From: mock@space.mit.edu (Patrick C. Mock)
Subject: Re: grav lensing
Originator: mock@benz
Reply-To: mock@space.mit.edu (Patrick C. Mock)
Organization: MIT Center for Space Research
Date: Thu, 20 Aug 1992 17:11:22 GMT


In article <Bt9Gn9.HA@well.sf.ca.us>, metares@well.sf.ca.us (Tom Van Flandern) writes:
> 
>      I guess we are making some progress here.  Pat has persuaded me that a
> gravitational lens is more like a convex lens than a concave one.  And I
> infer that he is no longer questioning the principle that, for a symmetric
> lens, source, image, observer, and lens axis must be co-planar.  But these
> are side issues to the main questions in this thread:  Of the three classes
> of hypothetical gravitational lenses [multiple quasar images, radio rings,
> and ring arcs in galaxy clusters], which are true gravitational lenses, and
> which are due to some other cause?  I have presented multiple lines of
> evidence that the first two are "other", but Pat has argued that this
> evidence is not conclusive.  The following points still need discussion:

Yes, I agree that we're making progress.  I've never disagreed that
the s-l-o plane model was appropriate for a spherically symmetric
lens.  Nonetheless, a gravitational lens, formed by approximately
convex elliptical mass distribution, will behave approximately like a
convex elliptical lens as described by Blandford et al.  Most galaxies
are sufficiently non-spherical that the s-l-o plane model is not
valid.

In any case I think we are coming to some agreement on the physics,
which has been my main goal here.  The observations are much more
controversial.   As has been pointed out in other posts, recent
quasar counts indicate that there are ~3e6 quasars detectable on the
sky.  We've studied only a small fraction of those.  Until much
more data is taken and analyzed I expect a bit of disagreement.

Given the current state of the observations I expect that some
lens candidates will turn out to be chance alignments or something else.
I also expect that we will discover unusual lenses due to unusual
objects.  For example, I think a merging pair of galaxies would
generate an unusual lens.  Given the number of quasars, I think it
is likely that we will eventually find a quasar behind a merging
pair of galaxies.

Personally I consider only one multiple imaging candidate as a
confirmed lens.  Quasar 0957+561, Lehar et al ApJ 384 p453, has a
measured time delay of 540 +/- 12 days.  This time delay was looked
for because it was predicted by the theory.  So far no other time
delays have been measured.  I will point out that this is an
exceedingly difficult measurement that requires a lot of observing
time.

The radio rings I find believable.  The MIT radio survey, which has
turned up most of the rings, has a series of interesting unpublished
objects that are still being studied.  Until the results of the survey
are finally published it is difficult to argue the about the number
statistics of radio rings.  According to James Annis there are
172 objects out of a total of 632 that have not been published.

> 
> Earlier, I wrote:
>                          ... stuff deleted ...
>      Construct a 2-argument table, x vs. y.  Let x be the number of
> gravitational lens candidates discovered to date.  Let y be the percentage of
> x with a more-than-2-magnitude component brightness difference required to
> falsify the hypothesis that almost all such cases are gravitational lenses,
> rather than some other mechanism.  Assuming gravitational lens candidates
> will continue to be discovered at an increasing rate, a reasonable table
> should be able to falsify the hypothesis within a decade, perhaps far less,
> if some other mechanism really is dominant; and conversely.
> 
>      Can you construct such a table?  If not, who could?

I think such a table would be difficult to construct for the currently
available data.  Much of what has been discovered has not been found
in any sort of complete survey and thus the statistics are dominated
by poorly understood selection effects.  Also I think the measured
time delay provides important confirmation for one gravitational lens
candidate.  

> >>>> Wambsganss (ApJ 392, pp. 424-431, 1992) showed that microlensing cannot
> >>>> be due to individual stars, but must be due to crossing caustics of the
> >>>> lens.  The lensing model just keeps getting weirder.
>
>      ... stuff deleted ...
>      Instead of the abstract phrase, "stretching the theory", let me
> substitute, "introducing a new 'ex post facto' hypothesis which does not
> automatically follow from the original model, solely for the purpose of
> insuring continued agreement with observational constraints."  IMO, when this
> becomes necessary, it is a dead giveaway of a model which is fundamentally
> wrong.
> 
>      You may argue that this new, complicated variant of the microlensing
> theory does automatically follow from the original model.  But one cannot
> make specific, quantitative, statistical predictions of the rough amplitude
> and frequency of microvariability.  Instead, one derives lensing galaxy mass
> density parameters which make the hypothesis work, invoking dark matter where
> needed.  No a priori constraints are placed on plausible values for the
> derived parameters, even when M/L values come out quite large.  Using such an
> approach guarantees that even a totally wrong model will appear to be
> working.  -|Tom|-
> 

I do argue that Wambganss's results follow automatically from the
original theory.  Furthermore, I think, in principle, one could
generate some predictions from his work via some sort of monte carlo
simulations.  It would take a lot of work and computer time.  His work
shows for the first time that these effects are important.  I expect
that someone will take up the task.

Regarding dark matter, as I've said before, dark matter is not
necessarily non-baryonic matter, except for the dark matter used to get
omega = 1.  Also we disagree on what constitutes a large M/L value.  I
am especially leery of relying on M/L values because L is so often
based on a B magnitude.  Much of the optically observed universe tends
to be red because of "cool" temperatures and/or obscuration that is
stronger in B than R.  I don't think M/L is a stable enought measurement
to use to prove or disprove any model unless the numbers get really extreme.

Pat

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