This new Planck result will get a little coverage, but much less than the BICEP2 news coverage last March, when it seemed like almost every cosmologist was popping a champagne cork in premature self-congratulation. It was a huge orgy of unwarranted credulous enthusiasm over a study with quite a few problems, problems I pointed out in a very skeptical blog post the day after the BICEP2 study was released (at a time when I seemed like a rare doubter, with few others voicing similar doubts at that time). My skepticism about BICEP2 was apparently warranted.
Given the new Planck result, it may be a good time to look at a basic question: should we actually believe in the theory of cosmic inflation? I will argue in this post that we should not.
The Difference Between the Big Bang Theory and the Cosmic Inflation Theory .
Before giving my case against the cosmic inflation theory, let me clarify the difference between the cosmic inflation theory and the Big Bang theory. No doubt many people get the two mixed up, because the concepts are fairly similar.
The Big Bang theory originated around the middle of the twentieth century. The Big Bang theory is the theory that about 13 billion years ago the universe originated from an extremely hot and dense state. It's basically the idea that the universe “exploded into existence” billions of years ago (or began to expand from a state so hot and dense that it was just as if the universe had exploded into existence). The theory was dramatically substantiated by the discovery of the cosmic background radiation around 1965, believed to be the “relic radiation” of the Big Bang. The Big Bang theory is also supported by the simple fact that the universe is expanding. When you “rewind the film” on an expanding universe all the way to the beginning, you are stuck with something like the Big Bang.
The cosmic inflation theory did not originate until about 1980. The cosmic inflation theory is actually a theory about a tiny fraction of the universe's first second. The theory maintains that when the universe was a fraction of a second old, the universe underwent exponential expansion, which is a type of expansion vastly quicker than the type of expansion we now observe. According to the cosmic inflation theory, this phase of super-fast exponential expansion lasted only a fraction of a second. The diagram below illustrates the cosmic inflation theory.
You can believe in the Big Bang theory without believing in the cosmic inflation theory, which is exactly what cosmologists generally did during the decade of the 1970's. However, you cannot believe in the cosmic inflation theory without believing in some form of the Big Bang theory.
The Reasons Many Cosmologists Believe in the Cosmic Inflation Theory .
The rationale given for believing in the cosmic inflation theory is that it supposedly solves some cosmic mysteries. The main such mystery is what is called the flatness problem. The flatness problem is a fine-tuning problem involving the Big Bang. According to cosmologists, when the universe began, it started to expand at just the right rate. If the universe had started to expand at a tiny bit faster rate, it would have expanded so quickly that galaxies would not have formed from gravitational contraction. If the universe had started to expand at a tiny bit slower rate, the gravitational attraction from the universe's matter would have caused the universe's matter to form into super-dense black holes rather than galaxies.
The
physicist Paul Davies puts it this way:
For
a given density of cosmic material, the universe has to explode from
the creation event with a precisely defined degree of vigor to
achieve its present structure. If the bang is too small, the cosmic
material merely falls back again after a brief dispersal, and
crunches itself to oblivion. On the other hand, if the bang is too
big, the fragments get blasted completely apart at high speed, and
soon become isolated, unable to clump together to form galaxies.
How
finely balanced did this expansion rate have to be in order for there
to be a universe like ours, in which galaxies exist? Scientists say
that it had to be balanced to at least one part in 10 to the
thirtieth power (1 part in
1,000,000,000,000,000,000,000,000,000,000). In other words, if the
universe had expanded at a rate only
.00000000000000000000000000000001 faster or slower, it would not have
galaxies, and would not have life.
The
calculation given here is not some oddball conclusion made by only
one or two scientists.A
statement like the statement above has been made in innumerable scientific
books and papers.
The cosmic inflation theory was created mainly to solve this problem. It seems that if the universe underwent the exponential phase of expansion imagined by the cosmic inflation theory, the universe's expansion would not need to be so fine-tuned.
Another reason given for believing in the cosmic inflation theory is that it offers an answer for what is called the horizon problem, which is basically the problem of why opposite ends of the universe have identical thermodynamic attributes, as viewed in the cosmic background radiation.
A third reason given for believing in the cosmic inflation theory is that it solves some “missing monopole” problem, although this is not a compelling reason because the problem only arises for those who believe in some family of theories called grand unification theories (and there seems to be no particular necessity in believing in such a theory).
Why Cosmic Inflation is Not a Good Way of Explaining These Problems .
