think the idea that light, time, decay rates, etc... have always been constant is not a correct assumption in a universe where space-time has presumably expanded from nothing
That's just it. They are not ad-hoc assumptions. They are tested experimentally and observationally, both directly and indirectly. SN 1987A is one of those observational tests.
You don't get away with ad-hoc assumptions in science, especially physics. Your assumptions have to be supported by the evidence and better than alternative assumptions are.
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