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Our future, our universe, and other weighty topics


Friday, May 12, 2017

Cosmic Inflation Clique Defends Its Tribal Folklore

Around about 1978, cosmologists (the scientists who study the universe as a whole) were puzzled by a problem of fine-tuning. They had figured out that the expansion rate of the very early universe (at the time of the Big Bang) must have been incredibly fine-tuned, apparently to one part in ten to the sixtieth power. This dilemma was known as the flatness problem.

Around then Alan Guth (an MIT professor) proposed a way to solve the flatness problem. Guth proposed that for a tiny fraction of its first second (for less than a trillionth of a trillionth of a second), the universe expanded at an exponential rate. The universe is not expanding at any such rate, but Guth proposed that after a very brief instant of exponential expansion, the universe switched back to the normal, linear expansion that it now has.

Even though there was no evidence for it, and the idea was very far-fetched from the beginning, Guth's idea became very popular among the small tribe of cosmologists. We can call this idea Guthism, and we can call the small tribe of cosmologists who adopted it Guthists. Guthism can also be called the cosmic inflation theory, although Guthism may be better term, to avoid confusion with the broader concept of the expansion of the universe (which does not require the idea of primordial cosmic inflation).

In January of 2017 Scientific American published an article attacking Guthism. The article was written by Princeton professor Paul Steinhardt and Harvard professor Abraham Loeb, along with Anna Iijas. The trio methodically dismantled the Guthist idea of primordial cosmic inflation.

Commenting on a Planck satellite data release that was proclaimed as being in support of Guthism, the article states, “If anything, the Planck data disfavored the simplest inflation models and exacerbated long-standing foundational problems with the theory, providing new reasons to consider competing ideas about the origin and evolution of the universe.”

Here is a quote from the paper:

Two improbable criteria have to be satisfied for inflation to start. First, shortly after the big bang, there has to be a patch of space where the quantum fluctuations of spacetime have died down and the space is well described by Einstein’s classical equations of general relativity; second, the patch of space must be flat enough and have a smooth enough distribution of energy that the inflationary energy can grow to dominate all other forms of energy. Several theoretical estimates of the probability of finding a patch with these characteristics just after the big bang suggest that it is more difficult than finding a snowy mountain equipped with a ski lift and well-maintained ski slopes in the middle of a desert.

A stunning statement – that a theory paraded around as a standard of cosmology is really more implausible than “finding a snowy mountain equipped with a ski lift and well-maintained ski slopes in the middle of a desert.”

The paper then goes on to explain why Guthism is basically worthless in terms of helping to explain the flatness problem. The authors state this:

More important, if it were easy to find a patch emerging from the big bang that is flat and smooth enough to start inflation, then inflation would not be needed in the first place. Recall that the entire motivation for introducing it was to explain how the visible universe came to have these properties; if starting inflation requires those same properties, with the only difference being that a smaller patch of space is needed, that is hardly progress.

The scientists also tell us, “Not only does inflation require starting conditions that are difficult to obtain, it also impossible to stop inflation once it gets going.” They are referring to the “graceful exit” problem. Appealing to an “inflaton field” that has never been discovered, Guthism tells the tale that exponential cosmic inflation lasted for less than a trillionth of a second, and that it both started and stopped during the universe's first second. But getting the stopping to occur requires fine-tuning and vastly improbable circumstances similar to the fine-tuning and vastly improbable circumstances needed to get this exponential cosmic inflation beginning in the first place. It's like trying to explain an elephant appearing out of nowhere and then disappearing in less than a second.

The three scientists suggest a “bouncing universe” theory as an alternative to Guthism, but there are powerful reasons (discussed here) for rejecting such a theory. Our three scientists have done a fine job at showing the low credibility of Guthism, but have not succeeded in presenting some viable alternative. But that's okay, because we can simply say that we do not understand the universe's beginning, rather than pretending to understand primordial mysteries beyond our comprehension and knowledge. Kudos to anyone who shows the weaknesses of a prevailing theory, even if they don't succeed in presenting a viable replacement for such a theory.

What do you call a theory that tells a “once upon a time” story describing unproven and unbelievable narrative details of the universe's first second – a theory hanging around because it has become a verbal story tradition of a little clique of scientists? The best term I can think of is: tribal folklore.

