arXiv subjects

Zong-Kuan Guo

Publications and source records attributed to Zong-Kuan Guo.

At least 55 records · Page 3Linked 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

Dynamics of $Z_N$ domain walls with bias directions

The spontaneous breaking of a discrete symmetry can lead to the formation of domain walls in the early Universe. In this work, we explore the impact of bias directions on the dynamics of $Z_N$ domain walls, mainly focusing on the $N = 3$ model with a biased potential. Utilizing the Press-Ryden-Spergel method, we numerically investigate the dynamics of domain walls with lattice simulations. We find notable differences in the dynamics of domain walls due to bias directions. Our results indicate that the annihilation time depends not only on the vacuum energy difference $δV$ but also on bias directions described by the relative potential difference $ ζ$.

astro-ph.CO↗

arXiv2025

Gravitational Waves from Gauge Quanta Produced during Inflation

A fast-rolling axion can transfer its kinetic energy to a gauge field via the Chern-Simons coupling, leading to copious production of gauge quanta, which can act as a source of gravitational waves (GWs) with potentially observable amplitudes. In this work, we investigate GW production in a spectator axion model when strong backreaction is taken into account. We find that decreasing the decay constant of the axion enhances GW production. Since the initial value of the axion is larger than its quantum fluctuations, such a condition imposes a lower bound on the axion dacay constant, which sets an upper bound on the amplitude of the energy spectrum of GWs. As a result, the amplitude of the predicted GW energy spectrum is lower than $10^{-10}$ in the nHz to mHz frequency range.

hep-ph↗

arXiv2025

Post-Newtonian dynamics of compact binaries with mass transfer

Taking into account the mass transfer effect, we derive the equations of motion of a compact binary system at the second-half post-Newtonian order. Applying such equations of motion to quasi-circular orbits, we obtain the time derivative of the orbital frequency, which is consistent with the angular momentum balance equation. Numerical estimates of the phase of gravitational waves are provided for typical mass transfer rates. Our result can be used to improve the waveforms of gravitational waves emitted by compact binaries with mass transfer.

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

One-loop corrections to infrared GWs is forbidden by symmetries

Small-scale scalar perturbations amplified during inflation can induce primordial gravitational waves through tensor-scalar interactions. A long-standing controversial issue is whether the one-loop corrections to tensor perturbations exist on large scales. Firstly, we demonstrate through direct one-loop calculations that one-loop corrections cancel each other out on large scales. We then proceed from the symmetry of the interacting system and directly prove, based on the Ward identity, the absence of one-loop corrections on large scales-without the need for specific loop diagram calculations. This is consistent with the results we previously obtained for scalar perturbations.

gr-qc↗

arXiv2025

Influence of cosmic expansion on gravitational waveforms

Gravitational waves undergo redshift as they propagate through the expanding universe, and the redshift may exhibit time-dependent drift. Consequently, for any isolated gravitational wave sources, the mass parameter $\mathcal{M}$ and the redshift $z$ exhibit an observational degeneracy, typically manifesting in the waveform as the redshifted mass $\mathcal{M}(1+z)$. Matching together the wave propagation and the wave generation solutions, we show that dimensionless source parameters depending on mass $\mathcal{M}$ can break this degeneracy. Notably, the postmerger signal from binary neutron stars contains several dimensionless parameters that satisfy this condition, including the quality factors of different frequency components and their frequency ratios. Considering the observations of solely the postmerger signal by the Neutron star Extreme Matter Observatory or the Einstein Telescope, based on the Fisher analysis, we find that the redshift can be measured with fractional uncertainties of $\sim30\%$ for sources at $0.01<z<0.09$. Additionally, we present a corrected derivation of the waveform phase correction due to the redshift drift effect, rectifying a sign error in previous studies.

gr-qc↗

arXiv2025

Incorporating Backreaction in One-Loop Corrections in Ultra-Slow-Roll Inflation

We investigate the one-loop quantum correction to the power spectrum of primordial curvature perturbations in the ultra-slow-roll (USR) inflationary scenario, incorporating the backreaction effect from curvature perturbations. In the spatially-flat gauge, we expand the background inflaton field up to second order and identify the one-loop level backreaction term in the action. Utilizing a gauge transformation, we derive the comoving curvature interaction Hamiltonian in the presence of the backreaction term and calculate the one-loop correction using the in-in formalism. Our results reveal that the one-loop super-horizon corrections previously reported in the literature are canceled by the backreaction contributions. This finding underscores the importance of accounting for the backreaction effects in the analysis of quantum corrections during USR inflation.

