Extremal Kerr white holes as a source of ultrahigh energy particles

We consider a process where two identical massive particles fall inwards, starting from rest at infinity towards the extremal Kerr black hole, collide outside the event horizon in its vicinity, and produce two massless particles. The center-of-mass energy of collision between the two particles diverges if one of the particles admits a specific critical value of the angular momentum and if the collision takes place at a location arbitrarily close to the event horizon. Assuming the isotropic emission of particles in the center-of-mass frame, we show that one of the massless particles produced has divergent conserved energy comparable to the center-of-mass energy with probability slightly less than half. This particle enters the black hole event horizon which coincides with the Cauchy horizon, turns back, and emerges through the white hole event horizon into another asymptotic region in the maximal extension of Kerr spacetime. Since conserved energy is preserved in this process, it is perceived as a particle with divergent energy by the observer when it reaches infinity. Thus the extremal white hole appears to be a source of ultrahigh energy particles. Similar processes wherein collision takes place slightly inside the black hole event horizon or just outside or inside of the white hole event horizon also produce high energy particles.