BB theory and preferred frames

DaleSpam

Then it isn't about the size of the observer, it is about the stress-energy of the observer. Obviously if the observer is so massive that it significantly distorts the metric then the metric will be significantly distorted. That tautology hardly implies any of the schizophrenia you mentioned in the OP.

PeterDonis

*We* don't see the universe as homogeneous and isotropic, and we are observers. I think that counts as empirical.

Only observers who are at rest in the "comoving" frame used in the FRW models see the universe as homogeneous and isotropic. Observers who are not at rest in that frame don't. Earth is not at rest in that frame, because we see a dipole anisotropy in the CMBR. We don't have reports from astronomers in other galaxies, but based on what we can see of their motions, it appears that at least some of the nearby ones are not at rest in the "comoving" frame either (that is, their observed relative velocity to us is different from what it would need to be to cancel the dipole anisotropy we see in the CMBR). AFAIK it gets harder to tell as you go farther out.

TrickyDicky

You didn't understand the set up I guess, the stress-energy of the observer is idealized in such way in the FRW metric so that it doesn't distort the metric. The whole point was to explain that what is "dust" in the FRW metric refers in fact to objects of a very big size, so yes, it is about size, we haven't found so far objects of that size, up to now the biggest objects we can see are still at inhomogeneous scale (but it is expected that at some size scale we should find true homogeneity), and their distortion of spacetime doesn't significantly distort the FRW metric at all, on the contrary those objects are still considered as dust in FRW models. So it seems you might have have some confusions about the FRW cosmological model. Of course for observers below that size all kind of deviations from comoving motion should be observed (although if you take a look at the previous post about "de Vaucouleurs paradox", those deviations are only significantly observed at much smaller size scales (of a few Mpc).
And after all you already admitted that the age of the universe is purely conventional and arbitrary in a previous post, but very "reasonable" convention.
So you seem to have no problem to think two contradictory things at the same time , relativity of simultaneity and absolute simultaneity (all observers in the same space hyperslice share simultaneity of BB event no matter their spatial separation) can both coexist. Hey, if you see no problem with that who am I to drag you from your comfortable conviction.

DaleSpam

I understand the setup. You deliberately make your observer so super-massive that it would distort the metric if it were not comoving and then, under the assumption that the metric is not distorted, you reach the tautological conclusion that your super-massive observer must be comoving.

What I don't understand is why you think that kind of a setup is at all important. Why do you want to go out of your way to use your super-massive observers in this scenario when in most other GR discussions observers are considered to have an insignificantly miniscule amount of mass?

I am not thinking two contradictory things. There is no absolute simultaneity. As we agreed above it is simply an arbitrary convention. "Arbitrary convention" explicitly implies that it is not absolute.

Only the comoving observers in the space hyperslice agree on the proper time from the BB. Not all observers are comoving, and the non-comoving ones disagree about the proper time from the BB. All of your super-massive observers are (tautologically) comoving, but clearly not all observers are super-massive.

TrickyDicky

I'm not so sure, or maybe it is the FRW model you find problem with?
My "super massive" observers don't distort the metric, they are treated as "dust" in the FRW model.Tautologically or not they are comoving. My point is that for them the there is absolute simultaneity wrt the BB event and this observers can not change their state of motion or else the homogeneity assumption is lost, so for them certainly their frame is absolute and defines an absolute state for all the others observers, all non-comoving (smaller) observers refer their state of motion to that comoving frame. The fact that there is an absolute state of motion doesn't mean absolutely all objects and observers must have that motion (be comoving) but that they refer their state of motion to that comoving frame.
See above

DaleSpam

Understood. In your setup we are given that they do not distort the metric, and we are given that they are sufficiently super-massive that they would distort the metric if they were not comoving. Therefore the super-massive observers are comoving. What specifically do you think I am not understanding about the setup?

This is incorrect. While all non-comoving observers may adopt the reasonable convention of refering their state of motion to that comoving frame. Nothing physical forces them to do that. Furthermore, the super-massive observers may decide to adopt the strange convention of refering their state of motion to some non-comoving frame. Nothing physical prevents them from doing that. The simultaneity in the FRW coordinates is still relative, not absolute.

my_wan

I understand Weyl's postulate, and the difficulties with trying to define the physical meaning of Weyl gauges in general. I don't really buy Penrose's Weyl curvature hypothesis either or the entropy argument in general. Due to the relational character of entropy and the nonsense of talking about what's outside the Big Bang when defined as a closed system. Yet if we have an observable metric that is internally expanding from an internal perspective, such as the FRW metric, does this not imply that these internal degrees of freedom where supplied from an outside source? Yet in both the BB conception of enclosed system and relativistic symmetries this notion of degrees of freedom "from the outside" is nonsense. In what way then does the FRW metric expansion decouple itself from the spacetime metric under GR such that this metric is an observable expansion not tied to local clocks under GR?

So don't tell me that I only need to read "a few books" to grok what's happening when the debates on which those books are based can't comes to terms with even the basic physics. Fundamentally this is defined by conceptual split between classical thermodynamics and statistical mechanics. Most obviously in the "arrow of time issues" with time reversible foundations of both classical and quantum physics.

So my question stands, and handwaiving it with "read a few books" buys nothing.

