Title: A New Dimension Hidden in the Shadow of a Wall Authors: Nemanja Kaloper

We propose a new way to hide the fifth dimension, and to modify gravity in the far infra-red. A gravitating tensional membrane in five dimensions folds the transverse space into a truncated cone, stoppered by the membrane. For near-critical tension, the conical opening is tiny, and the space becomes a very narrow conical sliver. A very long section, of length comparable to the membrane radius divided by the remaining conical angle, of this sliver is well approximated by a narrow cylinder ending on the membrane. Inside this cylindrical throat we can reduce the theory on the circle. At distances between the circle radius and the length of the cylinder, the theory looks 4D, with a Brans-Dicke-like gravity, and a preferred direction, while at larger distances the cone opens up and the theory turns 5D. The gravitational light scalar in the throat can get an effective local mass term from the interplay of matter interactions and quantum effective potentials on the cone, which may sup

Astrophysicist Alexander Kashlinsky of the NASA Goddard Space Flight Centre tackles this question. The evolution of the universe is described by the physics of general relativity, which was discovered by Albert Einstein in the early 20th century. When compared to Newtonian physics, this theory provides a radically different framework for the physical description of the gravitational force.

Title: A Spinor Theory of Gravity and the Cosmological Framework Authors: M. Novello

Recently we have presented a new formulation of the theory of gravity based on an implementation of the Einstein Equivalence Principle distinct from General Relativity. The kinetic part of the theory - that describes how matter is affected by the modified geometry due to the gravitational field - is the same as in General Relativity. However, we do not consider the metric as an independent field. Instead, it is an effective one, constructed in terms of two fundamental spinor fields \Psi and \Upsilon and thus the metric does not have a dynamics of its own, but inherits its evolution through its relation with the fundamental spinors. In the first paper it was shown that the metric that describes the gravitational field generated by a compact static and spherically symmetric configuration is very similar to the Schwarzschild metric. In the present paper we describe the cosmological framework in the realm of the Spinor Theory of Gravity.

Title: Visualising spacetimes via embedding diagrams Authors: Stanislav Hledik, Zdenek Stuchlik, Alois Cipko (revised v2)

It is hard to imagine curved spacetimes of General Relativity. A simple but powerful way how to achieve this is visualising them via embedding diagrams of both ordinary geometry and optical reference geometry. They facilitate to gain an intuitive insight into the gravitational field rendered into a curved spacetime, and to assess the influence of parameters like electric charge and spin of a black hole, magnetic field or cosmological constant. Optical reference geometry and related inertial forces and their relationship to embedding diagrams are particularly useful for investigation of test particles motion. Embedding diagrams of static and spherically symmetric, or stationary and axially symmetric black-hole and naked-singularity spacetimes thus present a useful concept for intuitive understanding of these spacetimes' nature. We concentrate on general way of embedding into 3-dimensional Euclidean space, and give a set of illustrative examples.

As big questions go, it's hard to get much bigger. Does space go on forever? Some cosmologists suspect not. Rather than stretching off into infinity, space might be a much smaller, more manageable place. If we could cross the cosmos in a spaceship, then like sailors circumnavigating the globe, we might find ourselves back where we started.

Unlike the Earth, though, the universe does not have to be round. Its true shape is still a mystery. Is it flat like a sheet of paper? Is it curved? Is it tied in a knot, tangled by gravity at the very beginning of the big bang? Does it even make sense to talk about the shape of something as complicated as the universe? After years of detective work, cosmologists are at last beginning to get a little closer to some answers.

The power of what is known as the anthropic principle has been challenged by a claim that it cannot, after all, make useful predictions about the universe. Supporters of the anthropic principle - the idea that the universe has to be the way it is for us to be around to observe it - say that it can explain many otherwise mysterious properties of the cosmos. They tend to focus on the so-called cosmological constant, or lambda, which determines the strength of the force that appears to be pushing the universe apart. String theory, combined with the theory of inflation, implies that there could be an infinity of universes, each with a different value of lambda ranging from zero up to a ludicrously high number about 10120 times what we observe. Most of these universes would be sterile. A super-strong lambda, for example, would shred matter before it formed.

