Cosmic Inflation Theory Has Not Passed the Predictive Test

The cosmic inflation theory is often confused with the Big Bang theory, but it is really just a variation of the Big Bang theory. The Big Bang theory makes the very general assertion that the universe started to expand from an incredibly hot and dense beginning 13 billion years ago. The cosmic inflation theory makes a very specific claim that during a tiny fraction of the universe's first second, the universe underwent a burst of “exponential expansion.” You can believe in the Big Bang theory without accepting the theory of cosmic inflation.

On June 17^th cosmologist Ethan Siegel published a post entitled “Cosmic Inflation's Five Great Predictions.” The subtitle of the piece is “A 'speculative' theory no more; it’s had four of them confirmed.” But Siegel's reasoning is in error. The theory of cosmic inflation – an extremely speculative theory – has not passed the predictive test. In fact, as we will see, it flunks such a test, by predicting that our universe is a lifeless universe.

First, let's look at the whole idea of using predictions to verify that a theory is correct. There are two main ways in which predictions might justify the claim that a theory has a good chance of being correct:

1. A theory might make precise numerical predictions that are invariably found to be exactly correct, in thousands of different comparisons between the predictions of the theory and an observed reality.
2. A theory might correctly predict a long series of observations that are each very unlikely to be observed if the theory is not correct.

An example of such a theory is the theory of gravitation. The theory of gravitation makes very precise numerical predictions, such as predicting that there will be exactly some particular force of attraction between two particular objects in space. Such predictions have been verified countless times, both on Earth and in space. So in a case such as the theory of gravitation, we can truly say that it has made countless thousands (or even millions) of exact predictions that have been verified. We can say that the chance of all these predictions being true by mere coincidence is incredibly tiny – less than 1 in 1,000,000,000,000 presumably.

But what happens in the field of science is that proponents of some theory will often claim that their theory has passed a predictive test, even though the record of successful predictions is infinitely weaker than the success record of gravitation theory. For example, imagine I say that my theory T predicted X (when X, Y, and Z were the alternatives) , and my theory predicted a value of 7 for parameter P (when that parameter could have had a value between 5 and 10). Does that mean I can then claim that my theory T has been confirmed because it passed a predictive test? Not by a long shot. In this case you would have a probability of merely 1 in 15 that an incorrect theory would make the correct predictions, and such a probability isn't impressive at all.

To understand why such a number is not at all impressive, imagine yourself as a bumbling scientific theorist who produces a scientific theory once every two weeks, without putting much thought into your work. Within about a year you will be likely to produce a theory that purely by chance meets a predictive test with a chance probability of 1 in 15. So meeting such a test means virtually nothing, and gives no reason for confidence that such a theory is true.

Now let's look at the items that cosmologist Siegel claims as successful predictions by the cosmic inflation theory. First, I may note that all of the items are merely “one-shot” affairs. In no case does cosmic inflation theory make any predictions that are repeatedly verified over and over, in the same way that gravitation theory and quantum mechanics do. So right off the bat this puts in grave doubt any claim of predictive verification involving the cosmic inflation theory.

We also find that in no case do any of the claims of predictive success involve something that was extremely unlikely to be true if the theory was false. All of the claimed predictive successes are what we may call semi-obvious predictions. A semi-obvious prediction is a prediction of something that isn't very unlikely to be true by chance – rather like predicting that a random male's favorite sport is baseball.

Let's look at the cases cited by Siegel.

Prediction #1 cited by Siegel: a flat universe. Back when the cosmic inflation theory was first advanced around 1980, there were three possibilities involving the universe's spatial geometry: the universe might be open, the universe might be closed, and the universe might be flat. The cosmic inflation theory predicted that the universe was perfectly flat. Thus far it looks like the universe is close to being spatially flat. But that's not a very impressive prediction, as you would have had 1 chance in 3 of getting such a prediction right if you had guessed. So this is merely a semi-obvious prediction. From a prediction verification standpoint, such a predictive success is “peanuts,” too paltry to even be considered. A solid example of predictive confirmation might be a case when a theory predicted a particular alternative when there were a thousand or a million possible alternatives.

I may also note here that it is premature to even say that the prediction of cosmic inflation theory on this matter has even been verified. The actual prediction of the theory is that what is called the critical density and the actual density differ by less than 1 part in 10,000 (the same as saying that the universe is perfectly flat). But we have only verified so far that these two differ by less than 2 parts in 100 (that the universe is close to being flat). It is still quite possible that the prediction of a difference of less than 1 part in 10,000 will be proven false.

Prediction #2 cited by Siegel: some fluctuations predicted by inflation theory were found. This also does not at all qualify as anything very unlikely to have occurred by chance. What Siegel says is basically that either the fluctuations might have been found, or might not have been found; and they were found, as predicted by cosmic inflation theory. So what? If I predict something exists when it might or might not exist, my chance of success is about 50%. Again, this is “peanuts,” from the standpoint of predictive verification – not at all impressive. It's another semi-obvious prediction that doesn't count for much.

Prediction #3 cited by Siegel: some fluctuations that could have been adiabatic, isocurvature, or a mixture of the two, were found to be adiabatic as predicted by cosmic inflation theory. Again, this is “peanuts” from the standpoint of predictive verification – a meager success that is “small potatoes.” Siegel says that were three alternatives, and cosmic inflation theory predicted the right one. The odds of that occurring by chance are 1 in 3, so this is merely another semi-obvious prediction. Solid predictive verification comes when a theory predicts an observation with a chance likelihood of something like 1 in a 1,000,000.

