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Helicobacter
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What were they dependent on?
Helicobacter said:What 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"?hellfire said:In the LQG framework (loop quantum cosmology) the existing universe is assumed as a given homogeneous and isotropic space-time,
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.
...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.marcus said: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.marcus said: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)?marcus said: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
hellfire said:I will search for this Penrose lecture, thanks.
hellfire said: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.
The initial conditions of our universe refer to the state of the universe at the very beginning, before any significant events or changes occurred. These conditions include the distribution and density of matter and energy, the temperature and expansion rate of the universe, and the strength of fundamental forces.
Scientists use various methods, such as studying the cosmic microwave background radiation and observing the distribution of galaxies, to gather evidence and make predictions about the initial conditions of the universe. These predictions are then tested and refined through experiments and observations.
The cause of the initial conditions of our universe is still a subject of debate among scientists. Some theories suggest that the universe began with a singularity, while others propose the existence of multiple universes or a cyclical model of the universe.
The initial conditions of our universe have changed significantly over time due to various events, such as the Big Bang, cosmic inflation, and the formation of stars and galaxies. These changes have led to the complex and diverse universe that we observe today.
The initial conditions of our universe have played a crucial role in shaping the universe and allowing for the development of life. For example, the precise balance of fundamental forces and the distribution of matter and energy have allowed for the formation of planets, stars, and galaxies, which ultimately led to the emergence of life on Earth.