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What were they dependent on?
Have a look at this recent paperWhat were they dependent on?
several recent papers are engaged in weakening this assumption. I don't remember if Ashtekar mentioned this. Just curious, did you happen to have a look at "The Issue of the Beginning"?In the LQG framework (loop quantum cosmology) the existing universe is assumed as a given homogeneous and isotropic space-time,
I think this misses the main point. There is no way avoid the question of a model's basic conditions---prime fundamental assumptions---call it what you will. These conditions just don't have to be anchored to a particular moment in time. Every physical model has basic underlying assumptions. There can be a moment in time when they they suddenly take effect (a 'beginning' located in time) or they can have always been in effect.and the equations that follow from this reduction to minisuperspace allow to trace backwards the evolution of the universe beyond the classicalsingularity. It seams to me that for such a procedure no initial conditions are needed.
I think you are mistaken if you think that, to be fundamental, a model must address conditions at some particular instant of time that you think is 14 billion years ago. LQC has a bounce and continues on back past that time. Of course one can always ask "What were conditions like at the bounce?" and that would play somewhat the same role as asking what were the initial conditions....Personally I think that the question about initial conditions is a necessary one for every model that claims to be fundamental. If the model does avoid this question then it seams to me that something is wrong, at least, the model cannot be fundamental. ...
Yes, this is exacly the point. I do see reasons that make eternal models unphysical.There is no rule that says cosmological models must have a beginning located at some particular instant of time. Ashtekar's point is that the pendulum is swinging back.
I agree. I would be happy to see some experimental confirmation of any of the quantum gravity theories that currently postulate quantum cosmological models.I expect in this case we will just have to let Nature decide for us what is required for a model to be fundamental.
But you can always find a point in time farther away than any other. Consider that you "travel" with finite "speed" along the real line. In the same way that you will never reach infinity from any definite real value, how can you assume that any definite real value can be reached starting from (past) infinity (or having an infinite amount of time before it)?1. on an infinite timeline there is no point at infinity. all points are a finite distance away. the issue of something "at infinity" whose effects cannot reach us, and suchlike issues, don't seem to me to arise at all
Glad you are interested, hellfire!I will search for this Penrose lecture, thanks.
How could something atemporal (in the sense of a non-changing state) ever become temporal. How could the universe emerge from a non-existent state?Yes, this is exacly the point. I do see reasons that make eternal models unphysical.
First, eternal models seam not to be according to the second law of thermodynamics. But this might be a minor issue. If time streches back to an infinite past, and the causal sequence of events does not follow infinitely fast, how can present time have been reached? Consider for example an body located at infinite distance. It may start approaching us, but an infinite distance will never become shorter and even after infinite time the body will not reach us. To my eyes this situation with an infinite distance is a similar problem than an infinite time. Infinite time and infinite distance make no sense in physics.
Of couse this is my personal view and I agree with you that infinite models, spatial and temporal, are usually postulated without taking care about this issues. The flat infinite classical Friedmann model is the best example of this. However, I believe that this is only a model and I don't think the questions I addressed above are irrelevant. In my oppinion our cosmological models are just too simple. It is usually extrapolated from physics (general relativity of quantum gravity) to cosmology without taking care about the special conditions, initial conditions and boundary conditions, that must hold for a truly fundamental cosmological model.
From this point of view initial conditions are a fundamental issue in any fundamental cosmology. I believe that any fundamental cosmology must describe a universe that emerges from an atemporal state. Of course it may turn out that postulating initial conditions as an add-on to the laws is not necessary if the equations of motion provide a unique solution.