arXiv subjects

Zhe Chang

Publications and source records attributed to Zhe Chang.

At least 73 records · Page 4Linked to original sources

arXiv2015

Nearly scale-invariant power spectrum and quantum cosmological perturbations in the gravity's rainbow scenario

We propose the gravity's rainbow scenario as a possible alternative of the inflation paradigm to account for the flatness and horizon problems. We focus on studying the cosmological scalar perturbations which are seeded by the quantum fluctuations in the very early universe. The scalar power spectrum is expected to be nearly scale-invariant. We estimate the rainbow index $λ$ and energy scale $M$ in the gravity's rainbow scenario by analyzing the Planck temperature and WMAP polarization datasets. The constraints on them are given by $λ=2.933\pm0.012$ and $\ln (10^5M/M_p)= -0.401^{+0.457}_{-0.451}$ at the $68\%$ confidence level.

gr-qc↗

arXiv2015

A unified description for dipoles of the fine-structure constant and SnIa Hubble diagram in Finslerian universe

We propose a Finsler spacetime scenario of the anisotropic universe. The Finslerian universe requires both the fine-structure constant and accelerating cosmic expansion have dipole structure, and the directions of these two dipoles are the same. Our numerical results show that the dipole direction of SnIa Hubble diagram locates at $(l,b)=(314.6^\circ\pm20.3^\circ,-11.5^\circ\pm12.1^\circ)$ with magnitude $B=(-3.60\pm1.66)\times10^{-2}$. And the dipole direction of the fine-structure constant locates at $(l,b)=(333.2^\circ\pm8.8^\circ,-12.7^\circ\pm6.3^\circ)$ with magnitude $B=(0.97\pm0.21)\times10^{-5}$. The angular separation between the two dipole directions is about $18.2^\circ$.

gr-qc↗

arXiv2015

Anisotropic inflation in Finsler spacetime

We suggest the universe is Finslerian in the stage of inflation. The Finslerian background spacetime breaks rotational symmetry and induces parity violation. The primordial power spectrum is given for quantum fluctuation of the inflation field. It depends not only on the magnitude of wavenumber but also on the preferred direction. We derive the gravitational field equations in the perturbed Finslerian background spacetime, and obtain a conserved quantity outside the Hubble horizon. The angular correlation coefficients are presented in our anisotropic inflation model. The parity violation feature of Finslerian background spacetime requires that the anisotropic effect only appears in angular correlation coefficients if $l'=l+1$. The numerical results of the angular correlation coefficients are given to describe the anisotropic effect.

gr-qc↗

arXiv2015

GRB Spectra in the complex of synchrotron and Compton processes

Under the steady state condition, the spectrum of electrons is investigated by solving the continuity equation under the complex radiation of both the synchrotron and Compton processes. The resulted GRB spectrum is a broken power law in both the fast and slow cooling phases. On the basis of this electron spectrum, the spectral indices of the Band function in four different phases are presented. In the complex radiation frame, the detail investigation on physical parameters reveals that both the reverse shock photosphere model and the forward shock with strong coupling model can answer the $α\sim -1$ problem. A possible marginal to fast cooling phase transition in GRB 080916C is discussed. The time resolved spectra in different pulses of GRB 100724B, GRB 100826A and GRB 130606B are investigated. We found that the flux is proportional to the peak energy in almost all pulses. The phases for different pulses are determined according to the spectral index revolution. We found the strong correlations between spectral indices and the peak energy in GRB 100826A, which can be explained by the Compton effect in the fast cooling phase. However, the complex scenario predicts a steeper index for the injected electrons, which challenges the acceleration mechanism in GRBs.

astro-ph.HE↗

arXiv2014

Inflation in de Sitter spacetime and CMB large scales anomaly

The influence of cosmological constant type dark energy in the early universe is investigated. This is accommodated by a new dispersion relation in de Sitter spacetime. We perform a global fitting to explore the cosmological parameters space by using the CosmoMC package with the recently released Planck TT and WMAP Polarization datasets. Using the results from global fitting, we compute a new CMB temperature-temperature spectrum. The obtained TT spectrum has lower power compared with the one based on $Λ$CDM model at large scales.

