Fact1: In the model we are living in one bubble, and that is what we call the universe (in this specific case).
Fact2: when we say that the universe had a beginning, in the KCA, we can mean that one bubble.
Fact3: There is nothing that lies at the past of that bubble.
Fact4: The universe/bubble had a beginning.
Fact5: This can be said about any other universe/bubble.
Fact6: Vilenkin argues that the low-entropy boundary conditions are imposed (by hand) at the bounce, which is different from other models where such boundary conditions naturally arise from quantum cosmology of the model (example Hartle-Hawking model) wave function of the universe. The AG model bounce hypersurface breaks de Sitter invariance (the laws are no longer invariant in the region) and this represents the beginning of the universe in this said model.
In other words: the mirror universe on the other side of the boundary in no sense represents a past out of which our current universe evolved, nor vice versa, and, our universe (this one bubble) began-to-exist
Thus, our universe (and any other universe) had a cause I will comment on the cause, later on.
Fact 7: The model offers no additional predictions.
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On laws and initial condition status:
The common use of the concept of physical laws is called , by some, Governability model:
2 views:
1. Laws govern, because a governor breaths fire into them.
2. Laws are primitives, that have the power to govern.
*** on 2, given an A theory of time, one has to ask laws are primitives of what??
When I say that it is a presumption, it is in fact a philosophical presumption; one either chooses 1 or 2 (both, philosophical presumptions).
Historically, this view was developed by Francis Bacon, then, echoed by Decartes, Newton and others, on the basis of God as a law giver (view no. 1) .
3 philosophical accounts of physical LawsIn this video a 3rd option is presented, but, the Governability (which is the one basically currently in use, in physics) account is also commented. Enjoy.
Tim Mudlin is a very recognized philosopher of science who defends the Governability account No. 2.
The direction of time is embedded so deeply into our language and concepts that it is impossible to expunge. Most verbs are time-directed: the difference between a rock falling and the rock rising is determined by which direction of time is toward the future and which toward the past. The words “to” and “from” have their usual application by reference to time: processes run from earlier states to later ones. We remember the past and anticipate the future. Our present actions can influence the course of the future but are impotent to influence the past. To say that the past-to-future direction of time is intrinsically different from the future-to -past direction is not the same as saying that the future must be qualitatively different from the past. The physicist Fred Hoyle once proposed a “steady state” theory of the universe: the universe as a whole is and always has been expanding, with galaxies moving ever farther apart from one another. But in Hoyle's theory, the average density of matter in the universe does not go down: new matter is created in the empty spaces at just the right rate to keep the density constant. In this model, all times— future, present, and past —are qualitatively the same: hence “steady state.” But there is still a fundamental direction of time, in virtue of which it is correct to say that the universe is expanding and new matter being created, rather than contracting with matter being destroyed. Our world is filled with processes that have an evident time direction. Ice cubes in hot water, isolated from outside influence, melt and produce lukewarm water. Lukewarm water, isolated from outside influence, never spontaneously segregates into ice cubes and hot water. But the laws of physics seem to allow the latter process just as much as the former. Once again, our very description of the two processes presupposes a time direction: the only difference between melting and freezing is the direction of time. Similarly, our description of a black hole in figure 27 presupposes a direction of time: it is because the direction to the future is represented by up on the diagram rather than down that we say things fall into and never escape the event horizon rather than are ejected from and cannot remain within it. Just as the fundamental laws seem to allow both the melting and spontaneous freezing of ice, so General Relativity allows for both black holes and their time reverses. And just as ice often melts but never spontaneously forms in lukewarm water, so there seem to be many black holes in the universe but no time-reverses of them. There are, then, two distinct questions to ask about the direction of time. One is why there seem to be processes that never occur even though their time-reverses regularly occur. When asked in terms of, say, spontaneous melting and spontaneous freezing, this question presupposes that there is a fundamental difference between the past-to-future and future-to-past direction. The question of the de facto directionality of processes— why ice cubes spontaneously melt in hot water but lukewarm water does not spontaneously segregate itself into ice cubes and hot water— is addressed by statistical physics. The idea that time has no intrinsic directionality at all is harder to evaluate . One could be lulled into this idea by an unreflective use of space-time diagrams, which are purely spatial objects with no intrinsic directionality. The past-to-future direction represented by such a diagram must be indicated by some extra convention, since the medium of the representation itself has no such asymmetry. But equally, the difference between the representation of space-like and time-like directions on the diagram must be indicated by a convention. Similarly, purely mathematical representations of space-time require many conventions, such as whether time-like Intervals are to be represented by real or imaginary numbers, or whether a time coordinate increases or decreases in the past-to-future direction. None of this even vaguely suggests that the two time directions are not fundamentally different, any more than it suggests that time-like directions do not differ intrinsically from space-like directions.
Maudlin, Tim (2012-07-22). Philosophy of Physics: Space and Time (Princeton Foundations of Contemporary Philosophy) (p. 166). Princeton University Press.
According to Barry Leower, Mudling argues, and he agrees, that, a Governing version of Laws implies an intrinsic direction of time.
Furthermore, the question was the plausibility of a reversal of the arrow of time.
Plausibility , philosophically speaking, is roughly understood as being worthy of belief. Above Maudlin explains how embedded is the arrow of time, in our lives, at all levels, in our models, etc.
