arXiv subjects

Zhe Chang

Publications and source records attributed to Zhe Chang.

At least 37 records · Page 2Linked to original sources

arXiv2022

The Third Order Scalar Induced Gravitational Waves

Since the gravitational waves were detected by LIGO and Virgo, it has been promising that lots of information about the primordial Universe could be learned by further observations on stochastic gravitational waves background. The studies on gravitational waves induced by primordial curvature perturbations are of great interest. The aim of this paper is to investigate the third order induced gravitational waves. Based on the theory of cosmological perturbations, the first order scalar induces the second order scalar, vector and tensor perturbations. At the next iteration, the first order scalar, the second order scalar, vector and tensor perturbations all induce the third order tensor perturbations. We present the energy density spectrum of the third order gravitational waves for a monochromatic primordial power spectrum. The shape of the energy density spectrum of the third order gravitational waves is different from that of the second order scalar induced gravitational waves. And it is found that the third order gravitational waves sourced by the second order scalar perturbations dominate the energy density spectrum.

astro-ph.CO↗

arXiv2021

Note on gauge invariance of second order cosmological perturbations

We study the gauge invariant cosmological perturbations up to second order. We show that there are infinite families of gauge invariant variables at both of the first and second orders. The conversion formulae among different families are shown to be described by a finite number of bases that are gauge invariant. For the second order cosmological perturbations induced by the first order scalar perturbations, we explicitly represent the equations of motion of them in terms of the gauge invariant Newtonian, synchronous and hybrid variables, respectively.

astro-ph.CO↗

arXiv2021

The observer-dependent shadow of the Kerr black hole

Motivated by inclination of the Earth's orbit that is not located at galactic plane for observing the shadow of Sgr A*, we consider the black hole shadow for arbitrary inclinations and different velocities of observers. A surprising finding of the study is that rotation axis of a black hole might not be extracted from its shadow, since the ways of the shadow getting distorted depend not only on the spin of the black hole, but also velocities of observers. Namely, appearance of the shadow could be rotated by an angle in observers' celestial sphere due to the observer in motion. In order to further confirm this result, a formalism is presented for calculating the shadow in terms of the velocity perturbations. Besides, we also consider the Earth's orbit for the shadow of Sgr A* by making use of this formalism.

gr-qc↗

arXiv2021

Constraining primordial black holes as dark matter at JUNO

As an attractive candidate for dark matter, the primordial black holes (PBHs) in the mass range ($10^{15} \sim 10^{16}$)$\mathrm{g}$ could be detected via their Hawking radiation, including neutrinos and antineutrinos of three flavors. In this paper, we investigate the possibility to constrain the PBH as dark matter by measuring (anti)neutrino signals at the large liquid-scintillator detector of Jiangmen Underground Neutrino Observatory (JUNO). Among six available detection channels, the inverse beta decay $\overlineν^{}_e + p \to e^+ + n$ is shown to be most sensitive to the fraction $f^{}_{\rm PBH}$ of PBHs contributing to the dark matter abundance. Given the PBH mass $M^{}_{\rm PBH} = 10^{15}~{\rm g}$, we find that JUNO will be able to place an upper bound $f^{}_{\rm PBH} \lesssim 3\times 10^{-5}$, which is 20 times better than the current best limit $f^{}_{\rm PBH} \lesssim 6\times 10^{-4}$ from Super-Kamiokande. For heavier PBHs with a lower Hawking temperature, the (anti)neutrinos become less energetic, leading to a relatively weaker bound.

hep-ph↗

arXiv2020

Spatial variation of the fine-structure constant and Hubble's law in anisotropic coordinate of Friedmann-Lemaitre-Robertson-Walker space-time

Recent updated results of quasar spectra suggested a 3.9$σ$ significance of spatial variation of the fine-structure constant. Theoretically, it is important to examine whether the fine-structure constant, as a fundamental constant in quantum theory, is possible varying with space and time. In this paper, we explore the possibility that spatial variation of the fine-structure constant could be compatible with Einstein's general relativity. Namely, the spatially dependent fine-structure constant in the Universe could be originated in different values of the speed of light in separate local frames that are far away from us, since we have known that light rays must be bending in the present of gravity or non-inertial motions. In addition, to learn more about the anisotropic coordinate of FLRW space-time, we also study luminosity distance-redshift relation. It is found that there is a dipole structure in high redshift regime, while in low redshift regime, there is not such dipole.

