Is the inflationary universe a scientific theory?

[mitosis]

This is the provocative question posed by Sabine Hossenfelder in her article in Forbes, covering the ongoing debate of inflation critics like Paul Steinhardt (once one of the theory founders) and scientists who develop models of inflation. Link to her article: https://www.forbes.com/sites/starts...universe-a-scientific-theory-not-anymore/amp/

 

[Andrew Mason]

A very interesting article. I have to be careful because I got formally reprimanded for suggesting something similar a few years ago. It is not that cosmology is not a legitimate and fascinating subject of scientific inquiry. It is. The problem, as the article states, is that cosmology does not lend itself well to the scientific method. It is not that that theories, such as inflation, are not capable of being disproved or tested. It is the extreme difficulty and long time spans needed to acquire evidence to test them. So various models and theories abound.

AM

 

[George Jones]

Scientific theories can be tested but not proved i.e., the best we can do is is to have experimental observations consistent with theoretical predictions. An observation test of inflation would be the discovery (or not) of B-mode polarization of the Cosmic Microwave Background that is not caused by intervening dust.

https://www.cfa.harvard.edu/~cbischoff/cmb/

BICEP found B-mode polarization, but could not determine that it was not caused by dust. Scientists hope to do this soon.

https://www.nasa.gov/feature/goddar...on-seeking-evidence-of-cosmological-inflation

This is a good example of cutting-edge scientific research.

This is the provocative question posed by Sabine Hossenfelder in her article in Forbes, covering the ongoing debate of inflation critics like Paul Steinhardt (once one of the theory founders) and scientists who develop models of inflation. Link to her article: https://www.forbes.com/sites/starts...universe-a-scientific-theory-not-anymore/amp/

The original motivations for inflation are no longer the main motivation for inflation.

The most exciting aspect of the inflationary cosmological theories described in chapter 4 is that they provide a natural quantum mechanical mechanism for the origin of the cosmological fluctuations observed in the cosmic microwave background and in the large scale structure of matter, and that may in the future be observed in gravitational waves.

In the modern view, by far the most important function of inflation is to generate the primordial curvature perturbation ... It may generate other primordial perturbations too, including the isocurvature and tensor perturbations ... However, the historical motivation for inflation was rather different, and arose largely on more philosophical grounds concerning the question of whether the initial conditions required for the unperturbed Big Bang seem likely or not.

Originally inflationary scenarios were suggested as a pseudo-solution to certain pseudo-problems; these are only of historical interest today and the only reason to take the possibility of an inflationary phase in the early universe seriously is because it provides a mechanism for generation the initial perturbations.

It is not that cosmology is not a legitimate and fascinating subject of scientific inquiry. It is. The problem, as the article states, is that cosmology does not lend itself well to the scientific method.

Steven Weinberg strongly disagrees.

The new excitement in cosmology came as if on cue for elementary particle physicists. By the 1980s the Standard Model of elementary particles and fields had become well established. Although significant theoretical and experimental work continued, there was now little contact between experiment and new theoretical ideas, and without this contact, particle physics lost much of its liveliness. Cosmology now offered the excitement that particle physicists had experienced in the 1960s and 1970s.

 

[zonde]

An observation test of inflation would be the discovery (or not) of B-mode polarization of the Cosmic Microwave Background that is not caused by intervening dust.

It does not seem such a clear case. There is a link to Inflation supporters article in Hossenfelder's article: https://blogs.scientificamerican.com/observations/a-cosmic-controversy/
They (Inflation supporters) say:
Fourth and finally, the models also make predictions for the patterns of polarization in the CMB, which can be divided into two classes, called E-modes and B-modes. The predictions for the E-modes are very similar for all standard inflationary models, whereas the levels of B-modes, which are a measure of gravitational radiation in the early universe, vary significantly within the class of standard models.

So it seems that Hossenfelder's criticism is quite relevant here:
The scientific approach to the situation would be to choose a model, determine the parameters that best fit observations, and then revise the model as necessary – i.e., as new data comes in. But that’s not what cosmologists presently do. Instead, they have produced so many variants of models that they can now “predict” pretty much anything that might be measured in the foreseeable future.
It is this abundance of useless models that gives rise to the criticism that inflation is not a scientific theory.

It reminds of Texas sharpshooter fallacy