Direction Dependence of the Deceleration Parameter (1109.0941v6)

Abstract: In this paper we study the possibly existing anisotropy in the acceleration expansion by use of the full sample of Union2 data. Using the hemisphere comparison method to search for a preferred direction, we take the deceleration parameter q0 as the diagnostic to quantify the anisotropy level in the wCDM model. We find that the maximum accelerating expansion direction is $(l,b)=(314_{+20^{\circ}}^{\circ-13^{\circ}},28_{+11^{\circ}}^{\circ-33^{\circ}})$,with the maximum anisotropy level of $\delta q_{0,max}/\bar{q}0=0.79{+0.27}^{-0.28}$, and that the anisotropy is more prominent when only low redshift data ($z\leq0.2$) are used. We also discuss this issue in the $CPL$ parameterized model, showing a similar result.

Direction Dependence of the Deceleration Parameter: An Analysis from the Union2 Dataset

This paper explores the potential anisotropy in the accelerating expansion of the Universe by leveraging the extensive Union2 dataset composed of 557 type Ia supernovae (SNIa). The investigation focuses on directional dependence in the cosmological deceleration parameter $q_0$ within the $wCDM$ framework, along with a brief analysis using the CPL dark energy parametrization.

Methodological Approach

The analysis employs the hemisphere comparison method to identify potential preferred directions in the cosmological data. This method involves selecting random vectors, dividing the sky into hemispheres, and computing cosmological parameters separately for opposing hemispheres. The paper uses the deceleration parameter $q_0$ to quantify anisotropy and assess directional expansion differences.

In addition to the main $wCDM$ model, the authors also examine directional implications within the CPL dynamical dark energy framework. The robustness of results across different theoretical models and redshift bins is tested to provide comprehensive insights into anisotropy.

Key Findings

1. Anisotropy Detection: The research identifies a maximum anisotropy direction at Galactic coordinates $$(l, b) = (314^{\circ\, +20^\circ}_{\,\, -13^\circ}, 28^{\circ\, +11^\circ}_{\,\, -33^\circ})$$ with a significant anisotropy level of $$\Delta q_{0, \text{max}}/\bar{q}_0 = 0.79_{+0.27}^{-0.28}$$ within the $wCDM$ model.
2. Redshift Dependence: The anisotropy is found to be more pronounced at low redshifts ($z \leq 0.2$), suggesting possible redshift dependence in the degree of cosmological anisotropy.
3. Model Sensitivity: Findings are consistent with previous analyses and show minor sensitivity to the underlying dark energy model. The preferred direction and anisotropy magnitude do not vary drastically when employing the CPL parametrization, thereby supporting the robustness of the detected anisotropy.
4. Comparison with Previous Studies: These observations align with other studies indicating a preferred cosmological direction, which are supported by additional astrophysical phenomena like the CMB dipole and quasar polarization alignments, although the paper emphasizes that the results remain statistically consistent with isotropy.

Implications and Future Directions

The potential anisotropy indicated by this analysis fundamentally challenges the cosmological principle, which posits isotropy and homogeneity at grand scales. If a preferential direction is genuine, this would suggest deviations from the standard $\Lambda$CDM cosmic model, inviting theories that incorporate vector fields or spatially anisotropic equations of state.

Future analyses must confirm these findings using larger, more diverse datasets, possibly including forthcoming observations from missions like the Joint Dark Energy Mission (JDEM). Moreover, improved modeling techniques that account for greater complexities in dark energy behavior could shed light on whether these apparent anisotropies are truly cosmological or artifacts of observational constraints.

In conclusion, while the evidence suggests possible cosmos anisotropy, it remains critical to approach these results with cautious interpretation and pursue further robust statistical methodologies to substantiate these intriguing claims.