BB theory and preferred frames

In summary: I don't think it can ever be "translated" into a single time that applies globally across all observers, it's more like a coordinate in spacetime. Two things, if t=0 is a real singularity, that is "outside physical theories realm", an exact age can't be that well-defined since we can't know how our theory behaves at the limit when t tends to zero, as far as we ñnow time could behave weirdly there and became asymptotic or...whatever, that what a true singularity does.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
  • #1
TrickyDicky
3,507
27
I can't manage to understand the quasi-schizofrenic way we should believe that our universe started at a certain time point called Big-bang 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" frame-even if many people uses "preferred frame" as the one where physics is different, which is a notion forbidden by reativity.
I think it is one way or the other, if we must think something extraordinary happened 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.
 
Physics news on Phys.org
  • #2
TrickyDicky said:
I can't manage to understand the quasi-schizofrenic way we should believe that our universe started at a certain time point called Big-bang 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" frame-even if many people uses "preferred frame" as the one where physics is different, which is a notion forbidden by reativity.
There is nothing schizophrenic about it. It is straight out of the Einstein Field Equations and differential geometry. The problem is trying to describe the math in english.

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_poems_of_John_Godfrey_Saxe/The_Blind_Men_and_the_Elephant
 
  • #3
DaleSpam said:
There is nothing schizophrenic about it. It is straight out of the Einstein Field Equations and differential geometry. The problem is trying to describe the math in english.

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_poems_of_John_Godfrey_Saxe/The_Blind_Men_and_the_Elephant

Thanks for the poem but I don't think the confusion comes in this case from the verbal description, I know the math and is precisely from the math that I gather the verbal description of my last post, in any case you could try and describe (in english or mathematically) which way of the two I described in the second paragraph do you think is the correct one.
 
  • #4
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 preferred frame.
5) Therefore, there the big bang does not imply a preferred frame.
 
  • #5
DaleSpam said:
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 preferred frame.
5) Therefore, there the big bang does not imply a preferred frame.

I agree with those 5 points, that's the problem.
So is the "age" of the universe just an arbitrary figure?
 
  • #6
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 well-defined.
 
  • #7
DaleSpam said:
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 well-defined.

Two things, if t=0 is a real singularity, that is "outside physical theories realm", an exact age can't be that well-defined since we can't know how our theory behaves at the limit when t tends to zero, as far as we ñnow time could behave weirdly there and became asymptotic or...whatever, that what a true singularity does.
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 non-centric or non-special location in any way, right? (if you respect the cosmological and Copernican principles at least).
 
  • #8
TrickyDicky said:
Two things, if t=0 is a real singularity, that is "outside physical theories realm", an exact age can't be that well-defined since we can't know how our theory behaves at the limit when t tends to zero, as far as we ñnow time could behave weirdly there and became asymptotic or...whatever, that what a true singularity does.
Agreed, similar issues happen with defining the proper time of an observer falling to the singularity of a black hole. Usually the singularity is simply excluded from the manifold, which makes the limits all behave nicely (and is actually required mathematically). This is what is meant by the proper time of an observer going into or coming out of a singularity.

Of course, GR probably diverges from reality well before reaching the singularity.

TrickyDicky said:
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 non-centric or non-special location in any way, right? (if you respect the cosmological and Copernican principles at least).
Science is full of bad terminology that persists despite good reasons for it to be dismissed. It sounds like you actually understand everything, and simply object to the terminology. Your objection is reasonable, but changing common terminology is nearly impossible.

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.
 
Last edited:
  • #9
DaleSpam said:
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.
As I said I disagree that it is simply a terminology issue, it is about logical consistency and avoiding contradictions.
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
TrickyDicky said:
As I said I disagree that it is simply a terminology issue, it is about logical consistency and avoiding contradictions.
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).
I'm no cosmologist but the only maybe useful observation I can make is that as real universe is lumpy not spatially homogeneous, temporal 'lumpiness' is also present to some extent. So higher gravitational potential inside a cosmic void region implies experiencing more relative total time since BB than inside a clumped region. And of course considerably less time has passed at the surface of an ancient neutron star say. But guessing that is not your concern here, as it seems from your earlier remarks we are assuming homogeneity.
 
