The Extragalactic Distance Scale

What do we mean by distance?

Luminosity distance, angular size distance, etc all reduce to "normal" geometric distances at low redshift (i.e., nearby universe). For small z, Taylor expansion of comoving coordinate distance and luminosity distance, for example, looks like this:

and similar for dA(z), etc. so as z-->0, all these distances converge to cz/H0, or Hubble's law: v=H0d.

So why do we need to get this right?

Measure H0:
Measure peculiar motions in the universe:

Important Complications:

The Distance Scale Ladder

Cepheid Distances via Hubble (example: M101, of course!)

    from Kelson etal 1997:

Cepheid (only) H0 Estimate

from Freedman etal 2001

The purpose of the HST Cepheid Key Project was not to get H0 directly, but to improve calibration of other distance indicators!