Re: Galaxies without dark matter halos?

Ralph Hartley <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> 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.

The key word being "eventually."  Here there is only one "data" point -- the
prior.  Which is why Bayesian statistics are invalid here.

> > 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.

Regardless of the probabilities that you set on the theories that *you* have
seen, they do exist.  By simply dismissing them without thought, you assume
the BB is correct.  A priori, the 'correct' theory of the cosmos may not
have yet been found. Yet it's probability must be allowed for.

If you want, you can 'rate' the probability of one known theory against
another known theory.  But you can't claim the BB is a 'near certainty' --
unless you assume it by attempting to apply a Bayesian approach.

> > 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.

Non-big bang theories do not require 'dark matter.'  This ever-shifting dark
matter is a requirement only for big-bang theories.

> 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.

The primary difference for the BB is the difference between non-baryonic
'dark matter' (required by the BB to keep the light elements 'cooked' right)
and just as-yet-unseen-because-it's-not-highly-luminous normal matter
(sometimes confusingly called dark matter) that would be used by non-BB
theories.

I consider (not 'believe') dark matter to be a phantom.  It is an ad hoc
speculation made simply to 'rescue' a popular theory from observation.

> > 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"?

Care to play some 3-card monte?  I'll give you the Bayesian odds.  You
*think* you know the odds (at worst 1/3, better if your eye is quick) --
because your theory doesn't (yet) include reality.  So you set up your
Bayesian prior, and happily wait for the payoff.  Somehow, you ALWAYS lose.
Because you didn't have the correct theory ready to hand.  You had *a*
theory.  Not *all* theories.

That is the difference between reality and Bayesian priors.

> > 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?

We are applying probability to cosmological *theories* -- not to a
theoretical event contained within only SOME of those theories.  If I use
the other theories, then the Bayesian probability of the BB event is zero.
(The BB event is not unique if it never occurred.)

Again, we can compare any two theories by relative probability (this does
not require Bayesian probability).  And we can arbitrarily select the 'most
probable' theory and call it a '1.0' probability.  But these probabilities
are all *relative* and will always be relative.  One cannot invoke a
Bayesian prior (which is an arbitrary choice) and then use this arbitrary
choice to claim an event within the theories is a 'near certainty.'

> >>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.

It obviously doesn't include the 'non-round' Earth.  :)

'Pretty much round' is not a theory.  I see no cause-and-effect, which is
required for a scientific theory.  It's not even quantified.

> 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).

Do I get to define 'more or less round?'  Without quantification on your
part, you've set up a sucker bet.  (I really would like to deal for some
three card monte.)

> I was not born with this belief, but having seen lots of evidence for it,
> holdit 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.

There are 'non-round' solutions without edges.

> >>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*.

If the BB describes the shape of the universe, then observations of the
shape of the universe (the value of omega) are the fundamental means for
checking the theory.  Therefore, they make a difference.

> > 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.

As described above, to be 'convinced' you used a Bayesian prior of 1.0.  You
are not allowing for your (or our) ignorance.

greywolf42
ubi dubium ibi libertas