154 There are different ways to measure distances in cosmology?
Description
This article is from the Astronomy FAQ, by Joseph Lazio (jlazio@patriot.net) with numerous contributions by others.
154 There are different ways to measure distances in cosmology?
By Joseph Lazio <jlazio@patriot.net>
Yes!
There are at least three ways one can measure the distance to objects:
* parallax;
* angular size; or
* brightness.
The parallaxes of cosmologically-distant objects are so small that
they will remain impossible to measure in the foreseeable future (with
the possible exception of some gravitationally-lensed quasars).
Suppose there exists an object (or even better a class of objects)
whose intrinsic length is known. That is, the object can be treated
as a ruler because its length known to be exactly L (e.g., 1 m, 100
light years, 10 kiloparsecs, etc.). When we look at it, it has an
*angular diameter* of H. Using basic geometry, we can then derive its
distance to be
L
D_L = ---
H
Suppose there exists an object (or even better a class of objects)
whose intrinsic brightness is known. That is, the object can be
treated as a lightbulb because the amount of energy it is radiating is
known to be F (e.g., 100 Watts, 1 solar luminosity, etc.). When we
look at it, we measure an *apparent* flux of f. The distance to the
object is then
F
D_F =sqrt( ------ )
4*pi*f
In general, D_L *is not equal to* D_F!
For more details, see "Distance Measures in Cosmology" by David Hogg,
<URL:http://xxx.lanl.gov/abs/astro-ph/9905116>, and references within.
Plots showing how to convert redshifts to various distance measures
are included in this paper, and the author will provide C code to do
the conversion as well. Even more details are provided in "A General
and Practical Method for Calculating Cosmological Distances" by Kayser
et al., <URL:http://xxx.lanl.gov/abs/astro-ph/9603028> or <URL:
http://multivac.jb.man.ac.uk:8000/helbig/Research/Publications/info/angsiz.html>.
Fortran code for calculating these distances is provided by the second
set of authors.
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