A Formula for Confusion by Phillip Helbig

Phillip Helbig

The Observatory, 138, 1262, 22–25 (February 2018)


The book contains a formula for cosmological "redshifts larger than one", namely the relativistic Doppler formula. It should be obvious that that formula cannot be correct, because it gives the velocity as a function of just the redshift; there are no cosmological parameters! Even the Hubble constant is absent. This implies that, even at arbitrarily high redshift, the velocity is independent of the cosmological model, and does not depend on even the Hubble constant (despite the fact that the latter is the proportionality constant in the velocity--distance relation!) while at the same time it is common knowledge that, except in the limit of small redshifts, the relation between observed and `absolute' or `physical' quantities depends on the cosmological model. Why does this misconception still persist? One reason might be that in practice cosmological velocities play no role in observational cosmology.


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