Ellis's review is not actually that useful, if one wants to judge Tipler's theory on physical grounds alone. Mostly he voices objections to Tipler's reductionist definitions of concepts like life, mind, and God.
But here is an example of a poor physics argument from Tipler. He has decided to seek a cosmology in which information processing goes on forever. For some reason I forget (but it might have been to avoid "information loss"), he decided it was important that black holes should not ever actually form. That part is a little less nonsensical than it might sound to an expert (and Tipler is an expert in general relativity), since the standard definition of a black hole is "global", there have to be regions inside the black hole which never become visible to any future observer. From this he jumps to the conclusion that the evolution of all potential black holes are managed by future advanced civilizations in order to prevent the formation of event horizons, and this is part of his argument that life doesn't die out in the universe.
Then there's his description of how information processing continues for a subjective eternity during the collapse towards the Big Crunch (since it's a closed universe). It's a finite "proper time" to the Big Crunch, so to get a subjective eternity, the rate of information processing has to keep increasing, supra-exponentially (e.g. hyperbolically). His starting point here is a kind of
chaotic oscillation in which the metric is alternately squeezed and stretched in different directions, which I think was originally proposed as a model of space-time emerging from the Big Bang. In any case, I believe Tipler's proposal is that as the universe collapses towards the Big Crunch, these oscillations are induced and/or managed by the final civilizations, in the form of flows of energy in the final plasma (because the universe becomes so hot and dense, that it passes through the stages of the early universe in reverse).
Independently of Tipler's physical theology, this is an interesting proposal for cosmic engineering. You could compare it with Freeman Dyson's "Dyson spheres", or Dyson's own proposal for how information processing could go on forever in an expanding universe,
just slower and slower. (Unfortunately that proposal doesn't work in a universe whose expansion is accelerating, although the string theorist Ashoke Sen wrote a paper on surviving in that kind of universe
for as long as possible.)
Tipler turns this engineering proposal into a theology by proposing that the asymptotic state of his collapsing universe has the attributes of God (e.g. it's all-knowing). One may debate the metaphysical legitimacy of reifying the asymptotic state, and giving it these attributes. But if we focus just on the path to the final state, there's a lot of details that are omitted too. In the final epochs, there are no solid structures, space is filled with a dense plasma of elementary particles. So how is "information processing" happening? It must somehow be in the flows of the plasma. The spacetime is chaotically oscillating, the plasma flows in one direction and then another, and the flows must be information processing events, thoughts squeezed out like musical notes from an accordion. Well, if we think that electrical currents in the brain can carry information, then why not flows in an ultimate plasma too? Nonetheless, as I recall he's completely vague about how this final incarnation of the universe, as an oscillating plasma accordion managing its own descent into infinite density, works in any physical sense. He just has the bare idea of chaotic oscillations during the approach to the final singularity.