It is everyone´s prerogative and within their capacity to elucidate a judgment as to how plausible is that everything we seem to know about the world, including our scientific knowledge of the world which includes statistical physics and the more.
Another reason or evidence to consider it implausible is that Aguirre and Gratton inserted the conditions at the null surface by hand, inducing the reversal, it does not come out of the equations by itself.
This is different than it would come out of the equations, and a far cry from accepting it due to real experimental evidence in favor of a reversal of the arrow of time, or, deductive argument to that conclusion, for instance.
No other motivation was given.
Calling it a boundary does not change the fact that it is a set of initial conditions, a non generic low entropy condition, put there by hand See from min1 to 1:44
Even more, the model is not accompanied by any experimentally verifiable predictions.
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Assuming brute facts are possible go even against the practice of science, which you purport to use.
I copy here an argument that I find quite reasonable against such view , and words things much better than I could.
“Robert Koons (1997): Start with the observation that once we admit that some contingent states of affairs have no explanations, a completely new skeptical scenario becomes possible: No demon is deceiving you, but your perceptual states are occurring for no reason at all, with no prior causes.
Moreover, objective probabilities are tied to laws of nature or objective tendencies, and so if an objective probability attaches to some contingent fact, then that situation can be given an explanation in terms of laws of nature or objective tendencies. Hence, if the PSR is false of some contingent fact, no objective probability attaches to the fact.
Thus we cannot even say that violations of the PSR are improbable if the PSR is false. Consequently, someone who does not affirm the PSR cannot say that the skeptical scenario is objectively improbable. It may be taken to follow from this that if the PSR were false or maybe even not known a priori, we wouldn't know any empirical truths. But we do know empirical truths. Hence, the PSR is true, and maybe even known a priori.”
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“Again, geodesic incompleteness only shows that, if classical rules hold, our description of the early universe will necessarily reduce to a mathematical paradox known as a "singularity". This does not intrinsically raise the probability that the universe had a beginning. It does, however, raise the probability that our classical description of reality is fundamentally incomplete, and therefore requires further development. An altogether incomplete model simply cannot tell us anything truly fundamental about the nature of the cosmos or its origins.”
On the PH, BV and BVG singularity theorems: “… Singularity theorems, certainly, don´t tell you that there is a singularity, they tell that you there is geodesic incompleteness over the space time regions over which the theorem assumptions hold…singularity theorems tell you there is a finite geodesic, or an incomplete geodesic. “ (Anthony Aguirre)
"Possible" perhaps, but "probable"? The fact that a thoroughly classical model happens to show itself ill equipped to describe what the early universe was doing prior to the planck epoch (which is a thoroughly quantum regime) does not therefore suggest that an absolute beginning is probable, or at the very least more probable than an eternal cosmos.
I find strange your interpretation, If that was all there was to it, Aguirre, Gratton, Carroll, Chen, would not be have spent their efforts in circumventing the assumptions.
Aguirre, for instance, thinks that the theorems don´t show that the universe had a beginning (not necessarily), not because he thinks the models are ill equipped, but, because he thinks he can circumvent the assumptions of the theorems.
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The AG model does not make predictions, which just goes to under write its character as a conceptual probe, and, not as a serious model of the world.
Putting credence over such probe models , above the main stream models, seems unwarranted.
A couple of things more, one can argue for the existence of brute facts, but, that is truly inconsistent, with the practice of science, in its very core (see Popper´s The logic of scientific discovery).
another thing, nobody is arguing that eternal universe are ruled out, much less intrinsically (what ever that amounts to) by our current cosmology, merely that given the evidence and analysis, it appears to be less probable that those are the correct models of our universe, so far.
I think you mentioned something to the effect that our current models are incomplete, or, something on those lines. Scientific physical models by their own nature will never be complete models of our universe. Let me give you some examples.
Some quantum loop theories still use the same Newton constant, with out ever showing where it comes from.
The infamous Carroll´s quantum eternity theorem is hoax, must quantum cosmologists think the energy of the universe is zero, while Carroll passed that one under the table, saying that if the universe had a non zero energy, the universe would then be infinite to the past and future. Even more, that the energy is zero, under one of the most accepted interpretations just means that the dynamic equations that govern the universe, are rather arbitrary. etc etc etc.
there will never be such a thing as a complete model of the universe, and, that is fine.
Guth beliefs the universe might be eternal based on certain criteria by which he prefers eternal models, according to Carroll ( read it from his blog ), not because he analysed and gave arguments in favor of such conclusion, like Vilenkin and Mithanis have done. Carroll´s showing of his pictures on screen is nothing more that a fallacious appeal to authority, since no arguments nor analysis of the models was put forward, in behalf of Guth.
I am probably wrong, after all, I am no one (since you asked), but in principle Guth´s , Carroll´s , Aguirre´s argument of the eternality of the universe is missguided. Using Penrose diagrams where the models of the universe are compressed from t=-oo to t=oo , for example, sort of makes people forget that such t=oo is an ever increasing time, not an actual infinite. As George Ellis argues, (very broadly) paraphrasing: one can substitute the "oo" by a very large number, to get a more physical picture of what is going on. And in the cases one gets something like an actual infinite one should reject the implications.
On a side note: there is as far as I know zero experimental corroboration that the universe reaches planck density, into the past. I am not saying it does not, just that we are all presuming it does.
Thanks for the exchange.
