arXiv subjects

Yungui Gong

Publications and source records attributed to Yungui Gong.

At least 55 records · Page 3Linked to original sources

arXiv2024

Constant-roll inflation with non-minimally derivative coupling

We investigate the constant-roll inflation with non-minimally kinetic coupling to the Einstein tensor. With the slow-roll parameter $η_ϕ= -\ddotϕ/(H\dotϕ)$ being a constant, we calculate the power spectra for scalar and tensor perturbations, and derive the expressions for the scalar spectral tilt $n_s$, the tensor spectral tilt $n_T$, and the tensor-to-scalar ratio $r$. We find that the expressions for $n_s$ are different with different ordering of taking the derivative of the scalar power spectrum with respect to the scale $k$ and the horizon crossing condition $c_sk=aH$ in the constant-roll inflation, the consistency relation $r=-8n_T$ does not hold if $|η_ϕ|$ is not small, and the duality of the tensor-to-scalar ratio between the slow-roll inflation and ultra-slow-roll inflation does not exist in inflationary models with non-minimally derivative coupling. The result offers a fresh perspective on the understanding of the inflationary models with non-minimally derivative coupling and is helpful for the production of scalar induced gravitational waves in the framework of ultra-slow-roll inflation with non-minimally derivative coupling.

gr-qc↗

arXiv2024

Limiting the Number of Extra Dimensions with Shortcuts

In higher-dimensional theories, a graviton propagating in the bulk can follow a shorter path, known as a shortcut, compared to a photon propagating in a four-dimensional spacetime. Thus by combining the observations of gravitational waves and their electromagnetic counterparts, one can gain insights into the structure and number of extra dimensions. In this paper, we construct a braneworld model that allows the existence of shortcuts in a $D(=4+d)$-dimensional spacetime. It has been proven that the equations for modeling brane cosmology recover the standard Friedmann equations for the late universe. We derive analytically the graviton and photon horizon radii on the brane under the low-energy limit. With the event GW170817/GRB 170817A, we find that the number of extra dimensions has an upper limit of $d\leq9$. Because of the errors in the source redshift and time delay, this upper limit can be shifted to $d\leq4$ and $d\leq12$. Although with the joint constraint on the $\text{AdS}_{D}^{~}$ radius from torsion balance measurements, theories with large $d$ are not yet ruled out, our work provides a new way to limit the number of extra dimensions.

gr-qc↗

arXiv2024

Primordial black holes and scalar-induced gravitational waves from the polynomial attractor model

Primordial black holes (PBHs) generated in the early Universe are considered as one of the candidates for dark matter. To produce PBHs with sufficient abundance, the primordial scalar power spectrum needs to be enhanced to the order of 0.01. Considering the third-order polynomial potential with polynomial $α$ attractors, we show that PBHs with the mass about $10^{17}$g can be produced while satisfying the constraints from the cosmic microwave background observations at the 2$σ$ confidence level. The mass of PBHs produced in the polynomial $α$ attractors can be much bigger than that in the exponential $α$ attractors. By adding a negative power-law term to the polynomials, abundant PBHs with different masses and the accompanying scalar-induced gravitational waves (SIGWs) with different peak frequency are easily generated. The PBHs with masses around $10^{-15}-10^{-12}$ $M_\odot$ can account for almost all dark matter. The SIGWs generated in the nanohertz band can explain the recent detection of stochastic gravitational-wave background by the pulsar timing array observations. The non-Gaussianity of the primordial curvature perturbations in the squeezed and equilateral limits are calculated numerically. We find that the non-Gaussianity correction greatly enhances the PBH abundance which makes the production of PBHs much easier, but the effect of non-Gaussianity on the generation of SIGWs is negligible.

gr-qc↗

arXiv2024

On the duality in constant-roll inflation

There is a duality in the observables $n_s$, $r$ and the inflaton potential between large and small $η_H$ for the constant-roll inflation if the slow-roll parameter $ε_H$ is negligible. In general, the duality between $η_H$ and $\barη_H$ does not hold for the background evolution of the inflation. For some particular solutions for the constant-roll inflation with $η_H$ being a constant, we find that in the small field approximation, the potential takes the quadratic form and it remains the same when the parameter $η_H$ changes to $\barη_H=3-η_H$. If the scalar field is small and the contribution of $ε_H$ is negligible, we find that there exists the logarithmic duality and the duality between large and small $η_H$ for the primordial curvature perturbation in inflationary models with the quadratic potential.