Last edited:
  • #11
TrickyDicky said:
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).

The FRW model does not say that every observer sees the same "universe age" regardless of their state of motion. It only says that *comoving* observers all see the same "universe age". We here on Earth are not comoving observers, so the universe age we measure will actually be slightly different than the universe age that would be measured by a comoving observer just passing through the Solar System right now. Our instantaneous hypersurface of simultaneity on Earth right now is also not the same as the hypersurface of constant comoving time passing through Earth right now.
 
  • #12
TrickyDicky said:
(you are basically demanding absolute simultaneity for the BB event for every point in the universe no matter how distant).
Huh? Any event (including the BB) is in fact simultaneous with itself absolutely. So yes, I am demanding that, and that is noncontroversial.

I think I must be severely misunderstanding your point.
 
  • #13
DaleSpam said:
Huh? Any event (including the BB) is in fact simultaneous with itself absolutely. So yes, I am demanding that, and that is noncontroversial.

I think I must be severely misunderstanding your point.

Ok, I guess you mean that the BB is supposed to have occurred not in a single location but in every location, and that's what you find noncontroversial. So it's no problem for you to do without the relativity of simultaneity for this particular event.
You do realize that all this implies an absolute frame, right?
 
  • #14
PeterDonis said:
The FRW model does not say that every observer sees the same "universe age" regardless of their state of motion. It only says that *comoving* observers all see the same "universe age". We here on Earth are not comoving observers, so the universe age we measure will actually be slightly different than the universe age that would be measured by a comoving observer just passing through the Solar System right now. Our instantaneous hypersurface of simultaneity on Earth right now is also not the same as the hypersurface of constant comoving time passing through Earth right now.

Sure, since we are using the FRW model I'm idealizing a little and assuming perfect homogeneity (wich is expected anyway at certain scale) so that "all observers are basically comoving".
 
  • #15
TrickyDicky said:
Sure, since we are using the FRW model I'm idealizing a little and assuming perfect homogeneity (wich is expected anyway at certain scale) so that "all observers are basically comoving".
We are observers and we are not comoving.

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
DaleSpam said:
We are observers and we are not comoving.

I said I'm assuming the idealization of the FRW model, the universe age we as observers measure will actually be slightly different than the universe age that would be measured by a comoving observer, if that is a problem for you here I guess you wouldn't accept the FRW model because we as observers are not comoving and therefore the universe must not be homogeneous. Frankly, that's feeble, we are looking at the big picture here.
 
  • #17
DaleSpam said:
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.

Ok, so if we can choose a different methd for assigning coordinates, you then admit there is nothing special about the age (13.7 bly) we usually assign to the universe right?
 
  • #18
TrickyDicky said:
I said I'm assuming the idealization of the FRW model
I am assuming the same idealization of the FRW model. In the ideal FRW model we observe a Doppler shift to the CMB, therefore we are not comoving observers.

TrickyDicky said:
if that is a problem for you here I guess you wouldn't accept the FRW model because we as observers are not comoving and therefore the universe must not be homogeneous. Frankly, that's feeble, we are looking at the big picture here.
This is certainly not correct. I have no problem with the FRW model as a very close approximation. What you do not seem to realize is that the FRW model, at each spatial location, identifies one particular worldline as the worldline of a co-moving observer, and it is not our worldline.
 
  • #19
TrickyDicky said:
Ok, so if we can choose a different methd for assigning coordinates, you then admit there is nothing special about the age (13.7 bly) we usually assign to the universe right?
Correct. I said as much back in post 6. It is a very reasonable convention, but a convention nonetheless.
 
  • #20
TrickyDicky said:
Sure, since we are using the FRW model I'm idealizing a little and assuming perfect homogeneity (wich is expected anyway at certain scale) so that "all observers are basically comoving".

No, all observers are *not* comoving, not even in an idealized, perfectly homogeneous and isotropic model. Only observers who *see* the universe as homogeneous and isotropic are comoving. Even in the idealized model, there are plenty of observers who do not see the universe as homogeneous and isotropic and so are not comoving.
 
