Quote:
Originally Posted by Robh
How can you differentiate between a doppler red shift and a cosmological red shift or, as distances get further, a combination of the two? Use of standard candles such as type Ia supernovae depends on them performing the same way back into the early Universe. Galaxies can vary enormously in both size and surface brightness and nearby galaxies will have older stars than those further away, so just how reliable is any distance measurement back into the early Universe? Seems like a lot of assumed generalizations.
Regards, Rob.
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A Doppler redshift is a function of the velocity of an object
in space relative to the observer. If there was only Doppler redshift then we wouldn't be able to explain the isotropic nature of the Universe. For example redshift surveys of distant galaxies produce the same distribution irrespective of the direction of observation.
This is explained through cosmological redshift caused by metric expansion. Since expansion is scale related every object that is not gravitationally bound is moving away from every other object.
Gravitationally bound galaxy clusters produce a higher Doppler component for the individual galaxies due to motion in space. However since the cosmological component of redshift increases with distance, the contribution of the Doppler effect becomes progressively smaller as the cluster is more distant.
A type Ia supernova follows a common recipe, a white dwarf in a binary star system captures matter from a companion star. When the mass of the white dwarf exceeds 1.4 solar masses it explodes due to carbon fusion. There is no reason to doubt the constancy of the mechanism over time.
Regards
Steven