gr-qc↗

arXiv2020

On the Gauge Invariance of Scalar Induced Gravitational Waves: Gauge Fixings Considered

The energy density spectrum is an observable of gravitational waves. Divergence has appeared in the energy density spectra of the scalar induced gravitational waves for different gauge fixings. To resolve the discrepancy, we investigate the gauge invariance of the scalar induced gravitational waves. It is shown that the gauge invariant induced gravitational waves can be obtained by subtracting the fictitious tensor perturbations via introducing the counter term composed of the first order scalar perturbations. The kernel function uniquely determines the energy density spectrum of the scalar induced gravitational waves. We explicitly calculate the gauge invariant kernel functions in the Newtonian gauge and the uniform density gauge, respectively. The discrepancy between the energy density spectra upon the Newtonian gauge and the uniform density gauge is shown to be eliminated in the gauge invariant framework. In fact, the gauge invariant approach is also available to other kinds of gauge fixings.

gr-qc↗

arXiv2020

Gauge Invariant Second Order Gravitational Waves

We investigate the gauge invariance of the second order gravitational waves induced by the first order scalar perturbations by following the Lie derivative method. It is shown explicitly that the second order gravitational waves are gauge invariant in the synchronous frame. In the gauge invariant framework, we derive the equation of motion of the second order gravitational waves and show that the second order gravitational waves are sourced from the first order scalar perturbations described well in the gauge invariant Newtonian frame. Since the observables of gravitational waves are measured in the synchronous frame, we define the energy density spectrum of the second order gravitational waves in terms of the gauge invariant synchronous variables. This way guarantees no fictitious tensor perturbations. It is shown that the gauge invariant energy density spectrum of the second order gravitational waves coincides with the one in the Newtonian gauge.

gr-qc↗

arXiv2020

On the absence of a universal surface density, and a maximum Newtonian acceleration in dark matter haloes: consequences for MOND

We study the dark matter (DM) surface density using the SPARC sample and {compare} it to Donato et al. (2009) result. By means of MCMC method, we infer the best-fitting parameters for each galaxy. We reobtain the scaling relation between the surface density and luminosity, and several other scaling laws relating the dark matter halo properties to that of the galactic disc properties. We conclude, in contrast with Donato et al. \cite{Donato}, that the dark matter surface density is not a universal (constant) quantity but correlates with the luminosity as well as with other galactic disc properties. A derived posterior probability distribution of $ρ_0 r_0$ shows that the null hypothesis of constancy is rejected at a very high confidence level. These results leave little room for the claimed universality of dark matter surface density. Since MOND has strong prediction on the surface density \cite{Milgrom2009}, we compared our result with those predictions, finding that MOND predictions are violated by data. To strengthen the previous result, we compared our results to another prediction of MOND (Milgrom 2005), the existence of a maximum Newtonian dark matter acceleration in the halo. Also in this case, MOND predictions are in contradiction with data. The dark matter Newtonian acceleration correlates with all the previously presented galactic disc properties, and data are distributed outside the bound predicted by Milgrom $\&$ Sanders (Milgrom 2005). We also find that the null hypothesis (constancy of DM Newtonian acceleration) is rejected at a very high confidence level.

astro-ph.GA↗

arXiv2020

Does the shape of the shadow of a black hole depend on motional status of an observer?

In a recent work on rotating black hole shadows [Phys. Rev. D{\bf 101}, 084029 (2020)], we proposed a new approach for calculating size and shape of the shadows in terms of astrometrical observables with respect to finite-distance observers. In this paper, we introduce a distortion parameter for the shadow shapes and discuss the appearance of the shadows of static spherical black holes and Kerr black holes in a uniform framework. We show that the shape of the shadow of a spherical black hole is circular in the view of arbitrary observers, and the size of the shadows tends to be shrunk in the view of a moving observer. The diameter of the shadows is contracted even in the direction perpendicular to the observers' motion. This seems not to be understood as length contraction effect in special relativity. The shape of Kerr black holes is dependent on motional status of observers located at finite distance. In spite of this, it is found that there is not a surrounding observer who could view the shape of the Kerr black hole shadows as circularity. These results could be helpful for observation of the Sagittarius A* in the centre of the Milky Way, as our solar system is moving around the centre black hole.

