The Principle of Contingent Causation: The Impossibility of Infinite Regress.

[polymath257]

I think the  problem is in the word 'first'. The usual argument only gives the existence of an *uncaused* cause. As you point out, there is nothing that says that uncaused cause is unique.

There is also nothing that says that multiple uncaused causes have to be simultaneous in any reference frame. This allows for a resolution to  your paradox.

I'd also like to point  out that, in cosmology, there *is* an absolute frame of reference for each event: that in which the universe looks isotropic and expands to retain that isotropy. The difficulty, of course, is that absolute framers at different events are not the same.

The problem is that simulteneity is required to avoid a first, for if there is a first from some reference frame, the rest are contingent from that perspective.

Why would that be the case if there was no causal link? And, in fact, it is quite easy in a relativistic scenario, to have multiple events, even ordered in some reference frames, that have no causal links.