  • #21
PeterDonis said:
No, all observers are *not* comoving, not even in an idealized, perfectly homogeneous and isotropic model. Only observers who *see* the universe as homogeneous and isotropic are comoving. Even in the idealized model, there are plenty of observers who do not see the universe as homogeneous and isotropic and so are not comoving.

I'm not sure if you like to nitpick for the sake of it, or you don't fully understand what I'm saying or the FRW model itself. I'm perfectly aware that in our universe there is departure from homogeneity at our size scale, the FRW model is an idealization expected for objects hypercluster size, that yet works with little deviation (slightly different measures for the Earth in your own words) since we are using this model -it is the concordance model- for objects Earth or solar system size. I'll try again, in an perfect idealized FRW model, so that we can consider ourselves and all observers like at least hyperclusters size observers, all observers must be basically comoving since we should see the universe as isotropic and homogeneous.
BTW this also implies that we should be observing that slight deviation from Hubble law in the nearby universe region, but we are actually not observing it, for reference this paradox is known as the "de Vaucouleurs paradox" in honor of the brilliant cosmologist that pointed it out, and has not yet been explained.
 
  • #22
Q-reeus said:
I'm no cosmologist but the only maybe useful observation I can make is that as real universe is lumpy not spatially homogeneous, temporal 'lumpiness' is also present to some extent. So higher gravitational potential inside a cosmic void region implies experiencing more relative total time since BB than inside a clumped region. And of course considerably less time has passed at the surface of an ancient neutron star say. But guessing that is not your concern here, as it seems from your earlier remarks we are assuming homogeneity.
It is exactly as you say, the FRW model is (like most models) an imperfect idealization since so far at all scales we observe there is departure from homogeneity, but this departure is considered small enough so that the FRW model is considered a valid working model.
For the sake of the better understanding of my argument I'm ignoring the small deviation at small scales, and taking the FRW model idealization as depicting realistically our universe at our scale too, I think it is a liberty I can take, but apparently peterdonis is not catching it.
 
  • #23
DaleSpam said:
Correct. I said as much back in post 6. It is a very reasonable convention, but a convention nonetheless.

Great, we are getting there. So you would agree (if you have read my last posts) that the reason it is a very reasonable convention is that we (no special observers) are very closely approximating the idealized FRW model in which all observers using those coordinates are comoving and we are expecting perfect homogeneity at a certain scale so all observers of a certain big enough size exactly should share being comoving observers, for those comoving observers the comoving frame must be absolute rather than conventional right?
 
  • #24
TrickyDicky said:
So you would agree (if you have read my last posts) that the reason it is a very reasonable convention is that we (no special observers) are very closely approximating the idealized FRW model in which all observers using those coordinates are comoving and we are expecting perfect homogeneity at a certain scale so all observers of a certain big enough size exactly should share being comoving observers, for those comoving observers the comoving frame must be absolute rather than conventional right?
No, I would not agree with that. The size of the observer is irrelevant, only it's velocity wrt the CMB determines if it is comoving or not. A cluster sized observer moving relative to the CMB would not be comoving, and a neutrino sized observer at rest wrt the CMB would be comoving.

IMO, the reason it is a very reasonable convention is because it can be applied in the identical fashion at each spatial location.
 
  • #25
DaleSpam said:
No, I would not agree with that. The size of the observer is irrelevant, only it's velocity wrt the CMB determines if it is comoving or not. A cluster sized observer moving relative to the CMB would not be comoving, and a neutrino sized observer at rest wrt the CMB would be comoving.

IMO, the reason it is a very reasonable convention is because it can be applied in the identical fashion at each spatial location.

Then you'll have to agree that in GR at certain distances velocity is not well- defined (meaning it is a path dependent parameter, and if you consider the LCDM model a valid one you will have to agree that at a certain scale the universe is homogeneous in the way FRW model predicts, therefore in this context size (or scale) is not irrelevant, there can't be a hypercluster (of whatever the size it is required by the LCDM model that the universe is truly homogeneous) that is not comoving with CMBR, if you don't agree with these two facts let me know.
 