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↗

arXiv2025

Spectrum instability and greybody factor stability for parabolic approximation of Regge-Wheeler potential

We investigate the stability of QNM spectra and greybody factors in the Schwarzschild black hole by approximating the Regge-Wheeler potential with a piecewise parabolic form and treating the deviation as a perturbation. We find that QNM spectra are sensitive to small perturbations, while greybody factors remain stable. This piecewise parabolic approximated potential gives rise to the long-lived modes whose imaginary parts remain close to zero and decrease slowly with overtone number increasing. The reflection coefficient shows distinct resonance feature in the high-frequency regime that are absent in the original R-W case. For the calculation of greybody factors, we employ an analytic method based on transfer matrix technique, and this approach can also be effectively used in other effective potential cases.

gr-qc↗

arXiv2025

Strong backreaction of gauge quanta produced during inflation

During inflation an axion field coupled to a gauge field through a Chern-Simons term can trigger the production of gauge quanta due to a tachyonic instability. The amplification of the gauge field modes exponentially depends on the velocity of the axion field, which in turn slows down the rolling of the axion field when backreaction is taken into account. To illustrate how the strength of the Chern-Simons coupling and the slope of the axion potential influence the particle production, in this paper we consider a toy model in which the axion field is a spectator with a linear potential. In the strong backreaction regime, the energy density of the gauge field quasiperiodically oscillates. The steep slope of the axion potential linearly increases the peak amplitude of the energy density while the strong coupling linearly decreases the peak amplitude. Additionally, we calculate the energy spectrum of gravitational waves.

hep-ph↗

arXiv2025

Spectrum of third-order tensor perturbations induced by excited scalar fields

We calculate for the first time the third-order spectrum of gravitational waves sourced by the amplified scalar field perturbations during inflation using the in-in formalism, and discuss the conditions for the third-order spectrum to be smaller than the second-order one. Assuming an exponential growth of the sub-horizon modes of the scalar field perturbations, we find that the third-order spectrum increases faster than the second-order one as the amplification of the field perturbations increases, and thus the third-order spectrum dominates for detectable gravitational waves, which indicates that the perturbation theory breakdowns.

gr-qc↗

arXiv2025

The pseudospectrum for the Kerr black hole with spin $s=0$ case

We investigate the pseudospectrum of the Kerr black hole, which indicates the instability of the spectrum of quasinormal modes (QNMs) of the Kerr black hole. Methodologically, we use the hyperboloidal framework to cast the QNM problem into a two-dimensional eigenvalue problem associated with a non-self-adjoint operator, and then the spectrum and pseudospectrum are solved by imposing the two-dimensional Chebyshev collocation method. The (energy) norm is constructed by using the conserved current method for the spin $s=0$ case. For the finite rank approximation of the operator, we discuss the convergence of pseudospectra using various norms, each involving different orders of derivatives. The convergence of the pseudospectrum improves as the order of the derivatives increases. We find that an increase in the imaginary part of complex frequency can deteriorate the convergence of the pseudospectrum under the condition of the same norms.

gr-qc↗

arXiv2025

Gravitational wave signatures and detectability of the mass transfer effect in compact binaries

The mass transfer process is prevalent during the inspiral phase of compact binary systems. Our study focuses on systems comprising low-mass white dwarfs, particularly in neutron star-white dwarf binaries and double white dwarf binaries, where a stable mass transfer process occurs at low frequencies. By analyzing the evolution of gravitational wave frequencies in the presence of mass transfer within quasi-circular orbits, we derive an analytical expression for the time-dependent frequency across different frequency bands and the waveforms emitted by compact binaries. Considering gravitational waves emitted by compact binaries in the $1\thicksim10$ mHz band, based on the Fisher analysis, we find that the mass transfer rate can be measured as accurately as $10^{-7} M_\odot/\text{year}$ by space-based gravitational-wave detectors with a signal-to-noise ratio of the order of $10^3$. Including the mass transfer effect in the waveforms provides a new possibility to measure the individual masses of double white dwarf binaries. The relative error of measured white dwarf masses can be down to the order of $0.01$.

