Modified Gravity: living without Birkhoff I. DGP

We consider the consequences of the absence of Birkhoff's theorem in theories of modified gravity. As an example, we calculate the gravitational force on a test particle due to a spherical mass shell in the Dvali-Gabadaze-Porrati model (DGP). We show that unlike in General Relativity, the force depends on the mass distribution. In particular, the gravitational force within a spherical mass shell depends on the geometric structure of the bulk, and is likely non-zero.Comment: 5 pages, 3 figure

Non-topological solitons in brane world models

We examine some general properties of a certain class of scalar filed theory models containing non-topological soliton solutions in the context of brane world models with compact large extra dimensions. If a scalar field is allowed to propagate in extra space, then, beside standard Kaluza-Klein type excitations, a whole new class of very massive soliton-type states can exist. Depending on their abundance, they can be important dark matter candidates or give significant contribution to entropy and energy density in our universe. .Comment: version accepted for publication in Physical Review

Bulk Fermion Stars with New Dimensions

Many efforts have been devoted to the studies of the phenomenology in particle physics with extra dimensions. We propose degenerate fermion stars with extra dimensions and study what features characterized by the size of extra dimensions should appear in its structure. We find that Kaluza-Klein excited modes arise for the larger scale of extra dimensions and examine the conditions on which different layers should be caused in the inside of the stars. We expound how the extra dimensions affect on physical quantities.Comment: 20 pages, 14 figure

Brane Localization and Stabilization via Regional Physics

Extra-dimensional scenarios have become widespread among particle and gravitational theories of physics to address several outstanding problems, including cosmic acceleration, the weak hierarchy problem, and the quantization of gravity. In general, the topology and geometry of the full spacetime manifold will be non-trivial, even if our ordinary dimensions have the topology of their covering space. Most compact manifolds are inhomogeneous, even if they admit a homogeneous geometry, and it will be physically relevant where in the extra-dimensions one is located. In this letter, we explore the use of both local and global effects in a braneworld scenario to naturally provide position-dependent forces that determine and stabilize the location of a single brane. For illustrative purposes, we consider the 2-dimensional hyperbolic horn and the Euclidean cone as toy models of the extra-dimensional manifold, and add a brane wrapped around one of the two spatial dimensions. We calculate the total energy due to brane tension and bending (extrinsic curvature) as well as that due to the Casimir energy of a bulk scalar satisfying a Dirchlet boundary condition on the brane. From the competition of at least two of these effects there can exist a stable minimum of the effective potential for the brane location. However, on more generic spaces (on which more symmetries are broken) any one of these effects may be sufficient to stabilize the brane. We discuss this as an example of physics that is neither local nor global, but regional.Comment: 4 pages, 2 figures. PRL submitte

Psychological problems resulting from parent-to-adolescent renal transplantation

This paper deals with the ways in which the psychological responses of renal transplant donors and recipients interfere with the developmental tasks of adolescence. The characteristic responses of donors and recipients in general are briefly reviewed. The special problems of the adolescent who has received a transplant from a parent are considered. Interference with the separation-individuation process is discussed. Also considered is the difficulty posed to the development of sexual identity. Excerpts from brief therapy are presented that illustrate the characteristic psychological conflicts described.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23095/1/0000014.pd

Flavordynamics with Conformal Matter and Gauge Theories on Compact Hyperbolic Manifolds in Extra Dimensions

We outline a toy model in which a unique mechanism may trigger a dynamical chain resulting in key low-energy regularities. The starting points are a negative cosmological term in the bulk and conformally invariant nongravity sector. These elements ensure compactification of the extra dimensional space on a compact hyperbolic manifold (with the negative and constant scalar curvature). The overall geometry is then M_4 x B_n. The negative curvature on B_n triggers the formation of the four-dimensional defect which provides in turn a dynamical localization of ordinary particles. It also leads, simultaneously, to a spontaneous breaking of gauge symmetry through a Higgs mechanism. Masses of the fermions, gauge bosons and scalars all derive from the curvature of the internal manifold such that the Higgs boson is generally heavier than the gauge bosons. The factorizable geometry M_4 x B_n and flatness of M_4 require fine-tuning.Comment: 16 pp, added references and a figure with improvements in text; journal versio

Measuring the Topology of the Universe

Observations of microwave background fluctuations can yield information not only about the geometry of the universe, but potentially about the topology of the universe. If the universe is negatively curved, then the characteristic scale for the topology of the universe is the curvature radius. Thus, if we are seeing the effects of the geometry of the universe, we can hope to soon see signatures of the topology of the universe. The cleanest signature of the topology of the universe is written on the microwave sky: there should be thousands of pairs of matched circles. These circles can be used to determine the precise topology and volume of the universe. Since we see hundreds of slices through the fundamental domain of the universe, we can use the microwave observations to reconstruct the initial conditions of the entire universe on the scale of a few Megaparsecs.Comment: 11 pages, LaTex, Talk at NAS Cosmology Conference, Irvine, CA, March 199

Radion Stabilization in Compact Hyperbolic Extra Dimensions

We consider radion stabilization in hyperbolic brane-world scenarios. We demonstrate that in the context of Einstein gravity, matter fields which stabilize the extra dimensions must violate the null energy condition. This result is shown to hold even allowing for FRW-like expansion on the brane. In particular, we explicitly demonstrate how one putative source of stabilizing matter fails to work, and how others violate the above condition. We speculate on a number of ways in which we may bypass this result, including the effect of Casimir energy in these spaces. A brief discussion of supersymmetry in these backgrounds is also given.Comment: 16 pages, 1 figur