arXiv subjects

Yungui Gong

Publications and source records attributed to Yungui Gong.

At least 91 records · Page 5Linked to original sources

arXiv2020

Gauge transformation of scalar induced gravitational waves

The gauge dependence of the scalar induced gravitational waves (SIGWs) generated at the second order imposes a challenge to the discussion of the secondary gravitational waves generated by scalar perturbations. We provide a general formula that is valid in any gauge for the calculation of SIGWs and the relationship for SIGWs calculated in various gauges under the coordinate transformation. The formula relating SIGWs in the Newtonian gauge to other gauges is used to calculate SIGWs in six different gauges. We find that the Newtonian gauge, the uniform curvature gauge, the synchronous gauge and the uniform expansion gauge yield the same result for the energy density of SIGWs. We also identify and eliminate the pure gauge modes that exist in the synchronous gauge. In the total matter gauge and the comoving orthogonal gauge, the energy density of SIGWs increases as $η^2$. While in the uniform density gauge, the energy density of SIGWs increases as $η^6$.

gr-qc↗

arXiv2020

On the waveform of the scalar induced gravitational waves

The scalar induced gravitational waves (SIGWs) is a useful tool to probe the physics in the early universe. To study inflationary models with this tool, we need to know how the waveform of SIGWs is related to the shape of the scalar power spectrum. We propose two parameterizations to approximate the scalar power spectrum with either a sharp or a broad spike at small scales, and then use these two parameterizations to study the relation between the shapes of $Ω_{GW}$ and the scalar power spectrum. We find that the waveform of SIGWs has a similar shape to the power spectrum. Away from the peak of the spike, the frequency relation $Ω_{GW}(k)\sim \mathcal{P}_ζ^2(k)$ holds independent of the functional form of the scalar power spectrum. We also give a physical explanation for this general relationship. The general relation is useful for determining the scalar power spectrum and probing inflationary physics with the waveform of SIGWs.

gr-qc↗

arXiv2020

Helical phase inflation and its observational constraints

We consider a class of helical phase inflation models from the ${\mathcal N}=1$ supergravity where the phase component of a complex field acts as an inflaton. This class of models avoids the eta problem in supergravity inflation due to the phase monodromy of the superpotential. We study the inflationary predictions of this class of models in the context of both standard and large extra dimensional brane cosmology, and find that they can easily accommodate the Planck 2018 and BICEP2 constraints. We find that the helical phase inflation has $α$-attractors and the attractors depend on one model parameter only.

hep-ph↗

arXiv2020

The TianQin project: current progress on science and technology

TianQin is a planned space-based gravitational wave (GW) observatory consisting of three earth orbiting satellites with an orbital radius of about $10^5~{\rm km}$. The satellites will form a equilateral triangle constellation the plane of which is nearly perpendicular to the ecliptic plane. TianQin aims to detect GWs between $10^{-4}~{\rm Hz}$ and $1~{\rm Hz}$ that can be generated by a wide variety of important astrophysical and cosmological sources, including the inspiral of Galactic ultra-compact binaries, the inspiral of stellar-mass black hole binaries, extreme mass ratio inspirals, the merger of massive black hole binaries, and possibly the energetic processes in the very early universe or exotic sources such as cosmic strings. In order to start science operations around 2035, a roadmap called the 0123 plan is being used to bring the key technologies of TianQin to maturity, supported by the construction of a series of research facilities on the ground. Two major projects of the 0123 plan are being carried out. In this process, the team has created a new generation $17~{\rm cm}$ single-body hollow corner-cube retro-reflector which has been launched with the QueQiao satellite on 21 May 2018; a new laser ranging station equipped with a $1.2~{\rm m}$ telescope has been constructed and the station has successfully ranged to all the five retro-reflectors on the Moon; and the TianQin-1 experimental satellite has been launched on 20 December 2019 and the first round result shows that the satellite has exceeded all of its mission requirements.