The theory of cosmic inflation offers a way of explaining the flatness problem and a way of explaining the horizon problem. But both of these problems are examples of more general phenomena. The flatness problem is an apparent case of cosmic fine-tuning, and the horizon problem is an example of cosmic uniformity. The weakness in trying to solve these problems with a theory of cosmic inflation is that we have many other apparent cases of cosmic fine-tuning and many other cases of astonishing cosmic uniformity – but the cosmic inflation theory only offers a solution to one of the many cases of cosmic fine-tuning, and only one of the many cases of cosmic uniformity.
Cases of apparent cosmic fine-tuning are discussed here and here (in a blog post that includes a handy color-coded chart). Among the many astonishing cases of cosmic fine-tuning are the flatness problem, the fine-tuning of the Higgs to 1 part in 100,000,000,000,000.000, the fine-tuning of the cosmological constant to 1 part in 10 to the sixtieth power (or 10 to the 120th power, depending on how you look at it), the fine-tuning of the strong nuclear force, the fine tuning of atomic resonances, the fine-tuning of fundamental constants related to stellar nuclear reactions, and the fine-tuning of the proton charge and the electron charge (involving a match to one part in 1,000,000,000,000,000,000,000). There are also many similar cases. Now, how many of these cases of cosmic fine-tuning does the cosmic inflation theory claim to explain? Exactly one: the flatness problem. In this sense, the cosmic inflation theory is rather like a theory that tries to explain the origin of animal species, but only explains the origin of tigers rather than explaining the origin of the rest of the animals.
If we are to try to explain cosmic fine-tuning, we need a more general explanation – some particular principal or assumption that will explain all the cases (or most of the cases) of fine-tuning, rather than jumping on some “one-trick pony” that explains just one example of cosmic fine-tuning.
When we look at examples of cosmic uniformity, we find a very similar situation. There is not just one amazing case of cosmic uniformity (the horizon problem), but many others. Among the main cases of cosmic uniformity are the uniformity of fundamental constants in opposite regions of the universe. Scientists have determined that some fundamental constants such as the fine-structure constant are the same in opposite regions of the universe separated by a distance of more than twenty billion light-years (ten billion light-years in one direction, plus ten billion in another direction). This is particularly amazing because it is not just a uniformity over a vastness of space but also a uniformity over a vastness of time equal to almost the age of the universe. Other examples of cosmic uniformity are the uniformity of the universe's laws. The universe is like a vast machine that keeps on following the same set of rules (which we call the laws of nature), obeying those laws to the letter, with slavish obedience eon after eon.
If we were to list all of the cases of cosmic uniformity, it would be a long list. But how many on that list does the theory of cosmic inflation purport to explain? Exactly one: the horizon problem. Again, in this sense the cosmic inflation is like a theory of the origin of species that only explains the origin of tigers, without explaining the origin of any other species. If we are to start trying to explain cosmic uniformity, we need a more general explanation, rather than jumping on some “one-trick pony” that explains just one example of cosmic uniformity.
How the Cosmic Inflation Theory Robs Peter to Pay Paul .
The advocates of the cosmic inflation theory neglect to explain that as the price of explaining one example of cosmic fine-tuning (the flatness problem), the cosmic inflation theory requires its own fine-tuning, in not just one place, but multiple places. One has to imagine various types of fine-tuning to create a theory of cosmic inflation compatible with observations. You have to do fine-tuning so that the cosmic inflation can start at just the right instant, and more fine-tuning so that the cosmic inflation can end at just the right time (or else you end up with a universe that keeps inflating exponentially, which we know did not happen). It is not clear at all that when you add up all these types of fine-tuning needed for cosmic inflation to work, that you end up with less cosmic fine-tuning than if you don't believe in the theory. It's basically a case of robbing Peter to pay Paul.
The False Prediction of the Cosmic Inflation Theory
Before justifying my assertion that the cosmic inflation theory makes a false prediction, I must declare an interesting and important principle that is sometimes overlooked. This is the principle that we should never ignore the “gross predictions” of a theory, and never try to subtract counterfactual predictions, thereby judging a theory only on a set of “net predictions.”
Here is what some people think our procedure should be when evaluating a theory:
(1) Start with the “gross predictions” of a theory – everything it seems to predict, regardless of known facts.
(2) Subtract from these “gross predictions” anything known to be false.
(3) Then evaluate the theory on a smaller set of “net predictions.”
