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[sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9)
Section - There are different ways to measure distances in cosmology?

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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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Top Document: [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9)
Previous Document: I.17 Since energy is conserved, where does the energy of redshifted photons go?
Next Document: Copyright

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