When cosmic rays pass through space, they slow down. But why? The energy of the photons that hit these particles couldn't just be transferred to it, making it go even faster?
In other words: Why cosmic rays lose energy (thus speed) when photons from the CMB collides with it?
I would appreciate the help!
PS.: I'm not a physicist nor expert. Just a beginner who likes astrophysics. So if I said something stupid, please correct me.
Best Answer
It has to do with something called the Doppler effect. Looking at it from the cosmic ray's point of view, the light it hits head on has a really high energy, and the light that hits it from behind is even colder/lower energy than what we see ($2.7$ Kelvin).
If you want to stick to our point of view, then yes a photon hitting it from behind would boost it, but when you balance out the conservation of energy and momentum the head on hit takes away far more energy than the rear-ending gives. The way the math works on those conservation laws, it's basically changing perspective to the person who sees the cosmic ray and photon having the same momentum in opposite directions ("center of mass" frame), seeing what happens, and then translating back to our perspective. When the cosmic ray is being hit from behind there is very little energy left in either the photon or the cosmic ray after the change in perspective. When they hit head on, though, there is also less energy, overall, but far more than in the other case. This happens because in both cases the change in perspective needed is to be moving almost as fast as the cosmic ray in the same direction. By the Doppler effect, this will boost the frequency of the head on photon, and further reduce that of the rear-end collision photon.
This becomes really important when there's enough energy in the center of momentum frame to start doing something called pair production. For every type of particle the cosmic ray has enough energy to produce with the CMB, the faster it will lose energy to collisions with it. There is even a hypothesis that the distance cosmic rays can travel above a certain kinetic energy is severely limited by the fact that they should produce pions when it runs into the CMB. The name for this limit is the GZK cutoff. If you want to know why that is the limit, and not the production of electron-positron pairs, muon-anti muon pairs, or some other process, I don't know those details.