I think such an approach is badly mistaken. Rather than discarding “gross predictions” of a theory that are clearly counterfactual, we should in fact pay great attention to such predictions, because they are often very important indicators that the theory is false. It's rather like this: suppose a theory predicts that a factory makes only red phones, and you open a package from the factory, seeing a blue phone. What does the theory predict now? Exactly the same thing it predicted before you opened the package: that the factory makes only red phones.
So let's look at exactly what are the predictions are of the cosmic inflation theory, without discarding any counterfactual “gross predictions.” The predictions of the cosmic inflation theory are as follows:
(1) The universe is spatially flat, or very close to being spatially flat.
(2) There is a relatively small amount of what is known as cosmological non-Gaussianity.
(3) Our universe is a lifeless “small bubble” universe that is way too young and small for any galaxies to have formed in it.
The cosmic inflation theory actually makes the third of these predictions 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 billions and trillions and quadrillions of 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 vastly outnumber bubble 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 is one of those smaller, lifeless universes. It is not legitimate at all to subtract this counterfactual prediction because of some principle that we are allowed to subtract counterfactual predictions, reducing a set of “gross predictions” to a set of “net predictions.”
So how can an advocate of the cosmic inflation theory explain how we got lucky enough to be living in one of the rare life-compatible “bubble universes,” when it is almost infinitely more likely (under cosmic inflation theory) that our universe would be one of the young “bubble universes” too small for galaxies to form in it? He must resort to a “blind luck” explanation. But the luck needed is greater than the luck needed to have a successful universe without cosmic inflation. So nothing is accomplished, and the “miracle” of our existence is not made any less miraculous. In fact, the cosmic inflation theory seems to make our existence even more miraculous. How can such a result be described as scientific progress?
Cosmic Inflation: A “Cash Cow” for Lazy Cosmologists?
If the case for cosmic inflation is so weak, why do so many cosmologists support it? One answer can be found in groupthink effects, the tendency of modern cosmologists to travel in a herd because of sociological “go with the crowd” reasons. But another reason is that for decades the cosmic inflation theory has seemingly been an easy “meal ticket” for lazy cosmologists.
Producing a new paper on cosmic inflation is a cinch for a modern cosmologist. He merely has to write some new paper juggling some astrophysical numbers (perhaps thinking of some minor new speculative tweak), and doing the same type of calculations done by many earlier cosmologists. Since the cosmic inflation theory was introduced, cosmologists have published thousands of new papers discussing different flavors of the theory. A large fraction of this work has been funded by university grants or federal research grants. So for a cosmologist who is not particularly innovative, the cosmic inflation theory is a wonderful “cash cow.” Think of how easy it is: just produce “yet another cosmic inflation paper” (something like “cosmic inflation paper number 5,678”) without any real originality, and with zero risk that anyone will ever prove you wrong; and let the taxpayers or your university foot the bill. I'm reminded of the phrase in that Gershwin song: nice work if you can get it.
Given the existence of this convenient “cash cow” that pays well for easy speculative work, cosmologists are reluctant to bite the hand that feeds them, and admit how weak the cosmic inflation theory is. That would be like up ripping up their meal ticket.
Conclusion
There seems to be good evidence for the Big Bang and the expansion of the universe, so we should keep believing in such theories. There is no good evidence for the theory of cosmic inflation (the theory of exponential expansion in the universe's first second), and it should be dumped, in the sense of being relegated to a mere possibility rather than asserted as a likelihood. Scientists Ijjas, Steinhardt, and Loeb recently wrote a paper giving some powerful objections to the cosmic inflation theory.
Rather than embracing a theory that claims to explain only one case of cosmic fine-tuning (when there are many such cases to explain), we should look for a more general explanation. Rather than embracing a theory that claims to explain only one case of cosmic uniformity (when there are many such cases to explain), we should also look for a more general explanation. If scientists cannot think of such a more general explanation, they should simply say that they do not understand the explanation for the flatness problem and the horizon problem that originally motivated the cosmic inflation theory. It is an intellectual sin to claim to understand a cosmic mystery that you do not really understand. For 1000 years, astronomers embraced the Ptolemaic theory, and claimed to understand why the solar system behaves as it does, before they really understood that mystery. There's a lesson to be learned from such a long mistake: don't claim to understand a cosmic mystery based on some weak theory. Much better to simply candidly say: I don't understand this cosmic mystery.
No comments:
Post a Comment