I was reading these papers by Sean Carroll (https://arxiv.org/abs/1405.0298; https://arxiv.org/abs/1505.02780) in which, among other things, he argues against vacuum up-tunneling occurring in the universe. He only acknowledged that it would be possible in the first moments of the universe while it was heated, but after that the rate goes to zero.

Meanwhile, vacuum energy can be excited by high energetic events like high energy cosmic rays. I thought that cosmic rays, being massive particles, were not redshfited by the expansion of the universe as photons are; therefore, if cosmic rays were produced in the big bang or as a result of a previous false vacuum decay, then these particles could eventually excite the vacuum, essentially causing an up-tunneling.

However, apparently cosmic rays can get redshifted as well (https://astronomy.stackexchange.com/questions/43525/is-there-an-equivalent-of-the-red-shift-effect-for-cosmic-rays). Then, is it impossible that cosmic rays may excite the vacuum in the future? Is there any kind of energy that does not get "redshifted" and therefore could cause an up-tunneling of the vacuum (like perhaps matter-antimatter generation: https://www.sciencedaily.com/releases/2011/10/111028142508.htm)? Is it really utterly impossible to excite the vacuum?