For centuries, people have puzzled over how our Universe began. But the heat just got turned way up on a debate that's quietly been raging between cosmologists, with 33 of the world's most famous physicists publishing a letter angrily defending one of the leading hypotheses we have for the origin of the Universe.

The letter is in response to a Scientific American feature published back in February, in which three physicists heavily criticised inflation theory - the idea that the Universe expanded just like a balloon shortly after the Big Bang. The article went as far as claiming that the model "cannot be evaluated using the scientific method" - the academic equivalent of saying it isn't even real science.

In response, 33 of the world's top physicists, including Stephen Hawking, Lisa Randall, and Leonard Susskind, have fired back by publishing their own open letter in Scientific American. The Cliff's note version is this: they're really angry.

Inflation theory was first proposed by cosmologist Alan Guth, now at MIT, back in 1980. It's based on the idea that a fraction of a second after the Big Bang, the Universe expanded rapidly, spinning entire galaxies out of quantum fluctuations.

"By the time it slowed down, what had been a tiny, quivering quantum realm was stretched out until it looked smooth and flat, save for speckles of denser matter that later became galaxies, stars, and planets," writes Joshua Sokol for The Atlantic.

In the following years, Guth's original idea was improved and updated by Stanford physicists Andrei Linde, and they've since spent their careers refining the inflation model - which has become the leading theory for how the Universe was born.

In fact, most of us were taught inflation theory at high school and university when discussing the Universe's origins.

Guth and Linde, along with cosmologists David Kaiser and Yasunori Nomura, were the ones who recruited the other 29 signees behind this week's letter.

Interestingly, one of Guth and Linde's former colleagues, physicist Paul Steinhardt, is part of the trio they're rallying against. Guth, Linde, and Steinhardt all shared the prestigious Dirac prize "for development of the concept of inflation in cosmology" back in 2002.

But in the years since, Steinhardt has gone rogue, and has become an active critic of inflationary theory. He was one of the authors of Scientific American's February feature, originally titled "Pop goes the Universe", along with Princeton physicist Anna Ijjas, and Harvard astronomer Abraham Loeb.

Scientific American

That article highlighted recent research into the cosmic microwave background, which doesn't match up with the predictions of inflationary theory.

It also criticised the fact that inflation would have generated primordial gravitational waves, which have never been found.

"The data suggest cosmologists should reassess this favoured paradigm and consider new ideas about how the universe began," summarises an In Brief wrap up of the article.

That criticism in itself wasn't a huge deal - these kinds of arguments are healthy in the science world.

But what really pissed off Guth, Linde, and the 31 other signees, was the suggestion that inflationary theory couldn't actually be tested in the first place, and therefore wasn't really science.

"They [made] 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'," the physicists write in their open letter.

"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."

Their argument is that inflation theory is based on many models, and there's no illusion that all of these models are correct. Over the past 37 years, some of the models have made correct, testable predictions - including the average mass density of the Universe, and its flat shape. Many are still unresolved.

But either way, these models are all testable, which means they're proper science, and they can be proven or disproven depending on the evidence we find in the coming years.

Ryan F. Mandelbaum has done incredible coverage of this feud over at Gizmodo, and points to a blog entry by Sean Carroll, one of the physicists who signed the letter, on the controversy:

"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."

The authors of the original article have since responded to the letter with their own extended FAQ on the debate. And they maintain their position - that inflation was once testable, but "what began in the 1980s as a theory that seemed to make definite predictions has become a theory that makes no definite predictions".

Which takes us right back to where we started... some cosmologists have publicly slammed inflation theory, and others have angrily responded.

Unfortunately there's no neat resolution to this debate on the horizon, with both sides standing pretty firm. The one thing they both agree on is the fact that inflation theory isn't perfect, and we should all keep an open mind about what really happened at the birth of our Universe as new data comes in.

Or as Guth told Mandelbaum in the ultimate mic drop when asked what would happen next: "I think we'll all continue on with our research."

You can read the original article here, the responding open letter here, and the original authors' response here.