Big Bang and absolute simultaneity

[loleater]

I was wondering does the notion that the universe is expanding uniformly means there is no absolute rest, but there is absolute simultaneity?

 

[mgb_phys]

If you mean being aable to measure if two things in different places happen at the same time, this is nothing to do with the universe expanding - it's just special relativity.

 

[marcus]

I was wondering does the notion that the universe is expanding uniformly means there is no absolute rest, but there is absolute simultaneity?

the universe expanding more or less on average uniformly is called the "Hubble flow"
and it defines an idea of absolute rest
Cosmologists speak of an observer being "at rest with respect to the Hubble flow"

The solar system is moving about 380 km/second with respect to Hubble flow----in the direction of the constellation Leo.
For some purposes one has to compensate for that motion and take it out of the data.

Hubble himself already figured this out and estimated the motion (but not so accurately) probably back in the 1930s

When the CMB (cosmic microwave background) was discovered the idea of absolute rest was corroborated

Cosmologists now speak of being "at rest with respect to the CMB." It means exactly the same thing

The solar system is moving about 380 km/second with respect to the CMB----in the direction of the constellation Leo.
For some purposes one has to compensate for that motion and take it out of the data. It is called "removing the dipole".
The dipole in the temperature map of the CMB is the artificially hot spot in constellation Leo and artificially cold spot 180 degrees
from there which are caused the Doppler effect of the solar system motion.

Special relativity symmetries do not apply globally to solutions of GENERAL relativity, such as the standard LambdaCDM model cosmology that is normally used.
So some things you are taught to expect in SPECIAL do not carry over to cosmology.

There is, in cosmology, a notion of absolute simultaneity. For instance Ned Wright uses it in his tutorial when he defines and discusses the usual distance scale called COMOVING DISTANCE. Comoving distance is the distance to an object at the present moment-----you can think of a chain of observers all at rest with respect in the absolute sense, with all their clocks synchronized, who all at the same moment (now, today) measure the distance to their nearest neighbors. the chain of observers stretches from here to the object and you add it all up to get the comoving distance to the object at the present moment.

as long as you are posting at the Cosmology forum then it is OK for you to talk like a professional cosmologist----so you can have a concept of absolute rest and of simultaneity.

there are practical limitations to these ideas (limits to how precisely one can measure the CMB dipole) practical difficulties defining simultaneity precisely given the bumpy uneven nature of spacetime but they work to a useful degree of approximation

By contrast the spacetime of SPECIAL relativity is unrealistic because in it distances do not expand and there is no curvature due to gravity. One reason one can see that the spacetime of special relativity is unrealistic is because it LACKS an idea of absolute rest, which in the real world we get immediately by observing the CMB (or as Hubble did earlier by measuring redshifts at comparable distances in several directions)

Hope you enjoy historical irony---and tricks Nature plays on scientists. Someone named LOL-eater ought to. Have some LOL

 

[loleater]

Thanks lol, so there is both absolute rest and absolute simultaneity?
And I was wondering if you could clarify why "Special relativity symmetries do not apply globally to solutions of GENERAL relativity, such as the standard LambdaCDM model cosmology that is normally used."
Thanks.

 

[marcus]

Hi Lol-eater,
I will try to answer. What I said sounds complicated but is not actually very complicated.

the space of SR is called Minkowski spacetime. it is just R⁴ ordinary flat D4 euclidean but with a special metric. the symmetries are the Poincare group.
I wish the terminology didn't sound so technical

I am puzzled how to say something basically simple (which sounds complicated merely for language reasons).

I guess the point is that REAL spacetime is bumpy and irregular and has expanding distances. So if you take a piece of its geometry and move it over ("translate" it) then it doesn't match the geometry where you moved it too.

Minkowski spacetime doesn't have any of those realistic features. If you slide around it is the same everywhere, it doesn't have expanding distances. It is completely regular and it satisfies Poincare symmetry exactly and all over ("globally")

Real spacetime doesn't satisfy Poincare symmetry except approximately and locally.
A small region looks very flat and Minkowski-like. (except near very massive objects) But if you look at the largescale picture it is noticeably bent out of shape.
When I say largescale I mean not even any farther than, say, the nearest cluster of galaxies that is not connected to the Virgo supercluster our local group belongs to.

Real spacetime geometry is quite funny compared with flat Minkowski geometry of SR.
In the real world, for example, redshift z= 1.6 is not all that far away and yet if you go out past z = 1.6 then things start looking bigger the farther away they are.

So basically what I am saying is GR trumps SR. SR is just a local flat approximation. The real world geometry is a SOLUTION TO THE GR EQUATION and it does not have the precise regular uniformity of SR minkowski space.

Other people have made threads about this at PF, and may have said it better, but see if that explanation works for you. If not maybe someone will hunt down one of the threads of discussion we've had about this kind of thing.

 

[pervect]

As a purist, I would say that there is no absolute notion of simultaneity, even in cosmology. I would describe the notion of simultaneity in cosmology as "conventional", i.e. it is conventional to use cosmological time to define simultaneity in cosmology.

If you happen to be moving relative to the Hubble flow, though, you will still have your own notion of simultaneity due to SR. This SR notion of simultaneity will be different from the cosmological notion of simultaneity in the region where they both overlap, i.e. in your neighborhood. The existence of these different notions of simultaneity means that it's still relative, not absolute, according to my understanding of the common usage of "absolute".

 

[loleater]

@marcus: thankyou for clarifying it and yes I saw it hiding in another thread in the archives. @pervect: thanks for your explanation i can see the flaw in the idea I posted in the first post.
Thanks PF!