astro-ph.CO↗

arXiv2014

Gamma-ray polarization induced by cold electrons via Compton processes

The polarization measurement is an important tool to probe the prompt emission mechanism in gamma-ray bursts (GRBs). The synchrotron photons can be scattered by cold electrons in the outflow via Compton scattering processes. The observed polarization depends on both the photon energy and the viewing angle. With the typical bulk Lorentz factor $Γ\sim 200$, photons with energy $E>10$ MeV tend to have smaller polarization than photons with energy $E<1$ MeV. At the right viewing angle, i.e. $θ\sim Γ^{-1}$, the polarization achieves its maximal value, and the polarization angle changes $90^{\circ}$ relative to the initial polarization direction. Thus, the synchrotron radiation plus Compton scattering model can naturally explain the $90^{\circ}$ change of the polarization angle in GRB 100826A.

astro-ph.HE↗

arXiv2014

The change of GRB polarization angles in the magnetic-dominated jet model

The polarimetric measurement on the prompt phase of GRB 100826A shows that the polarization angle changes $\sim 90^{\circ}$ between two adjacent time intervals. We will show that this phenomenon can be naturally interpreted in the framework of the magnetic-dominated jet (MDJ) model. The MDJ model suggests that the bulk Lorentz factor of the outflow increases as $Γ\propto r^{1/3}$, until reaching a saturated value $Γ_{\rm sat}$. Electrons move in the globally ordered magnetic field advected by the jet from the central engine and produce synchrotron photons. The polarized synchrotron photons travel alone the jet direction and then collide with the cold electrons at the front of the jet. After the Compton scattering process, these photons escape from the jet and are detected by the observer locating slightly off-axis. If photons are emitted before the bulk Lorentz factor saturates, the change of polarization angle is a natural result of the acceleration of the outflow.

astro-ph.HE↗

arXiv2014

Comparison between hemisphere comparison method and dipole-fitting method in tracing the anisotropic expansion of the Universe use the Union2 dataset

Type-Ia supernovae (SNe Ia) are often used as the standard candles to probe the anisotropic expansion of the Universe. In this paper, we make a comprehensive comparison between the hemisphere comparison (HC) method and dipole-fitting (DF) method in searching for the cosmological preferred direction using the Union2 dataset, a compilation of 557 well-calibrated SNe Ia. We find that the directions of the faintest SNe Ia derived from these two methods are approximately opposite. Monte Carlo simulations show that the results of the HC method strongly depend on the distribution of the data points in the sky. The coincidence that the HC method and DF method give two completely opposite directions may be due to the extremely nonuniform distribution of the Union2 dataset.

astro-ph.CO↗

arXiv2014

Gamma-ray polarization of synchrotron-self-Compton process from a highly relativistic jet

The high polarization observed in the prompt phase of some gamma-ray bursts (GRBs) arouses extensive studies on the emission mechanism. In this paper, we investigate the polarization properties of the synchrotron-self-Compton (SSC) process from a highly relativistic jet. A magnetic-dominated, baryon-loaded jet ejected from the central engine travels with a large Lorentz factor. Shells with slightly different velocities collide with each other and produce shocks. The shocks accelerate electrons to power-law distribution, and at the same time, magnify the magnetic field. Electrons move in the magnetic field and produce synchrotron photons. The synchrotron photons suffer from the Compton scattering (CS) process and then are detected by an observer locating slightly off-axis. We derive analytically the formulae of photon polarization in the SSC process in two magnetic configurations: magnetic field in the shock plane and perpendicular to the shock plane. We show that photons induced by the SSC process can be highly polarized, with the maximum polarization $Π\sim 24\%$ in the energy band $[0.5,5]$ MeV. The polarization depends on the viewing angles, peaking in the plane perpendicular to the magnetic field. In the energy band $[0.05,0.5]$ MeV, in which most $γ$-ray polarimeters are active, the polarization is about twice of that in the Thomson limit, reaching to $Π\sim 20\%$. This implies that the Klein-Nishina effect, which is often neglected in literatures, should be carefully considered.