gr-qc↗

arXiv2025

Gravitational Waves from a Gauge Field Non-minimally Coupled to Gravity

An axion-like spectator during inflation can trigger a tachyonic instability which amplifies the modes of one of the helicities of the gauge field, resulting in the production of parity-violating gravitational waves (GWs). In this paper we investigate the impact of the coupling $RFF$ of the gauge field to gravity on the production of GWs. We find that such a coupling introduces a multiplicative factor to the tachyonic mass, which effectively enhances the amplitude of the gauge field modes. Produced GWs are expected to be observed by future space-based GW detectors. Additionally, we find that the strong backreaction due to particle production leads to multiple peaks in the energy spectrum of GWs.

astro-ph.CO↗

arXiv2024

Constraints on ultra-slow-roll inflation with the NANOGrav 15-Year Dataset

Ultra-slow-roll~(USR) inflation predicts an exponential amplification of scalar perturbations at small scales, which leads to a stochastic gravitational wave background~(SGWB) through the coupling of the scalar and tensor modes at the second-order expansion of the Einstein equation. In this work, we search for such a scalar-induced SGWB from the NANOGrav 15-year (NG15) dataset, and find that the SGWB from USR inflation could explain the observed data. The Bayes factors are $54\pm 5$ for the USR inflation model alone and $68\pm 6$ for the combined USR inflation plus supermassive black hole binaries (SMBHB) models. We place constraints on the amplitude of the scalar power spectrum to $P_{\mathrm{Rp}} > 10^{-1.95}$ at $95\%$ confidence level (C.L.) at the scale of $k\sim 20\, \mathrm{pc}^{-1}$. We find that $\log_{10} P_{\mathrm{Rp}}$ degenerates with the peak scale $\log_{10} k_{\mathrm{p}}$. We also obtain the parameter space allowed by the data in the USR inflationary scenario, where the $e$-folding numbers of the duration of the USR phase has a lower limit $ΔN > 2.80$ ($95\%$ C.L.) when the USR phase ends at $N\approx 20$. With astrophysically motivated priors, the NG15 dataset fits both the USR inflation model and SMBHB model equally well.

astro-ph.CO↗

arXiv2024

The pseudospectrum and transient of Kaluza-Klein black holes in Einstein-Gauss-Bonnet gravity

The spectrum and dynamical instability, as well as the transient effect of the tensor perturbation for the so-called Maeda-Dadhich black hole, a type of Kaluza-Klein black hole, in Einstein-Gauss-Bonnet gravity have been investigated in framework of pseudospectrum. We cast the problem of solving quasinormal modes (QNMs) in AdS-like spacetime as the linear evolution problem of the non-normal operator in null slicing by using ingoing Eddington-Finkelstein coordinates. In terms of spectrum instability, based on the generalised eigenvalue problem, the QNM spectrum and $ε$-pseudospectrum has been studied, while the open structure of $ε$-pseudospectrum caused by the non-normality of operator indicates the spectrum instability. In terms of dynamical instability, we introduce the concept of the distance to dynamical instability, which plays a crucial role in bridging the spectrum instability and the dynamical instability. We calculate such distance, named the complex stability radius, as parameters vary. Finally, we show the behaviour of the energy norm of the evolution operator, which can be roughly reflected by the three kinds of abscissas in context of pseudospectrum, and find the transient growth of the energy norm of the evolution operator.

gr-qc↗

arXiv2024

Tensor bispectrum mediated by an excited scalar field during inflation

We calculate the tensor bispectrum mediated by an excited scalar field during inflation and find that the bispectrum peaks in the squeezed configuration, which is different from that of gravitational waves induced by enhanced curvature perturbations re-entering the horizon in the radiation-dominated era. Measuring the bispectrum provides a promising way to distinguish the stochastic gravitational-wave background generated during inflation from that generated after inflation.

astro-ph.CO↗