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

Bartosz Lew Bartosz.Lew w
Pon, 30 Sie 2004, 08:31:18 CEST


On Fri, 27 Aug 2004, Boud Roukema wrote:

> 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.

plus it often happens that they fade away very quickly or change shape so
within a few months the recognition of the same blob mae sometimes
be difficult.

> My initial reaction is that in a statistical sample these should
> average out, provided that the errors are random and symmetrically
> distributed.
> 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.
hmm this  I quite don't understand ? why do you say so, that they are only
detected when they are superliuminal. in fact superluminality isn't a
condition "sine qua non" for doing this thing.

> 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.

from what I've done and played with the real data, I see that the final
value is quite sinsitive on precise measurments from the maps !! so
true - the big sample is needed and this should work.

> 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).

that's ok with me.
anytime (except for the one, that is needed for repreparation of updated
presentation) ;)

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