The article in Scientific America criticizing the theory of cosmic inflation was by three scientists (Anna Ijjas, Paul J. Steinhardt, Abraham Loeb), one a Harvard professor and another a Princeton professor. It was filled with very good points that should be read by anyone curious about the claims of the cosmic inflation theory. You can read the article on a Harvard web site here. Or you can go to this site by the article's authors, summarizing their critique of the cosmic inflation theory.
Recently a very long scientific paper appeared on the ArXiv physics paper server, a paper with the cute title “Cosmic Inflation: Trick or Treat?” In its very first words the paper's author (Jerome Martin) misinforms us, because he refers to cosmic inflation as something that was “discovered almost 40 years ago.” Discovery is a word that should be used only for observational results in science. Cosmic inflation (the speculation that the universe underwent an instant of exponential expansion) was never discovered or observed by scientists. In fact, it is impossible that this “cosmic inflation” or exponential expansion ever could be observed. During the first 300,000 years of the universe's history, the density of matter and energy was so great that all light particles were thoroughly scattered and shuffled a million times. It is therefore physically impossible that we ever will be able to observe any unscrambled light signals from the first 300,000 years of the universe's history. So we will never be able to get observations that might verify the claim of cosmic inflation theorists that the universe underwent an instant of exponential expansion.
At the end of the paper the author claims that the cosmic inflation theory has “all of the criterions that a good scientific theory should possess.” The author gives only two examples of such things: first, the claim that the cosmic inflation theory is falsifiable, and second that “inflation has been able to make predictions.” His claim that the theory is falsifiable is not very solid. He says that the cosmic inflation theory could be falsified if it were found that the universe did not have what is called a flat geometry, but then he refers us to a version of the cosmic inflation theory that predicted a universe without such a flat geometry. So cosmic inflation theory isn't really falsifiable at all. So many papers have been published speculating about different versions of cosmic inflation theory that the theory can be made to work with any future observations. Harvard astronomer Loeb says here the cosmic inflation theory "cannot be falsified."
It is not at all true that the cosmic inflation theory has “all of the criterions that a good scientific theory should possess,” or even most of those characteristics. Below is a list of some of the characteristics that are desirable in a good scientific theory. You can have a good scientific theory without having all of these characteristics, but the more of these characteristics that you have, the more highly regarded your scientific theory should be.
- The theory is
potentially verifiable. While falsification has been widely
discussed in connection with scientific theories, it should not be
forgotten that the opposite of falsification (verification) is
equally important. Every good scientific theory should be
potentially verifiable, meaning that there should always be some
reasonable hypothetical set of observations that might verify the
theory. In the case of the cosmic inflation theory, we can imagine
no such observations. The only thing that could verify the cosmic
inflation theory would be if we were to look back to the first
instant of the universe and observe exponential expansion occurring.
But, as I previously mentioned, there is a reason why such an
observation can never possibly occur, no matter how powerful future
telescopes are. The reason is that the density of the very early universe was
so great that all light signals from the first 300,000 years of the
history were hopelessly shuffled, scrambled and scattered millions of times.
- The theory merely requires us to believe in something very simple. A very desirable characteristic of a scientific theory is that it only requires that we believe in something very simple. An example of a theory with such a characteristic is the theory that the extinction of the dinosaurs was caused by an asteroid collision. Such a theory asks us only to believe in something very simple, merely that a big rock fell from space and hit our planet. Another example of a theory that meets this characteristic is the theory of global warming. In its most basic form, the theory asks us to merely believe in something very simple, that humans are putting more greenhouse gases in the atmosphere, and that such gases raise temperatures (as we know they do inside a greenhouse). But the cosmic inflation theory (the theory of primordial exponential expansion) does not have this simplicity characteristic. All versions of such a theory require complex special conditions in order for this cosmic inflation (exponential expansion) to begin, to last for only an instant, and then to end in less than a second so that the universe ends up with the type of expansion that it now has (linear expansion, not exponential expansion). We need merely look at the papers of the cosmic inflation theorists (all filled with complex mathematical speculations) to see that the theory fails very much to meet this simplicity characteristic of a good scientific theory. In a recent post, the cosmic inflation pitchman Ethan Siegel tells us, "If you have an inflationary Universe that's governed by quantum physics, a Multiverse is unavoidable." What that means is the cosmic inflation has the near-infinite baggage of requiring belief in some vast collection of universes. Of course, this is the exact opposite of the simplicity that is desirable in a good theory.
- There is no evidence
conflicting with the theory. A characteristic of a good scientific
theory is that there is no evidence conflicting with the theory.
