974 research outputs found
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 ZrZn
We report a study of the ferromagnetism of ZrZn, the most promising
material to exhibit ferromagnetic quantum criticality, at low temperatures
as function of pressure . We find that the ordered ferromagnetic moment
disappears discontinuously at =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 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, 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.
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