  • #26
TrickyDicky said:
Then you'll have to agree that in GR at certain distances velocity is not well- defined (meaning it is a path dependent parameter
Agree.

TrickyDicky said:
and if you consider the LCDM model a valid one you will have to agree that at a certain scale the universe is homogeneous in the way FRW model predicts,
Agree, meaning the stress energy tensor is homogenous.

TrickyDicky said:
therefore in this context size (or scale) is not irrelevant, there can't be a hypercluster (of whatever the size it is required by the LCDM model that the universe is truly homogeneous) that is not comoving with CMBR, if you don't agree with these two facts let me know.
Disagree. As long as the observers stress-energy is negligible (usually implied), then you certainly could have a hyper cluster sized non-comoving observer.
 
Last edited:
  • #27
Q-reeus said:
I'm no cosmologist but the only maybe useful observation I can make is that as real universe is lumpy not spatially homogeneous, temporal 'lumpiness' is also present to some extent. So higher gravitational potential inside a cosmic void region implies experiencing more relative total time since BB than inside a clumped region. And of course considerably less time has passed at the surface of an ancient neutron star say. But guessing that is not your concern here, as it seems from your earlier remarks we are assuming homogeneity.

Q-reeus brings up a point that I would like to see a more complete analysis of. Historically the initial condition problem began with the so called arrow of time issue that arose with the development of statistical mechanics. Poincare showed in 1893 that a time reversible dynamics would return to initial conditions given some impossible large number of years, which technically violates the second law. Long story short this lead to speculations about initial conditions we now call the Big Bang.

Although the FRW metric is certainly a valid solution to the EFE is is not the only possible solution. Such alternative speculations are unwarranted, though we can ask what the standard solution entails observationally. Given that gravitational time dilation is absolute, though not in degree, this implies the issue Q-reeus brought up.

The question is how dependent is the apparent age and size of the Universe on differing gravitational depths of a pair of observers watching the Universe grow and age? If it is entirely independent, as appears likely under under relativistic symmetries, it implies that the age/size factor of the Universe is independent of our local clock rates. This implies that counting out 14 billion years on our local clocks might not properly scale to the beginning of the Universe. If the age of the Universe is dependent on local inertial clock rates, with an isotropic expansion rate of space itself, it implies that the apparent age, density, etc, is dependent on the inertial state or the observer.

Trying to work through all the relativistic transforms for both general and special relativity under different initial and boundary condition assumptions is messy at best. Can anybody point at published work that explicitly compares the effects on comoving observers which do not share a common gravitational depth and/or inertial frame?
 
  • #28
TrickyDicky said:
I'll try again, in an perfect idealized FRW model, so that we can consider ourselves and all observers like at least hyperclusters size observers, all observers must be basically comoving since we should see the universe as isotropic and homogeneous.

No, it is *not* true that all observers see the universe as homogeneous and isotropic, even in the perfect idealized FRW model. Only a particular set of observers does, the set whose worldlines are orthogonal to the set of spacelike hypersurfaces which are homogeneous and isotropic. Are you trying to claim that *every single worldline* in the spacetime is orthogonal to that set of hypersurfaces? That's absurd. If that isn't what you're saying, then I don't understand what you're saying; the only other way I can parse your statements is that you're saying that comoving observers are comoving, which is a tautology.

Edit: It looks like DaleSpam is making similar objections to mine, I agree with his posts.

TrickyDicky said:
BTW this also implies that we should be observing that slight deviation from Hubble law in the nearby universe region, but we are actually not observing it, for reference this paradox is known as the "de Vaucouleurs paradox" in honor of the brilliant cosmologist that pointed it out, and has not yet been explained.

I'll look it up. Do you have any particular references?
 
  • #29
TrickyDicky said:
BTW this also implies that we should be observing that slight deviation from Hubble law in the nearby universe region, but we are actually not observing it, for reference this paradox is known as the "de Vaucouleurs paradox" in honor of the brilliant cosmologist that pointed it out, and has not yet been explained.

