Chaotic Inflation is currently one of the more popular models for eternal inflation. It was first proposed in 1986 by Andrei Linde with his paper "Eternally Existing Self-Reproducing Chaotic Inflationary Universe".

This model posits an unintelligent, infinite, eternal, chaotic energy. From this primordial energy, random fluctuations (governed by the laws of physics of course) give rise to an infinite number of separate, inflating island universes. It is assumed that somehow each island universe randomly has a different set of fundamental constants.

A major problem with any model of inflation which claims that inflation randomly occurs out of chaos, is that it is less probable to get a universe which look like ours ("good inflation"), than a universe which look nothing like ours ("bad inflation"). An even bigger problem with a model for inflation is when it predicts no inflation at all as the most likely explanation for our universe! Paul Steinhardt, in the Scientific America article, explains the problem this way:

Not only is bad inflation more likely than good inflation, but no inflation is more likely than either. University of Oxford physicist Roger Penrose first made this point in the 1980s. He applied thermodynamic principles, similar to those used to describe configurations of atoms and molecules in a gas, to count the possible starting configurations of the inflaton and gravitational fields. Some of these configurations lead to inflation and thence to a nearly uniform, flat distribution of matter and a geometrically flat shape. Other configurations lead to a uniform, flat universe directly—without inflation. Both sets of configurations are rare, so obtaining a flat universe is unlikely overall. Penrose’s shocking conclusion, though, was that obtaining a flat universe without inflation is much more likely than with inflation—by a factor of 10 to the googol (10^100) power!An even more fundamental problem with a model of inflation that predicts infinite universes is that there is no way to naturally make any predictions from this model at all.

For an infinite collection of coins, there are an infinite number of ways of sorting that produce an infinite range of probabilities. So there is no legitimate way to judge which coin is more likely. By the same reasoning, there is no way to judge which kind of island is more likely in an eternally inflating universe. Now you should be disturbed. What does it mean to say that inflation makes certain predictions that, for example, the universe is uniform or has scale-invariant fluctuations—if anything that can happen will happen an infinite number of times? And if the theory does not make testable predictions, how can cosmologists claim that the theory agrees with observations, as they routinely do?Steinhardt continues to explain that it is possible to model a theory of inflation in a

__finite__way that does allow the theory to make genuine predictions. These are the same predictions of the prior post that are confirmed by experiment and observations. This would allow us to maintain the theory of inflation which also solves the horizon problem and the flatness problem.

Some suggest trying to construct theories of inflation that are not eternal, to nip the infinity of universes in the bud. But eternality is a natural consequence of inflation plus quantum physics. To avoid it, the universe would have to start off in a very special initial state and with a special form of inflationary energy, so that inflation ended everywhere in space before quantum fluctuations had a chance to reignite it. In this scenario, though, the observed outcome depends sensitively on what the initial state is. That defeats the entire purpose of inflation: to explain the outcome no matter what conditions existed beforehand.Steinhardt rejects the model of finite inflation because it would require fine tuning for the initial state of the universe, which would defeat "the entire purpose of the theory of inflation" (in his mind), to explain the universe without having any fine tuning.

Steinhardt goes on to explain how eternal inflation theorists try to solve the problem (that their model doesn't make any actual predictions) by positing ad hoc measures for these infinite sets and thereby calculate probabilities on them.

An alternative strategy supposes that islands like our observable universe are the most likely outcome of inflation. Proponents of this approach impose a so-called measure, a specific rule for weighting which kinds of islands are most likely—analogous to declaring that we must take three quarters for every five pennies when drawing coins from our sack. The notion of a measure, an ad hoc addition, is an open admission that inflationary theory on its own does not explain or predict anything.

Worse, theorists have come up with many equally reasonable measures that lead to different conclusions. An example is the volume measure, which says that islands should be weighted by their size. At first glance, this choice makes common sense. The intuitive idea underlying inflation is that it explains the uniformity and flatness we observe by creating large volumes of space with those properties. Unfortunately, the volume measure fails...By this measure, it is unlikely we would even exist.

Measure enthusiasts take a trial-and-error approach in which they invent and test measures until, they hope, one produces the desired answer: that our universe is highly probable. Suppose they succeed someday. Then they will need another principle to justify using that measure instead of the others, yet another principle to choose that principle, and so on.Steinhardt ends up rejecting the entire theory of inflation (not just the specific models of eternal inflation or finite inflation):

In light of these arguments, the oft-cited claim that cosmological data have verified the central predictions of inflationary theory is misleading, at best. What one can say is that data have confirmed predictions of the naive inflationary theory as we understood it before 1983, but this theory is not inflationary cosmology as understood today...And if inflationary theory makes no firm predictions, what is its point?Steinhardt ultimately proposes his own cyclic cosmology theory which we won't go into in depth. He uses string theory to speculate that our universe is really living on a three dimensional

*p*-brane. These

*p*-branes are imagined to exist in the extra hidden dimensions of string theory. Our universe was made by one of an infinite number of collisions that happens every trillion years between two of these

*p*-branes. (See post 11 for more on the extra dimensions of string theory and the landscape problem.)

