Zhe Chang

Testing the isotropy of the Universe by using the JLA compilation of Type Ia supernovae

We probe the possible anisotropy of the Universe by using the joint light-curve analysis (JLA) compilation of Type Ia supernovae. We apply the Markov Chain Monte Carlo method to constrain the amplitude and direction of anisotropy in three cosmological models. For the dipole-modulated Lambda cold dark matter (Lambda CDM) model, the anisotropic amplitude is consistent with zero at 68 per cent cl, and has an upper bound AD < 1.98 x 10(-3) at 95 per cent cl. Regardless of much larger uncertainty, we amazingly find that the dipole direction of JLA is almost opposite to that of Union2. Similar results are found for the dipole-modulated wCDM and Chevallier-Polarski-Linder (CPL) models. Thus, the Universe is still well consistent with the isotropy according to the JLA compilation.

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-ℓ multipoles. This anomaly might imply certain deviations from statistical isotropy. To incorporate the Planck’s data into the 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.

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

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 data set, 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 non-uniform distribution of the Union2 data set.

Finslerian MOND versus observations of Bullet Cluster 1E 0657-558

It is known that theory of MOND (Modification of Newtonian Dynamics) with spherical symmetry cannot account for the convergence kappa-map of Bullet Cluster 1E 0657-558. In this paper, we try to set up a Finslerian MOND, a generalization of MOND in Finsler space-time. We use Ric = 0 to obtain the gravitational vacuum field equation in a 4D Finsler space-time. To leading order in the post-Newtonian approximation, we obtain the explicit form of the Finslerian line element. It is simply the Schwarzschilds metric except for the Finslerian rescaling coefficient f(v) of the radial coordinate r, i.e. R = f(v) r. By setting f (v) = root 1 - (GMa(0)/v(4)), we obtain the famous MOND in a Finslerian framework. Taking a dipole and a quadrupole term into consideration, we give the convergence kappa in gravitational lensing astrophysics in our model. Numerical analysis shows that our prediction is to a certain extent in agreement with the observations of Bullet Cluster 1E 0657-558. With the theoretical temperature T taking the observed value 14.8 keV, the mass density profile of the main cluster obtained in our model is of the same order as that given by the best-fitting King beta-model.

The significance of anisotropic signals hiding in the type Ia supernovae

We use two different methods, i.e. dipole-fitting (DF) and hemisphere comparison (HC), to search for the anisotropic signals hiding in the Union2.1 data set. We find that the directions of maximum matter density derived using these two methods are about 114A degrees away from each other. We construct four Union2.1-like mock samples to test the statistical significance of these two methods. It is shown that DF method is statistically significant, while HC method is strongly biased by the distribution of data points in the sky. Then we assume that the anisotropy of distance modulus is mainly induced by the anisotropy of matter density, which is modelled to be the dipole form Omega(M) = Omega(M0)(1 - cos theta). We fit our model to Union2.1, and find that the direction of maximum matter density is well consistent with the direction derived using DF method, but it is very different from the direction previously claimed. Monte Carlo simulations show that our method is statistically more significant than HC method, although it is not as significant as DF method. The statistical significance can be further improved if the data points are homogeneously distributed in the sky. Due to the low quality of present supernovae data, however, it is still premature to claim that the Universe has any preferred direction.

OPERA SUPERLUMINAL NEUTRINOS AND KINEMATICS IN FINSLER SPACETIME

The OPERA collaboration recently reported that the muon neutrinos could be superluminal. More recently, Cohen and Glashow pointed that such superluminal neutrinos would be suppressed since they lose their energies rapidly via bremsstrahlung. Here, we propose that the Finslerian nature of the space time could account for the superluminal phenomena of particles. The Finsler spacetime permits the existence of the superluminal behavior of particles while the causality still holds. A new dispersion relation is obtained in a class of Finsler spacetimes. It is shown that the superluminal speed is linearly dependent on the energy per unit mass of the particle. We find that such a superluminal speed formula is consistent with the data of the OPERA, MINOS and Fermilab-1979 neutrino experiments as well as the observations on the neutrinos from the SN1987A.

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 the accelerating cosmic expansion to have a dipole structure and the directions of these two dipoles to be the same. Our numerical results show that the dipole direction of the SnIa Hubble diagram locates at

Kinematics in Randers-Finsler geometry and secular increase of the astronomical unit

(l,b)=(314.6^\circ \pm 20.3^\circ ,-11.5^\circ \pm 12.1^\circ )