The big picture from the Dipole Repeller to the Great Attractor and beyond

AgentB
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Topic 205115

https://www.youtube.com/watch?v=NpV0GQo3P0c

Space may be hard, but is really really really big.

astro-marwil
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Hallo! Interesting,

Hallo!

Interesting, indeed!

So, our cosmos is not like a clumpy ball but like a clumpy pancake, as seen in some situations in the simulation. Astonishing finding!  

But isn´t this finding contradictory to the very basic postulate of cosmology, that on large scales the cosmos is homogeneous, independent of direction?

What forces do form our pancake universe?

 

Kind regards and happy crunching

Martin

astro-marwil
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Hallo!" isn´t this finding

Hallo!

" isn´t this finding contradictory to the very basic postulate of cosmology, that on large scales the cosmos is homogeneous, independent of direction? "

No, it isn´t, as its diameter of about 1,74E9[Lj] ist too small compared to the radius of 13,7E9[Lj] of the cosmos up to now. So this structure, we are belonging too, seems to me, to be part of the shell beween two very big voids in comos.  so no indication for a revolution in current cosmology.

Sorry for bothering you.

Kind regards an happy crunching

Martin

Mike Hewson
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Alas the horse falls at the

Alas the horse somewhat falls at the first fence here :

".... and it has been assumed that the excesses in the distribution of matter induce this motion."

Which sounds harmless enough until you realise that the prior history of the region - not only the existing velocity vectors - is required to make a dynamic comment. It is simply necessary to have a further time derivative involved. We haven't been around long enough to observationally generate those ( velocity ) differentials = accelerations. Other aspects are linguistic. If you use the modelling of spherical harmonics - monopoles attracting/repelling, and dipoles etc - then you will arrive at a narrative bound by that choice of reference system ie. ( anti-) flow lines and whatnot. So be careful about what one thinks is being described/deduced. Is it merely cosmography* which is a mapping of what is where, or going deeper to causal explanatory chains ? Note that the Dipole Repeller is an artifact of the analysis, not an observed object, and yes you can then derive equi-potential surfaces. I can't see anything wrong with what is presented, I am simply warning about over-reaching of conclusions.

{ Quick example of semantics : If I fall toward the ground, does that mean I am being 'repelled' from above ? :-) }

Nice graphics though ! :-)

Cheers, Mike.

* Hopefully useful counterpoints to contemplate : your local topography around your own house. Probably may be successfully modeled as basically a plane with bumps and dips from some chosen datum level. But beware. Exactly how did you decide to define the boundaries of that region ? How might your analysis change with increasing or decreasing scale ? We know that gotcha already : the Earth is flat ( enough ) until a larger bite indicates an inherent curvature. Note here the incredible cleverness of seaport lookouts from several centuries ago. The masts of ships were the first to appear on approach and the last to disappear from view on recession. For better warning then go higher eg. crows' nests on the top of ship masts. Does mapping that shape today decide either how it came to be or how it will evolve ? What might you not know that might matter ? ( shock/horror )

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

AgentB
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I read the first fence in

I read the first fence in total as

"The quest for the sources of that motion has dominated cosmography in recent decades .... and it has been assumed that the excesses in the distribution of matter induce this motion."

I don't understand what wrong conclusions could be made from that statement and the paper (my ignorance is bliss!).  I think basically they are pointing that voids push in an expanding universe (well known) and where a nearby void is located (the interesting thing).  To describe and model flows, then it seems reasonable to use appropriate maths and terminology for that. 

Mike Hewson wrote:
Note that the Dipole Repeller is an artifact of the analysis, not an observed object, and yes you can then derive equi-potential surfaces. I can't see anything wrong with what is presented, I am simply warning about over-reaching of conclusions.

Not sure what you mean here, are black holes (and voids) which can not be observed, just artefacts of analysis then?