gr-qc↗

arXiv2020

The evidence of cosmic acceleration and observational constraints

Directly comparing the 6 expansion rate measured by type Ia supernovae data and the lower bound on the expansion rate set by the strong energy conditions or the null hypothesis that there never exists cosmic acceleration, we see $3σ$ direct evidence of cosmic acceleration and the $Rh=ct$ model is strongly excluded by the type Ia supernovae data. We also use Gaussian process method to reconstruct the expansion rate and the deceleration parameter from the 31 cosmic chronometers data and the 6 data points on the expansion rate measured from type Ia supernoave data, the direct evidence of cosmic acceleration is more than $3σ$ and we find that the transition redshift $z_t=0.60_{-0.12}^{+0.21}$ at which the expansion of the Universe underwent the transition from acceleration to deceleration. The Hubble constant inferred from the cosmic chronometers data with the Gaussian process method is $H_0=67.46\pm4.75$ Km/s/Mpc. To understand the properties of cosmic acceleration and dark energy, we fit two different two-parameter models to the observational data, and we find that the constraints on the model parameters from either the full distance modulus data by the Pantheon compilation or the compressed expansion rate data are very similar, and the derived Hubble constants are consistent with the Planck 2018 result. Our results confirm that the 6 compressed expansion rate data can replace the full 1048 distance modulus data from the Pantheon compilation. We derive the transition redshift $z_t=0.61_{-0.16}^{+0.24}$ by fitting a simple $q(z)$ model to the combination of cosmic chronometers data and the Pantheon compilation, the result is consistent with that obtained from the reconstruction with Gaussian process. Abridged.

astro-ph.CO↗

arXiv2020

Full analytical formulas for frequency response of space-based gravitational wave detectors

The discovery of gravitational waves, which are ripples of space-time itself, opened a new window to test general relativity, because it predicts that there are only plus and cross polarizations for gravitational waves. For alternative theories of gravity, there may be up to six polarizations. The measurement of the polarization is one of the major scientific goals for future gravitational wave detectors. To evaluate the capability of the detector, we need to use the frequency dependent response functions averaged over the source direction and polarization angle. We derive the full analytical formulas of the averaged response functions for all six possible polarizations and present their asymptotic behaviors based on these analytical formulas. Compared with the numerical simulation, the full analytical formulas are more efficient and valid for any equal-arm interferometric gravitational wave detector without optical cavities in the arms and for a time-delay-interferometry Michelson combination.

gr-qc↗

arXiv2020

Primordial black holes and secondary gravitational waves from k/G inflation

The possibility that in the mass range around $10^{-12}\ M_\odot$ most of dark matter constitutes of primordial black holes (PBHs) is a very interesting topic. To produce PBHs with this mass, the primordial scalar power spectrum needs to be enhanced to the order of 0.01 at the scale $k\sim 10^{12}\ \text{Mpc}^{-1}$. The enhanced power spectrum also produces large secondary gravitational waves at the mHz band. A phenomenological delta function power spectrum is usually used to discuss the production of PBHs and secondary gravitational waves. Based on G and k inflations, we propose a new mechanism to enhance the power spectrum at small scales by introducing a non-canonical kinetic term $[1-2G(ϕ)]X$ with the function $G(ϕ)$ having a peak. Away from the peak, $G(ϕ)$ is negligible and we recover the usual slow-roll inflation which is constrained by the cosmic microwave background anisotrpy observations. Around the peak, the slow-roll inflation transiently turns to ultra slow-roll inflation. The enhancement of the power spectrum can be obtained with generic potentials, and there is no need to fine tune the parameters in $G(ϕ)$. The energy spectrum $Ω_{GW}(f)$ of secondary gravitational waves have the characteristic power law behaviour $Ω_{GW}(f)\sim f^{n}$ and is testable by pulsar timing array and space based gravitational wave detectors.

gr-qc↗

arXiv2020

$α$-attractor from superconformal E-models in brane inflation

In the large extra dimensional braneworld inflation, Friedmann equation is modified to include a quadratic term in energy density with an additional parameter $λ$ called brane tension in addition to the usual linear term. The high energy brane corrections modify the slow-roll parameters and affect the behaviour of inflation. We analyse the superconformal inflation for E-models and find that there exist $α$-attractors in brane inflation. The predictions for the scalar spectral index $n_s$ and the tensor-to-scalar ratio $r$ are computed numerically, and approximate analytic formulas in the high energy limit have been given for the observable $n_s$ and $r$. The constraints on the model parameters are obtained by using Planck 2018 and BICEP2 observational data.