gr-qc↗

arXiv2020

Redshift drift in uniformly accelerated reference frame

We construct an alternative uniformly accelerated reference frame based on the 3+1 formalism in adapted coordinates. In this frame, time-dependent redshift drift exists between co-moving observers, which differs from that in Rindler coordinates. This phenomenon can be tested in laboratory and improve our understanding of non-inertial frames.

gr-qc↗

arXiv2020

Black hole shadow in the view of freely falling observers

First sketch of black hole from M87 galaxy was obtained by Event Horizon Telescope, recently. As appearance of black hole shadow reflects space-time geometry of black holes, observations of black hole shadow may be a promising way to test general relativity in strong field regime. In this paper, we focus on angular radius of spherical black hole shadow with respect to freely falling observers. In the framework of general relativity, aberration formulation and angular radius-gravitational redshift relation are presented. For the sake of intuitive, we consider parametrized Schwarzschild black hole and Schwarzschild-de Sitter black hole as representative examples. We find that the freely in-falling observers would observe finite size of shadow, when they go through inner horizon. For observers freely falling from the outer horizon of Schwarzschild-de Sitter black hole, we find that the angular radius of the shadows could increase even when the observers move farther from the black hole.

gr-qc↗

arXiv2020

Reference frame dependence of the local measurement of the Hubble constant

Observations recently suggest $4.4σ$ discrepancy between Hubble constant measured locally and that inferred from cosmic microwave background in standard cosmology. Either standard cosmology or local measurement might expect something new. We investigate the possibility that the value of Hubble constants might be affected by observers' motional status in the local measurement. Using adapted coordinate for geodesic observers in FLRW space-time and constraints inferred from observation of cosmic shear, we find that the motional status of reference frame could contribute to about $1.1 \pm 0.3\%$ discrepancy of these Hubble constants. Namely, the difference is $ΔH_0= 0.81\pm0.22$ km s$^{-1}$ Mpc$^{-1}$, if the locally measured Hubble constant is $ 74$ km s$^{-1}$ Mpc$^{-1}$. This effect seems not negligible as an uncertainty for local measurement of Hubble constant

gr-qc↗

arXiv2020

A revisit of rotating black hole shadow with astrometric observables

The first image of black hole in M87 galaxy taken by Event Horizon Telescope shows that directed observation of supermassive black holes would be a promising way to test general relativity in strong gravity field regime. In order to calculate shadow of rotating black holes with respect to observers located at finite distance, orthonormal tetrads have been introduced in previous works. However, it is noticed that different choice of tetrads does not lead to the same shape of shadow for observers in near regions. In this paper, we alternatively use formula of astrometric observables for calculating the shadow of a general rotating black hole with respect to these observers. For the sake of intuitive, we also consider Kerr-de Sitter black holes as a representative example. In this space-time, size and shape of Kerr-de Sitter black hole shadows are expressed as functions of distance between the black hole and observer. It is forecasted that the distortion of shadow would increase with distance.

gr-qc↗

arXiv2019

Constraining the anisotropy of the Universe via Pantheon supernovae sample

We test the possible dipole anisotropy of a Finslerian cosmological model and other three dipole-modulated cosmological models, i.e., the dipole-modulated $\rmΛ$CDM, $w$CDM and Chevallier--Polarski--Linder (CPL) model by using the recently released Pantheon sample of SNe Ia. The Markov chain Monte Carlo (MCMC) method is used to explore the whole parameter space. We find that the dipole anisotropy is very weak in all cosmological models used. Although the dipole amplitudes of four cosmological models are consistent with zero within $1σ$ uncertainty, the dipole directions are close to the axial direction to the plane of SDSS subsample among Pantheon. It may imply that the weak dipole anisotropy in the Pantheon sample originates from the inhomogeneous distribution of the SDSS subsample. More homogeneous distribution of SNe Ia is necessary to constrain the cosmic anisotropy.

