What is the true purpose of inflation in the early universe?

[julcab12]

I'm no statistician (though I can use some basics). But I suspect that the above is not quite correct. Bayesian inferences are only promoted to probabilities that all agree on if the bets can be tested (HMM modeled).

Science use frequentist and bayesian probability methods of course, but mainly they use testing (as I described re the measurement theory that underlies it all), so likelihoods:

[ http://www-library.desy.de/preparch/books/vstatmp_engl.pdf ]

... In cosmology, Experimental means are quite limited. We cannot directly create, access,manipulate and infer a scenario with galactic bodies to extrapolate a better testing of the mechanics unlike local scientific methodology. Furthermore, regions in the universe are practically inaccessible by observation. So the only reliable approached is treat the dynamics--theory as an idealized and abstract mathematical model and materialize it by fitting the data using method of analysis and statistical techniques or approximation by prior assumption(like the cosmological principle) in the hope that it will be consistent all throughout. Or whether it can be tested through observations.

My point is, due to the limits of testing. We need an alternative reliable method in the Bayesian and statistical regime/spirit and clues in High energy physics to create an approximate judgement.

 

[Chronos]

I'm not aware of any statistical method that is not axiomatic [i.e., does not rely on prior assumptions].

 

[Torbjorn_L]

I'm not aware of any statistical method that is not axiomatic [i.e., does not rely on prior assumptions].

That method would be hypothesis testing. I'm not even sure if it is axiomatized after the fact, since people still argue about the old compromise that Fisher et al did when putting it on more solid ground.

 

[CKH]

Torbjorn_L,

The reason I call inflation speculative is that the hypotheses (which themselves are loosely defined since there are so many versions) are untested even though they may give the desired result. The level of validity is that a hypothetical explanation works, but the assumptions are untested. This is just the first step in forming a theory. My understanding of Bayesian reasoning is very weak, but it's hard to see how you can assign any measure of certainty at this stage.

 

[bapowell]

The reason I call inflation speculative is that the hypotheses (which themselves are loosely defined since there are so many versions) are untested even though they may give the desired result. The level of validity is that a hypothetical explanation works, but the assumptions are untested.

What are the assumptions that you think are untested?

My understanding of Bayesian reasoning is very weak, but it's hard to see how you can assign any measure of certainty at this stage.

Bayesian reasoning enables one to form a probability distribution over the model space of inflation. Given the data, we select the model that maximizes this probability distribution. What is missing "at this stage"? What else do you think is needed?

 

[CKH]

Many models can predict the same things. E.g. to use an extreme, the initial parameters may have been perfectly fine-tuned in the first place, perhaps due to some other mechanism. It could well be the whim of creation, if there is such a thing. How can you limit the possibilities of creation? (Without cause, it is outside the realm of physics.) Possibly some undiscovered self-balancing action prevents deviation from flatness. Maybe the universe never had the ultra-high density and inhomogeneity assumed, so there is no need for inflation. The need arises from an extrapolation of expansion backwards far beyond the point at which we have any direct evidence of expansion (more than 60 orders of magnitude beyond).

Inflation is one of a variety of models one can conjecture to explain things, limited only by imagination and getting the desired end result. Clearly some people think that this particular extension of physics (to the extent that it is a particular extension) is the right one.

The assumptions I'm aware of are: some as yet unknown field arose at about 10^-36 seconds post singularity, acted for a very short time and blew up the universe at an exponential rate by a factor of 10^60. This scalar(?) field arose and decayed in just the right manner to flatten out (almost?) whatever wrinkles the universe may have had at the point where known physics surely breaks down.

Bayesian reasoning might help if you knew a priori 1) something special must have happened to flatten the universe and 2) that you know what all possible models are and 3) you can somehow suggest which models are more probable.

Of course what you actually referred to was the "model space of inflation", a much narrower range of theories. In that space maybe there is some particular model or models that are preferred, but none has been settled on as correct.

If you think that we are forced to these conclusions (that in general an inflation model is required) by what we know, without untested assumptions, well that's not my understanding of the situation, but I could be wrong.

To be honest, now that I have a better idea of what motivates inflation, whether it is true or not seems to make little difference unless it predicts something new. It would be important if it changes the way we interpret observations of the universe post-CMB. I really don't understand what makes this theory so dear to cosmologist, but apparently they are quite worried about something(s) in BBT to bother proposing it in the first place.

In laboratory science, you don't take theories proposing new physics seriously until experiments demonstrate them (oops, I forgot supersymmetry and string theory). Cosmology is a whole nother ball game because all we have are passive observations.

Until some controlled experiments were done, the theory of spontaneous generation was thought correct.

 

[Drakkith]

Closed, pending moderation.