I think that ONLY the light is not part of Soul Society OR Hollow World. So right now I think it is possible that a shinigami can enter Hollow World because the Hollows can enter Soul Society. It wouldn't seem logical if Hollows could get into Soul Society and Death Gods can't. Remember that Aizen is INSIDE of the light, he isn't touching it. So if you were to touch the light you would be instantly destroyed but in this case you are either OUTSIDE the light or INSIDE the light and not ON the light.
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