Re: Galaxies without dark matter halos?

greywolf42 wrote:

> Ralph Hartley <[EMAIL PROTECTED]> wrote:

>>Huh??? The (Bayesian) probability of the big bang is very close to 1.
>>Depending on how you rate the evidence for inflation the relative
>>probabilities with and without inflation will vary, but still sum to near
>>1 (they are mutually exclusive by construction).
> 
> The Bayesian probability is whatever you set the prior.

Modified by the evidence you have seen so far. If you pick a good prior, 
the evidence will eventually win over your prejudices.

> Since you have
> defined your 'prior' as including only variants of the Big Bang.  (You
> didn't mention any non-Big Bang theories that you addressed -- thus setting
> the prior for all big-bang variants to 1.0.)

No. I'm saying that the *posteriori* probabilities are that way. I haven't 
seen any Non-BB theories that aren't contradicted by evidence. None have 
enough a-posteriori probability to be worth bothering with.

> Well, we've seen galaxies that have no hint of dark matter.  This would
> require yet another ad hoc revision to the model -- allowing one to randomly
> distribute 'dark matter' wherever one finds a discrepant observation -- and
> withhold dark matter where everything looks fine.

Non Big Bang theories (if there were any worth mentioning) would have to do 
the same thing, so it's a wash. If you don't believe in dark matter, you 
still need to explain why some galaxies behave *as* *if* they had dark 
matter and some do not. I don't see how eliminating the Big Bang helps you 
do that.

> I'm talking about non-Bayesian statistics.

I don't see what "the non-Bayesian probability of the Big Bang" even 
*means*! The Big Bang ether happened or it didn't. I can express my 
knowledge as a probability (basically what odds I would consider a fair 
bet), that's what Bayesian probability means. What do *you* mean by "the 
probability of the big bang"?

> Hence -- since it is 'open' -- we have no way to set the probability.
> Therefore, since one cannot define a real (non-Bayesian) probability for the
> (qualitatively) 'most likely' BB theory -- we cannot define a real
> (non-Bayesian) probability of "near certainty" (arbitrarily close to 1.0 in
> a NON-bayesian sense) to the existence of the Big Bang explosion.

NON-Bayesian probability doesn't apply *at* *all* to unique events, so why 
would I want to do such a thing?

>>Nor can evidence for or
>>against different theories for how the earth formed be taken as evidence
>>for or against the "round earth" theory.
> 
> It depends on which 'round Earth' theory you are championing.  There are
> many.  (Expanding Earth, plate tectonics, immoble Earth, shrinking
> crust....)  Some 'Earth formation' theories even include a non-round Earth.

I generally believe in the "earth is pretty much round" theory, which 
includes all of those.

I will bet you a dollar that, when all is said and done, it will turn out 
that the earth is more or less round. I will give you 100:1 odds (i.e. my 
odds for a round earth are more than 0.99).

I was not born with this belief, but having seen lots of evidence for it, 
hold it fairly strongly, and would only be convinced otherwise by sailing 
over the edge. Show me the edge, and I'll give you $100, but forgive me for 
asking to hold your $1 in the meantime.

>>Similarly, the Big Bang describes the rough shape of the universe. Neither
>>arguments about its exact shape (e.g. the value of Omega), nor about its
>>origin (inflation or not) make any difference.
> 
> Of course they make a difference to the overall probability!

How? Why? And if you reject Bayesian probability what do you even *mean*.

> The problem is
> that you are starting with a Bayesian prior that the probability of the Big
> Bang is arbitrarily close to 1.0.

No. I had to be convinced. I was.

Ralph Hartley