950 research outputs found
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
Eddington-Born-Infeld action for dark energy and dark matter
We argue that Einstein gravity coupled to a Born-Infeld theory provides an
attractive candidate to represent dark matter and dark energy. For cosmological
models, the Born-Infeld field has an equation of state which interpolates
between matter, w=0 (small times), and a cosmological constant w=-1 (large
times). On galactic scales, the Born-Infeld field predicts asymptotically flat
rotation curves.Comment: A sign mistake in section on galactic scales is pointed out. This
sign invalidates the content of that section. See comment on manuscrip
Quadratic alpha' corrections to T-duality
The quadratic alpha' corrections to the two-dimensional black hole and to its
T-dual are calculated. These backgrounds are used to write the covariant form
of the quadratic alpha' corrections to the T-duality for general time-dependent
backgrounds of dilaton and diagonal metric in the bosonic string theory.Comment: 15 pages, JHEP; typos corrected, references adde
Multiple first-order metamagnetic transitions and quantum oscillations in ultrapure
We present measurements on ultra clean single crystals of the bilayered
ruthenate metal Sr3Ru2O7, which has a magnetic-field-tuned quantum critical
point. Quantum oscillations of differing frequencies can be seen in the
resistivity both below and above its metamagnetic transition. This frequency
shift corresponds to a small change in the Fermi surface volume that is
qualitatively consistent with the small moment change in the magnetisation
across the metamagnetic transition. Very near the metamagnetic field, unusual
behaviour is seen. There is a strong enhancement of the resistivity in a narrow
field window, with a minimum in the resistivity as a function of temperature
below 1 K that becomes more pronounced as the disorder level decreases. The
region of anomalous behaviour is bounded at low temperatures by two first-order
phase transitions. The implications of the results are discussed. PACS:
68.35.Rh, 71.27.+a, 72.15.-v, 74.70.PqComment: 12 pages 4 figures, submitte
The `s-rule' exclusion principle and vacuum interpolation in worldvolume dynamics
We show how the worldvolume realization of the Hanany-Witten effect for a
supersymmetric D5-brane in a D3 background also provides a classical
realization of the `s-rule' exclusion principle. Despite the supersymmetry, the
force on the D5-brane vanishes only in the D5 `ground state', which is shown to
interpolate between 6-dimensional Minkowski space and an -invariant
geometry. The M-theory analogue of these results is briefly
discussed.Comment: 25 pages, 9 figures, LaTeX JHEP styl
Electron Transport through Disordered Domain Walls: Coherent and Incoherent Regimes
We study electron transport through a domain wall in a ferromagnetic nanowire
subject to spin-dependent scattering. A scattering matrix formalism is
developed to address both coherent and incoherent transport properties. The
coherent case corresponds to elastic scattering by static defects, which is
dominant at low temperatures, while the incoherent case provides a
phenomenological description of the inelastic scattering present in real
physical systems at room temperature. It is found that disorder scattering
increases the amount of spin-mixing of transmitted electrons, reducing the
adiabaticity. This leads, in the incoherent case, to a reduction of conductance
through the domain wall as compared to a uniformly magnetized region which is
similar to the giant magnetoresistance effect. In the coherent case, a
reduction of weak localization, together with a suppression of spin-reversing
scattering amplitudes, leads to an enhancement of conductance due to the domain
wall in the regime of strong disorder. The total effect of a domain wall on the
conductance of a nanowire is studied by incorporating the disordered regions on
either side of the wall. It is found that spin-dependent scattering in these
regions increases the domain wall magnetoconductance as compared to the effect
found by considering only the scattering inside the wall. This increase is most
dramatic in the narrow wall limit, but remains significant for wide walls.Comment: 23 pages, 12 figure
Cosmology as Geodesic Motion
For gravity coupled to N scalar fields with arbitrary potential V, it is
shown that all flat (homogeneous and isotropic) cosmologies correspond to
geodesics in an (N+1)-dimensional `augmented' target space of Lorentzian
signature (1,N), timelike if V>0, null if V=0 and spacelike if V<0.
Accelerating cosmologies correspond to timelike geodesics that lie within an
`acceleration subcone' of the `lightcone'. Non-flat (k=-1,+1) cosmologies are
shown to evolve as projections of geodesic motion in a space of dimension
(N+2), of signature (1,N+1) for k=-1 and signature (2,N) for k=+1. This
formalism is illustrated by cosmological solutions of models with an
exponential potential, which are comprehensively analysed; the late-time
behviour for other potentials of current interest is deduced by comparison.Comment: 26 pages, 2 figures, journal version with additional reference
Radiation-dominated area metric cosmology
We provide further crucial support for a refined, area metric structure of
spacetime. Based on the solution of conceptual issues, such as the consistent
coupling of fermions and the covariant identification of radiation fields on
area metric backgrounds, we show that the radiation-dominated epoch of area
metric cosmology is equivalent to that epoch in standard Einstein cosmology.
This ensures, in particular, successful nucleosynthesis. This surprising result
complements the previously derived prediction of a small late-time acceleration
of an area metric universe.Comment: 23 pages, no figures; references adde
Determinant-Gravity: Cosmological implications
We analyze the action as a possible alternative or addition to the Einstein gravity.
Choosing a particular form of we can restore the
Einstein gravity and, if , we obtain the cosmological constant
term. Taking and expanding the action in , we obtain as a leading term the Einstein Lagrangian with a cosmological
constant proportional to and a series of higher order operators. In
general case of non-vanishing and new cosmological
solutions for the Robertson-Walker metric are obtained.Comment: revtex format, 5 pages,8 figures,references adde
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