|Annu. Rev. Astron. Astrophys. 1997. 35:
Copyright © 1997 by . All rights reserved
1.2. Distances and Hubble Law
The Hubble law between distance and cosmological redshift is a blessing for the cosmographer. A great motivation for investigation of the distance scale, it is also helpful for tackling the problems mentioned above.
Systematic errors in obtained distances are often recognized as a deviation from the linear Hubble law, and the reality and speed of galaxy streams, for example, closely depend on how well distances to galaxies are known. When one speaks about the choice of one or another distance scale, this is intimately connected with the Hubble constant Ho. In the Friedmann model, Ho together with qo allows one to extend the distance scale to high cosmological redshifts where classical distance indicators are lost. However, this extension is not the topic of my review [for discussion of such questions, see e.g. Rowan-Robinson (1985)].
To discuss biases in extragalactic distances, one might like to know what "distance" represents. As McVittie (1974) says, distance is a degree of remoteness; in some sense or another, faint galaxies must be remote. Only a cosmological model gives the exact recipe for calculating from the observed properties of an object its distance (which may be of different kinds). Because our basic data consist of directions and fluxes of radiation, we are usually concerned with luminosity or diameter distances. An error, say, in the luminosity distance in a transparent space means that if one puts a genuine standard candle beside the distance indicator in question, its photometric distance modulus is not the same.
Among the variety of distance concepts, one would like to think that there exists a fundamental one, corresponding to meter sticks put one after another from the Sun to the center of a galaxy. For instance, in the Friedmann universe, the theoretical and not directly measurable "momentary" proper distance is often in the background of our minds as the basic one (Harrison 1993), and the luminosity and other types of distances are the necessary workhorses. This review refers to the local galaxy universe where the different distance types are practically the same; in any case, the tiny differences between them are overwhelmed by selection biases and other sources of error. Another allowance from Nature is that in this distance range, evolutionary effects can be forgotten: In an evolving universe, distance indicators often require that the look-back time should be much less than the global evolutionary time scale.