Thirty-three of the world's most respected scientists, including renowned physicist Stephen Hawking, have signed an open letter responding to a controversial article that branded popular views on the origins of the universe unscientific.

The letter was published in response to an article in the February issue of the magazine Scientific American, in which three physicists criticized the popular inflation theory.

The idea is that the universe started expanding exponentially after the Big Bang, with quantum fluctuations translating into stars and galaxies. First proposed in the 1980s, it is now taught as standard in most schools and universities, and is being explored through several related competing models.

"Pop Goes The Universe," written by Princeton University's Paul Steinhardt and Anna Ijjas, and Harvard University's Abraham Loeb, argues that recent research into cosmic microwave background - radiation left over from the time of the Big Bang - does not support the theory of a rapid expansion. Instead, it posits an alternate theory, the "big bounce," in which the Big Bang was not the beginning of the universe, but rather, "a transition from some preceding cosmological phase to the present expanding phase."

"The data suggest cosmologists should reassess this favored paradigm and consider new ideas about how the universe began," the article's In Brief summary reads.

The problem this article had within the scientific community was not in challenging the inflation theory per se, but the claim that in certain aspects it is untestable.

"Inflationary cosmology, as we currently understand it, cannot be evaluated using the scientific method," Steinhardt, Ijjas, and Loeb state at the end of the article.

This bold statement, which basically brands the theory speculative, is what prompted physicists Alan Guth and Andrei Linde, two of the pioneers of inflation theory, David Kaiser from MIT, and Yasunori Nomura from Berkeley, to write a response, which was also published in Scientific American.

"They 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,'" their letter reads.

"We have no idea what scientists they are referring to. We disagree with a number of statements in their article, but in this letter, we will focus on our categorical disagreement with these statements about the testability of inflation."

Guth and Linde are former colleagues of Steinhardt, with whom they shared the prestigious Dirac prize in 2002 for their work in developing the concept of inflation in cosmology. Their letter is co-signed by 29 other scientists, including four Nobel Prize winners and Stephen Hawking.

"We were particularly in strong disagreement with the statements they made about the testability of inflation which we thought were completely without justification," Guth told the site Gizmodo. "We thought it was about time someone answered those objections."

The physicists countered that inflation theory was able to make several predictions that have been proven correct, such as the average mass density of the universe. The writers also acknowledge that inflation is far from being proven as truth, and some of its models have been discarded as wrong, but say that this does not make it unscientific.

"Like any scientific theory, inflation need not address all conceivable questions. Inflationary models, like all scientific theories, rest on a set of assumptions," the letter reads. "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."

The three dissident scientists have stuck to their guns and have published an FAQ in response, in which they maintain that "what began in the 1980s as a theory that seemed to make definite predictions has become a theory that makes no definite predictions."

It seems this heated debate is far from over.