 

Mike Hewson
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Despite the ( humanly ) huge

Despite the ( humanly ) huge scale under discussion, it is still not large enough to include dark-energy/lambda/vacuum-energy as acting significantly on the objects being discussed here, to produce the given flows. That is, if you take the reasonable range of lambda as deduced from comparing all-universe models to observed findings ( CMB et al ) then that lambda won't produce 'focal repulsion', for want of a better phrase, over this study's region. So yeah, space is really big .... :-)

As for the Repeller per se : well, we believe gravity to have infinite range and so what mass distributions might lie beyond the studied region that could give rise to the same observed motions ?  To my mind at least, using the words attractor/repeller imply a force/acceleration mapping. What is seen is a velocity field, the rest is modelling because we haven't been around long enough to observe another derivative. For example if I see an object zoom past my second floor window with some instantaneous speed then what floor did it originate from ? What non-gravitational forces are acting ? I have to look at a time series to even begin that analysis. The window's aperture literally frames my opportunity for knowledge. So yes there is a void, but a 'repelling' void ? My point was not to decry the assumption that "excesses in the distribution of matter induce this motion" but what matter set or sets does one contemplate ? The specific technical point here is the boundary conditions placed on the walls of the box, which one must have to solve a differential scheme. Again, the difference here is b/w description vs. causative comment.

So my explanation was aimed at someone - yourself here - that might attribute ( as you did ) a mere local churn for something of vastly greater importance. I judged that the language of attractor/repellers etc induced by their choice of analysis might do this. This is not to denigrate the authors or their hard work etc. Or yourself. It was a scale comment.

A good analogy may help : a tiny/just-visible vortex in a cup of my coffee still would not span the required scale difference. It is really hard to hold the true - or best guess - whole universe scale in the one cranium. In general Martin's issue raises a key point. We only have one viewpoint of measurement and that for only a brief time period in the evolution of the thing we try to comprehend.

BTW : so called dark energy acts within your lounge room ! You aren't going to notice that because it might induce an acceleration of the order of 10^[-45] m/s2 , whereas gravity as produced by Earth's inertial mass is about 10 m/s. So even if you scale up by a massive factor of 10^[40] then you still only get a wee small amount, acting over and above local matter induced effects. The reason why 'vacuum energy' is called that is : mathematically - in Einstein's GR equations - it is included separately from the typical mass, kinetic-energy etc terms. Regions of the universe with mass, kinetic energy etc are still posited as having a vacuum term. Think of it as the baseline above which we place all the other traditionally labelled stuff.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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Actually this discussion

Actually this discussion brings up another aspect. More general than what this study reveals. Classically - from Newton to Einstein if you like - it was assumed what the authors stated. That is : measured/visible mass/objects are the inputs to theory, and no other gravitational sources need be considered. You say : of course ! :-)

But SR's E = mc and GR's Principle of Equivalence require all forms of energy whatsoever to be accounted for. The masses m1 and m2 we annotate in Newton's Law of gravity :

|F| = G mm2 / r

is only one component of a full GR treatment*. Thus a hot ball of gas generates more gravity than a cold ball of gas, each with the same rest mass. A star whizzing past me has more gravity ( admittedly as a gravito-magnetic component ) than the same rest mass star not moving with respect to me. A spinning star has more gravity than a non-spinning one of the same rest mass etc.

A curly bit here is an accounting issue. One must be careful not to double count, in that we may express some energies in terms of others. Thus a pressure term is really the kinetic energy of smaller components, but for an arbitrary collection of things we can decompose their motions as that of the centre of mass plus those around the centre of mass. The base currency, as it were, is energy but each form may be 'painted' or annotated differently.

There is the Principle Of General Covariance which roughly states that the physical result predicted ought be independent of a chosen accounting categorisation/method. Now when applied to entire universes small scale effects are glossed over. The modelling typically used in cosmology is that of interacting 'dust'. The innards of each dust particle are ignored. These local features of attractors/repellers would be just one speck ..... :-)

Cheers, Mike. 

* So in a sense what I have objected to is the language of 'repulsion' used in a non-relativistic analysis. Neither fish nor fowl.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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