Modified Gravity and the Origin of the Excess Radio Galaxy Number-Count Dipole

TL;DR

The paper tackles the observed excess in the radio galaxy number-count dipole, which exceeds the purely kinematic expectation by a factor of about 3–4. It proposes STVG-MOG, a scalar-tensor-vector gravity theory with a scale-dependent effective gravitational coupling $G_{ m eff}(k,a)=G_N[1+\alpha_{ m eff}(k,a)]$, to boost ultra-large-scale structure and coherent bulk flows while preserving small-scale and early-universe phenomenology. The analysis shows that the LSS contribution to the dipole can be amplified by a factor $R_d(a)\approx[1+\alpha_{ m eff}(k_*,a_*)]^p$ on the scales probed by radio surveys, explaining the observed $d_{ m obs}\approx(3-4)\,d_{ m kin}$ without contradicting constraints from the CMB, $P(k)$, weak lensing, or BAO. The authors argue that the dipole anomaly provides a unique, testable window into gravity on the largest cosmological scales and emphasize future tests with SKA, kSZ cross-correlations, and relativistic clustering observables to discriminate between modified gravity and standard dark-matter scenarios.

Abstract

Recent analyses of wide-area radio-galaxy surveys have reported a statistically significant excess in the cosmic number-count dipole, with an amplitude exceeding the purely kinematic expectation of the standard $Λ$CDM model by a factor of $\sim 3$--$4$, quoted at a significance level of up to $5.4σ$. While residual observational systematics and local-structure effects cannot be definitively excluded, this result motivates the exploration of alternative physical interpretations beyond the minimal $Λ$CDM framework. We investigate whether Scalar--Tensor--Vector Gravity (STVG-MOG) can provide a consistent explanation for an enhanced large-scale anisotropic dipole without violating existing constraints from early-universe cosmology, the cosmic microwave background (CMB) dipole, galaxy dynamics, weak lensing, or the observed late-time matter power spectrum. The radio number-count dipole probes ultra-large-scale, anisotropic structure and coherent gravitational response, rather than virialized dynamics or linear growth alone. In STVG-MOG, a scale- and time-dependent effective gravitational coupling preserves standard cosmological evolution at early times and on small to intermediate scales, while amplifying gravitational response on gigaparsec scales. This scale-selective enhancement can increase the large-scale structure contribution to the radio dipole without overproducing power on smaller scales. If the observed dipole excess reflects a physical cosmological signal rather than residual systematics, STVG-MOG offers a viable and testable alternative interpretation. It is demonstrated that the radio dipole anomaly provides a novel probe of gravitational physics on the largest observable scales.