Inflation from String/M-Theory Compactification?

We present some exact scalar potentials for the dimensionally reduced theory and examine the possibility of obtaining accelerating 4d cosmology from String/M-theory, more generally, hyperbolic and flux compactification. In the hyperbolic case, even in the zero-flux limit, the scalar potential is positive for the 4d effective theory as required to get an accelerating universe, and thereby evading the ``no-go theorem'' given for static internal space. When we turn on the gauge fields as source terms at the cosmological background with potential V\propto exp(-2c\phi), we find eternally accelerating cosmologies when the 4d space-time is flat and c\geq 1, or hyperbolic and 1<c<\sqrt{2}.Comment: 3 pp. espcrc2.sty. Minor typos corrected and Ref. added. To appear in proceedings of Lattice 2003 (Gravity), Tsukuba, Japan, July 200

Brans-Dicke geometry

We reveal the non-metric geometry underlying omega-->0 Brans-Dicke theory by unifying the metric and scalar field into a single geometric structure. Taking this structure seriously as the geometry to which matter universally couples, we show that the theory is fully consistent with solar system tests. This is in striking constrast with the standard metric coupling, which grossly violates post-Newtonian experimental constraints.Comment: 8 pages, v2 with additional comment and reference

Quantum Phase Transitions in the Itinerant Ferromagnet ZrZn2_2

We report a study of the ferromagnetism of ZrZn$_{2}$, the most promising material to exhibit ferromagnetic quantum criticality, at low temperatures $T$ as function of pressure $p$. We find that the ordered ferromagnetic moment disappears discontinuously at $p_c$=16.5 kbar. Thus a tricritical point separates a line of first order ferromagnetic transitions from second order (continuous) transitions at higher temperature. We also identify two lines of transitions of the magnetisation isotherms up to 12 T in the $p-T$ plane where the derivative of the magnetization changes rapidly. These quantum phase transitions (QPT) establish a high sensitivity to local minima in the free energy in ZrZn$_{2}$, thus strongly suggesting that QPT in itinerant ferromagnets are always first order

Geometry for the accelerating universe

The Lorentzian spacetime metric is replaced by an area metric which naturally emerges as a generalized geometry in quantum string and gauge theory. Employing the area metric curvature scalar, the gravitational Einstein-Hilbert action is re-interpreted as dynamics for an area metric. Without the need for dark energy or fine-tuning, area metric cosmology explains the observed small acceleration of the late Universe.Comment: 4 pages, 1 figur

Cosmological constraints on a classical limit of quantum gravity

We investigate the cosmology of a recently proposed deformation of Einstein gravity, emerging from quantum gravity heuristics. The theory is constructed to have de Sitter space as a vacuum solution, and thus to be relevant to the accelerating universe. However, this solution turns out to be unstable, and the true phase space of cosmological solutions is significantly more complex, displaying two late-time power-law attractors -- one accelerating and the other dramatically decelerating. It is also shown that non-accelerating cosmologies sit on a separatrix between the two basins of attraction of these attractors. Hence it is impossible to pass from a decelerating cosmology to an accelerating one, as required in standard cosmology for consistency with nucleosynthesis and structure formation and compatibility with the data inferred from supernovae Ia. We point out that alternative models of the early universe, such as the one investigated here might provide possible ways to circumvent these requirements.Comment: 14 pages, 2 figures, REVTeX

Sectional Curvature Bounds in Gravity: Regularisation of the Schwarzschild Singularity

A general geometrical scheme is presented for the construction of novel classical gravity theories whose solutions obey two-sided bounds on the sectional curvatures along certain subvarieties of the Grassmannian of two-planes. The motivation to study sectional curvature bounds comes from their equivalence to bounds on the acceleration between nearby geodesics. A universal minimal length scale is a necessary ingredient of the construction, and an application of the kinematical framework to static, spherically symmetric spacetimes shows drastic differences to the Schwarzschild solution of general relativity by the exclusion of spacelike singularities.Comment: 20 pages, 1 figure, REVTeX4, updated reference

Reflection-Free One-Way Edge Modes in a Gyromagnetic Photonic Crystal

We point out that electromagnetic one-way edge modes analogous to quantum Hall edge states, originally predicted by Raghu and Haldane in 2D gyroelectric photonic crystals possessing Dirac point-derived bandgaps, can appear in more general settings. In particular, we show that the TM modes in a gyromagnetic photonic crystal can be formally mapped to electronic wavefunctions in a periodic electromagnetic field, so that the only requirement for the existence of one-way edge modes is that the Chern number for all bands below a gap is non-zero. In a square-lattice gyromagnetic Yttrium-Iron-Garnet photonic crystal operating at microwave frequencies, which lacks Dirac points, time-reversal breaking is strong enough that the effect should be easily observable. For realistic material parameters, the edge modes occupy a 10% band gap. Numerical simulations of a one-way waveguide incorporating this crystal show 100% transmission across strong defects, such as perfect conductors several lattice constants wide, larger than the width of the waveguide.Comment: 4 pages, 3 figures (Figs. 1 and 2 revised.