superluminal motions & H_0 - comments from Ajk - Bartek's IAP talk

Boud Roukema boud w
Pią, 27 Sie 2004, 14:07:37 CEST

Witam wszyscy,
   Andrzej Kus is sceptical about using superluminal motions for
estimating (local) cosmological parameters.  Good :).

Andrzej K - please correct me if i'm wrong, but what i understand of
your arguments are something like:

-> jets are bent, not systematically, nobody knows why,
sometimes helical, very complex

-> superluminal expansion speed is highly variable

-> depends on which blobs are in which place, some blobs
are fast, some are slow
--> especially in mm data

-> on month scale changes very rapidly - some blobs just stop for
some time, and then start up again.

My initial reaction is that in a statistical sample these should
average out, provided that the errors are random and symmetrically

Alternatively: can we try to statistically correct for this?

Given that blobs stop and then start again, or change direction, but
are only *detected* when they are moving superluminally, then it would
seem to me that the true average speed is slower than estimated from
the doppler factor.

Most practical cosmology is about statistical corrections - it requires
careful modelling and correct statistical analysis, but it can be done.

Is it reasonable to use the observational data which *shows* the
erratic behaviour of these objects to statistically model this?

If yes, then we would have the necessary correction factor.

Anyway, here's a link to Bartek's IAP talk:

Andrzej K suggested that Bartek give a presentation on this (a Monday
talk?) - seems a fine idea to me, and doesn't require much preparation
from Bartek's side (except to take account of Andrzej's comments).

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