Cosmic inflation debate bleeds into popular science media
Subtle temperature variations in the cosmic microwave background (seen here in data collected by the Planck satellite) are thought to be a result of cosmic inflation.
ESA and the Planck Collaboration
When three physicists published “Cosmic inflation theory faces challenges ” earlier this year in Scientific American, the article itself constituted a challenge. Its subhead declared, “The latest astrophysical measurements, combined with theoretical problems, cast doubt on the long-cherished inflationary theory of the early cosmos and suggest we need new ideas.” The trio’s aggressive reappraisal of a scientific consensus inspired an energetic rebuttal , also in Scientific American, from 33 prominent physicists, including four Nobel laureates.
In this astrophysical and cosmological debate, arguments about the first instants of our cosmos have been framed and pressed by experts not only for consideration by their physics peers, but by the public. The Atlantic , Newsweek , and others have published overviews for lay readers. Commentaries have appeared from physicists who popularize science. Wired went so far as to propose that the debate invites nonscientists “to decide what it means to Do Science.”
The three inflation-theory critics, sometimes referred to as IS&L for their surnames’ initials, are Anna Ijjas , the John A. Wheeler postdoctoral fellow at the Princeton Center for Theoretical Science; Paul Steinhardt , Albert Einstein Professor in Science at Princeton University, director of the Princeton Center for Theoretical Science, and an original architect of cosmic inflation theory; and Abraham Loeb , Harvard University’s astronomy chair, founding director of Harvard’s Black Hole Initiative, and director of the Institute for Theory and Computation at the Harvard–Smithsonian Center for Astrophysics. IS&L’s article originally carried the title “Pop goes the universe.” As the editors of Scientific American put it , the three argued “against the dominant idea that the early cosmos underwent an extremely rapid expansion called inflation,” advocating instead “for another scenario—that our universe began not with a bang but with a bounce from a previously contracting cosmos.”
A forerunner debate—presented for physicists only, and extending existing discussions—had already been held in Physics Letters B. There in 2013, IS&L published “Inflationary paradigm in trouble after Planck2013,” with the title referring to data from the European Space Agency’s Planck satellite. In 2014, three of this year’s 33 rebutters answered. Alan Guth (another architect of the theory who coined the term inflation) and David Kaiser of MIT, along with Yasunori Nomura of the Berkeley Center for Theoretical Physics, argued that IS&L’s “conclusions rest on several problematic assumptions” and concluded that “cosmic inflation is on a stronger footing than ever before.”
Steinhardt had already begun questioning that footing in earlier issues of Scientific American. His article “The inflation debate: Is the theory at the heart of modern cosmology deeply flawed?” appeared in 2011. In 2014 the magazine published a second such article from him and later a Q&A that opened by referring to him as an inflation-theory “apostate.”
This year, IS&L opened their critique by noting that predictions based on inflation do not match up with the pattern of temperature variation observed in the cosmic microwave background. They added that primordial gravitational waves, another predicted consequence of inflation, have not been found.
As the public debate progressed, IS&L came to urge special attention to their final paragraphs. That section emphasized the consternation-causing dimension under the possibly insightful, but certainly inciteful, heading “Nonempirical Science?”:
Given the issues with inflation and the possibilities of bouncing cosmologies, one would expect a lively debate among scientists today focused on how to distinguish between these theories through observations. Still, there is a hitch: 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.
Some scientists accept that inflation is untestable but refuse to abandon it. They have proposed that, instead, science must change by discarding one of its defining properties: empirical testability. This notion has triggered a roller coaster of discussions about the nature of science and its possible redefinition, promoting the idea of some kind of nonempirical science.
It’s probably futile to try to sample judiciously from the list of 33 high-stature scientists incited to rebut, but here’s one stab at doing so anyway: Stephen Hawking, Lisa Randall of Harvard, Martin Rees of Cambridge University and the Royal Society, LIGO cofounder Rainer Weiss, and physics Nobel laureates John Mather, George Smoot, Steven Weinberg, and Frank Wilczek. Their rebuttal ‘s opening paragraph moved directly to the empiricism issue:
[IS&L] close by making the extraordinary claim that inflationary cosmology “cannot be evaluated using the scientific method” and go on to assert that some scientists who accept inflation have proposed “discarding one of [science’s] defining properties: empirical testability,” thereby “promoting the idea of some kind of nonempirical science.” We have no idea what scientists they are referring to.
The 33 declared that “inflation is not only testable, but it has been subjected to a significant number of tests and so far has passed every one.” They framed that statement with the rhetorical appeal to authority: “According to the high-energy physics database INSPIRE, there are now more than 14,000 papers in the scientific literature, written by over 9,000 distinct scientists, that use the word ‘inflation’ or ‘inflationary’ in their titles or abstracts. By claiming that inflationary cosmology lies outside the scientific method, IS&L are dismissing the research of not only all the authors of this letter but also that of a substantial contingent of the scientific community.”
That theme of inflation theory’s dominance dominates the rebuttal’s ending:
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!
Appended to the rebuttal in Scientific American are reply comments from IS&L. The three critics charged that the rebuttal “misses our key point”— namely, “that we should be talking about the contemporary version of inflation, warts and all, not some defunct relic.” IS&L asserted that the “claim that inflation has been confirmed refers to the outdated theory before we understood its fundamental problems.” They elaborated further in yet another reply that they posted elsewhere.
An article at Undark dwelled on heated personal language heard among the debaters. In a posting at Scientific American, technopundit John Horgan used strong language in siding with IS&L. He wrote: “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.”
But in the end, science in any case has a reputation for deliberateness and patience, as prescribed for this debate by physics professors and Forbes.com columnists Ethan Siegel and Chad Orzel and by the physics-trained Ryan F. Mandelbaum at Gizmodo. Orzel advises neutrality, surmising, “Experimental evidence for or against inflation will come along eventually.” Mandelbaum excerpts part of this passage from a blog posting by Caltech’s Sean Carroll, one of the 33 rebuttal authors:
Inflation is indubitably science. It is investigated by scientists, used to make scientific predictions, and plays a potentially important explanatory role in our understanding of the early universe. The multiverse [a potential consequence of inflation] is potentially testable in its own right, but even if it weren’t that wouldn’t affect the status of inflation as a scientific theory. We judge theories by what predictions they make that we can test, not the ones they make that can’t be tested. It’s absolutely true that there are important unanswered questions facing the inflationary paradigm. But the right response in that situation is to either work on trying to answer them, or switch to working on something else (which is a perfectly respectable option). It’s not to claim that the questions are in principle unanswerable, and therefore the field has dropped out of the realm of science.
Three years ago, a Soapbox Science posting at Nature engaged the science-outreach views of astrophysicist and science popularizer Neil deGrasse Tyson. Nothing like this public debate came up, but an apt phrase to describe it did: the “full mainstreaming of science.”
Steven T. Corneliussen is Physics Today‘s media analyst. He has published op-eds in the Washington Post and other newspapers, has written for NASA’s history program, and was a science writer at a particle-accelerator laboratory.