in many beyond standard model physics. In this Letter, we propose to solve the domain wall problem with the first-order phase transition. We numerically study the phase transition dynamics, and for the first time show that the domain walls reached scaling regime can be diluted through the interaction with vacuum bubbles during the first-order phase transition. We find that the amplitude of the gravitational waves produced by the second-stage first-order phase transition is several orders higher than that from the domain walls evolution in the scaling regime. The scale of the first-order phase transition that dilute the domain walls can be probed through gravitational waves detection.

hep-ph↗