But there are many valid systems of normative rules. If there is an absolute morality, that means that one such system must be identified as special in some way. The thing that makes correct physics special over other possible descriptive theories is that, in this universe, it accurately predicts events. What about absolute morality makes it special as compared to other systems of normative rules?
As you know, there are different "valid" set of theories regarding the physical reality: the biblical view, the theories underlying TCM, the theories underlying homeopathy, the theories underlying chiropractise and the scientific view. The scientific view is well-established because there is an intersubjective consensus on the usefulness of the methodology.
The methods used in moral discussions are by far not so rigidly defined as in science, it's called civil discourse. The arguments must be logical consistent and the outcomes and conclusions of...
Level 1: Algorithm-based Intelligence
An intelligence of level 1 acts on innate algorithms, like a bacterium that survives using inherited mechanisms.
Level 2: Goal-oriented Intelligence
An intelligence of level 2 has an innate goal. It develops and finds new algorithms to solve a problem. For example, the paperclip maximizer is a level-2 intelligence.
Level 3: Philosophical Intelligence
An intelligence of level 3 has neither any preset algorithms nor goals. It looks for goals and algorithms to achieve the goal. Ethical questions are only applicable to intelligence of level 3.