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Can metric junctions look like gravitational fields?But these aren't
#1
Metric junctions have been studied off and on since the 1930's. As far as I know this isn't an active field of research now. But, maybe it should be. W. Israel showed how to treat an infinitely thin shell relativistically;his "thin shell" method was significant because singularities were known to raise their ugly heads if the shell was allowed any structure. But these singularities aren't just mathematical, but have physical effects because they arise in the space time maths.

In his treatment he allowed for boundary conditions that separated two equivalent spaces. But in our Universe it may be argued that we have at least two differing space times. We have the static space times within bound gravitational structures and we have the expanding space times associated with the Voids. And, thus we can conclude that we have existing metric junctions wherever gravitational structures abut Voids. In effect, everywhere!

So why don't we observe singularities at these junctions? The whole Universe is in flux,in motion. Galaxies swell and contract as they rotate;with varying mass arms presenting themselves to differing gravitational fields within cluster and supercluster formations. This motion ensures that thin shell space time boundaries between matter structures and expanding Voids never occur. 

It seems reasonable to ask " do these junctions look like gravitational fields to the rest of the universe?" It is certainly reasonable to investigate these space time junctions. When singularities cannot form does it preclude the existence of any type of potential? It seems to me that without the "pressure" applied by the expanding Voids against the static space times of bound matter structures
There would be no significant boundary structure. And thus, no dark matter.
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#2
If I understand correctly, you're saying the boundary between void (where universe is expanding) and a gravitationally bound structure develops some sort of "sheet" or topological defect which distorts spacetime causing effects like gravity. But individual galaxies don't abut void, in the usual model; entire clusters are gravitationally bound. Thus your idea wouldn't be able to address the flat spiral galaxy rotation curves, or even inter-cluster motions which violate virial theorem. Perhaps you postulate that the edges of galaxies do, in fact, have such effects. Seems you're implying that when you say "with varying mass arms presenting themselves to differing gravitational fields within cluster and supercluster formations".

Unlike most "personal theories" the idea isn't unreasonable on the face of it. Get to work, develop the math, maybe it's worth something.
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#3
Infinitely thin shells are, clearly, idealizations which contain, from the start, by construction, infinite elements. To have an own non-zero energy-momentum tensor, which would be able to influence something, one would have to assign some infinite energy-momentum density to it. Ok, usually the situation is much more harmless, we have, say, a surface of the star, and the energy-momentum tensor only changes, in a discontinuous way, near the surface. This is usually an un-problematic approximation, but, of course, one has to care about using the correct boundary conditions.

But even these discontinuities are not real discontinuities, but approximations. And there is a standard way to handle them - to use a smooth approximation, compute everything, and then take the limit where the smooth function becomes discontinuous.
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#4
Secure raises a significant point. This is a point I've already recognized and admittedly have no ready answer for. What role does position play for any individual galaxy within a structure? So far my thinking has led me to wonder about the intergalactic spaces and to think of them in terms of an analogy. Sandstones are porous rocks with huge potential as reservoirs for liquid water. The water resides in the interstitial spaces between the SiO2 granules because they are loosely bound. The key point being that interstices exist throughout the main body. I would suggest that a similar analogy exists for galactic structures and that expansion does occur within the interstitial spaces in some cases. For support I would point out that peculiar velocities are recognized. The predominant paradigm however, ascribes these as all due to gravitational collapse; whereas I would ask one to at least recognize the "snowplow" effect of expanding space times and ask whether some of this movement arises from the juxtaposition of Voids and a consequent 'squeezing' of matter structures.

In response to Schmelzer's post I come from a slightly different angle. The maths required have to speak to the idea being proposed. Without a specific model first I wonder if the existing maths are adequate to illustrate my idea. So, another analogy.... In any two body system the gravitational fields combine to create volumes where the gravitational potential influences the behavior of some third body...the Lagrangian Points. The key here is that two fields, with the same metric signatures combine on a planar surface. If we consider that a gravitational field is only a type of space time configuration then we have to ask, "can any space time configuration be considered a field?" I think it's reasonable to affirm this and to recognize that expanding space times within Voids are another type of field.
If we acknowledge this we can then ask " is there a corresponding Lagrangian response between two different, abutting space times and how do we model this"? I think the answer is yes a Lagrangian can be expected.
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