Cosmologic philosophy: inflation or not? (Introduction)

by David Turell , Sunday, March 06, 2022, 19:36 (993 days ago) @ David Turell

Hassenpfeffer comments:

http://backreaction.blogspot.com/2022/03/did-early-universe-inflate.html

"In the popular science media, inflation is sometimes presented as if it was established fact. It isn’t. Its status is similar to that of particle dark matter. They are both unconfirmed hypotheses. But while most physicists agree that particle dark matter has yet to be empirically confirmed, opinions about inflation are extremely polarized.

"On the one hand you have people like Alan Guth, one of the inventors of inflation theory, arguing that the theory has made many correct predictions and that evidence speaks for it. On the other hand, you have people like Paul Steinhardt, interestingly enough also one of the inventors of inflation, who argue that inflation doesn’t make any predictions and isn’t even science. In an essay some years ago, Steinhardt together with Anna Ijjas and Avi Loeb wrote “inflationary cosmology, as we currently understand it, cannot be evaluated using the scientific method.”

"Which side is right? They’re both right and they’re both wrong. Stay with me for some minutes and I hope it’ll start making sense.

"Guth, and most of his colleagues really, argue that physicists have used models of inflation to make predictions which were later confirmed, such as some properties of the large scale structure and the cosmic microwave background, notably the scalar spectral index which is somewhat smaller than one, and that space is on average flat, to good precision. This agrees with observation and they think this is evidence in favor of inflation. Steinhardt’s side holds against this that inflation models really predicted anything so that *those predictions which turned out to fit to observations can’t speak in favor of inflation.

"On that count, Steinhardt’s people are clearly correct. Just because someone made a correct prediction doesn’t mean they have a good scientific theory.

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"What Steinhardt’s people are arguing in an nutshell is that inflation is such a flexible hypothesis that it can be made to fit any data.

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"Steinhardt and his people now argue, that regardless of what we observe, you can always fumble together an inflationary model that would fit to the observations. Therefore, the idea has no predictive power.

"Guth and his side have two answers to this which actually contradict each other. First, you often hear them claim that inflation has made unambiguous predictions, as I said earlier, that the spectral index is somewhat smaller than one and that the curvature density of space is small today, indeed so small that at present it’s consistent with zero.

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"It is correct of course that for a theory to be testable not all its predictions have to be independent of the parameters. But it does require that you predict more data points than you have parameters. A scientific theory requires that you get more out than you put in, otherwise you don’t explain anything, you’re overfitting data.

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"The most impressive data which simple inflationary models explain is a peculiar correlation in the cosmic microwave background, that between the temperature and the E modes, called the ET correlation. Doesn’t really matter if you don’t exactly know what this is, the point is it’s something which has been observed, and it a non-trivial correlation in the data which you can calculate from bunch of simple inflationary models. These models are good explanations for observations.

***

"So to summarize. Guth is right in saying that inflation is good science. But he is wrong with the reason for why that’s the case. Steinhardt is right with pointing out that Guth’s argument doesn’t hold up. But his conclusion is wrong because there are other reasons for why inflation is good science.

"However, that doesn’t mean inflation is right. Physicists have proposed many other theories for the early universe, for example cyclic cosmology, and those can also explain observations. And maybe in the end one of those other theories will be the better explanation."

Comment: so her point is inflation is pure theory, not fact. And for that matter the Big Bang is also pure theory, not fact. But the BB has religious and philosophic implications. See the next entry