gr-qc↗

arXiv2019

Gravitational Waves in Scalar-Tensor-Vector Gravity Theory

In this paper, we study the properties of gravitational waves in the scalar-tensor-vector gravity theory. The polarizations of the gravitational waves are investigated by analyzing the relative motion of the test particles. It is found that the interaction between the matter and vector field in the theory leads to two additional transverse polarization modes. By making use of the polarization content, the stress-energy pseudo-tensor is calculated by employing the perturbed equation method. Besides, the relaxed field equation for the modified gravity in question is derived by using the Landau-Lifshitz formalism suitable to systems with non-negligible self-gravity.

gr-qc↗

arXiv2019

Constraints on primordial curvature perturbations from primordial black hole dark matter and secondary gravitational waves

Primordial black holes and secondary gravitational waves can be used to probe the small scale physics at very early time. For secondary gravitational waves produced after the horizon reentry, we derive an analytical formula for the time integral of the source and analytical behavior of the time dependence of the energy density of induced gravitational waves is obtained. By proposing a piecewise power law parametrization for the power spectrum of primordial curvature perturbations, we use the observational constraints on primordial black hole dark matter to obtain an upper bound on the power spectrum, and discuss the test of the model with future space based gravitational wave antenna.

gr-qc↗

arXiv2019

Frequency response of time-delay interferometry for space-based gravitational wave antennas

Space-based gravitational wave detectors cannot keep rigid structures and precise arm length equality, so the precise equality of detector arms which is required in a ground-based interferometer to cancel the overwhelming laser noise is impossible. The time-delay interferometry method is applied to unequal arm lengths to cancel the laser frequency noise. We give analytical formulas of the averaged response functions for tensor, vector, breathing and longitudinal polarizations in different TDI combinations, and obtain their asymptotic behaviors. At low frequencies, $f\ll f_*$, the averaged response functions of all TDI combinations increase as $f^2$ for all six polarizations. The one exception is the averaged response functions of $ζ$ for all six polarizations increase as $f^4$ in the equilateral-triangle case. At high frequencies, $f\gg f_*$, the averaged response functions of all TDI combinations for the tensor and breathing modes fall off as $1/f^2$, the averaged response functions of all TDI combinations for the vector mode fall off as $\ln(f)/f^2$ , and the averaged response functions of all TDI combinations for the longitudinal mode fall as $1/f$. We also give LISA and TianQin sensitivity curves in different TDI combinations for tensor, vector, breathing and longitudinal polarizations.

gr-qc↗

arXiv2019

Frequency response of space-based interferometric gravitational-wave detectors

Gravitational waves are perturbations of the metric of space-time. Six polarizations are possible, although general relativity predicts that only two such polarizations, tensor plus and tensor cross are present for gravitational waves. We give the analytical formulas for the antenna response functions for the six polarizations which are valid for any equal-arm interferometric gravitational-wave detectors without optical cavities in the arms.The response function averaged over the source direction and polarization angle decreases at high frequencies which deteriorates the signal-to-noise ratio registered in the detector. At high frequencies, the averaged response functions for the tensor and breathing modes fall of as $1/f^2$, the averaged response function for the longitudinal mode falls off as $1/f$ and the averaged response function for the vector mode falls off as $\ln(f)/f^2$.

gr-qc↗

arXiv2019

Strong Equivalence Principle and Gravitational Wave Polarizations in Horndeski Theory

The relative acceleration between two nearby particles moving along accelerated trajectories is studied, which generalizes the geodesic deviation equation. The polarization content of the gravitational wave in Horndeski theory is investigated by examining the relative acceleration between two self-gravitating particles. It is found out that the longitudinal polarization exists no matter whether the scalar field is massive or not. It would be still very difficult to detect the enhanced longitudinal polarization with the interferometer, as the violation of the strong equivalence principle of mirrors used by interferometers is extremely small. However, the pulsar timing array is promised to relatively easily detect the effect of the violation as neutron stars have large self-energy. The advantage of using this method to test the violation of the strong equivalence principle is that neutron stars are not required to be present in the binary systems.