gr-qc↗

arXiv2024

On the improved dynamics approach in loop quantum black holes

In this paper, we consider the Böhmer-Vandersloot (BV) model of loop quantum black holes obtained from the improved dynamics approach. We adopt the Saini-Singh gauge, in which it was found analytically that the BV spacetime is geodesically complete. We show that black/white hole horizons do not exist in this geodesically complete spacetime. Instead, there exists only an infinite number of transition surfaces, which always separate trapped regions from anti-trapped ones. Comments on the improved dynamics approach adopted in other models of loop quantum black holes are also given.

gr-qc↗

arXiv2023

Scalar induced gravitational waves in light of Pulsar Timing Array data

The power-law parametrization for the energy density spectrum of gravitational wave (GW) background is a useful tool to study its physics and origin. While scalar induced secondary gravitational waves (SIGWs) from some particular models fit the signal detected by NANOGrav, Parkers Pulsar Timing Array, European Pulsar Timing Array, and Chinese Pulsar Timing Array collaborations better than GWs from supermassive black hole binaries (SMBHBs), we test the consistency of the data with the infrared part of SIGWs which is somewhat independent of models. Through Bayesian analysis, we show that the infrared parts of SIGWs fit the data better than GW background from SMBHBs. The results give tentative evidence for SIGWs.

gr-qc↗

arXiv2023

Supernova calibration by gravitational wave

Hubble tension is one of the most important problems in cosmology. Although the local measurements on the Hubble constant with Type Ia supernovae (SNe Ia) are independent of cosmological models, they suffer the problem of zero-point calibration of the luminosity distance. The observations of gravitational waves (GWs) with space-based GW detectors can measure the luminosity distance of the GW source with high precision. By assuming that massive binary black hole mergers and SNe Ia occur in the same host galaxy, we study the possibility of re-calibrating the luminosity distances of SNe Ia by GWs. Then we use low-redshift re-calibrated SNe Ia to determine the local Hubble constant. We find that we need at least 7 SNe Ia with their luminosity distances re-calibrated by GWs to reach a 2\% precision of the local Hubble constant. The value of the local Hubble constant is free from the problems of zero-point calibration and model dependence, so the result can shed light on the Hubble tension.

gr-qc↗

arXiv2023

A note on supergravity inflation in braneworld

We discuss supergravity inflation in braneworld cosmology for the class of potentials $V(ϕ)=αϕ^n\rm{exp}(-β^m ϕ^m)$ with $m=1,~2$. These minimal SUGRA models evade the $η$ problem due to a broken shift symmetry and can easily accommodate the observational constraints. In the high energy regime $V/λ\gg 1$, the numerical predictions and approximate analytic formulas are given for the scalar spectral index $n_s$ and tensor-to-scalar ratio $r$. The models with smaller $n$ are preferred while the models with larger $n$ are out of the $2σ$ region. Remarkably, the $ρ^2/λ$ correction to the energy density in Friedmann equation results in sub-Planckian inflaton excursions $Δϕ<1$.

hep-th↗

arXiv2023

Testing alternative theories of gravity with space-based gravitational wave detectors

We use gravitational waves (GWs) from binary black holes (BBHs) and neutron stars inspiraling into intermediate-mass black holes to evaluate how accurately the future space-based GW detectors such as LISA, Taiji and TianQin and their combined networks can determine source parameters and constrain alternative theories of gravity. We find that, compared with single detector, the detector network can greatly improve the estimation errors of source parameters, especially the sky localization, but the improvement of the constraint on the graviton mass $m_g$ and the Brans-Dicke coupling constant $ω_{BD}$ is small. We also consider possible scalar modes existed in alternative theories of gravity and we find the inclusion of the scalar mode has little effect on the constraints on source parameters, $m_g$, and $ω_{BD}$ and the parametrized amplitude $A_B$ of scalar modes are small. For the constraint on the graviton mass, we consider both the effects in the GW phase and the transfer function due to the mass of graviton. With the network of LISA, Taiji and TianQin, we get the lower bound on the graviton Compton wavelength $λ_g\gtrsim 1.24 \times 10^{20}$ m for BBHs with masses $(10^6+10^7)M_\odot$, and $A_B< 5.7\times 10^{-4}$ for BBHs with masses $(1+2)\times 10^5M_\odot$; $ω_{BD}>6.11\times10^{6}$ for neutron star-black hole binary with masses $(1.4+400)M_{\odot}$.