Title: Are High-Redshift Quasars Blurry? Authors: Eric Steinbring

It has been suggested that the fuzzy nature of spacetime at the Planck scale may cause lightwaves to lose phase coherence, and if severe enough this could blur images of distant point-like sources sufficiently that they do not form an Airy pattern at the focal plane of a telescope. Blurring this dramatic has already been observationally ruled out by images from Hubble Space Telescope (HST), but I show that the underlying phenomenon could still be stronger than previously considered. It is harder to detect, which may explain why it has gone unseen. A systematic search is made in archival HST images of among the highest known redshift quasars. Planck-scale induced blurring may be evident, but this could be confused with partially resolved sources.

Title: Spinor Theory of Gravity Authors: M. Novello Version 3

The proposal of this work is to provide an answer to the following question: is it possible to treat the metric of space-time - that in General Relativity (GR) describes the gravitational interaction - as an effective geometry? In other words, to obtain the dynamics of the metric tensor as a consequence of the dynamics of other fields. In this work we will use a slight modification of the non-linear equation of motion of a spinor field proposed some years ago by Heisenberg, although in a completely distinct context, to obtain a field theory that provides a framework equivalent to the way GR represents the gravitational interaction. In particular we exhibit a solution of the equations of motion that represents the gravitational field of a compact object and compare it with the corresponding Schwarzschild solution of General Relativity.

Roni Harnik is an explorer who likes to think big. Foreign continents hold no sway over him. Not even the outer reaches of the solar system appeal. Harnik is a theoretical physicist at the Stanford Linear Accelerator Centre in California and his expeditions are on a far grander scale: he goes in search of other universes.

Title: Folded Supersymmetry and the LEP Paradox Authors: Gustavo Burdman, Z. Chacko, Hock-Seng Goh, and Roni Harnik

We present a new class of models that stabilise the weak scale against radiative corrections up to scales of order 5 TeV without large corrections to precision electroweak observables. In these ‘folded supersymmetric’ theories the one loop quadratic divergences of the Standard Model Higgs field are cancelled by opposite spin partners, but the gauge quantum numbers of these new particles are in general different from those of the conventional superpartners. This class of models is built around the correspondence that exists in the large N limit between the correlation functions of supersymmetric theories and those of their non-supersymmetric orbifold daughters. By identifying the mechanism which underlies the cancellation of one loop quadratic divergences in these theories, we are able to construct simple extensions of the Standard Model which are radiatively stable at one loop. Ultraviolet completions of these theories can be obtained by imposing suitable boundary conditions on an appropriate supersymmetric higher dimensional theory compactified down to four dimensions. We construct a specific model based on these ideas which stabilises the weak scale up to about 20 TeV and where the states which cancel the top loop are scalars not charged under Standard Model colour. Its collider signatures are distinct from conventional supersymmetric theories and include characteristic events with hard leptons and missing energy.

Title: A Universe Without Weak Interactions Authors: Roni Harnik, Graham D. Kribs, and Gilad Perez

A universe without weak interactions is constructed that undergoes big-bang nucleosynthesis, matter domination, structure formation, and star formation. The stars in this universe are able to burn for billions of years, synthesize elements up to iron, and undergo supernova explosions, dispersing heavy elements into the interstellar medium. These definitive claims are supported by a detailed analysis where this hypothetical “Weakless Universe” is matched to our Universe by simultaneously adjusting Standard Model and cosmological parameters. For instance, chemistry and nuclear physics are essentially unchanged. The apparent habitability of the Weakless Universe suggests that the anthropic principle does not determine the scale of electroweak breaking, or even require that it be smaller than the Planck scale, so long as technically natural parameters may be suitably adjusted. Whether the multi-parameter adjustment is realised or probable is dependent on the ultraviolet completion, such as the string landscape. Considering a similar analysis for the cosmological constant, however, we argue that no adjustments of other parameters are able to allow the cosmological constant to raise up even remotely close to the Planck scale while obtaining macroscopic structure. The fine-tuning problems associated with the electroweak breaking scale and the cosmological constant therefore appear to be qualitatively different from the perspective of obtaining a habitable universe.