What is so perverse about this reasoning is that Steinhardt rejects the entire theory of inflation because he is not interested in any universe which implies fine tuning (and thereby an Intelligent Agent). He was one of the major contributors to inflationary theory which, when modeled in a finite manner, yields testable results confirmed by observation. Steinhardt ends up rejecting a very plausible scientific theory of inflation, in exchange for a speculative theory of colliding membranes, simply because of a bias against fine tuning; a fact that is obvious throughout the universe, not just in the fine tuning required for inflation.

In the next post, we will illustrate how

*any*multiverse theory that tries to explain the special initial conditions of the big bang by positing infinite randomness, runs into even deeper problems than those already discussed.

Click here to continue to Stage 2c

On a humorous note, we would like to share one of our favorite multiverse scenarios which is also a model of eternal inflation. We think it is a far more plausible model than chaotic eternal inflation. We call our model the "Original Central Banker" (OCB).

ReplyDeleteThis model basically proposes that an Original Central Banker randomly fluctuated out of chaos and started printing an infinite quantity of money. We call this process Quantitative Easing to Infinity (QEI). As a natural result of the infinite money printing of the Original Central Banker, our model predicts eternal inflation.

We think three pillars of empirical support for our model are the Federal Reserve Bank, the ECB, and the long term price trend of gold since 1971.

Our theory predicts the the actions of the OCB will be repeated again by later central bankers (ECB), and the universe will go through another period of infinite inflation. The Weimar Republic is excellent empirical support for this prediction.

In addition, many Austrian economists believe that we are entering another phase of infinite inflation on all fiat currencies. But what do they know? They're economists, not scientists.

Like.

DeleteI think you just took "reductio ad absurdum" ad absurdum. Twilight zones stuff.

ReplyDeleteDr_Manhattan

I've been thoroughly enjoying (and re-publishing) your insightful essays on the multiverse, in the Jewishtimes magazine: www.mesora.org/jewishtimes

ReplyDeleteThank you both for your hard work to teach the truth.

Rabbi Moshe Ben-Chaim, Founder

www.mesora.org

Thanks! We've been enjoying the process as well, and we hope your readers do too.

DeleteRAZ/REF / REF/RAZ,

ReplyDeleteCorrect me if I'm wrong, but it seems to me that prior to Guth's proposal of inflation, we could have explained the uniformity and flatness of the universe by saying that the intial distribution of matter and energy was fine-tuned. Looking back, however, inflation is a more elegant explanation, since it only requires the fine-tuning of inflationary energy, while the distribution of all the matter and energy in the universe would not need to be fine-tuned (at least to the point where it would produce unformity and flatness). So at what point do we stop and assume fine-tuning, and when do we say "this looks pre-arranged, but let's look for something deeper and simpler"?

Great question. Your question also relates to why this proof does not kill the activity of science.

DeleteScience has to trace back the ordered complexity all the way back to the most simple concepts. That is the task of science.

At every stage of reducing the complexity of the universe to its more simple elements, we find more order and not chaos. Complex life, cells, molecules, the periodic table of atoms, protons and neutrons, elementary particles like electrons and quarks which have fine tuned masses.

The same process of unification hold for the forces of nature that we trace back to the most fundamental, simple forces. This lead to fine tuning of the electromagnetic force (fine structure constant).

The same holds true when we trace the low entropy state of the universe all the way back to the even lower entropy state of the initial conditions of the big bang at the very beginning of creation.

So to answer your question more directly, science should not stop until it reaches what we believe to be the most basic created existences. Science's job only ends once it comes to the most fundamental aspects of physical reality.

What we find when we observe this process is that at no point in the chain does the fine tuning go away. Lo and behold, the most simple principles themselves (the constants and initial conditions) are themselves fine tuned.

My apologies for commenting on an old post, but it remains a deeply interesting and well thought out one that really explains the subject. I was wondering if I could ask a quick question regarding recent news.

ReplyDeleteAm I correct that the recent (March 2014) confirmation of inflation only confirms (in your words) the model of inflation, but not the theory? Most news articles seem to be leaning towards the "possibility of a multiverse" line. However, I don't see any evidence for this.

Am I correct, and are they committing the fallacy you describe? Or is there something about the recent evidence that points them towards eternal inflation, vs. hybrid/finite?

We're glad you still keep coming back to these posts. It is important to try to keep them updated with the most current science.

Deletehttp://www.csmonitor.com/Science/2014/0317/Big-bang-waves-direct-evidence-of-universe-s-extraordinary-expansion-video

It seems that you might have confused the term "theory" with "model" in your question. The recent findings of B-mode polarization show strong support for the theory of inflation in our universe. Namely, that our one universe had a rapid period of expansion soon after the big bang. Moreover, the specific findings would seem to out rule certain models of inflation that cannot account for B-mode polarization.

However, in your main point you are correct. There is nothing about B-mode polarization that points to eternal inflation as opposed to a finite inflation. The observation of B-mode polarization in our universe has no bearing on the question of an infinite number of other universes, and it most certainly is no evidence for the laws of nature being different in the other imagined universes.