In the twins paradox, the twin that travels away from earth comes back to find that his (her) twin has aged much more due to relativity's effect on time. Why can't we posit that it is not the one twin moving away from Earth at the speed of light, but it is the entire universe, the earth and his twin included, who are accelerating and moving away at the speed of light and then coming back? In this case shouldn't be the other twin to have aged less? (and, in fact, the whole universe) What is wrong with this way of thinking?
This has NOTHING to do with the other question. I am asking what is wrong in my thought-experiment in which the whole universe accelerates away at the speed of light and then comes back. There clearly is an asymmetry here I am missing, and asking what it is. The other question is asking to clarify the twins paradox, it is not addressing the asymmetry.
Answer
One of the premises of special relativity is that observations made in different non-accelerating (or "inertial") frames are equally valid, and that there are certain quantities that all such observers agree on.
One such quantity is the proper acceleration $\vec{a}^2-a_0^2$. All inertial observers agree on whether an object is accelerating or not.
This is the asymmetry. If one twin stays in an inertial frame and the other one accelerates, the accelerated one ages less. Incidentally, if we make it truly symmetrical, and accelerate the whole universe one way and the twin the other way the same amount, both twins age the same.
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