astro-ph.HE↗

arXiv2014

CosmoMC Installation and Running Guidelines

CosmoMC is a Fortran 95 Markov-Chain Monte-Carlo (MCMC) engine to explore the cosmological parameter space, plus a Python suite for plotting and presenting results (see http://cosmologist.info/cosmomc/). This document describes the installation of the CosmoMC on a Linux system (Ubuntu 14.04.1 LTS 64-bit version). It is written for those who want to use it in their scientific research but without much training on Linux and the program. Besides a step-by-step installation guide, we also give a brief introduction of how to run the program on both a desktop and a cluster. We share our way to generate the plots that are commonly used in the references of cosmology. For more information, one can refer to the CosmoCoffee forum (http://cosmocoffee.info/viewforum.php?f=11) or contact the authors of this document. Questions and comments would be much appreciated.

astro-ph.IM↗

arXiv2014

Constraining the Anisotropy of the Universe from Supernovae and Gamma-ray Bursts

Recently, an anisotropic cosmological model was proposed. An arbitrary 1-form, which picks out a privileged axis in the universe, was added to the Friedmann-Robertson-Walker line element. The distance-redshift relation was modified such that it is direction dependent. In this paper, we use the Union2 dataset and 59 high-redshift gamma-ray bursts to give constraints on the anisotropy of the universe. The results show that the magnitude of anisotropy is about $D=-0.044\pm0.018$, and the privileged axis points towards the direction $(l_0,b_0)=(306.1^{\circ}\pm 18.7^{\circ},-18.2^{\circ}\pm 11.2^{\circ})$ in the galactic coordinate system. The anisotropy is small and the isotropic cosmological model is an excellent approximation.

astro-ph.CO↗

arXiv2014

Implications of primordial power spectra with statistical anisotropy on CMB temperature fluctuation and polarizations

Both the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck observations reported the hemispherical asymmetry of the cosmic microwave background (CMB) temperature fluctuation. The hemispherical asymmetry might be stemmed from the primordial statistical anisotropy during the inflationary era of the universe. In this paper, we study possible implications of the primordial power spectra with dipolar anisotropy on the CMB temperature fluctuation and polarizations. We explicitly show that the statistical dipolar anisotropy may induce the off-diagonal ($\ell'\neq\ell$) $TT$, $EE$, $BB$, and $TE$ correlations, as well as the diagonal ($\ell'=\ell$) $TB$ and $EB$ spectra. In particular, these correlation coefficients are expected to be $m$-dependent generically. These signals of statistical anisotropy might be tested by CMB observations in future.

astro-ph.CO↗

arXiv2014

Constraining anisotropy of the universe from different groups of type-Ia supernovae

Recent released Planck data and other astronomical observations show that the universe may be anisotropic on large scales. Inspired by this, anisotropic cosmological models have been proposed. We note that the Finsler-Randers spacetime provides an appropriate framework for the anisotropic cosmology. By adding an arbitrary 1-form to the Friedmann-Robertson-Walker (FRW) line element, a privileged axis in the universe is picked out. The distance-redshift relation is modified to be direction-dependent. We wish that the anisotropic cosmological model may be tested crossly by independent observations. Type-Ia supernovae (SNe Ia) calibrated from four different light curve fitters are used to constrain possible anisotropy of the universe. The magnitudes of anisotropy are all between 2% --- 5%, but the systematic uncertainty cannot be excluded. The directions of privileged axis seem to differ from each other. The statistical significance is not high enough to make a convincing conclusion. Nevertheless, the $1σ$ contours in the $(l,b)$ plane obtained from four groups of SNe Ia have an overlap, centering at $(l,b)\approx (170^{\circ},0^{\circ})$. Monte Carlo simulation shows that the anisotropy is unlikely to be caused by selection effect.