The theory of electromagnetism and the theory of plate tectonics are
very good theories, and there is no evidence against them. But there
are quite a few observations conflicting with the cosmic inflation
theory (the theory of exponential expansion in the universe's first
instant). Such observations (sometimes called CMB anomalies) are discussed in this post. The
observations are mainly cases in which the cosmic background
radiation has some characteristic that we would not expect to see if
the cosmic inflation theory were true. A scientific paper says,
“These are therefore clearly surprising, highly statistically
significant anomalies — unexpected in the standard inflationary
theory and the accepted cosmological model.”
- The theory makes precise
numerical predictions that have been exactly verified to several
decimal places very many times. This characteristic is one that
the best theories in physics have, theories such as the theory of
general relativity, the theory of quantum mechanics, and the theory
of electromagnetism. For example, the theory may predict that some
unmeasured quantity will be 342.2304, and scientists will measure
that quantity and find that it is exactly 342.2304. Or the theory
may predict that some asteroid will hit the Moon on exactly
10:30 PM EST on May 23, 2026, and it will then be found (10 days
later) that the asteroid did hit the Moon on exactly 10:30
PM EST on May 23, 2026. The cosmic inflation theory does not have
this characteristic of a good scientific theory. It makes no exact
numerical predictions at all. There have been published several hundred different versions of the cosmic inflation theory, each of
which is a different scientific model. Each of those hundreds of models can
predict 1000 different things, because the numerical parameters used with the
equations can be varied. So the predictions of the cosmic inflation
theory are pretty much all over the map, and it is impossible to
point to any case in which it made a good precise successful
prediction. When advocates of the cosmic inflation theory talk about
predictive success, they are talking about woolly kind of
predictions (like “the universe will be pretty flat”) rather
than exact numerical predictions, and they are talking about
one-shot affairs rather than cases in which predictions are
repeatedly verified. Many a wrong theory can have an equal degree
of predictive success. For example, a bad economic theory may
predict various things, and may vaguely predict correctly that the
stock market will go up next year.
- We continue to get
observational signs that the theory is correct. A desirable
characteristic of a good scientific theory is that we continue to
observe signs suggesting that theory is correct. The theory of plate
tectonics has such a characteristic. Every time there is an
earthquake in the “Ring of Fire” region that marks the
boundaries of continental plates, that's an additional
observational sign that the plate tectonics theory is correct. The
theory of gravitation continues to send us observational signals every day that the theory is correct. But we do not get any observational
signs from the universe that it once underwent an instant of
exponential expansion, nor can we logically imagine how such signs
could ever come or keep coming from such a primordial event.
So it is clear that Martin's claim that the theory of cosmic inflation has “all of the criterions that a good scientific theory should possess” is not at all true. Saying something similar to what I said above, a New Scientist article puts it this way:
But no measurement will rule out inflation entirely, because it doesn’t make specific predictions. “There is a huge space of possible inflationary theories, which makes testing the basic idea very difficult,” says Peter Coles at Cardiff University, UK. “It’s like nailing jelly to the wall.”
The tall tale of cosmic inflation (exponential expansion at the beginning of the universe) is a modern case of a tribal folktale, told by a small tribe of a few thousand cosmologists. Below is the basic piece of folklore of the cosmic inflation theory:
"At the very beginning, the universe started out with just the right conditions for it to start expanding at a super-fast exponential rate. So for the tiniest fraction of a second, the universe did expand at this explosive exponential rate. Then, BOOM, the universe suddenly switched gears, did a dramatic change, and started expanding at the much slower, linear rate that we now observe."
Why would anyone believe such a story that can never be verified? The answer is: because they have a strong motivation. The arguments given for the cosmic inflation theory are examples of what is called motivated reasoning. Motivated reasoning is reasoning that people engage in not because they have premises or evidence that demand particular conclusions, but because they have a motivation for reaching the conclusion.
The motivation for the cosmic inflation theory was that people wanted to get rid of some apparent fine-tuning in the Big Bang. At about the time the cosmic inflation theory appeared, scientists were saying that the universe's initial expansion rate was just right, and that if it had differed by less than 1 part in 1,000,000,000,000,000,000,000,000,000,000,000,000,000, we would not have ended up with a universe that would have allowed life to exist in it. That type of extremely precise fine-tuning at the very beginning of Time bothers those who want to believe in a purposeless universe.