gr-qc↗

arXiv2023

Detecting new fundamental fields with Pulsar Timing Arrays

Strong evidence of the existence of the Stochastic Gravitational-Wave Background (SGWB) has been reported by the NANOGrav, PPTA, EPTA and CPTA collaborations. The Bayesian posteriors of the Gravitational-Wave Background (GWB) amplitude and spectrum are compatible with current astrophysical predictions for the GWB from the population of supermassive black hole binaries (SMBHBs). In this paper, we discuss the corrections arising from the extra scalar or vector radiation to the characteristic dimensionless strain in PTA experiments and explore the possibility to detect charges surrounding massive black holes, which could give rise to SGWB with vector or scalar polarizations. The parametrized frequency-dependent characteristic dimensionless strain is used to take a Bayesian analysis and the Bayes factor is also computed for charged and neutral SMBHBs. The Bayesian posterior of GWB tensor amplitude is $\log_{10} A_T=-14.85^{+0.26}_{-0.38}$ and spectral exponent $α=-0.60^{+0.32}_{-0.36}$. The Bayesian posterior for vector or scalar amplitude $A_{V, S}$ is nearly flat and there is nearly no constraint from the current observation data. The Bayesian factor is $0.71$ far less than 100, so the current observation can not support the existence of the charged SMBHB.

gr-qc↗

arXiv2023

Detecting vector charge with extreme mass ratio inspirals onto Kerr black holes

Extreme mass ratio inspirals (EMRIs) are excellent sources for space-based observatories to explore the properties of black holes and test no-hair theorems. We consider EMRIs with a charged compact object inspiralling onto a Kerr black hole in quasi-circular orbits. Using the Teukolsky and generalized Sasaki-Nakamura formalisms for the gravitational and vector perturbations about a Kerr black hole, we numerically calculate the energy fluxes for both gravitational and vector perturbations induced by a charged particle moving in equatorial circular orbits. With one-year observations of EMRIs, we apply the Fisher information matrix method to estimate the charge uncertainty detected by space-based gravitational wave detectors such as the Laser Interferometer Space Antenna, TianQin, and Taiji, and we find that it is possible to detect vector charge as small as $q\sim 0.0049$. The results show that EMRIs composed of a Kerr black hole with a higher spin $a$ and lighter mass $M$, and a secondary charged object with more vector charge give smaller relative error on the charge, thus constrain the charge better. The positive spin of the Kerr black hole can decrease the charge uncertainty by about one or two orders of magnitude.

gr-qc↗

arXiv2023

Gravitational waves from eccentric extreme mass-ratio inspirals as probes of scalar fields

We study eccentric orbits of the Schwarzschild spacetime for extreme mass ratio system (EMRI) in modified gravity theories with additional scalar fields. Due to the additional energy and angular momentum carried away by the scalar field, the orbit of the EMRI in modified gravity decays faster than that in general relativity. The time that it takes the eccentricity $e$ to reach the minimum is smaller and the values of the semi-latus rectum $p$ and $e$ at the turning point when $e$ reaches the minimum are bigger for larger scalar charge $d$. In addition to the calculation of energy fluxes with numerical method, we also use the Post-Newtonian expansion of the rate of energy carried away by the scalar field in eccentric orbits to understand the behaviors of the energy emission. By adding the scalar flux to the open code FastEMRIWaveforms of the Black Hole Perturbation Toolkit, we numerically generate fast gravitational waveforms for eccentric EMRIs with scalar fields and use the faithfulness between waveforms with and without the scalar charge to discuss the detection of scalar charge $d$. The detection error of the scalar charge is also estimated with method of Fisher information matrix.

gr-qc↗

arXiv2023

Detecting the secondary spin with extreme mass ratio inspirals in Scalar-Tensor theory

In this paper, we investigate the detectability of secondary spin in the extreme mass ratio inspirals system within a modified gravity model coupled with a scalar field. The central black hole, which reduces to a Kerr one, is circularly spiralled by a scalar-charged spinning secondary body on the equatorial plane. The analysis reveals that the presence of the scalar field amplifies the secondary spin effect, allowing for a lower limit of the detectability and an improved resolution of the secondary spin when the scalar charge is sufficiently large. Our findings suggest that the secondary spin detection is more feasible when the primary mass is not large, and TianQin is the optimal choice for detection.

gr-qc↗

arXiv2023

Primordial black holes and scalar induced gravitational waves from Higgs inflation with non-canonical kinetic term