astro-ph.CO↗

arXiv2014

Gamma-ray burst polarization via Compton scattering process

Synchrotron radiation and Compton scattering are widely accepted as the most likely emission mechanisms of some astrophysical phenomena, such as gamma-ray bursts (GRBs) and active galactic nuclei (AGNs). The measurement on polarization of photons provides a useful tool to distinguish different emission mechanisms and structures of the emission region. Based on the differential cross section of a polarized photon scattered by an unpolarized electron of any initial momentum, we derive analytical formula of polarization for beamed photons scattered by isotropic electrons with a power law distribution. Numerical calculations are carried out in four special cases: electrons at rest, Thomson limit, head-on collision and monochromatic electrons. It is found that the maximum polarization can be as high as $100\%$ for low energy photons, if the electrons are at rest. Although polarization is highly suppressed due to the isotropic electrons, a maximum value of $\sim 10\% \-- 20\%$ can still be achieved. Compton scattering process can be used to explain the polarization of GRB 041219A and GRB 100826A.

astro-ph.HE↗

arXiv2013

Quadrupole--octopole alignment of CMB related to primordial power spectrum with dipolar modulation in anisotropic spacetime

The WMAP and Planck observations show that the quadrupole and octopole orientations of the CMB might align with each other. We reveal that the quadrupole--octopole alignment is a natural implication of the primordial power spectrum in an anisotropic spacetime. The primordial power spectrum is presented with a dipolar modulation. We obtain the privileged plane by employing the ``power tensor'' technique. At this plane, there is the maximum correlation between quadrupole and octopole. The probability for the alignment is much larger than what in the isotropic universe. We find that this model would lead to deviations from the statistical isotropy only for low--$\ell$ multipoles.

astro-ph.CO↗

arXiv2013

Inflation and primordial power spectra at anisotropic spacetime inspired by Planck's constraints on isotropy of CMB

Recently, the Planck 2013 results showed possible evidence for a dipolar power modulation of the CMB temperature fluctuations at low--(\ell) multipoles. This anomaly might imply certain deviations from statistical isotropy. To incorporate the Planck's data into standard cosmological model, we propose an inflation model of the very early universe in an anisotropic spacetime. A generalized Friedmann-Robertson-Walker (FRW) metric is presented in the Randers-Finsler spacetime. We obtain the primordial power spectrum of the scalar perturbation with direction dependence, such as the dipolar modulation. This is consistent with the dipolar power modulation of the CMB anisotropy signaled by the Planck observation.

astro-ph.CO↗

arXiv2013

$Λ$CDM model with a scalar perturbation vs. preferred direction of the universe

We present a scalar perturbation for the $Λ$CDM model, which breaks the isotropic symmetry of the universe. Based on the Union2 data, the least-$χ^2$ fit of the scalar perturbed $Λ$CDM model shows that the universe has a preferred direction $(l,b)=(287^\circ\pm25^\circ,11^\circ\pm22^\circ)$. The magnitude of scalar perturbation is about $-2.3\times10^{-5}$. The scalar perturbation for the $Λ$CDM model implies a peculiar velocity, which is perpendicular to the radial direction. We show that the maximum peculiar velocities at redshift $z=0.15$ and $z=0.015$ equal to $73\pm28 \rm km\cdot s^{-1}$ and $1099\pm427 \rm km\cdot s^{-1}$, respectively. They are compatible with the constraints on peculiar velocity given by Planck Collaboration.

gr-qc↗

arXiv2013

Polarization of photons scattered by electrons in any spectral distribution

Based on the quantum electrodynamics, we present a generic formalism of the polarization for beamed monochromatic photons scattered by electrons in any spectral distribution. The formulae reduce to the components of the Fano matrix when electrons are at rest. We mainly investigate the polarization in three scenarios, i.e., electrons at rest, isotropic electrons with a power law spectrum and thermal electrons. If the incident beam is polarized, the polarization is reduced significantly by isotropic electrons at large viewing angles, and the degree of polarization due to thermal electrons is about one times less than that of electrons in a power law. If the incident bean is unpolarized, soft $γ$-rays can lead to about 15% polarization at viewing angles around $π/4$. For isotropic electrons, one remarkable feature is that the polarization as a function of the incident photon energy always peaks roughly at 1 MeV, this is valid for both the thermal and power law cases. This feature can be used to distinguish the model of the inverse Compton scattering from that of the synchrotron radiation.

astro-ph.HE↗