A brouhaha has erupted over the theory of cosmic creation known as inflation. The theory holds that in the first instant of the big bang, the universe underwent a tremendous, exponential growth spurt before settling down to the slower rate of expansion observed today.
First conceived in the early 1980s, inflation quickly became popular, because it seemed to account for puzzling features of the observable universe. Inflation explains, supposedly, why the universe looks quite similar in all directions and yet isn’t entirely uniform, since it contains galaxies and other clumps of matter.
By the early 1990s, some cosmologists were beginning to doubt inflation. “I like inflation,” David Schramm, a prominent contributor to the big bang theory, told me in 1993. But he worried that inflation does not offer any unique, definitive predictions, which cannot be explained in any other way.
“You won’t see that for inflation, Schramm said, “whereas for the big bang itself you do see that. The beautiful, cosmic microwave background and the light-element abundances tell you, ‘This is it.’” In other words, inflation cannot be falsified. According to philosopher Karl Popper, a theory that doesn’t offer predictions specific and precise enough to be proven false isn’t really scientific.
In my 1996 book The End of Science I derided inflation as “ironic science,” which can never be proven true or false and hence isn’t really science at all. I have continued whacking inflation since then, because as with string theory, another example of ironic science, the problems of inflation have only worsened over time.
There are many different versions of string theory and inflation, which offer many different predictions. Both theories imply, moreover, that our cosmos is just one of many universes, none of which can be observed. (For more criticism of strings and multiverses, see my recent Q&A with string critic Peter Woit.)
I was thus gratified when physicists Anna Ijjas, Paul Steinhardt and Abraham Loeb presented a stinging critique of inflation in Scientific American in February and urged cosmologists to “consider new ideas about how the universe began.”
Steinhardt’s authorship is especially significant, since he is credited with inventing inflation together with Alan Guth and Andrei Linde. Steinhardt has been voicing qualms about inflation for years. See for example my 2014 Q&A with him on this blog, in which Steinhardt says: “Scientific ideas should be simple, explanatory, predictive. The inflationary multiverse as currently understood appears to have none of those properties.” Ijjas et al. expand on Steinhardt’s long-standing concerns. The authors assert that
inflationary cosmology, as we currently understand it, cannot be evaluated using the scientific method. As we have discussed, the expected outcome of inflation can easily change if we vary the initial conditions, change the shape of the inflationary energy density curve, or simply note that it leads to eternal inflation and a multimess. Individually and collectively, these features make inflation so flexible that no experiment can ever disprove it.
I love the term “multimess.” Now a group of 33 scientists has pushed back hard against the critique of Ijjas, Steinhardt and Loeb. The group includes inflation pioneers Alan Guth and Andrei Linde as well as Steven Weinberg, Edward Witten and Stephen Hawking. In a letter published in Scientific American, they insist that inflation is testable and hence scientific. They conclude:
During the more than 35 years of its existence, inflationary theory has gradually become the main cosmological paradigm describing the early stages of the evolution of the universe and the formation of its large-scale structure. No one claims that inflation has become certain; scientific theories don’t get proved the way mathematical theorems do, but as time passes, the successful ones become better and better established by improved experimental tests and theoretical advances. This has happened with inflation. Progress continues, supported by the enthusiastic efforts of many scientists who have chosen to participate in this vibrant branch of cosmology. Empirical science is alive and well!
That last sentence strikes me as whistling past the graveyard, but read the letter and judge for yourself. In their response, Ijjas, Steinhardt and Loeb stand firm, especially on their argument that inflation is not empirically testable. They note that
if inflation produces a multiverse in which, to quote a previous statement from one of the responding authors (Guth), “anything that can happen will happen”—it makes no sense whatsoever to talk about predictions… any inflationary model gives an infinite diversity of outcomes with none preferred over any other. This makes inflation immune from any observational test.
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?
See also my Q&As with physicists Edward Witten, Steven Weinberg, George Ellis, Carlo Rovelli, Scott Aaronson, Stephen Wolfram, Sabine Hossenfelder, Priyamvada Natarajan, Garrett Lisi, Paul Steinhardt and Lee Smolin.