Hawking and his colleagues firmly believe in the widely-accepted theory of inflation, which describes how the universe rapidly expanded following the Big Bang. But three physicists, Anna Ijjas, Paul J. Steinhardt, Abraham Loeb, argued in the article that another theory, the "Big Bounce," was more likely an explanation for the universe's origins.
Hawking and his peers did not entertain the idea.
"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 wrote to Scientific American.
"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," the letter added.
Ijjas, Steinhardt and Loeb were disappointed with the scientists' response.
"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 responded.