Is the Aguirre-Gratton time-reversed de Sitter model true?

“No, after all…
  • The bounce gets thwarted (into a big crunch)

      During the past-infinite contracting phase, the whole space becomes thermalized1 This is relevant because if a space is entirely thermalized, it is no longer de Sitter; instead of a bounce and expansion, the thermalized space ends in a big crunch.2

      1. Arvin Borde, Alan Guth, Alexander Vilenkin:“If thermalized regions were able to form all the way to past infinity in the contracting space-time, the whole universe would have been thermalized before inflationary expansion could begin.” [“Inflationary spacetimes are not past-complete.” Physical Review Letters 90, 151301 (2003)]
      2. Alexander Vilenkin: “Then bubbles nucleating at τ → −∞ will fill the space, the energy in the bubble walls will thermalize, and the universe will contract to a big crunch and will never get to the bounce and to the expanding phase.” [“Arrows of time and the beginning of the universe.” Physical Review D 88, 043516 (2013)]
  • The model requires acausal fine-tuning

      The model requires “acausal fine-tuning.” (That is to say, the entropic evolution is extraordinarily fine-tuned1, and yet logically eliminates any hope of explaining it.) This is relevant because it means the so-called prior probability of the model being true is irresolvably something like 1 over infinity; i.e. the chances of it being true prior to examining the evidence for and against it are virtually zero, and so the model would correspondingly require almost incomprehensively good evidence to overcome its implausibility.

      1. The model puts the low-entropy problem in the middle of the hourglass, and yet they provide no mechanism to bring about this unnatural state. In fact, worse, the model in principle removes any hope of ultimately increasing the probability of the highly improbable state.

        Alexander Vilenkin: “The Aguirre-Gratton model can avoid singularities by postulating a small “initial” closed universe and then allowing it to evolve in both directions of time. I put "initial" in quotation marks, because Aguirre and Gratton do not think of it that way. But this model requires that a very special condition is enforced at some moment in the history of the universe. At that moment, the universe should be very small and have very low entropy. Aguirre and Gratton do not specify a physical mechanism that could enforce such a condition.” E-mail to William Lane Craig
        Andreas Albrecht: “In the Aguirre-Gratton construction, the arrow of time is put in by hand. One simply declares 'the universe is in this state,' and it happens to have an arrow of time. [389.] ... “The eternally inflation state that emerges from that approach has specific global properties that reflect an arrow of time. In particular, an array of regions of reheating (or decay of the potential-dominated state) must be organized coherently to be pointing in a commonly agreed ‘forward direction.’ In fact, there are actually two different ‘back-to-back’ coherent domains in this picture, with arrows of time pointing in opposite directions. The coherence must extend over infinitely many reheated regions, distributed throughout an infinitely large spacetime volume.” [Science and Ultimate Reaity (Cambridge, 2004), 388.]
        Sean Carroll: “Why is the entropy so low in the middle of the history of the universe? In other words, the bouncing-entropy model doesn’t, by itself, explain anything at all about the arrow of time. Rather, it takes the need for a Past Hypothesis and replaced it with the need for a “Middle Hypothesis.” There is just as much fine-tuning as ever; we are still stuck trying to explain why the configuration of our coming patch of space found itself in such a low-entropy state near the cosmological bounce.” [From Eternity to Here (), none-given.]

  • It undercuts causal reasoning (and science)

      The Aguirre-Gratton model undercuts causal reasoning (in life and science) (After all, the model yields two mirror halves, so there's a 50% chance we are the half experiencing time in thermodynamic reverse.) This is relevant because its an absurd conclusion and even undercuts justification for belief in the model itself.1

      1. To see how the Aguirre-Gratton model may be self-defeating, consider that knowledge requires justified true belief (or something like it), and justification (or anything like it) requires the belief to be formed in the right way. However, believing the Aguirre-Gratton model means having a belief which entails a 50% likelihood that your belief in the model itself was formed in a terrible way: by spontaneously forming and being grounded in a reasoning process you haven't done yet, and on the basis of false memories of events you haven't experienced yet.