The forerunners utilize slipspace rifts to send energy across galaxies instantly. Bam, done. That is, of course, implying the halo rings need to follow the real rules, which they don't. They exist as they do in their universe, therefore the rules of their universe must be such that the halo rings can exist as they do. It doesn't need to follow the rules of reality because it isn't real. I'm not sure why you can't comprehend this.