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 cosmologicallydistant objects are so small that they will remain impossible to measure in the foreseeable future (with the possible exception of some gravitationallylensed 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/astroph/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/astroph/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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