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The GZK limit stops high energy protons from propagating long distances because of the interaction with the CMB. But as the universe ages the CMB will become more red shifted. Will this mean the the GZK limit will be higher in the future?

If it does, will there be a point where conventional radiation is so low that the vacuum energy density will become the main limiter of long distance, high energy proton propagation and therefore form a new GZK limit that is independent of the age of the universe?

edit: Upon reflection it may be that energy loss due to the Unruh effect from nearby massive objects may form a barrier long before the vacuum energy does but my question still stands. Can a high energy proton interact with vacuum energy to slow itself down?

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  • $\begingroup$ If you can define the interaction of the proton with the vacuum energy, you probably could compute it much the same way one computed the GZK limit via the interaction $p+\gamma_\text{CMB}\to\Delta^+\to p+\pi^0$ (or other channels). $\endgroup$
    – Kyle Kanos
    Commented May 23 at 23:17

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