Found this web page by David Wiltshire at the University of Canterbury in New Zealand:

http://www2.phys.canterbury.ac.nz/~dlw24/universe/general.html

Here's how he describes the paradox:

SANDAGE-DE VAUCOULEURS PARADOX

... In the standard way of thinking about cosmological averages, if you take a box of the size of average homogeneity, then you should expect galaxies to have large peculiar velocities if you average on scales much smaller than the homogeneous box, which I mentioned before was of order 170 Mpc. In particular, if you look at very small scales the statistical scatter of peculiar velocities should be so great that no linear Hubble law between redshift and distance can be extracted. Yet Hubble discovered his law on nearby scales of 20Mpc, 10% of the scale of homogeneity. By standard thinking this does not make sense.

As he describes it, the paradox is that on scales small compared to the size of the universe, we should not be able to see a Hubble law at all. However, he goes on to say that, while we do observe a Hubble law even on small distance scales, the Hubble constant we observe on those scales is *smaller* than the one we observe on larger scales:

As far as the argument of Sandage and de Vaucouleurs is concerned; if we measure the Hubble constant in an ideal "bubble wall" where the average clock rate is close to ours, we will get a low value of the Hubble constant, 48 km/s/Mpc. If we measure it to the other side of a void of the dominant size of 48Mpc across, we will get a higher value, 76 km/s/Mpc - because space appears to be expanding faster there by our clocks - which are going slower than the clocks in the voids. Once we average on the scale of apparent homogeneity, our average includes as many bubble walls as voids as the average in the observable universe, then we converge to a "global average" Hubble constant between the two extremes, of 62 km/s/Mpc.

As he goes on to note, the value of the Hubble constant has been an area of some dispute; the current "consensus" value is around 72 km/s/Mpc, but the Sandage team claims a value of around 62 km/s/Mpc, the "global average" value given above. But a variation of the "expansion rate" of the universe with size scale would remove the apparent discrepancy. The reason for the apparent variation in "expansion rate", he says, is simply that galactic clusters are bound systems, and we are inside one: so our local clocks run slower than clocks in the voids, outside the bound systems, for the simple reason that we are inside the gravity well of our local cluster and the voids are not.

Wiltshire has a paper on arxiv in which he claims to be able to account for the observations currently attributed to "dark energy" by this method:

http://arxiv.org/abs/0809.1183

I don't know if others on PF have seen this paper and can give any input. In particular, I don't know if his proposed model fits the other observations that have led to the current consensus Lambda-CDM model, which are well described, for example, by Ned Wright in his cosmology tutorial here:

http://www.astro.ucla.edu/~wright/cosmo_01.htm
 
  • #30
DaleSpam said:
Disagree. As long as the observers stress-energy is negligible (usually implied), then you certainly could have a hyper cluster sized non-comoving observer.

PeterDonis said:
Are you trying to claim that *every single worldline* in the spacetime is orthogonal to that set of hypersurfaces? That's absurd. If that isn't what you're saying, then I don't understand what you're saying;
No, not every worldline, if you read carefully you'll notice I'm talking about a certain sized objects.
In this case the observer stress-energy can't be neglected because it is an important part of the assumption.
Usually it is implied to be negligible precisely because it is understood that the FRW model is an idealization, thus galaxies and clusters are considered "dust" even if we know their stress energy is pretty great in reality.
My set up is just a "cosmological rescaling" so to speak.
 
  • #31
PeterDonis said:
No, it is *not* true that all observers see the universe as homogeneous and isotropic, even in the perfect idealized FRW model. Only a particular set of observers does, the set whose worldlines are orthogonal to the set of spacelike hypersurfaces which are homogeneous and isotropic. Are you trying to claim that *every single worldline* in the spacetime is orthogonal to that set of hypersurfaces? That's absurd.

This is related to the question I asked. In what way do you support this claim of absurdity? My cosmology may be weak but I know that as you get farther out the peculiar motions settle out and and the isotropy begins to dominate. Though here is a 2007 New Astronomy paper that claims to have observed some anisotropy in the overall Hubble expansion the first sentence in the abstract says:
[PLAIN]http://arxiv.org/abs/astro-ph/0703556 said:
Based[/PLAIN] [Broken] on general relativity, it can be argued that deviations from a uniform Hubble flow should be thought of as variations in the Universe's expansion velocity field, rather than being thought of as peculiar velocities with respect to a uniformly expanding space.