Darwinists have constantly been telling us, “Nature does not make leaps.” But the cosmic inflation theorists want you to believe that nature made two gigantic leaps in its first second. According to them, the first of the leaps when was this exponential period of cosmic inflation started; and the second leap, a fraction of a second later, was when this exponential period of cosmic inflation ended (with the universe returning to the normal, linear expansion rate we now observe).

cosmic inflation

Is there any chance that this folk tale will ever be verified? No, because we can never hope to look back to the beginning and see what happened. The cosmic density preceding the “recombination era” scattered photons from the very beginning of the Big Bang, and prevents us from looking back to a time earlier than about 380,000 years after the Big Bang.

If you do a Google search for “cosmic inflation,” you will find quite a few web links proclaiming that observations have confirmed the theory. No such thing has happened. The web links date from 2014, when the BICEP2 team announced observations which it claimed found something (primordial b-modes) that the cosmic inflation theory had predicted. But later in the year, a consensus emerged that the team had done no such thing, and that the observations were just as likely to have been the result of ordinary dust. This was all a great big embarrassment to modern cosmology, since a giant false alarm had been raised. At a time in the spring of 2014 when most major scientific web sites were toasting the supposed glorious success of BICEP2, this blog was one of only a handful of web sites raising doubts about the claim (my 2014 posts on the topic are here). By the end of the year, things had reversed, and the scientific world was pretty much saying this about BICEP2: “Never mind.”

The Scientific American article by the three scientists has provoked an unusual response. The main supporters of Guthism (including Alan Guth and Andrei Linde) along with about 30 other cosmologists have published a rebuttal article called “A Cosmic Controversy.” It is kind of an authoritarian power play, designed to impress the reader by listing authorship by some of the top names in cosmology. The list of authors is very impressive, but the reasoning of the rebuttal is very unimpressive.

The core of the rebuttal is the claim that cosmic inflation theory (Guthism) has made several predictions that have proven true. The predictions listed are that the universe is geometrically flat, that ripples in the cosmic background radiation should be nearly “scale invariant,” that these ripples should be “adiabatic,” and that these ripples should be “Gaussian.”

Let me explain why such things cannot be cited as good evidence that Guthism is correct. First, we must recognize that many a false theory may make a true prediction. Some person may have a false theory that the Freemasons are secretly plotting to take over the world. That theory may predict (among other things) that interest rates will rise, that the stock market will go down, and that real estate prices will not change much. If it then happens that each of these three things happens, it does nothing to prove that such a theory of a Freemason conspiracy is correct. There would be about 1 chance in 10 of such predictions being right accidentally, and a 1 in 10 coincidence is not an impressive one.

In physics the theories of gravitation, electromagnetism, and quantum mechanics are regarded very highly because they make very precise predictions that turn out to be exactly correct. For example, a theory of gravitation may predict that a particular small asteroid will crash into the moon at exactly 10:35 PM EST on January 23, 2025. When such exact predictions turn out to be precisely true, it is very impressive, because it's hard to see how the theory could be so precisely right if it were not true. If the theory were not true, such a prediction would seem to require perhaps a 1 in a billion coincidence.

Conversely, there is nothing impressive about a theory being correct with a few predictions that may be coincidentally correct with a likelihood of about 1 in 2 or 1 in 3. The overall unlikelihood of such a level success is only about 1 in 6 or 1 in 9, which is not very unlikely at all.

In the case of the predictions of Guthism (primordial cosmic inflation), the items mentioned by the “A Cosmic Controversy” rebuttal are items that would have not been very unlikely for a false theory to have predicted. There are three possible geometries of space (flat, open, and closed), so you have 1 chance in 3 of being right if you pick one of those. If you guess that primordial ripples were Gaussian, you have about 1 chance in 2 of being correct. If you guess that primordial ripples were scale-invariant, you also have 1 chance in 2 of being correct. If you guess that fluctuations were adiabatic, you also have about 1 chance in 2 of being correct.

We may also note the fact that over the past 35 years there have been many hundreds of papers published presenting different versions of Guthist cosmic inflation theories. The predictions of these models have been “all over the map.” Typically a particular version of the theory will present a model consisting of equations, with lots of free parameters that can be plugged into the equations. A particular model (one version of the cosmic inflation theory) may predict a million different things, depending on what is chosen for the free parameters. And hundreds of such Guthist models have been published, each with slightly different equations.

