Last year, an unexpected dip in the Cosmic Microwave Background (CMB) temperature around nearby galaxies was reported. The CMB was found to be colder than the average up to distances of 1Mpc around galaxies in the 2MRS redshift catalogue, a distance which is several times larger than the galaxy itself.
A follow-up paper found a remarkable correlation between the large scale distribution of nearby (< 100Mpc) galaxies and the large scale temperature fluctuations of the CMB. This imprint was found to possibly explain most of the so-called statistical anomalies of the CMB fluctuations.
I will review the findings and speculate about the possible origin of the galactic signal: is it connected to physical processes in the inter-galactic medium? Dark matter? Or is it all a statistical fluke?
We can see the temperature map of the Cosmic Microwave Background (CMB) as observed with the Planck satellite (upper left) and the density of nearby spiral galaxies represented as negative temperature where the density is high (lower left). Similarities between the CMB and the galaxy distribution are shown as circles. On the right, we can see a close-up of the so-called 'cold spot', an unexpectedly large low temperature outlier in the CMB. The lower right plot shows galaxy density in the cold spot area. Credit: Frode K. Hansen
