A Test of the Cosmological Principle with Quasars

Abstract: We study the large-scale anisotropy of the Universe by measuring the dipole in the angular distribution of a flux-limited, all-sky sample of 1.36 million quasars observed by the Wide-field Infrared Survey Explorer (WISE). This sample is derived from the new CatWISE2020 catalog, which contains deep photometric measurements at 3.4 and 4.6 $\mu$m from the cryogenic, post-cryogenic, and reactivation phases of the WISE mission. While the direction of the dipole in the quasar sky is similar to that of the cosmic microwave background (CMB), its amplitude is over twice as large as expected, rejecting the canonical, exclusively kinematic interpretation of the CMB dipole with a p-value of $5\times10^{-7}$ ($4.9\sigma$ for a normal distribution, one-sided), the highest significance achieved to date in such studies. Our results are in conflict with the cosmological principle, a foundational assumption of the concordance $\Lambda$CDM model.

• The paper reports a quasar dipole nearly twice the amplitude predicted by kinematic CMB measurements.
• It employs a flux-limited, all-sky quasar sample from the CatWISE2020 catalog to probe large-scale anisotropy.
• The findings question the standard cosmological principle, suggesting the need to revise FLRW isotropy assumptions.

An Analysis of Large-Scale Anisotropy in the Universe through Quasar Dipole

The paper "A Test of the Cosmological Principle with Quasars" authored by Secrest et al. conducts a comprehensive analysis of large-scale anisotropy in the Universe using a flux-limited, all-sky quasar sample observed by the Wide-field Infrared Survey Explorer (WISE). This paper leverages data from the CatWISE2020 catalog, which boasts significant advancements in data depth and completeness, providing a robust dataset for examining the universe's large-scale structure.

The Cosmological Principle and Its Examination

The cosmological principle, a bedrock of modern cosmology, posits that the Universe is homogeneous and isotropic on a large scale. This assumption underlies the concordance $\Lambda$CDM model and is supported by observations of the Cosmic Microwave Background (CMB) which exhibits minor temperature fluctuations.

The peculiarity of the CMB dipole anisotropy, being asymmetric on larger scales than higher multipoles, is traditionally interpreted as a kinematic effect arising from the Earth's motion relative to the CMB's rest frame. According to current measurements, this motion attributes a velocity of approximately 369.82 km/s to the Solar System, inferred from specific directions in galactic coordinates.

Investigation with the CatWISE Quasar Sample

Secrest et al.'s analysis utilizes an extensive sample of 1.36 million quasars from the CatWISE2020 catalog to evaluate whether the distribution of this cosmic matter shares the same dipole feature observed in the CMB. Quasars, due to their significant luminosity and populous nature in the high-redshift universe, provide an excellent medium to test deviations from isotropy on cosmological scales.

The study meticulously constructs this quasar sample by applying various selection criteria, such as mid-infrared photometric cuts, ensuring that the sample covers a high redshift range, which is relatively free from local isotropic deviations.

Results from the Quasar Dipole Measurement

The results demonstrate a dipole in the quasar sky distribution compatible in direction but significantly larger in amplitude than the CMB dipole. The observed amplitude is approximately more than twice that predicted by the kinematic interpretation of the CMB dipole. This significant deviation calls into question the isotropy of the universe dictated by the standard cosmological model.

With a statistical significance exemplified by a p-value of $5 \times 10^{-7}$, corresponding to a $4.9\sigma$ significance level, these findings challenge the assumptions underpinning the FLRW metric assumed in the $\Lambda$CDM model.

Implications and Future Directions

The findings presented by Secrest et al. present a compelling argument against the standard cosmological principle when it comes to the large-scale distribution of quasars. They suggest that the rest frame defined by quasars may differ from that defined by the CMB, which could have profound implications for our understanding of cosmic expansion and structure formation.

From a broader perspective, these results introduce the potential need for reconsidering aspects of cosmological models, possibly invoking new physics beyond the standard model, such as scenarios involving pre-inflationary universe dynamics or modifications to gravitational theory.

Future research might focus on corroborating these findings with other independent astronomical surveys to confirm the anisotropy and explore potential theoretical frameworks that can accommodate such discrepancies. With the upcoming advanced radio telescopes and deep field surveys, there is scope for achieving enhanced precision in cosmic dipole measurements, offering a path to resolving these ambiguities in cosmological investigations.