Prediction #4 cited by Siegel: something called the scalar spectral index was predicted by cosmic inflation theory to be between .92 and .98, and was found to have a value of about .97. Once again, this is “peanuts” from the standpoint of predictive verification, a paltry success. If we assume that such a “scalar spectral index” parameter could have had a value between .7 and 1.3, then the chance likelihood of you predicting a value between .92 and .98 and being correct is something like 1 in 7. That's nothing like what you need for real predictive verification, which might involve successfully predicting something that has a chance likelihood of only 1 in a 1000, 1 in a million, or 1 in a billion.

In short, Siegel is not able to come up with one “smashing success” of cosmic inflation theory, a case in which it predicted something incredibly unlikely to be true unless the theory is true. He's merely able to list some semi-obvious predictions that each would have had a decent chance of being true by pure coincidence.

But do the four “little successes” cited by Siegel add up to a decent record of predictive success? No, not when one considers that one could probably list 20 things that have been incorrectly predicted by cosmic inflation theorists over the past 35 years. Since the cosmic inflation idea became popular around 1980, physicists have cranked out more than 1000 papers spelling out countless different flavors of the theory. Countless numbers of these variations have been discarded, because their predictions turned out to be false. So should we have any confidence in cosmic inflation because some of these variations or flavors have some minor predictive successes not very unlikely to have occurred by chance? No, we shouldn't.

If I were to create a computer program that automatically generates variations of some physics theory of mine, I might find that 99% predicted the wrong things, and a few lucky ones had some predictive successes. But that should give you no real confidence that the lucky ones were some indication that the underlying theory was correct.

Imagine the following conversation.

Jim: Do you know my dog can read your mind? I can tell what he's thinking by watching how his tail wags.

Jane: Oh, really? Well, let's give it a try.

Jim: First, think about whether you're a Republican or a Democrat. Let's see .. my dog tells me by his tail wagging that you're a Republican.

Jane: Yes, that's correct.

Jim: Now, think about which season you were born in. Let's see...my dog tells me by his tail wagging that you were born in summer.

Jane: Yes, that's correct.

Jim: Now, think about your favorite holiday. Let's see...my dog tells me by his tail wagging that your favorite holiday is Christmas.

Jane: Yes, that's correct.

Jim: So that's it. I've proven that my dog can read your mind.

Of course, this would not prove any such thing, because it is not all that unlikely that such successes might be purely due to chance. The same thing can be said about the semi-obvious predictive successes listed by Siegel in regard to cosmic inflation.

There are also cases in which cosmic inflation theory seems to flunk the predictive test. One such case involves what is called the cold spot in the cosmic microwave background radiation, which is shown in the circled portion of the image below. 

 The CMB Cold Spot (credit:NASA)

A cosmologist at Cambridge University suggests that the existence of this cold spot conflicts with what the cosmic inflation theory predicts:

[The inflationary model] “predicts that today’s universe should appear uniform at the largest scales in all directions. That uniformity should also characterize the distribution of fluctuations at the largest scales. But these anomalies, which Planck confirmed, such as the cold spot, suggest that this isn’t the case… This is very strange. And I think that if there really is anything to this, you have to question how that fits in with inflation…. It’s really puzzling.

Siegel fails to mention the most important thing about the predictions of cosmic inflation theory: that it incorrectly predicts that we should not even exist. The cosmic inflation theory predicts that our universe should be a lifeless “small bubble” universe that is way too young and way too small for any galaxies to have formed in it.

The cosmic inflation theory actually makes this prediction because it predicts that each universe that undergoes exponential expansion produces many other “bubble universes,” and that each of these bubble universes themselves produce many other bubble universes, and so on and so forth. According to the predictions of the theory, the number of these bubble universes too small to contain any galaxies (and any life) should be more than 1,000,000,000,000,000 times larger than the number of bubble universes large enough for galaxies to form. As cosmic inflation proponent Alan Guth describes here (in a discussion of this “youngness paradox”), “The population of pocket universes is therefore an incredibly youth-dominated society, in which the mature universes are vastly outnumbered by universes that have just barely begun to evolve.”

Given such a situation (in which small bubble universes are many trillions of times more common than universes large enough for galaxies to form), and given that predicting one thing is trillions of times more likely than another thing is equivalent to predicting the first thing, it must be said that the cosmic inflation theory predicts that our universe should be one of those smaller, lifeless universes.

I may note that we should not all “subtract” one of a theory's predictions from the list of its predictions simply because we have always known that prediction to be false. We are not at all entitled to “cross out” one of a theory's predictions because we have always observed the contrary. In such a case we should note that such a prediction is a falsified prediction. So it is with the existence of life and the cosmic inflation theory. The theory predicts that our universe should be a young, lifeless, tiny bubble universe. Rather than crossing out or ignoring this prediction because we know it to be false, we should carefully note it as a falsified prediction.

An inflationary theorist might try to counter this argument by claiming that a theory's predictions consists only of what has been publicly predicted by advocates of that theory, and no such advocate has predicted that the universe is lifeless. But such a general claim about theory predictions is not true. A theory's predictions consists not of what has been publicly predicted by advocates of that theory, but instead a theory's predictions consist of whatever predictions are logically or physically implied by the theory.

In short, the predictive record of the cosmic inflation theory consists of a few unimpressive semi-obvious minor successes (none of them very unlikely to have occurred by chance), a likely failure in regard to the cold spot in the microwave background radiation, and one huge, gigantic predictive failure – that our universe should be a lifeless thing too small to have galaxies. Overall, that adds up to a failed predictive record.

For more on the shortcomings of the cosmic inflation theory, see this post.