# Missing dust signature in the cosmic microwave background

## TL;DR

This paper investigates the potential spectral distortion of the cosmic microwave background caused by galactic and intergalactic dust absorption, suggesting that even minimal dust opacity could significantly affect CMB observations and challenge current cosmological models.

## Contribution

It introduces a model predicting measurable CMB distortions due to dust absorption, highlighting the importance of intergalactic dust opacity in cosmological observations.

## Key findings

- Intergalactic dust opacity can cause detectable CMB distortions.
- Predicted distortions exceed current measurement sensitivities.
- Absence of observed distortion raises questions about dust properties or cosmological models.

## Abstract

I examine a possible spectral distortion of the Cosmic Microwave Background (CMB) due to its absorption by galactic and intergalactic dust. I show that even subtle intergalactic opacity of $1 \times 10^{-7}\, \mathrm{mag}\, h\, \mathrm{Gpc}^{-1}$ at the CMB wavelengths in the local Universe causes non-negligible CMB absorption and decline of the CMB intensity because the opacity steeply increases with redshift. The CMB should be distorted even during the epoch of the Universe defined by redshifts $z < 10$. For this epoch, the maximum spectral distortion of the CMB is at least $20 \times 10^{-22} \,\mathrm{Wm}^{-2}\, \mathrm{Hz}^{-1}\, \mathrm{sr}^{-1}$ at 300 GHz being well above the sensitivity of the COBE/FIRAS, WMAP or Planck flux measurements. If dust mass is considered to be redshift dependent with noticeable dust abundance at redshifts 2-4, the predicted CMB distortion is even higher. The CMB would be distorted also in a perfectly transparent universe due to dust in galaxies but this effect is lower by one order than that due to intergalactic opacity. The fact that the distortion of the CMB by dust is not observed is intriguing and questions either opacity and extinction law measurements or validity of the current model of the Universe.