So even the meager predictive successes mentioned in the “A Cosmic Controversy” rebuttal are not at all something that can be trotted out as some sign that “the predictions of cosmic inflation theory have been verified.” The few predictions mentioned are predictions cherry picked from a large family of models, which made predictions all over the map.

If I advanced some economic theory, and got disciples to grind out hundreds of different flavors of my theory, I would no doubt be able to find among some of these works some successful predictions that each had a chance probability of maybe 1 in 2 or 1 in 3 of being correct. But that would not be evidence that my theory was correct. Similarly, a few successful predictions (none very numerically exact) among the huge number of Guthist cosmic inflation models is not at all impressive, and something we might well expect to occur by chance.

The “successful predictions” cited in the “A Cosmic Controversy” rebuttal are also things that have been produced as the evidence was gradually coming in. It's not too hard to get successful predictions if you are predicting as the evidence is coming in. For example, if it's 2007 and the evidence is starting to come in that the housing market is collapsing, it's not hard to predict in that year that the housing market will collapse.

As Steinhardt, Loeb, and Iijas state in their rebuttal to the “Cosmic Controversy” rebuttal, “Any inflationary model gives an infinite diversity of outcomes with none preferred over any other.”

At this link Steinhardt, Loeb, and Iijas cite various Guthist cosmic inflation models over the years which have made false predictions, including some models predicting an open universe, some models predicting non-guassianity, some models predicting large tilt, some models predicting deviations from isotropy, some models predicting bumps and wiggles in the primordial spectrum, and some models that “predict B-modes with amplitudes that should have been detected by the WMAP and Planck satellites.”

It would actually be a gigantic project probably requiring years of work for someone to analyze whether the predictions of cosmic inflation models have been mainly successful or mainly unsuccessful. You would have to do something like put together an incredibly complex computer program that included thousands of equations that were extracted from more than 500 cosmic inflation papers published in the past 35 years. You would then have to vary the input parameters on all of these models, and see how well the results matched up with a large variety of cosmological observations. No one has ever done such a thing, partially because of the runaway complexity of such a project, which might be thousands of time more difficult than checking the accuracy of IPCC predictions on global warming.

In short, it is not right to claim that predictive successes show some likelihood that Guthist cosmic inflation actually occurred. We must also consider that the Cold Spot in the cosmic background radiation seems to be inconsistent with what such a theory predicts (as discussed here).

Imagine a salesman who knocks on your door and tells you he can make your kitchen look real nice if you spend only a few dollars to do some simple work. You let him in to do some work, thinking it will be a simple affair. Imagine the guy starts fiddling with the pipes and electricity, and then eventually tells you the job is going to cost you many thousands of dollars. You think to yourself: I never would have let this guy in if he had told me that at the beginning.

The tale of Guthism is a similar tale. It was originally pitched as something pretty simple. So cosmologists welcomed it. As time progressed, and the simplest versions kept failing, our Guthists eventually indicated that the theory required a whole multiverse (a huge collection of universes). The theory never would have been initially welcomed if this gargantuan requirement had been confessed at the very beginning. But by the time the multiverse requirements of Guthism had become apparent, the Guthist thought virus had infected so many cosmologists that they were reluctant to say, “This thing has got out of hand – we've been going in the wrong direction.” It's rather like a husband who takes the wrong turn trying to get from New York City to Philadelphia. If you point out his mistake the first few miles, he may turn around. But by the time he's gone a hundred miles down the wrong road, he may never admit he made a wrong turn. And even if he starts seeing signs saying, “Welcome to Massachusetts,” he may still swear he made the right turn. 

Postscript: Scientific American columnist John Horgan writes the following:

Almost 40 years after their inception, inflation and string theory are in worse shape than ever. The persistence of these unfalsifiable and hence unscientific theories is an embarrassment that risks damaging science’s reputation at a time when science can ill afford it. Isn’t it time to pull the plug? 

Here is a relevant previous post of mine, entitled "Let's Keep the Big Bang but Dump the Cosmic Inflation Theory."