Cosmic filaments confirm unexplained cooling of CMB photons in two independent redshift ranges

Recent papers have reported an unexplained cooling of CMB photons passing through galaxies in nearby cosmic filaments $z<0.02$ at the $>5σ$ level. Here we show for the first time that this effect is also present at higher redshifts $0.02<z<0.04$. Instead of calculating the CMB temperature around individual galaxies as in previous works, we analyze mean CMB temperature profiles associated to cosmic filaments in three dimensions. We have considered different thresholds in the linear K-band luminosity density of the filaments as a proxy to mass density. Furthermore, we have analyzed the dependence of the results on the average orientation of filaments with respect to the line of sight. These studies were implemented to test the expected dependence on mass density as well as on photon trajectory length within the cosmic filaments. We find a $3-4σ$ detection of a temperature decrement trend towards the spine of the filaments, the larger the mass and the more radially oriented the filament, the stronger the temperature decrement. This trend is seen independently in both redshift ranges $0.004<z<0.02$ and $0.02<z<0.04$. We therefore conclude that our results provide strong evidence for a lower CMB temperature along massive cosmic filaments in the nearby universe $z<0.04$.

💡 Research Summary

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The authors investigate a recently reported anomalous cooling of Cosmic Microwave Background (CMB) photons that had been observed around massive nearby spiral galaxies (z < 0.02). Instead of focusing on individual galaxies, they extend the analysis to the three‑dimensional network of cosmic filaments identified in the 2MASS Redshift Survey (2MRS). Using the DisPerSE algorithm, they extract filamentary structures in two redshift shells, 0.004 < z < 0.02 and 0.02 < z < 0.04, obtaining 278 and 908 filaments respectively.

Filament mass density is approximated by the linear K‑band luminosity density: the K‑band luminosities of all galaxies within a 4 Mpc perpendicular distance from the filament spine are summed, assuming a roughly constant stellar‑to‑total‑mass ratio. Filament orientation is quantified by the angle ϕ between the filament’s end‑to‑end vector and the line of sight; smaller ϕ corresponds to a longer photon path through the structure.

CMB data are taken from Planck PR3 SMICA maps, masked to 78 % of the sky, and high‑pass filtered by removing multipoles ℓ ≤ 32 for the higher‑z shell (ℓ ≤ 5 for the lower‑z shell). The authors also test a range of cut‑offs (ℓ > 5, 10, 16, 32, 64, 128) to assess robustness. For each filament they construct a rectangular “tube” around each segment, divide the tube into twelve 0.5 Mpc transverse bins (extending up to 6 Mpc from the spine), and stack the CMB temperature values of all pixels falling in each bin across all filaments. The stacked profile ⟨ΔT⟩(d⊥) is then compared to zero using 100 Planck‑based simulations that include realistic noise and systematics.

The main findings are:

1. Mass dependence – Filaments in the top 10 % (or decile) of K‑band luminosity density show a temperature decrement of ≈ –20 µK at the filament spine, roughly 1.5 × stronger than the signal from lower‑density filaments.

2. Orientation dependence – Filaments with ϕ < 30° (more aligned with the line of sight) exhibit the strongest decrement, while those with ϕ > 60° show no significant signal.

3. Radial dependence – The decrement peaks within 0–1 Mpc of the spine and declines toward 3 Mpc, consistent with a photon‑path‑length effect.

These trends are observed independently in both redshift ranges, with statistical significances of 3–4 σ based on the simulated null distribution.

The authors argue that the thermal Sunyaev‑Zel’dovich (tSZ) effect can contribute at most ≈ 2 µK at the Planck frequencies used, far below the observed ≈ 20 µK decrement, and that the Integrated Sachs‑Wolfe (ISW) effect should produce a positive temperature shift at these low redshifts. Consequently, the observed cooling cannot be explained by standard secondary anisotropies within ΛCDM.

Methodological caveats are acknowledged: the K‑band luminosity‑to‑mass conversion assumes a uniform stellar‑to‑total‑mass ratio; redshift‑space distortions and peculiar velocities may bias filament orientation estimates, especially at the lowest redshifts; high‑pass filtering removes large‑scale modes that could contain part of the genuine signal; and the 100 Planck simulations may not fully capture the true filamentary density field.

The paper concludes that the detection of a CMB temperature decrement along massive, radially oriented filaments provides strong empirical evidence for a previously unaccounted‑for interaction between CMB photons and nearby large‑scale structure. However, distinguishing between new physics (e.g., exotic photon‑dark‑matter couplings) and residual systematic effects will require deeper surveys (extending to z ≈ 0.1), multi‑frequency CMB data (including bands above 217 GHz to isolate tSZ/kSZ contributions), and more sophisticated modeling of filament mass, geometry, and redshift‑space effects.