Stephen Hawking and 32 of his fellow scientists have written an angry letter responding to a recent Scientific American article about how the universe began. In it, they declare their “categorical disagreement” with several of the statements made, and explain why the theory of inflation is still one of the best models for the origin of the cosmos.
The article in question was published in February. Titled “Pop Goes the Universe,” physicists Anna Ijjas, Paul J. Steinhardt, Abraham Loeb examine the latest measurements from the European Space Agency relating to cosmic microwave background (CMB).
CMB is the oldest light in the universe—light emitted just after the Big Bang around 13.7 billion years ago. In 2013, a map of the CMB appeared to show how the universe inflated extremely fast, before settling down to become the universe we see today. This, many experts said, backed up models relating to inflation theories, where the universe expanded exponentially fast a fraction of a second after the Big Bang.
However, Ijjas, Steinhardt and Loeb disagreed with this interpretation. “If anything, the Planck data disfavored the simplest inflation models and exacerbated long-standing foundational problems with the theory, providing new reasons to consider competing ideas about the origin and evolution of the universe,” they write.
The three physicists argue that since the 2013 map was produced, more precise data has been gathered. And this data, they say, adds more evidence to the argument that the Big Bang and inflation do not adequately explain how the universe started. “Yet even now the cosmology community has not taken a cold, honest look at the big bang inflationary theory or paid significant attention to critics who question whether inflation happened,” they say.
Instead, they claim the idea of a “big bounce” is a more likely scenario. In this theory, the universe works on a cyclical basis of expansion and contraction. At the moment, it is expanding. However, when it runs out of energy (or whatever happens to stop its expansion), it will start contracting. Eventually, it will get to the point where it is so small it starts expanding again.
They point to several flaws in inflation theory, including that we are yet to discover primordial gravitational waves—ripples in spacetime created by the Big Bang. Another problem is that inflation requires the existence of “inflationary energy,” for which there is no direct evidence.
“Given all these problems, the prospect that inflation did not occur deserves serious consideration,” they write. “Today we are fortunate to have sharp, fundamental questions imposed on us by observations. The fact that our leading ideas have not worked out is a historic opportunity for a theoretical breakthrough. Instead of closing the book on the early universe, we should recognize that cosmology is wide open.”
Responding to the article, 33 scientists, including Hawking, have written a letter of response to Scientific American in which they dismantle the arguments made by Ijjas, Steinhardt and Loeb.
In the letter, they say there is “no disputing” the fact that inflation is the dominant theory when it comes to cosmology. They point out that there are over 14,000 scientific papers by over 9,000 scientists relating to inflation: “By claiming that inflationary cosmology lies outside the scientific method, IS&L [the authors of the earlier article] are dismissing the research of not only all the authors of this letter but also that of a substantial contingent of the scientific community,” they write. “Moreover, as the work of several major, international collaborations has made clear, inflation is not only testable, but it has been subjected to a significant number of tests and so far has passed every one.”
They say there are many models of inflation and no one believes they are all correct. Instead, the theory is something of a work in progress, where scientists are working to find one that fits all the experiments and observations.
The scientists refer to a multitude of reasons why inflation is, at present, the best model for the origin of the universe. This includes there being testable models—including the observations from the 2013 CMB data.
“Like any scientific theory, inflation need not address all conceivable questions. Inflationary models, like all scientific theories, rest on a set of assumptions, and to understand those assumptions we might need to appeal to some deeper theory,” they say. “This, however, does not undermine the success of inflationary models.”
“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.”
Ijjas, Steinhardt and Loeb responded to the letter with disappointment. They say they have “great respect for the scientists” who signed, but say they missed the key point of the article—how inflation theory has changed over time.
“We firmly believe that in a healthy scientific community, respectful disagreement is possible and hence reject the suggestion that by pointing out problems, we are discarding the work of all of those who developed the theory of inflation and enabled precise measurements of the universe,” they write.
Their main point, they said, is that “we should be talking about the contemporary version of inflation, warts and all, not some defunct relic.” They argue that even when different parameters are taken into account, there are an infinite number of outcomes relating to any model of inflation.
“Our article was not intended to revisit old debates but to discuss the implications of recent observations and to point out unresolved issues that present opportunities for a new generation of young cosmologists to make a lasting impact. We hope readers will go back and review our article’s concluding paragraphs. We advocated against invoking authority and for open recognition of the shortcomings of current concepts, a reinvigorated effort to resolve these problems and an open-minded exploration of diverse ideas that avoid them altogether. We stand by these principles.”
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