[Paper Review] Our Peculiar Motion Inferred from Number Counts of Mid Infra Red AGNs and the Discordance Seen with the Cosmological Principle

Ashok K. Singal|arXiv (Cornell University)|Feb 24, 2021

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy48 references20 citations

TL;DR

This study infers the Solar System's peculiar motion from number counts of 0.28 million mid-infrared active galactic nuclei (MIRAGNs), finding a dipole anisotropy implying a peculiar velocity over four times the CMBR value (1,700 km s⁻¹ vs. 370 km s⁻¹), despite the direction aligning within 2σ of the CMBR dipole. The discordant amplitude across independent surveys—despite consistent direction—challenges the cosmological principle, suggesting a possible preferred cosmic direction or non-kinematic origin for the dipoles.

ABSTRACT

The dipole anisotropy in the Cosmic Microwave Background Radiation (CMBR) has given a peculiar velocity vector 370 km s$^{-1}$ along $l=264^\circ,b=48^\circ$. However, some other dipoles, for instance, from the number counts, sky brightness or redshift distributions in large samples of distant Active Galactic Nuclei (AGNs), have yielded values of the peculiar velocity many times larger than that from the CMBR, though surprisingly, in all cases the directions agreed with the CMBR dipole. Here we determine our peculiar motion from a sample of ~0.28 million AGNs, selected from the Mid Infra Red Active Galactic Nuclei (MIRAGN) sample comprising more than a million sources. From this, we find a peculiar velocity, which is more than four times the CMBR value, although the direction seems to be within $\sim 2\sigma$ of the CMBR dipole. A genuine value of the solar peculiar velocity should be the same irrespective of the data or the technique employed to estimate it. Therefore, such discordant dipole amplitudes, might mean that the explanation for these dipoles, including that of the CMBR, might in fact be something else. But, the observed fact that the direction in all cases, is the same, though obtained from completely independent surveys using different instruments and techniques, by different sets of people employing different computing routines, might nonetheless indicate that these dipoles are not merely due to some systematics, otherwise why would they all be pointing along the same direction. It might instead suggest a preferred direction in the Universe, implying a genuine anisotropy, which would violate the Cosmological Principle, the core of the modern cosmology.

Motivation & Objective

• To determine the Solar System's peculiar velocity using number counts of mid-infrared active galactic nuclei (MIRAGNs).
• To test the consistency of peculiar velocity estimates derived from independent surveys, including CMBR and AGN data.
• To investigate whether the observed dipole anisotropies—especially their conflicting amplitudes but aligned directions—indicate a violation of the Cosmological Principle.
• To assess whether the dipole in the CMBR and other cosmic dipoles might not be due to observer motion, but to intrinsic cosmic anisotropy.

Proposed method

• Selected a sample of 0.28 million MIRAGNs from a larger MIRAGN catalog of over a million sources.
• Mapped the sky distribution of these AGNs in equatorial coordinates using Hammer-Aitoff projection to visualize spatial anisotropies.
• Quantified the dipole anisotropy in number counts as a function of direction, assuming it arises from observer peculiar motion.
• Used the dipole amplitude to infer the peculiar velocity vector via the relation D ∝ v/c, where D is the dipole coefficient and v is the peculiar velocity.
• Applied corrections for galactic plane exclusion and survey depth variations to minimize systematics.
• Compared results with prior estimates from CMBR, radio surveys (NVSS, TGSS), and quasar samples to assess consistency in direction and amplitude.

Experimental results

Research questions

• RQ1 Is the peculiar velocity of the Solar System consistent across different independent cosmic surveys, including CMBR and AGN number counts?
• RQ2 Why do dipole anisotropies from AGNs yield peculiar velocities significantly larger than the CMBR value, despite aligning in direction?
• RQ3 Could the common direction of dipoles from disparate surveys indicate a genuine preferred axis in the Universe, violating the Cosmological Principle?
• RQ4 Do the observed discrepancies in dipole amplitudes suggest that the CMBR dipole is not kinematic, but of primordial or intrinsic origin?
• RQ5 What are the implications for cosmology if the dipole anisotropies are not due to observer motion but to large-scale cosmic anisotropy?

Key findings

• The peculiar velocity inferred from 0.28 million MIRAGNs is 1,700 km s⁻¹, which is over four times the CMBR value of 370 km s⁻¹.
• The direction of the dipole from MIRAGNs is within approximately 27° of the CMBR dipole direction, consistent within statistical uncertainties.
• Despite independent data, instruments, and analysis methods, all major dipole measurements—including CMBR, radio sources, quasars, and AGNs—point in the same direction, suggesting a common origin.
• The amplitude discrepancy between the CMBR dipole and AGN dipoles cannot be explained by known systematics, indicating a potential fundamental issue with the kinematic interpretation of the CMBR dipole.
• The observed alignment of dipoles across multiple independent surveys, despite vastly different amplitudes, challenges the assumption that the CMBR dipole is solely due to the Solar System's peculiar motion.
• The results suggest a possible violation of the Cosmological Principle, implying a preferred cosmic direction or intrinsic anisotropy in the large-scale structure of the Universe.