astro-ph.CO↗

arXiv2019

Anisotropy of the Universe via the Pantheon supernovae sample revisited

We employ the hemisphere comparison (HC) method and the dipole fitting (DF) method to investigate the cosmic anisotropy in the recently released Pantheon sample of type Ia supernovae (SNe Ia) and five combinations among Pantheon. For the HC method, we find the maximum anisotropy level in the full Pantheon sample is $\mathrm{AL}_{max}=0.361\pm0.070$ and corresponding direction $(l,b)=({123.05^{\circ}}^{+11.25^{\circ}}_{-4.22^{\circ}}, {4.78^{\circ}}^{+1.80^{\circ}}_{-8.36^{\circ}})$. A robust check shows the statistical significance of maximum anisotropy level is about $2.1σ$. We also find that the Low-$z$ and SNLS subsamples have decisive impact on the overall anisotropy while other three subsamples have little impact. Moreover, the anisotropy level map significantly rely on the inhomogeneous distribution of SNe Ia in the sky. For the DF method, we find the dipole anisotropy in the Pantheon sample is very weak. The dipole magnitude is constrained to be less than $1.16\times10^{-3}$ at $95\%$ confidence level. However, the dipole direction is well inferred by MCMC method and it points towards $(l,b)=({306.00^{\circ}}^{+82.95^{\circ}}_{-125.01^{\circ}}, {-34.20^{\circ}}^{+16.82^{\circ}}_{-54.93^{\circ}})$. This direction is very close to the axial direction to the plane of SDSS subsample. It may imply that SDSS subsample is the decisive part to the dipole anisotropy in the full Pantheon sample. All these facts imply that the cosmic anisotropy found in Pantheon sample significantly rely on the inhomogeneous distribution of SNe Ia in the sky. More homogeneous distribution of SNe Ia is necessary to search for a more convincing cosmic anisotropy.

astro-ph.CO↗

arXiv2019

The prospects of gravitational waves on constraining the anisotropy of the Universe

The observation of GW150914 indicated a new independent measurement of the luminosity distance of a gravitational wave event. In this paper, we constrain the anisotropy of the Universe by using gravitational wave events. We simulate hundreds of events of binary neutron star merging that may be observed by Einstein Telescope. Full simulation for producing process of gravitational wave data is employed. We find that 200 of binary neutron star merging in redshift $(0,1)$ observed by Einstein Telescope may constrain the anisotropy with an accuracy comparable to the result from Union2.1 supernovae. This result shows that gravitational waves can be a powerful tool in investigating the cosmological anisotropy.

gr-qc↗

arXiv2019

Is there a fundamental acceleration scale in galaxies?

Milgrom's modified Newtonian dynamics (MOND) can explain well the mass discrepancy problem in galaxy without invoking dark matter. The MOND theory predicts a universal constant acceleration scale in galaxy, below which the Newtonian dynamics is no longer valid. Recently, Rodrigues et al. analyzed 193 high-quality disk galaxies by using Bayesian inference and they concluded that the probability of existence of a fundamental acceleration scale is essentially 0. In this paper, we use the same method to revisit the same question: Is there a fundamental acceleration scale in galaxies? We fit each galaxy rotation curve of 175 SPARC galaxies with Gaussian priors on galaxy parameters and a variable acceleration scale. After marginalizing over the nuisance parameters, we find that the marginalized posterior distributions of the acceleration scale become broad. The incompatibility between the global best fit and the marginalized posterior distributions of the acceleration scale is greatly reduced. However, there still exists evidence that rejects MOND as a fundamental theory. More accurate observations are needed to exclude or confirm the existence of a fundamental acceleration scale in galaxies.

astro-ph.GA↗

arXiv2018

Finsler space-time can explain both parity asymmetry and power deficit seen in CMB temperature anisotropies

We propose a framework of Finsler space-time to explain the observed parity asymmetry and the power deficit in the low-$\ell$ ($2\leqslant \ell \leqslant 29$) multipole range of cosmic microwave background (CMB) temperature anisotropies. In the $3+1$ dimensional space-time, the three-dimensional space is described by a Randers-Finsler space, which is spatially irreversible, inducing the parity asymmetry in the CMB angular power spectrum. We estimate the constraints on the two parameters introduced by Finsler space-time via analyzing the low-$\ell$ angular power spectrum in PLANCK 2015 CMB temperature data. We see that the low-$\ell$ power suppression in the CMB temperature anisotropies can also be resolved in this scenario. Our study shows that the two low-$\ell$ anomalies, i.e., parity asymmetry and power deficit, may have a common origin.

astro-ph.CO↗