Top Document: [sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9) Previous Document: H.04 How are galaxy distances measured? Next Document: H.06 What are QSO's ("quasars")? See reader questions & answers on this topic! - Help others by sharing your knowledge far away when the light left them? Author: William Keel <keel@bildad.astr.ua.edu> Distance is indeed a slippery thing in an expanding universe such as ours. There are at least three kinds of distances: * angular-diameter distance---the one you need to make the usual relation sine(angular size) = linear size/distance work; * luminosity distance---makes the typical relationship observed flux = luminosity / 4 pi (distance**2) work; and * proper distance---the piece-by-piece distance the light actually travelled. Of the three, the proper distance is perhaps the most sensible of the three. In this case, distance doesn't mean either when the light was emitted or received, but how far the light travelled. Since the Universe expands, we have been moving away from the emitting object so the light is catching up to us (at a rate set by the rate of expansion and our separation from the quasar or whatever at some fiducial time). You can of course turn this distance into an extrapolated distance (where the quasar or it descendant object is "today") but that gets very slippery. Both special and general relativity must be taken into account, so simultaneity, i.e., "today," has only a limited meaning. Nearby galaxies are pretty much where we see them; for example, the light from the Andromeda galaxy M31 has been travelling only about 0.01% of the usually estimated age of the Universe, so its distance from us would have changed by about that fraction, if nothing but the Hubble expansion affected its measured distance (which is not the case, because gravitational interactions between the Andromeda galaxy and our Galaxy affect the relative velocity of the two galaxies). To muddy the waters further, observers usually express distances (or times) not in light-years (or years) but by the observable quantity the redshift. The redshift is, by definition, the amount by which light from an object has been shifted divided by the emitted or laboratory wavelength of the light and is usually denoted by z. For an object with a redshift z, one can show that (1+z) is the ratio of the scale size of distances in the Universe between now and the epoch when the light was given off. Turning this into an absolute distance (i.e., some number of light-years) requires us to plug in a rate for the expansion (the Hubble constant) and its change with time (the deceleration parameter), neither of which is as precisely known as we might like. As a result ages and distances are usually quoted in fairly round numbers. If the expansion rate has remained constant (the unrealistic case of an empty Universe), the age of the Universe is the reciprocal of the Hubble constant. This is from 10--20 billion (US, 10^9) years for the plausible range of Hubble constants. If we account for the matter in the Universe, the Universe's age drops to 7--15 billion years. A quick estimate of the look-back time (i.e., how long the light from an object has been travelling to us) for something at redshift z is t = (z/[1+z])*1/H0 for Hubble constant H0. For example, the author has published a paper discussing a cluster of galaxies at z=2.4. For the press release we quoted a distance of 2.4/3.4 x 15 billion light-years (rounded to 11 since that 15 is fuzzy). User Contributions:Top Document: [sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9) Previous Document: H.04 How are galaxy distances measured? Next Document: H.06 What are QSO's ("quasars")? 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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