gr-qc↗

arXiv2019

On the constant-roll inflation with large and small $η_H$

We study the seemingly duality between large and small $η_H$ for the constant-roll inflation with the second slow-roll parameter $η_H$ being a constant. In the previous studies, only the constant-roll inflationary models with small $η_H$ are found to be consistent with the observations. The seemingly duality suggests that the constant-roll inflationary models with large $η_H$ may be also consistent with the observations. We find that the duality between the constant-roll inflation with large and small $η_H$ does not exist because both the background and scalar perturbation evolutions are very different. By fitting the constant-roll inflationary models to the observations, we get $-0.016\leη_H\le-0.0078$ at the 95\% C.L if we take $N=60$ for the models with increasing $ε_H$ in which inflation ends when $ε_H=1$, and $3.0135\le η_H\le 3.021$ at the 68\% C.L., and $3.0115\le η_H\le 3.024$ at the 95\% C.L. for the models with decreasing $ε_H$.

gr-qc↗

arXiv2018

Reconstruction of extended inflationary potentials for attractors

We give the procedure to reconstruct the extended inflationary potentials for attractors and use the $α$ attractor and the constant-roll model as examples to show how to reconstruct the class of extended inflationary potentials in the strong coupling limit. We also derive the strong coupling condition which the coupling constant $ξ$ satisfies. The class of extended inflationary potentials has the same attractor, and the reconstructed extended inflationary potentials are consistent with the observational constraints. The existence of the attractors from extended inflationary models further challenges the model discrimination just by the observables $n_s$ and $r$.

gr-qc↗

arXiv2018

Gravitational waves and the polarizations in Hořava gravity after GW170817

The gravitational waves of Hořava gravity, their polarization states and their possible observational signatures are discussed. Using the gauge-invariant variable formalism, we found the three polarization modes in Hořava gravity excited by the three physical degrees of freedom contained in this theory. In particular, the scalar degree of freedom excites a mix of the transverse breathing and the longitudinal polarizations. The constraints from the previous experimental observations are taken into account, especially including the speed bound from the observations of GW170817 and GRB 170817A. It was found that Hořava theory is highly constrained. Within the experimentally allowed parametric space, we studied whether the pulsar timing arrays and the Gaia mission can be used to distinguish the different polarizations. After calculating the cross-correlation functions between the redshifts of photons and the astrometric positions of stars, one concludes that it is possible to tell whether there exits the scalar polarization using pulsar timing arrays and the Gaia mission.

gr-qc↗

arXiv2018

Gauss-Bonnet inflation and swampland

The two swampland criteria are generically in tension with the single field slow-roll inflation because the first swampland criterion requires small tensor to scalar ratio while the second swampland criterion requires large tensor to scalar ratio. The challenge to the single field slow-roll inflation imposed by the swampland criteria can be avoided by modifying the relationship between the tensor to scalar ratio and the slow-roll parameter. We show that the Gauss-Bonnet inflation with the coupling function inversely proportional to the potential overcomes the challenge by adding a constant factor in the relationship between the tensor to scalar ratio and the slow-roll parameter. For the Gauss-Bonnet inflation, while the swampland criteria are satisfied, the slow-roll conditions are also fulfilled, so the scalar spectral tilt and the tensor to scalar ratio are consistent with the observations. We use the potentials for chaotic inflation and the E-model as examples to show that the models pass all the constraints. The swampland criteria may imply Gauss-Bonnet coupling.

gr-qc↗

arXiv2018

Inflation with Gauss-Bonnet coupling

We consider inflationary models with the inflaton coupled to the Gauss-Bonnet term assuming a special relation $δ_1=2λε_1$ between the two slow-roll parameters $δ_1$ and $ε_1$. For the slow-roll inflation, the assumed relation leads to the reciprocal relation between the Gauss-Bonnet coupling function $ξ(ϕ)$ and the potential $V(ϕ)$, and it leads to the relation $r=16(1-λ)ε_1$ that reduces the tensor-to-scalar ratio $r$ by a factor of $1-λ$. For the constant-roll inflation, we derive the analytical expressions for the scalar and tensor power spectra, the scalar and tensor spectral tilts, and the tensor-to-scalar ratio to the first order of $ε_1$ by using the method of Bessel function approximation. The tensor-to-scalar ratio is reduced by a factor of $1-λ+λ\tilde η$. Comparing the derived $n_s$-$r$ with the observations, we obtain the constraints on the model parameters $\tildeη$ and $λ$.

gr-qc↗