Caption: A cartoon of a photon gas in darkness.

If the heat death of the universe arrives---which is an era that may extend to infinite cosmic time---photons forever cosmological redshifting will be among the last constituents of the universe---or, if the eternal inflation version of multiverse paradigm is right, our pocket universe.

Credit/Permission: © User:Zalasur, 2005 (uploaded to Wikipedia by User:Maksim, 2006) / Creative Commons CC BY-SA 3.0.
Image link: Wikipedia: File:Photon_waves.png.
Local file: local link: heat_death_photons.html.
File: Cosmology file: heat_death_photons.html.