[Altheasy]

"What we know is that gamma-ray photons from extragalactic sources travel in the universe toward Earth, where they can be absorbed by interacting with the photons from starlight," Ajello said. "The rate of interaction depends on the length that they travel in the universe. And the length that they travel depends on expansion. If the expansion is low, they travel a small distance. If the expansion is large, they travel a very large distance. So the amount of absorption that we measured depended very strongly on the value of the Hubble Constant. What we did was turn this around and use it to constrain the expansion rate of the universe."

[TeMPOraL]

Does that quote say photons absorb photons? I haven't heard about something like this before.

[kmm]

Because they are electrically neutral, there is no direct interaction between photons, which is good, because it allows us to see each other even when there is light coming from all directions. At low energies, that's the end of the story as the interactions via intermediate particles are incredibly unlikely.

However, think about how an electron and a positron can annihilate into two photons. As the electromagnetic interaction is symmetric under time reversal, the reverse reaction should be possible, as long as sufficient energy is available. So an extremely energetic photon can combine with a photon from the background starlight and turn into an electron-positron pair, essentially being absorbed.

[cjfd]

https://en.wikipedia.org/wiki/Two-photon_physics