In this paper written by physicist Max Tegmark (https://arxiv.org/pdf/0704.0646.pdf) it talks about "External Reality Hypothesis". Specifically, he says:
Although many physicists subscribe to the ERH and dedicate their careers to the search for a deeper understanding of this assumed external reality, the ERH is not universally accepted
We will see that, although it sounds innocuous, the ERH has sweeping implications for physics if taken seriously. Physics theories aim to describe how this assumed external reality works. So it seems that there are theories in physics which accept the existence of this so-called "External Reality".
Apparently in the paper no example of these types of physical theories appear. So, do you know of any theory which assumes this? What theories is Tegmark talking about? Do you know of any physics theory that would assume a Tegmark's External Reality where his Mathematical Universe Hypothesis and universes described by different matematical structures could be real?