33 Of The World's Top Physicists Penned A Letter About The Origin Of The Universe

Scientists are no strangers to healthy debate. You need criticism to strengthen your ideas, and when debate is done right, both parties leave knowing more than they did when they started. But there are some things that will just make a scientist mad. One of those things? Saying their scientific theory isn't scientific. That's what a trio of physicists did in a 2017 article they published in Scientific American which stated that the idea of an expanding universe simply isn't testable. The response from other physicists? Oh, it's on.

Diagram of the inflation model of the universe.

Oh No They Didn't

The inflation model of the universe says that all matter began as a tiny quantum speck that exploded, expanded, and grew into the planets, stars, and galaxies of the universe we know today—a universe that's still expanding. In a February 2017 Scientific American article entitled "Pop Goes The Universe," Princeton cosmologist Paul Steinhardt, Princeton physicist Anna Ijjas, and Harvard astronomer Avi Loeb began their takedown of that idea by recalling the 2013 press conference the European Space Agency held to announce that the Planck satellite had mapped the cosmic microwave background (CMB) radiation. "The principal message of the press conference was that the Planck data perfectly fit the predictions of the simplest inflationary models," they wrote, "reinforcing the impression that the theory is firmly established. The book on cosmology seemed to be closed, the team suggested."

The trio begged to differ with the conclusion of that press conference, and penned a lengthy article pointing out the limitations and problems with current models of inflation. Among them are that the patterns seen in the CMB aren't only explained by inflation, and the idea of the "multiverse" (or "multimess" as they called it) means that inflation just predicts every possible outcome instead of the physics of our own universe. Oh, and that "inflationary cosmology, as we currently understand it, cannot be evaluated using the scientific method." In other words, it's not science.

Artist's conception of what's known as the cosmic distance ladder, a series of stars and other objects within galaxies that have known distances.

"Those Words Angered Me."

"They really made the accusation that the inflationary community understands that the theory is not testable," MIT's Alan Guth, an original architect of inflation, told The Atlantic. "Those words angered me." In response, Guth and inflation pioneer Andrei Linde of Stanford (two of Paul Steinhardt's former colleagues, incidentally) along with cosmologists David Kaiser and Yasunori Nomura penned an open letter in Scientific American. But writing the letter wasn't enough. They also circulated it among the world's top physicists, garnering signatures from 29 other experts in the field that included Stephen Hawking, four Nobel Prize winners, a Fields Medal winner, and the head researchers on some of the most groundbreaking experiments into inflation theory.

In the letter, the authors explain that the original trio was essentially bashing a straw man. "Inflation is not a unique theory but rather a class of models based on similar principles," they wrote. "Of course, nobody believes that all these models are correct, so the relevant question is whether there exists at least one model of inflation that [...] correctly describes the measurable properties of our universe." They compared it to the Standard Model of particle physics, which started with a bunch of different models but eventually landed on one that fit every experiment.

Speaking of experiments, the authors ran down a number of them that successfully predicted the model of inflation—then dropped this bomb: "So if inflation is untestable, as IS&L would have us believe, why have there been so many tests of it and with such remarkable success?" Burn.

In the end, they make it clear that science's adherence to inflation isn't blind faith; it's a long, slow period of discovery that we're still in the midst of. "No one claims that inflation has become certain; scientific theories don't get proved the way mathematical theorems do," they write, "but as time passes, the successful ones become better and better established by improved experimental tests and theoretical advances. This has happened with inflation." The authors of the original article have since published their own response to the response.

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