
#1
Dec1711, 05:14 AM

P: 2,900

I can't manage to understand the quasischizofrenic way we should believe that our universe started at a certain time point called Bigbang while at the same we must never admit that in order to say that it follows that an absolute time (and an absolute frame) must be distinguished, (the CMB rest comoving frame). We at most can call it "preferred" frameeven if many people uses "preferred frame" as the one where physics is different, wich is a notion forbidden by reativity.
I think it is one way or the other, if we must think something extraordinary happend exactly 13.7 bly (with fractions of a second precision) ago then we are using an absolute clock, and therefore we shoud admit an absolute frame. If no such frame exists and that frame is only preferred in an arbitrary way, those 13.7 bly are also an arbitrary number and there's not anything absolute associated to that "age" and we could as well say something singular happened an infinite (if our universe was infinite) time ago or yesterday or an infinite number of different ages ago fom some other point in the universe. Please someone take me out this interpretational swamp. 



#2
Dec1711, 06:48 AM

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P: 16,485

If you are frustrated and confused by the verbal descriptions then you really need to learn the math. Otherwise you are "blind" to the overall picture. http://en.wikisource.org/wiki/The_po...d_the_Elephant 



#3
Dec1711, 07:10 AM

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#4
Dec1711, 07:39 AM

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P: 16,485

BB theory and preferred frames
If you know the math then what is the problem?
1) The big bang is a feature of the FRW metric. 2) The FRW metric is a solution of the EFE. 3) The EFE are manifestly covariant. 4) Covariance implies the lack of a prefered frame. 5) Therefore, there the big bang does not imply a prefered frame. 



#5
Dec1711, 07:42 AM

P: 2,900

So is the "age" of the universe just an arbitrary figure? 



#6
Dec1711, 07:50 AM

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P: 16,485

The "age of the universe" refers specifically to the proper time experienced by an observer which is passing next to us now and has been at rest in the FRW coordinates since the big bang. It is arbitrary, but welldefined.




#7
Dec1711, 08:01 AM

P: 2,900

Second, I agree with the " refers specifically to the proper time experienced by an observer which is passing next to us now and has been at rest in the FRW coordinates since the big bang", but a proper time in relativity is a purely local measure (with the caveat I referred to above on top of it), it can never be generalized to a "universe age" in any sense other than our local noncentric or nonspecial location in any way, right? (if you respect the cosmological and Copernican principles at least). 



#8
Dec1711, 08:22 AM

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P: 16,485

Of course, GR probably diverges from reality well before reaching the singularity. However, I personally don't think that the terminology is as bad as you seem to feel it is. The above definition certainly can be generalized to a "universe age" simply by adopting the same convention at every point in the universe. Since the FRW metric is homogenous it is reasonable to do, IMO. 



#9
Dec1911, 01:45 PM

P: 2,900

The FRW metric is homogeneous only spatially and to adopt the same convention at every point in te universe wrt a "universe age" i.e, sharing the same clock for the BB event, it would be necessary to either have a not FRW universe spatially and temporally homogeneous which seems not to be the case or do without the relativity of simultaneity (you are basically demanding absolute simultaneity for the BB event for every point in the universe no matter how distant). 



#10
Dec1911, 02:48 PM

P: 1,115





#11
Dec1911, 03:03 PM

Physics
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#12
Dec1911, 03:53 PM

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I think I must be severely misunderstanding your point. 



#13
Dec1911, 04:48 PM

P: 2,900

You do realize that all this implies an absolute frame, right? 



#14
Dec1911, 04:56 PM

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#15
Dec1911, 04:58 PM

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P: 16,485

Let me describe a slightly different way of constructing this coordinate system. First, recall that in order to define a geodesic you need an event and a tangent vector (i.e. velocity). So, take any event in the universe, and at that event find the one unique velocity at which there is no CMB Doppler shift. Using that event and tangent vector construct a geodesic that goes back in time to the BB, called the comoving geodesic. At each event along the geodesic calculate the length of that geodesic from the BB to the event (with the aforementioned caveats). Assign that number as the t coordinate of each event along the geodesic. Because the universe is spatially homogenous you can do this procedure at every point in the universe, assigning 3 different coordinates to identify the different comoving geodesics. This will result in the standard FRW coordinate system, with corresponding hypersurfaces of simultaneity. However, there is nothing that requires you to use that procedure. You could just as well choose a different method for assigning coordinates. You can have simultaneity without having absolute simultaneity. 



#16
Dec1911, 05:05 PM

P: 2,900





#17
Dec1911, 05:09 PM

P: 2,900





#18
Dec1911, 05:41 PM

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P: 16,485




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