In the article it states:
[PLAIN]http://arxiv.org/PS_cache/astro-ph/pdf/0703/0703556v1.pdf said:
The[/PLAIN] [Broken] Cosmological Principle—that the Universe is homogeneous and isotropic—is generally assumed to hold, since averaged over large enough scales the Universe will appear homogeneous. However, general relativity is needed to understand not only small dense systems, but large diffuse systems such as the Universe, and according to Einstein’s field equations, the spacetime corresponding to a homogeneous universe can not be used to represent a spatially averaged inhomogeneous universe.

Qualitatively it is not hard to see how under special relativity maintains a homogeneous expansion under a boost. If you have two equidistant galaxies some light years away in opposite direction they share essentially the same Hubble shift. Now boost an observer in the direction of one of the galaxies, say A. The redshift of galaxy A is decreased while the other increases. However, under SR this observer now measures the distance to galaxy A as shorter, hence galaxy A has proper distance as defined by that observer that warrants labeling the decreased redshift as a constant indicator of that proper distance. Visa versa for galaxy B. Hence under boost the Hubble law remains a valid constant. The homogeneity of the Hubble constant is not frame dependent under special relativity.

Under GR, under a change of gravitation depth, the observational effects are essentially the same for both galaxies. Given that light speed defines both time and distance for each observer, contains the very definition of relativistic simultaneity, the isotropy remains even more generally. This doesn't conflict with the quoted paper since this does not entail a statement of how homogeneous the Universe actually is, only how boost and gravitational depths can effect an observers measure of that homogeneity.

So I ask, how do you empirically justify that only a particular set of observers see the universe as homogeneous and isotropic? It seems to me that if what you claim is actually true then we should be able to measure distances just by the amount of anisotropy we can induce with a local boost. Not seeing that go anywhere. So explain?
 
Last edited by a moderator:
  • #32
PeterDonis said:
Found this web page by David Wiltshire at the University of Canterbury in New Zealand:

http://www2.phys.canterbury.ac.nz/~dlw24/universe/general.html

Here's how he describes the paradox:



As he describes it, the paradox is that on scales small compared to the size of the universe, we should not be able to see a Hubble law at all. However, he goes on to say that, while we do observe a Hubble law even on small distance scales, the Hubble constant we observe on those scales is *smaller* than the one we observe on larger scales:



As he goes on to note, the value of the Hubble constant has been an area of some dispute; the current "consensus" value is around 72 km/s/Mpc, but the Sandage team claims a value of around 62 km/s/Mpc, the "global average" value given above. But a variation of the "expansion rate" of the universe with size scale would remove the apparent discrepancy. The reason for the apparent variation in "expansion rate", he says, is simply that galactic clusters are bound systems, and we are inside one: so our local clocks run slower than clocks in the voids, outside the bound systems, for the simple reason that we are inside the gravity well of our local cluster and the voids are not.

Wiltshire has a paper on arxiv in which he claims to be able to account for the observations currently attributed to "dark energy" by this method:

http://arxiv.org/abs/0809.1183

I don't know if others on PF have seen this paper and can give any input. In particular, I don't know if his proposed model fits the other observations that have led to the current consensus Lambda-CDM model, which are well described, for example, by Ned Wright in his cosmology tutorial here:

http://www.astro.ucla.edu/~wright/cosmo_01.htm

Wiltshire is a bit controversial and certainly his "solutions" (to the paradox or to dark energy) are not considered mainstream. AFAIK is an open problem in cosmology, one that is rarely discussed or known though.
 
  • #33
I would find it far easier to deal with these kinds of questions and learn far more about the science involved without all the model specific assumptions built in. Unfortunately wading through this or that model trying to explain the observations produces a large work load to try and separate out the observations from the model specific assumptions.

