Cosmic inflation's biggest criticisms can't undermine its success

"In the 1920s, we first measured the distances to objects beyond our own Milky Way, and swiftly discovered that the Universe was expanding: consistent with how Einstein's General Relativity tells us the Universe would evolve. If the Universe was expanding today, that implies it was smaller - and hotter, and denser, and more uniform - in the past, leading to the idea of the hot Big Bang."

"In the early 1980s, a revolutionary new theory was proposed, intending to solve those puzzles: cosmic inflation. Theorists rushed to work on inflation, figuring out which "flavors" of inflation successfully led to a hot Big Bang and then making a series of predictions that could be compared and tested against the differing predictions of the Big Bang without inflation. Over the 80s, 90s, and 00s, those predictions were tested and confirmed, with inflation passing every observational test thus far."

"However, inflation still has its critics, and its major criticisms fall into two categories: the measure problem, which concerns the difficulty of quantifying the likelihood of inflation describing our Universe, and the fact that "inflation could have predicted literally anything," which argues that inflation lacks generic predictions and that any observation could be considered "success" for inflation."

In the 1920s, measurements of galaxies revealed cosmic expansion, implying a hotter, denser, and more uniform past and leading to the hot Big Bang. By the 1960s, key Big Bang predictions were confirmed, yet puzzles about singular origins and arbitrarily high initial temperatures persisted. In the early 1980s, cosmic inflation was proposed to solve those problems. Theorists developed many inflationary models and derived specific, testable predictions that distinguish inflation from non-inflationary scenarios. From the 1980s through the 2000s, observations tested those predictions and found agreement, with inflation passing available empirical tests. Major criticisms focus on the measure problem and claims of excessive flexibility.

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