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 See reader questions & answers on this topic! - Help others by sharing your knowledge 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. User Contributions: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 Part0 - Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
Last Update March 27 2014 @ 02:11 PM
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