Lookback time vs. true distance (Hubble Law)
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From: Lax on 20 Apr 2010 00:33 If we use the redshift of light from a far galaxy to calculate its radial velocity away from us then use the Hubble Law to find a distance, what is this distance really? Is it A: distance from position of galaxy when it emitted its light to position of us AT THAT TIME or B: distance from position of galaxy when it emitted its light to position of us RIGHT NOW? Or does it give the distance between the galaxies right now?
From: Peter Webb on 20 Apr 2010 07:42 "Lax" <lax.clarke(a)gmail.com> wrote in message news:877951b0-25bd-466c-acd8-8ea7cc1c9dc8(a)f17g2000vbd.googlegroups.com... > If we use the redshift of light from a far galaxy to calculate its > radial velocity away from us then use the Hubble Law to find a > distance, what is this distance really? Is it A: distance from > position of galaxy when it emitted its light to position of us AT THAT > TIME or B: distance from position of galaxy when it emitted its light > to position of us RIGHT NOW? Or does it give the distance between > the galaxies right now? I was hoping that somebody would post a good answer; its a slippery question. Firstly, the Universe is expanding, and the distance is actually increasing as the light travels here. So the distance the light travels is intermediate in a sense between these two. This is very complicated, and I don't understand it well myself. Even if we drop that problem, its still slippery. You use the term "RIGHT NOW", but due to Special Relativity what is right now depends on the reference frame you use for measurement, and if the recession velocity is a significant fraction of c then this matters. Calculations as to distances and times between events (the light being emitted by the galaxy and seen in a telescope on earth) will depend on whether these are measured in our Galaxy's frame of reference or that of the distant galaxy. Neither can claim to be the centre of the Universe. Ignoring this complication, by decreeing that the Milky Way is stationary and all the other Galaxies are moving away from us , then the light emitted by a galaxy 1 billion light years away shows where it was 1 billion years ago, and it will have moved since then.
From: Sam Wormley on 20 Apr 2010 09:03 On 4/19/10 11:33 PM, Lax wrote: > If we use the redshift of light from a far galaxy to calculate its > radial velocity away from us then use the Hubble Law to find a > distance, what is this distance really? Is it A: distance from > position of galaxy when it emitted its light to position of us AT THAT > TIME or B: distance from position of galaxy when it emitted its light > to position of us RIGHT NOW? Or does it give the distance between > the galaxies right now? One need to know absolute brightness. Type Ia supernovae make reasonable "standard candles".
From: Uncle Al on 20 Apr 2010 17:41 Lax wrote: > > If we use the redshift of light from a far galaxy to calculate its > radial velocity away from us then use the Hubble Law to find a > distance, what is this distance really? Is it A: distance from > position of galaxy when it emitted its light to position of us AT THAT > TIME or B: distance from position of galaxy when it emitted its light > to position of us RIGHT NOW? Or does it give the distance between > the galaxies right now? A relativistic universe has four distinct distances: luminosity (inverse square), angular diameter, parallax, and proper motion. No two of them need agree to maintain consistency. Clocks can only be synchronized by being local. Which distance would you like? -- Uncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/qz4.htm
From: John Polasek on 20 Apr 2010 19:24
On Mon, 19 Apr 2010 21:33:23 -0700 (PDT), Lax <lax.clarke(a)gmail.com> wrote: >If we use the redshift of light from a far galaxy to calculate its >radial velocity away from us then use the Hubble Law to find a >distance, what is this distance really? Is it A: distance from >position of galaxy when it emitted its light to position of us AT THAT >TIME or B: distance from position of galaxy when it emitted its light >to position of us RIGHT NOW? Or does it give the distance between >the galaxies right now? It is B: distance from position of galaxy when it emitted its light to position of us RIGHT NOW. Unfortunately in the current cosmology there is not a defined geometry* of the universe so it might be hard to prove this but it is true nevertheless using a more cogent construction for the universe. *If you apply Hubble's principal a few times you will see why cosmologists have defaulted to a primitive measure. 'a', as a size coefficient for a dust-like universe. John Polasek |