Horizon and the question whether galaxies that recede faster than light are observable.

Details Horizon and the question whether galaxies that recede faster than light are observable. Horizon and the question whether galaxies that recede faster than light are observable.

Jul 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997acaps..17..225k&link_type=abstract

Acta Astrophys. Sin., Vol. 17, No. 3, p. 225 - 238

Mathematics

Logic

Cosmology: Redshifts

Scientific paper

To the question "Can we observe galaxies that recede faster than light?" the great majority of cosmologists would answer "No". Underlying this answer is the idea that velocity in relativistic cosmology has to be defined by the relativistic Doppler shift formula. But in cosmology, redshift is "cosmological" and not "Doppler". There is available an independent definition of velocity. Thanks to the Cosmological Principle, there is a distance-independent, universal time t and a time-dependent, instantaneous distance l, and velocity can naturally be defined as dl/dt. With this definition and the cosmological interpretation of redshift, it is shown: (1) That "horizon", which owes its role as the limit of observation to its association with infinite redshift, is irrelevant to the question. (2) That the answer must depend on the particular cosmological model. Specifically, the answer is NO for the steady state model, and YES for all three types (k = 0,-1,+1) of the big bang model; in the k = 0 model, all sources with redshifts greater than 1.25 would have had their recession velocities at the time of emission greater than 1 light velocity. It has been found useful to contrast the character of time and distance in cosmology and black hole physics.

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