Distance measures (cosmology)
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Distance measures are used in physical cosmology to give a natural notion of the distance between two objects or events in the universe. They are often used to tie some observable quantity (such as the luminosity of a distant quasar, the redshift of a distant galaxy, or the angular size of the acoustic peaks in the CMB power spectrum) to another quantity that is not directly observable, but is more convenient for calculations (such as the comoving coordinates of the quasar, galaxy, etc). The distance measures discussed here all reduce to the naïve notion of Euclidean distance at low redshift.
In accord with our present understanding of cosmology, these measures are calculated within the context of general relativity, where the Friedmann-Robertson-Walker solution is used to describe the universe.
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[edit] Types of Distance Measures
- Angular diameter distance or Proper motion distance
- Luminosity distance
- Comoving distance
- Lookback time (this is how long ago light left an object of given redshift)
- Naive Hubble's law, taking z = H0d/c, with H0 today's Hubble constant, z the redshift of the object, c the speed of light, and d the "distance."
[edit] Comparison of Distance Measures
[edit] See also
[edit] References
- P. J. E. Peebles, Principles of Physical Cosmology. Princeton University Press (1993)
- Scott Dodelson, Modern Cosmology. Academic Press (2003).
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