Saying that the universe's initial expansion rate was fine-tuned is equivalent to saying that the density was fine-tuned, for the requirement is a very precise balancing involving an expansion rate that is just right for a particular density (or, to state the same idea, a density that is just right for a particular expansion rate). In a recent very long cosmology paper, scientist Fred Adams notes on page 41 the requirement for a very precise fine-tuning of the universe's initial density (something like 1 in 10 to the sixtieth power, which is a trillionth of a trillionth of a trillionth of a trillionth of a trillionth). On page 42 Adams states that, "The paradigm of inflation was developed to alleviate this issue of the sensitive fine-tuning of the density parameter." That was the motivation of the cosmic inflation theory -- to sweep under the rug or get rid of a dramatic case of fine-tuning in nature.
The folklore mongers who sell cosmic inflation stories may believe that they have got rid of this fine-tuning at the beginning. But they actually haven't. They've merely “robbed Peter to pay Paul,” by getting rid of fine-tuning in one place (in regard to the universe's initial expansion rate) at the price of requiring lots of fine-tuning in lots of other places. That's because all theories of cosmic inflation themselves require enormous amounts of fine-tuning. But with a cosmic inflation theory it may be rather less noticeable, because the required fine-tuning occurs in lots of different places rather than in one place.
Judging from a 2016 cosmology paper, the cosmic inflation theory requires not just one type of fine-tuning, but three types of fine-tuning. The paper says, “Provided one permits a reasonable amount of fine tuning (precisely three fine tunings are needed), one can get a flat enough effective potential in the Einstein frame to grant inflation whose predictions are consistent with observations.” How on Earth does it represent progress to try to get rid of one case of fine-tuning by introducing a theory that requires three cases of fine-tuning? And the estimate of three fine-tunings in the paper is probably an underestimate, as other papers I have read suggest that 7 or more precise fine-tunings are needed.
This is not theoretical progress
We may compare the cosmic inflation pitchman to some person who wants to sell someone in Manhattan a car. “Think of all the money you'll save!” says the pitchman. “You won't have to pay $40 on subways each week.” But what the pitchman fails to tell you is that when you add up the cost of the monthly car payments, the cost of car insurance, and the cost of a garage parking space (because there's so few parking spaces in Manhattan), the total cost of the car is much more than the cost of the subway. Similarly the pitchmen of cosmic inflation theory tell us that the theory is great because it reduces fine-tuning in one place (in regard to the universe's initial expansion rate), and neglect to tell you that the total amount of fine-tuning (adding up all of the special requirements and fine-tuning needed for cosmic inflation to work) is probably far “worse” if you believe that cosmic inflation occurred.
What has been going on with the cosmic inflation theory is very similar to what went on for decades with the supersymmetry theory, a theory which physicists have been fruitlessly laboring on for decades. Like the cosmic inflation theory, supersymmetry was motivated by a desire to sweep under the rug some fine-tuning. In the case of supersymmetry, the fine-tuning scientists wanted to get rid of was the apparent fact of the Higgs boson or Higgs field being fine-tuned very precisely ("like a pencil standing on its point" is an analogy sometimes given). An article on the supersymmetry theory discusses the fine-tuning that motivated the theory:
One logical option is that nature has chosen the initial value of the Higgs boson mass to precisely offset these quantum fluctuations, to an accuracy of one in 1016. However, that possibility seems remote at best, because the initial value and the quantum fluctuation have nothing to do with each other. It would be akin to dropping a sharp pencil onto a table and having it land exactly upright, balanced on its point. In physics terms, the configuration of the pencil is unnatural or fine-tuned.
Similarly, a paper on an MIT server entitled "Motivation for Supersymmetry" states the following (referring to the many new types of hypothetical particles called "supersymmetric partners" imagined by the supersymmetry theory):
Thus in order to get the required low Higgs mass, the bare mass must be fine-tuned to dozens of significant places in order to precisely cancel the very large interaction terms....However, if supersymmetric partners are included, this fine-tuning is not needed.
Physicists erected the ornate theory of supersymmetry, thinking that they were explaining away this very precise fine-tuning in nature, "to dozens of significant places." But they failed to see that they were just “robbing Peter to pay Paul,” because the total amount of fine-tuning required by the supersymmetry theory (given all of its many different things that had to be just right) was as great as the fine-tuning that it tried to explain away. So there was no net lessening of fine-tuning even if the supersymmetry theory was true.
The MIT paper above says "many thousands" of science papers have been written about supersymmetry. Most of them spun out ornate webs of speculation, as ornate and unsubstantiated as the gossamer speculations of cosmic inflation theorists. Supersymmetry has failed all observational tests, and now many physicists are lamenting that they wasted so many years on it. Our cosmic inflation theorists have failed to heed the lesson of the supersymmetry fiasco: that trying to explain away fine-tuning in the universe is a waste of time.