Modified Gravity and Cosmology

We give a general introduction to quantum phase transitions in strongly-correlated electron systems. These transitions which occur at zero temperature when a non-thermal parameter $g$ like pressure, chemical composition or magnetic field is tuned to a critical value are characterized by a dynamic exponent $z$ related to the energy and length scales $\Delta$ and $\xi$. Simple arguments based on an expansion to first order in the effective interaction allow to define an upper-critical dimension $D_{C}=4$ (where $D=d+z$ and $d$ is the spatial dimension) below which mean-field description is no longer valid. We emphasize the role of pertubative renormalization group (RG) approaches and self-consistent renormalized spin fluctuation (SCR-SF) theories to understand the quantum-classical crossover in the vicinity of the quantum critical point with generalization to the Kondo effect in heavy-fermion systems. Finally we quote some recent inelastic neutron scattering experiments performed on heavy-fermions which lead to unusual scaling law in $\omega /T$ for the dynamical spin susceptibility revealing critical local modes beyond the itinerant magnetism scheme and mention new attempts to describe this local quantum critical point.

https://arxiv.org/pdf/cond-mat/0102119.pdf

Quantum Phase Transitions

Massive gravity has seen a resurgence of interest due to recent progress which has overcome its traditional problems, yielding an avenue for addressing important open questions such as the cosmological constant naturalness problem. The possibility of a massive graviton has been studied on and off for the past 70 years. During this time, curiosities such as the van Dam, Veltman, and Zakharov (vDVZ) discontinuity and the Boulware-Deser ghost were uncovered. These results are rederived in a pedagogical manner and the St\"uckelberg formalism to discuss them from the modern effective field theory viewpoint is developed. Recent progress of the last decade is reviewed, including the dissolution of the vDVZ discontinuity via the Vainshtein screening mechanism, the existence of a consistent effective field theory with a stable hierarchy between the graviton mass and the cutoff, and the existence of particular interactions which raise the maximal effective field theory cutoff and remove the ghosts. In addition, some peculiarities of massive gravitons on curved space, novel theories in three dimensions, and examples of the emergence of a massive graviton from extra dimensions and brane worlds are reviewed.

Theoretical Aspects of Massive Gravity

Kaluza-Klein Supergravity

Super Collider Physics

Towards a realistic Kaluza-Klein cosmology

Evolving Lorentzian wormholes with the required matter satisfying the energy conditions are discussed. Several different scale factors are used and the corresponding consequences derived. The effect of extra, decaying (in time) compact dimensions present in the wormhole metric is also explored and certain interesting conclusions are derived for the cases of exponential and Kaluza-Klein inflation. \textcopyright{} 1996 The American Physical Society.

Evolving Lorentzian wormholes.

We suggest that the reactions $\mathrm{pp}\ensuremath{\rightarrow}{W}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\gamma}X$ and $p\overline{p}\ensuremath{\rightarrow}{W}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\gamma}X$ are good candidates for measuring the magnetic moment parameter $\ensuremath{\kappa}$ in ${\ensuremath{\mu}}_{W}=(\frac{e}{2{M}_{W}})(1+\ensuremath{\kappa})$. The angular distribution of the $W$ bosons in $p\overline{p}\ensuremath{\rightarrow}{W}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\gamma}X$ is particularly sensitive to this parameter. For the gauge-theory value of $\ensuremath{\kappa}=1$, we have found a peculiar zero in $\frac{d\ensuremath{\sigma}(d\overline{u}\ensuremath{\rightarrow}{W}^{\ensuremath{-}}\ensuremath{\gamma})}{d}cos\ensuremath{\theta}$ at $cos\ensuremath{\theta}=\ensuremath{-}\frac{1}{3}$, the location of this zero depending on the quark charge through $cos\ensuremath{\theta}=\ensuremath{-}(1+2{Q}_{d})$. A similar zero occurs in $\frac{d\ensuremath{\sigma}(u\overline{d}\ensuremath{\rightarrow}{W}^{+}\ensuremath{\gamma})}{d}cos\ensuremath{\theta}$. We can offer no explanation for this behavior.

Magnetic moment of weak bosons produced in pp and pp-bar collisions

We augment a previous discussion of the production of pairs of gauge bosons with a study of the reactions pp and p-barp --> WZ/sup 0/X or WgammaX As before, these depend upon trilinear boson couplings and the high-energy behavior is controlled by gauge cancellations In particular, the (hard) photon production is sensitive to the magnetic-moment parameter kappa for the W We also discuss the related neutrino reactions, nue --> WZ/sup 0/ or Wgamma, which may be of interest in very-high-energy cosmic-ray physics

W/sup + -/Z/sup 0/ and W/sup + -/. gamma. pair production in. nu. e, pp, and p-barp collisions

The dimensional reduction of massless gauge-invariant free Lagrangians for totally symmetric and totally antisymmetric tensors of arbitrary integral spins is carried out for the covariant and light-cone gauges. The structure of the auxiliary fields for the massive Lagrangians so obtained is elucidated.

Deshdeep Sahdev

Papers

Towards a realistic Kaluza-Klein cosmology

Evolving Lorentzian wormholes.

Magnetic moment of weak bosons produced in pp and pp-bar collisions

W/sup + -/Z/sup 0/ and W/sup + -/. gamma. pair production in. nu. e, pp, and p-barp collisions

Dimensional reduction of symmetric higher spin actions i: bosons