The effect of gravitational lensing on the Cosmic Microwave Background
This image shows the density of the large-scale distribution of matter in the Universe as seen projected along the line of sight and estimated using two different types of data.
The grey-scale map (shown in both panels) is based on observations of the Cosmic Microwave Background (CMB) performed with the National Science Foundation's South Pole Telescope (SPT) at a frequency of 150 GHz. White areas indicate regions where the matter density is higher than average, whereas black areas indicate regions with lower than average density.
The colour map (plotted over the grey-scale one in the right panel) is based on observations of the Cosmic Infrared Background (CIB) obtained with ESA's Herschel Space Observatory. The reconstruction is based on data collected at a wavelength of 500 microns using the SPIRE instrument on board Herschel. Red areas indicate regions where the matter density is higher than average, whereas blue areas indicate regions with lower than average density.
In contrast to the CMB, which is the diffuse light that permeated the very early Universe, the CIB is a cumulative background, and arose with the formation of stars and galaxies.
Gravitational lensing, the bending of light caused by massive objects, also affects the CMB as it propagates across the large-scale distribution of structure that started populating the Universe a few hundred million years after the Big Bang. Massive bodies, such as galaxies, galaxy clusters and the dark matter halos in which these are embedded, act as lenses and deflect the path of photons, causing distortions to the image of distant sources. For this reason, there is a very strong correlation between the gravitationally-lensed CMB and the CIB detected by Herschel, as the latter traces the lenses responsible for the deflection.
This correlation is apparent in the right panel, which shows the gravitational potential of the galaxies that are distorting the CMB estimated from the gravitationally-lensed CMB (grey-scale map) and from the distribution of galaxies (colour map).
Like ordinary glass lenses, a gravitational lens is most effective when located half way between the source of light and the observer. In a cosmological context, the galaxies that most contribute to lens the CMB are those located at a redshift z~2. These galaxies are best probed through the longest-wavelength band on the SPIRE instrument on Herschel, which is centred on 500 microns.