A p < 0.0001 detection of cosmic microwave background cooling in galactic halos and its possible relation to dark matter

¹ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
² Instituto de Astronomía Teórica y Experimental (IATE), CONICET-UNC, Córdoba, Argentina
³ Observatorio Astronómico de Córdoba, UNC, Córdoba, Argentina
⁴ Comisión Nacional de Actividades Espaciales (CONAE), Córdoba, Argentina

^⋆ Corresponding author; frodekh@astro.uio.no

Received: 21 November 2024
Accepted: 9 March 2025

Abstract

We confirm, at the 5.7σ level, previous studies reporting cosmic microwave background (CMB) temperatures being significantly lower around nearby spiral galaxies than expected from the ΛCDM model. Results from our earlier work were disputed in a recent paper, however, that analysis included areas far beyond the galactic halos, while disregarding the neighbourhood of the galaxies where the main signal is seen. Here, we limit the present study to pixels that are well within the galactic halos, focussing on galaxies in dense cosmic filaments and improving the signal-to-noise ratio (S/N), as compared to previous studies. The average CMB temperature in discs around these galaxies is always much lower in Planck data than in any of the 10 000 Planck-like CMB simulations. Even when correcting for the look-elsewhere effect, the detection is still at the 3 − 4σ level. We further show that the largest scales (ℓ < 16) of the Planck CMB fluctuations are more correlated with the distribution of nearby galaxies than 99.99% of simulated CMB maps. We argue that the existence of a new CMB foreground cannot be ignored and a physical interaction mechanism should be sought, which could possibly involve dark matter and could also be linked to intergalactic magnetic fields.