ohwilleke said:
The filp side, of course, and the reason that so much attention is focused on quasars is that quasars are outliers.
Indeed, but there are other reasons too.
They are distributed overwhelmingly at very high redshift (z>2 on average and in some instances z>5). If that redshift entirely implies distance, then quasars must be extremely massive (on the order of 100s galaxies)
Why?
and yet must have excedingly narrow radii to account for the rapid variations in luminosity (assumed to be due to rotation).
To be pedantic, 'small' would be a better term than 'have excedingly narrow radii'
Seeming interactions of quasars and more local objects must all be optical illusions.
If you count gravitational lensing as 'optical illusions', then yes; if you're thinking of Arp's lists, then I'm not aware of any for which the apparent association with a 'local' object remains strong.
In contrast, if some of the redshift is due to something other than distance, than quasars could be much smaller in size (perhaps on the order of magnitude of a typical star), would have radii typical to stars, and would have luminosity on the order of magnitude of stars and can be in systems with stars and galaxies.
Indeed; like AGN, or BL Lac objects, or Seyferts, or ...
Suddenly, tens of thousands of extreme outliers become one more class of star like objects that just happens to exhibit spectra which can be confused for highly redshifted spectra.
Well, no; all manner of good observations would then need a different model ... such as the quasars which appear to be just where galactic nuclei would be ... of the galaxies they seem to be in (IIRC, very soon after the first quasars were observed, some astonomers - Sandage? - noted that several appeared to have 'fuzz' around them, and others appeared to be not quite point sources; the latest in this line is, of course, the HST observations ... pity that the coronograph won't fly

), the lensed quasars, ALL aspects of the Lyman forest (not just the existence of sets of absorption lines), the lack of anything even remotely like quasars 'locally', the well-observed 'pure luminosity evolution' of quasars, quasars whose underlying galaxy has the same redshift as the quasar, Lyman forest lines that correspond to the redshift of galaxy (clusters) through which they seem to pass, ...
Yet, if the early universe is dominated by huge objects with masses of 100 galaxies+ why does the modern universe lack such huge objects?
So what are AGNs? What's the correspondence between SMBH in galaxy nuclei (well observed) and quasars? (can you provide a link to models which demand that quasars have masses >10^11 sol?)
Also, while in the case of Cepheid standard candles we can observe such stars close up and use them to calibrate our redshift yardstick, we don't have the same luxury with quasars. We don't have any close quasars to use as a reference that could be used to compare distant quasars, and in the standard theory we also lack any non-quasar objects closely associated with quasars that we can used to say, e.g. a quasar at this known distance (through means other than redshift) looks like x. We can certainly say that one quasar is shifted relative to another quasar, but the original spectrum is theory dependent.
It's fairly easy to list observations that could fill this gap ... I expect JWST, SNAP, perhaps even some of the AO and optical interferometric instruments+scopes being planned at '8m class' observatories (e.g. Keck, VLT) will shed some light on this.
Also, quasar theory lacks the seasoning of many other parts of modern cosmology. General Relativity and Quantum Mechanics are ideas that have been kicking around since before 1920. The first quasar was observed in 1963, and large data sets (now in the mid to high thousands), have been around for a much shorter time period.
Because the phenomena posited to be behind quasars is more extraordinary than that behind other phenomena in astronomy,
Er, no; it's the same as that for AGNs, Seyferts, BL Lac objects, ...
it requires more extraordinary proof and alternatives are more paletable.
Taste is a rather personal thing ... and 'proof' surely not to be found anywhere in any science ... but for some, the mountain of good observations consistent with quasars' redshifts being cosmological goes down well.
Also, while quasars are meaningful details in theories of early cosmology, some form of intrinsic redshift in quasars by some previously unconsidered by plausible means would not shake up any other realm of physics unduly.
I guess that depends on what the 'previously unconsidered by plausible means' is (or are)!
