# Redshift z=40 and gravitational waves

Why z=40 is the critical value of the redshift (more or less) to decide find out primordial gravitational waves around it? I have read that is related to the following issue: black hole mergers should NOT be detected at certain cosmic age, so stellar black holes should not be found at times where there is no stars (Pop III included, I presume). If black hole mergers are found beyond z=40 (I would like to understand how this critical z is guessed or estimated), either:

1. Primordial black holes from inflation do exist.
2. The early Universe evolved from a different way to that of the LCDM model.
3. A non-gaussian Universe.

Moreover, I can not see how could we differentiate 1 from 2 or so, and how is 3, related to the remaining two ideas...I mean, since I asked recently as well, we can not differentiate in principle stellar black holes merging from primordial black holes yet. That is, from current knowledge, there is no test to know if a black hole is primordial or it is from stellar origin unless we observe the BH formation event with multimessenger astronomy. And why new physics should depart from gaussianity at this point (z=40 or beyond)?