We resolve the potential-restriction problem in K/G inflation by introducing nonminimal coupling. In this context, Higgs field successfully drives inflation satisfying CMB observations while enhancing curvature perturbations at small scales, which in turn accounts for primordial black holes (PBHs) and scalar induced gravitational waves (SIGWs). We then uncover the effect of the non-canonical kinetic coupling function in more detail and study its the observational constraint. Besides, we also give the gauge invariant expression for the integral kernel of SIGWs, which is related to terms propagating with the speed of light. Finally, the non-Gaussian effect on PBH abundance and SIGWs is studied. We find that non-Gaussianity makes PBHs form more easily, but its effect on the energy density of SIGWs is negligible.

gr-qc↗

arXiv2022

Constraint on Brans-Dicke theory from intermediate/extreme mass ratio inspirals

Intermediate/Extreme mass ratio inspiral (I/EMRI) system provides a good tool to test the nature of gravity in strong field. Based on the method of osculating orbits, we compute the orbital evolutions of I/EMRIs on quasi-elliptic orbits in both Einstein's general relativity and Brans-Dicke theory. The extra monopolar and dipolar channels in Brans-Dicke theory accelerate the orbital decay, so it is important to consider the effects of monopolar and dipolar emissions on the waveform. With the help of accurate orbital motion, we generate waveform templates which include both monopolar and dipolar contributions for I/EMRIs on eccentric orbits in Brans-Dicke theory. With a two-year observation of gravitational waves emitted from I/EMRIs by LISA, we get the most stringent constraint on the Brans-Dicke coupling parameter $ω_0>10^6$.

gr-qc↗

arXiv2022

Intermediate Mass-Ratio Inspirals with Dark Matter Minispikes

The dark matter (DM) distributed around an intermediate massive black hole (IMBH) forms an overdensity region called DM minispike. We consider the binary system which consists of an IMBH with DM minispike and a small black hole inspiralling around the IMBH in eccentric orbits. The factors which affect the evolution of the orbit include the gravity of the system, the dynamical friction and accretion of the small black hole caused by the DM minispike, and the radiation reaction of gravitational waves (GWs). Using the method of osculating orbit, we find that when the semilatus rectum p<<10^5 Rs (Rs is the Schwarzschild radius of the IMBH) the dominated factors are the dynamical friction and accretion from the DM minispike, and the radiation reaction. When p>>10^5 Rs, the gravity from the DM minispike dominates the orbital evolution. The existence of DM minispike leads to the deviation from the Keplerian orbit, such as extra orbital precession, henceforth extra phase shift in the GW waveform. By calculating the signal-to-noise ratio for GWs with and without DM minispikes and the mismatch between them, we show that the effect of the DM minispike in GW waveforms can potentially be detected by future space-based GW detectors such as LISA, Taiji, and Tianqin.

gr-qc↗

arXiv2022

Detection of scalar fields by Extreme Mass Ratio Inspirals with a Kerr black hole

We study extreme mass ratio inspirals occurring in modified gravity, for which the system is modeled by a small compact object with scalar charge spiraling into a supermassive Kerr black hole. Besides the tensorial gravitational waves arising from the metric perturbations, radiations are also induced by the scalar field. The relevant metric and scalar perturbations are triggered by the orbital motion of the small object, which give rise to a system of inhomogeneous differential equations under the adiabatic approximation. Such a system of equations is then solved numerically using Green's function furnished by the solutions of the corresponding homogeneous equations. To explore the present scenario from an observational perspective, we investigate how the pertinent observables are dependent on specific spacetime configurations. In this regard, the energy fluxes and the gravitational wave dephasing accumulated during the process are evaluated, as functions of the scalar charge, mass ratio, and spin of the central supermassive black hole. In particular, the presence of additional scalar emission leads to a more significant rate of overall energy loss, which, in turn, decreases the total number of orbital cycles before the small object plunges into the central black hole. Moreover, for a central black hole with a higher spin, the imprints of the scalar charge on the resultant gravitational radiations are found to be more significant, which indicates the possibility of detecting the scalar charge.

gr-qc↗

arXiv2022

Source localizations with the network of space-based gravitational wave detectors

The sky localization of the gravitational wave (GW) source is an important scientific objective for GW observations. A network of space-based GW detectors dramatically improves the sky localization accuracy compared with an individual detector not only in the inspiral stage but also in the ringdown stage. It is interesting to explore what plays an important role in the improvement. We find that the angle between the detector planes dominates the improvement, and the time delay is the next important factor. A detector network can dramatically improve the source localization for short signals and long signals with most contributions to the signal-to-noise ratio (SNR) coming from a small part of the signal in a short time, and the more SNR contributed by smaller parts, the better improvement by the network. We also find the effects of the arm length in the transfer function and higher harmonics are negligible for source localization with the detector network.

gr-qc↗