As one might expect, metabolic rate within the portal would point also in the opposite-backward direction. Hence, one would become increasingly younger during the journey as metabolic functioning flows in the opposite direction. Therefore, it can be understood that the speed of one's body relative to the speed of light, governs also one's metabolic functioning rate and metabolic rate in-turn governs time. This is an important point worthy of note. This is left as an open question in this model and is, thus, subject to interpretation.
Time is personal.
One's grandmother may still be dead and she doesn't come back to life, although one can personally become younger by entering the portal. This description, and that in the previous paragraph, are some of the time paradox's that can happen if the astronaut does return home. Perhaps, the aliens know how to do this. For this idea to work at all, let us hope that they do. The blockage of background radiation creates a void, an area of very high space-time curvature.