The significance of anisotropic signals hiding in the Type Ia supernovae

Abstract

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 114• 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 modeled to be the dipole form Ω M = Ω M0 (1 − cos θ). 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.

The prospects of using gravitational waves for constraining the anisotropy of the Universe *

“…µ ΛCDM is the distance modulus in fiducial ΛCDM model. The fiducial parameters in this model is given by the result of Union 2.1 [35], which are d = 0.0012, l = 310.6 • , b = −13 • . Next, we use the curve of Eq.…”

“…It can be seen that the inferred direction becomes more and more accurate as the number of observed BNS merging events increases. When BNS merging events observed by ET reaches 400, the determination of the anisotropic direction will be able to achieve the accuracy of Union2.1 [35]. Fig.…”

“…It can be seen that when the number of BNS merging events observed increase, the uncertainty of the inferred amplitude becomes smaller. When 200 BNS merging events are observed in redshift (0, 1) by ET, the determination of the amplitude d will be able to achieve the accuracy of Union2.1 [35]. When the number of BNS merging events becomes larger, the incline of uncertainty reduction is gentle.…”

Testing the anisotropy of the universe using the simulated gravitational wave events from advanced LIGO and Virgo

“…In step 7, when calculating the anisotropic distance modulus, we choose the fiducial dipole amplitude to be the same to Union2.1, i.e. d = 1.2 × 10 −3 [24]. The fiducial direction is arbitrarily chosen to be (α 0 , δ 0 ) = (310.6 • , − 13.0 • ), which is also the same to Union2.1 but changed from the galactic coordinates to equatorial coordinates.…”

“…If the universe is indeed anisotropic, it implies that there are new physics beyond the standard model. Whether these anisotropic signals come from the intrinsic property of the universe or merely the statistical fluctuation is extensively debated [19][20][21][22][23][24].…”