1,781 research outputs found
Coupled Quintessence in a Power-Law Case and the Cosmic Coincidence Problem
The problem of the cosmic coincidence is a longstanding puzzle. This
conundrum may be solved by introducing a coupling between the two dark sectors.
In this Letter, we study a coupled quintessence scenario in which the scalar
field evolves in a power law potential and the mass of dark matter particles
depends on a power law function of . It is shown that this scenario has a
stable attractor solution and can thus provide a natural solution to the cosmic
coincidence problem.Comment: 9 pages, 3 figure
Cosmological Imprint of an Energy Component with General Equation of State
We examine the possibility that a significant component of the energy density
of the universe has an equation-of-state different from that of matter,
radiation or cosmological constant (). An example is a cosmic scalar
field evolving in a potential, but our treatment is more general. Including
this component alters cosmic evolution in a way that fits current observations
well. Unlike , it evolves dynamically and develops fluctuations,
leaving a distinctive imprint on the microwave background anisotropy and mass
power spectrum.Comment: revised version, with added references, to appear in Phys. Rev. Lett.
(4 pages Latex, 2 postscript figures
Stretching the Inflaton Potential with Kinetic Energy
Inflation near a maximum of the potential is studied when non-local
derivative operators are included in the inflaton Lagrangian. Such terms can
impose additional sources of friction on the field. For an arbitrary spacetime
geometry, these effects can be quantified in terms of a local field theory with
a potential whose curvature around the turning point is strongly suppressed.
This implies that a prolonged phase of slow-roll inflation can be achieved with
potentials that are otherwise too steep to drive quasi-exponential expansion.
We illustrate this mechanism within the context of p-adic string theory.Comment: 4 page
Interacting dark energy, holographic principle and coincidence problem
The interacting and holographic dark energy models involve two important
quantities. One is the characteristic size of the holographic bound and the
other is the coupling term of the interaction between dark energy and dark
matter. Rather than fixing either of them, we present a detailed study of
theoretical relationships among these quantities and cosmological parameters as
well as observational constraints in a very general formalism. In particular,
we argue that the ratio of dark matter to dark energy density depends on the
choice of these two quantities, thus providing a mechanism to change the
evolution history of the ratio from that in standard cosmology such that the
coincidence problem may be solved. We investigate this problem in detail and
construct explicit models to demonstrate that it may be alleviated provided
that the interacting term and the characteristic size of holographic bound are
appropriately specified. Furthermore, these models are well fitted with the
current observation at least in the low red-shift region.Comment: 20 pages, 3 figure
Angular Inflation from Supergravity
We study supergravity inflationary models where inflation is produced along
the angular direction. For this we express the scalar component of a chiral
superfield in terms of the radial and the angular components. We then express
the supergravity potential in a form particularly simple for calculations
involving polynomial expressions for the superpotential and Kahler potential.
We show for a simple Polonyi model the angular direction may give rise to a
stage of inflation when the radial field is fixed to its minimum. We obtain
analytical expressions for all the relevant inflationary quantities and discuss
the possibility of supersymmetry breaking in the radial direction while
inflating by the angular component.Comment: 7 pages, one figure. Final version. Title changed, two figures
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Measuring deviations from a cosmological constant: a field-space parameterization
Most parameterizations of the dark energy equation of state do not reflect
realistic underlying physical models. Here, we develop a relatively simple
description of dark energy based on the dynamics of a scalar field which is
exact in the limit that the equation of state approaches a cosmological
constant, assuming some degree of smoothness of the potential. By introducing
just two parameters defined in the configuration space of the field we are able
to reproduce a wide class of quintessence models. We examine the observational
constraints on these models as compared to linear evolution models, and show
how priors in the field space translate into priors on observational
parameters.Comment: 5 pages, 6 figures. Final versio
Precision Cosmology? Not Just Yet
The recent announcement from the Wilkinson Microwave Anisotropy Probe (WMAP)
satellite experiment combined with other recent advances in observational
cosmology verifies key components of the standard cosmological model. However,
we argue that there remain some significant open issues regarding the basic
history and composition of the Universe and uncertainties in some of the most
important parameters.Comment: 2 pages, 2 figures. Online journal version
http://www.sciencemag.org/cgi/content/full/299/5612/153
A phason disordered two dimensional quantum antiferromagnet
We examine a novel type of disorder in quantum antiferromagnets. Our model
consists of localized spins with antiferromagnetic exchanges on a bipartite
quasiperiodic structure, which is geometrically disordered in such a way that
no frustration is introduced. In the limit of zero disorder, the structure is
the perfect Penrose rhombus tiling. This tiling is progressively disordered by
augmenting the number of random "phason flips" or local tile-reshuffling
operations. The ground state remains N\'eel ordered, and we have studied its
properties as a function of increasing disorder using linear spin wave theory
and quantum Monte Carlo. We find that the ground state energy decreases,
indicating enhanced quantum fluctuations with increasing disorder. The magnon
spectrum is progressively smoothed, and the effective spin wave velocity of low
energy magnons increases with disorder. For large disorder, the ground state
energy as well as the average staggered magnetization tend towards limiting
values characteristic of this type of randomized tilings.Comment: 5 pages, 7 figure
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