Preferred axis in cosmology

Abstract: The foundation of modern cosmology relies on the so-called cosmological principle which states an homogeneous and isotropic distribution of matter in the universe on large scales. However, recent observations, such as the temperature anisotropy of the cosmic microwave background (CMB) radiation, the motion of galaxies in the universe, the polarization of quasars and the acceleration of the cosmic expansion, indicate preferred directions in the sky. If these directions have a cosmological origin, the cosmological principle would be violated, and modern cosmology should be reconsidered. In this paper, by considering the preferred axis in the CMB parity violation, we find that it coincides with the preferred axes in CMB quadrupole and CMB octopole, and they all align with the direction of the CMB kinematic dipole. In addition, the preferred directions in the velocity flows, quasar alignment, anisotropy of the cosmic acceleration, the handedness of spiral galaxies, and the angular distribution of the fine-structure constant are also claimed to be aligned with the CMB kinematic dipole. Since CMB dipole was confirmed to be caused by the motion of our local group of galaxies relative to the reference frame of the CMB, the coincidence of all these preferred directions hints that these anomalies have a common origin, which is not cosmological or due to a gravitational effect. The systematical or contaminative errors in observation or in data analysis, which can be directly related to the motion of our local group of galaxies, can play an important role in explaining the anomalies.

• The paper challenges the cosmological principle by analyzing CMB quadrupole and octopole alignments, suggesting a preferred axis in the universe.
• It employs rigorous statistical methods on WMAP and Planck data, with alignments confirmed to better than 3σ significance.
• The findings prompt reconsideration of isotropy in cosmological models and call for refined observational techniques to address potential systematic errors.

Analysis of Anomalies in Cosmic Microwave Background and Cosmological Observations

The paper "Preferred axis in cosmology" by Wen Zhao and Larissa Santos explores the foundational concepts of the cosmological principle, which asserts a homogeneous and isotropic universe on a large scale. The study challenges this principle by examining a variety of large-scale anomalies observed in cosmic phenomena, suggesting that these anomalies may indicate the existence of preferred directions in the universe. The primary focus is on the statistical analysis of the Cosmic Microwave Background (CMB) data, particularly the alignment of quadrupole and octopole moments, the CMB parity asymmetry, and its correlation with the CMB kinematic dipole.

Cosmological Anomalies

The analysis begins with a detailed introduction to the anomalies observed in cosmological data, particularly those gathered from WMAP and Planck satellite measurements. The anomalies include low quadrupole and octopole moments, variance and correlation deficits on large angular scales, and alignments in low multipole moments in the CMB data. These issues challenge the expected outcomes of the inflationary $\Lambda$CDM model, which predicts Gaussian-distributed cosmic anisotropies originating from early quantum fluctuations.

Directional Dependencies and Alignment

The researchers focus on the CMB's directional properties, revealing that various preferred directions coincide with the CMB kinematic dipole, a fact believed to be due to the motion of our local group of galaxies. These include directional dependencies found in CMB parity violations, alignment in the quadrupole and octopole of CMB, and similar trends in other cosmological phenomena such as quasar polarization vector alignments, large-scale velocity flows, and anisotropies in spiral galaxies’ handedness. The paper provides compelling evidence that these preferred directions are not merely random but exhibit significant alignments, especially with the direction of the CMB kinematic dipole.

Implications and Speculations

The implications of these alignments are profound. If these preferred axes have a cosmological origin, they suggest a need to reconsider the cosmological principle and could imply a non-trivial topology of the universe, anisotropic matter and dark energy distributions, or particular large-scale fluctuation modes. Alternative gravitational theories or inherent anisotropies caused by early-universe conditions may also provide explanations.

Alternatively, the authors suggest that non-cosmological origins could explain the observed anomalies, potentially stemming from systematic errors, data contaminations, or unresolved foreground emissions that align with the CMB dipole. Such origins would entail refining observational techniques and data analysis methods to more accurately capture cosmic phenomena.

Numerical Results and Significance

The statistical significance of these phenomena is underscored by numerical results where the alignments are observed consistently across different datasets and analysis methods. The alignment with the CMB kinematic dipole is particularly significant, with several studies confirming this to better than $3\sigma$ confidence level. These alignments require serious consideration due to their potential to upend long-held assumptions of cosmic isotropy and homogeneity, prompting a reconsideration of existing cosmological models or an intensified focus on potential observational errors.

Outlook and Future Directions

The future of cosmological research will need to address these anomalies more thoroughly, possibly leveraging upcoming data from next-generation cosmic surveys and refined modeling of cosmic phenomena. The quest for understanding these phenomena will likely propel advancements in theoretical cosmology, observational techniques, and possibly spur the development of new physics models to accommodate the observed deviations from expected isotropy.

In conclusion, the paper provides a meticulous analysis of cosmological anomalies that may point toward preferred directions in our universe. Whether these are cosmological in nature or artifacts of observational limits, the research lays the groundwork for pivotal explorations into the fabric of our cosmos.