TrickyDicky

I didn't sell you anything. Your question is too broad and you need to start your own thread.

TrickyDicky

You see nothing physical in the fact that super-size observers can only have one motion state (whether you wanna call it "rest frame" or "ether frame" or "CMB frame")?

DaleSpam

I agree. It feels like you are trying to hijack TrickyDicky's thread on a tangential question.

DaleSpam

No, I don't. It is a direct result of using super-massive observers and I see nothing physical about your super-massive observers.

The super massive observers are comoving, but even comoving observers are free to do physics calculations in a frame where they are not stationary if they wish. Simultaneity is not absolute. Refer back to post 4, your super-massive observers do not change that line of reasoning.

my_wan

That is rather obvious, though my description did not depend on any model containing and homogeneity or nonhomogeneity.

Ok, here's the problem once again. If we think in terms of the distribution of galaxies (CMBR aside) and their Hubble law relation to distance, then my description explain exactly why this same Hubble law would apply under all relativistic boost.

Now you bring in the CMBR. Here it is unique from the Hubble law of galaxies since the source is presumed to be at some equidistant boundary condition from a particular frame. In the galaxy distance/redshift case this merely corresponds to a frame in which both galaxies have the same redshift in opposite directions. Now I can't even count the number of things that can go wrong with this assumption, and the claim that a frame in which two redshifts from opposite directions exists such that the redshifts are equal is a tautology of any and all of those possibilities.

Therefore I'll ask for more detail on what you suppose this "prefered" isotropic frame means.
1) Does this mean the Universe is effectively older or younger for our frame, given our anisotropic frame with respect to CMBR?
2) Does this mean that the time since the BB is effectively older when in our frame it's measured in one direction and younger when measured in the other direction?

Now the major point from an earlier reference in this thread, but I'm only interested what Martin Rees is purported to have said here:
Now, here Martin Rees's justification was the "isotropic CMB". We see that it is not isotropic. We then choose a "preferred" frame in which it is isotropic and call that justification for isotropic. Yet for any two galaxies or rocks in the Universe, irrespective of any homogeneity or lack of, a "preferred" frame tautologically exist to define their redshifts as equal. Yet, due to the way relativity defines the spacetime metric, the Hubble law is generally valid for all galaxies irrespective of which frame you choose, no "preferred" frame needed. Hence defining a tautology that such a "preferred" frame exist is not evidence of squat.

This compounding of tautologies as if it was "the" evidence is aggravating. The only thing more aggravating is crackpots that remodel their own interpretation, not leaving much left where you can actually learn much about the raw empirical data without all the baggage.

TrickyDicky

Well, certainly, my observers are not physical, this is more like a gedanken experiment, very often observers are considered massless and nobody sees any problem with that. I think you are missing the important point, their velocity, their motion, is absolute, stationary (time invariant since it can't change his motion state) with respect to any other state of motion.
Otherwise you have to break the homogeneity assumption. That is not related to whether they are free to do calculations in any frame , the fact is that their state of motion is fixed.
Also the fact that a model is a solution of the EFE doesn't guarantee that the model is physical, and it only guarantees general covariance, not Lorentz covariance which is what here is being discussed.

Passionflower

The FRW solution is a spacetime that is assumed to be an approximate model of our spacetime.
I have serious reservations about that since we clearly do not see the matter distribution as homogeneous and isotropic. The assumption that it is homogeneous and isotropic on a large enough scale it is I think is sheer speculation. Matter clutters due to gravity even at large scales.

my_wan

I am significantly more interested in how to ask empirically answerable questions than in how such questions fit into this or that model, standard or otherwise. So the comfy chair is not for me and my issues are about how uncomfortable I am. Yet it seems to me that so long as the data is wrapped around a particular model a lot of people sit in comfy chairs with little concern about empirically askable questions. Especially those that are not explicitly formatted for their model of choice.

The notion that some idealization of a metric, such as the "stress-energy of the observer", can technically be chosen to validly justify a particular picture is a non-point to me. I can technically choose a valid frame and say that all observer must transform observations to this frame to see "the" valid picture of what is really happening to. So what. The technical validity is blindingly myopic. Before Einstein the ether played the same role the idealized metrics play in cosmology.

Passionflower

Not only that but if the data does not match up there is always invisible dark energy and invisible dark matter that comes to the rescue.

That that is equivalent to insisting that a theory is right but that the discrepancies are caused by invisible pink unicorns is something that seems to go right over the heads of many.

If the CERN-Grasso experiment turns out to be correct I would not be surprised that it is posed that the theory still stands but that the discrepancies are caused by undetectable dark spacetime fluxes or something like it, with the key being that it must be undetectable.

my_wan

The dark matter/energy issue is not that big a deal to me. Unknowns are a part of science. On the face of it, it's really no worse than MOND, which pulls a form fitted to specs equation out of their... What bugs me about MOND is that if it was simply designed to specs to fit a certain empirical data curve why is it so effective with such a large variety of disparate data on so many scales? So MOND suffers from a similar non-explanation. Yet the dark matter people can't justifiable just hand wave and simply say there's no point in answering this because we already know it's wrong without looking! So the whole thing just reaks of a battle of models rather than how to actually ask real questions.

As far as the CERN-Grasso experiment, I wouldn't hold my breath. But hey, at least their asking rather than defending a model turf.