Norman Albers Posted March 30, 2009 Share Posted March 30, 2009 Lakmilis, probably the most common distance measure in cosmology is the "comoving distance" which is the same as the "proper" or actual physical distrance if it could be measured today. (By a pre-arranged sequence of observers with radar devices stretched out between here and there all measuring simultaneously according to the universal timescale.) The Friedman equation model basic to all cosmology, and the Hubble Law are both stated in terms of that distance and depend on the same criterion of rest and simultaneity. Cosmology is slightly different from GR in having those features, it is a specialized and simplified form of GR. Our construction here demands a wide-ranging network of agreed simultaneity. Martin, you use provocative terms: "Cosmology is slightly different from GR..." I have been asking about this, as mathematically it is letting go of strict covariance. Link to comment Share on other sites More sharing options...
NowThatWeKnow Posted March 30, 2009 Share Posted March 30, 2009 Our construction here demands a wide-ranging network of agreed simultaneity. Martin, you use provocative terms: "Cosmology is slightly different from GR..." I have been asking about this, as mathematically it is letting go of strict covariance. Hi Norman, I am in over my head and I am definitely not answering for Martin. Don't most agree that GR will require some tweaking for simultaneity when considering some of the large distances in cosmology? It seems like I was reading about that somewhere. Link to comment Share on other sites More sharing options...
granpa Posted March 30, 2009 Share Posted March 30, 2009 imagine a line of clocks stretching completely around the universe, stationary and synchronized with respect to each other. imagine a rocket moving along this line. whether the clock on the opposite side of the universe is ahead of or behind the local clock depends on which direction you look along the line. is that what you meant? Link to comment Share on other sites More sharing options...
Norman Albers Posted March 30, 2009 Share Posted March 30, 2009 Yes granpa, and I don't know, NowThatWeKnow. Consider the implications for the far-out clocks. It takes most of the age of the universe for their signals to reach us, so synchrony becomes a challenge, no? Link to comment Share on other sites More sharing options...
lakmilis Posted March 30, 2009 Share Posted March 30, 2009 Well cheers Martin.. Yes I think I was in that site ages ago.. but I tend to not like 'calculators' etc but stick to the theory itself rather.. (so thanks for the input earlier, the link to calcs.. I am interested in knowing how we do it.. not actually calculating it without knowing ). (truth is I have been 'away'from cosmology and even just physics for ages.. forgot more than I remember xx) Merged post follows: Consecutive posts mergedWell cheers Martin.. Yes I think I was in that site ages ago.. but I tend to not like 'calculators' etc but stick to the theory itself rather.. (so thanks for the input earlier, the link to calcs.. I am interested in knowing how we do it.. not actually calculating it without knowing ). (truth is I have been 'away'from cosmology and even just physics for ages.. forgot more than I remember xx) Link to comment Share on other sites More sharing options...
Martin Posted March 30, 2009 Share Posted March 30, 2009 Hi Norman, I am in over my head and I am definitely not answering for Martin. Don't most agree that GR will require some tweaking for simultaneity when considering some of the large distances in cosmology? It seems like I was reading about that somewhere. Simultaneity and the existence of "universe time" in cosmology is no big deal. It comes straight out of the existence of the CMB. An observer is at rest relative to CMB if he sees no doppler hotspot (only the usual fluctuations in temperature which are like 1/1000 of one percent). Aside from gravitational effects (which can be made small) two stationary observers can agree---synchronize clocks and all that. Stationary observers are called "comoving" (some cosmo jargon). Before the CMB was discovered cosmologists already had the idea of universal rest---being at rest with respect to the expansion process or "Hubble flow". If you are at rest then the expansion looks symmetric. Same thing. The point is that using universe time and comoving distance (based on the simultaneity seen by observers at rest) is optional. It just happens that the statement of Hubble Law uses this idea in its statement. And the Friedman model (a couple of simple equations) uses it. It would be a lot more complicated to state the basic facts of Hubble Law and Friedman model if you used some other coordinate system, some other idea of distance, etc. and pretended you didn't have this natural idea of simultaneity. Ultimately using those things is optional and based on convenience. In pure GR you don't have the CMB and life is harder. Link to comment Share on other sites More sharing options...
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