So, although I am not terribly interested in particular model specific description, I will expand my original question of how do you "empirically justify that only a particular set of observers see the universe as homogeneous and isotropic". Expand this to: How can you even theoretically justify that only a particular set of observers see the universe as homogeneous and isotropic, under empirical constraints.
 
  • #34
my_wan said:
How can you even theoretically justify that only a particular set of observers see the universe as homogeneous and isotropic, under empirical constraints.
This would require reading a few books about cosmology and GR, but you can start looking up "Weyl's postulate" in wikipedia( and possibly "cosmological principle" and "FRW metric".
 
  • #35
TrickyDicky said:
In this case the observer stress-energy can't be neglected because it is an important part of the assumption.
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.
 
Last edited:
<h2>What is the BB theory?</h2><p>The BB theory, also known as the Big Bang theory, is a scientific explanation for the origin and evolution of the universe. It proposes that the universe began as a singularity, a point of infinite density and temperature, and has been expanding and cooling over billions of years.</p><h2>How does the BB theory explain the expansion of the universe?</h2><p>The BB theory suggests that the universe is constantly expanding, with galaxies moving away from each other due to the force of the initial explosion. This expansion is supported by evidence such as the redshift of light from distant galaxies and the cosmic microwave background radiation.</p><h2>What are preferred frames in relation to the BB theory?</h2><p>In the context of the BB theory, preferred frames refer to a hypothetical set of reference frames in which the laws of physics appear to be the same in all directions. This concept is important in understanding the distribution of matter and energy in the universe and the overall structure of the cosmos.</p><h2>Why is the concept of preferred frames controversial?</h2><p>The idea of preferred frames is controversial because it goes against the principle of relativity, which states that the laws of physics are the same in all inertial reference frames. Some scientists argue that the concept of preferred frames is unnecessary and that the laws of physics can be explained without it.</p><h2>How does the concept of preferred frames relate to the concept of a "center" of the universe?</h2><p>The concept of preferred frames is often associated with the idea of a "center" of the universe, but this is a misconception. Preferred frames do not imply that there is a specific point in the universe that can be considered the center. Rather, they refer to a set of reference frames that are preferred for studying the overall structure and expansion of the universe.</p>

What is the BB theory?

The BB theory, also known as the Big Bang theory, is a scientific explanation for the origin and evolution of the universe. It proposes that the universe began as a singularity, a point of infinite density and temperature, and has been expanding and cooling over billions of years.

How does the BB theory explain the expansion of the universe?

The BB theory suggests that the universe is constantly expanding, with galaxies moving away from each other due to the force of the initial explosion. This expansion is supported by evidence such as the redshift of light from distant galaxies and the cosmic microwave background radiation.

What are preferred frames in relation to the BB theory?

In the context of the BB theory, preferred frames refer to a hypothetical set of reference frames in which the laws of physics appear to be the same in all directions. This concept is important in understanding the distribution of matter and energy in the universe and the overall structure of the cosmos.

Why is the concept of preferred frames controversial?

The idea of preferred frames is controversial because it goes against the principle of relativity, which states that the laws of physics are the same in all inertial reference frames. Some scientists argue that the concept of preferred frames is unnecessary and that the laws of physics can be explained without it.

How does the concept of preferred frames relate to the concept of a "center" of the universe?

The concept of preferred frames is often associated with the idea of a "center" of the universe, but this is a misconception. Preferred frames do not imply that there is a specific point in the universe that can be considered the center. Rather, they refer to a set of reference frames that are preferred for studying the overall structure and expansion of the universe.

Similar threads

  • Special and General Relativity
Replies
15
Views
1K
  • Special and General Relativity
2
Replies
51
Views
2K
  • Special and General Relativity
Replies
2
Views
810
  • Special and General Relativity
2
Replies
50
Views
5K
  • Special and General Relativity
3
Replies
83
Views
3K
  • Special and General Relativity
2
Replies
51
Views
2K
  • Special and General Relativity
Replies
18
Views
2K
  • Special and General Relativity
Replies
13
Views
2K
Replies
10
Views
4K
  • Special and General Relativity
2
Replies
47
Views
5K
Back
Top