Capture d’écran 2023-10-15 000124

At late times, photons fall into gravitational structures like rich clusters or sparse voids, and then leave again. However, matter can flow in or out of these structures, and the expansion of the Universe can change the strength of that potential during the time a photon traverses it, creating a relative redshift or blueshift owing to what’s known as the integrated Sachs-Wolfe effect.