1,832 research outputs found
Asymmetric embedding in brane cosmology
We derive a system of cosmological equations for a braneworld with induced
curvature which is a junction between several bulk spaces. The permutation
symmetry of the bulk spaces is not imposed, and the values of the fundamental
constants, and even the signatures of the extra dimension, may be different on
different sides of the brane. We then consider the usual partial case of two
asymmetric bulk spaces and derive an exact closed system of scalar equations on
the brane. We apply this result to the cosmological evolution on such a brane
and describe its various partial cases.Comment: 10 page
Determination of Wave Function Functionals: The Constrained-Search--Variational Method
In a recent paper [Phys. Rev. Lett. \textbf{93}, 130401 (2004)], we proposed
the idea of expanding the space of variations in variational calculations of
the energy by considering the approximate wave function to be a
functional of functions rather than a function. The
space of variations is expanded because a search over the functions can
in principle lead to the true wave function. As the space of such variations is
large, we proposed the constrained-search-- variational method whereby a
constrained search is first performed over all functions such that the
wave function functional satisfies a physical constraint such as
normalization or the Fermi-Coulomb hole sum rule, or leads to the known value
of an observable such as the diamagnetic susceptibility, nuclear magnetic
constant or Fermi contact term. A rigorous upper bound to the energy is then
obtained by application of the variational principle. A key attribute of the
method is that the wave function functional is accurate throughout space, in
contrast to the standard variational method for which the wave function is
accurate only in those regions of space contributing principally to the energy.
In this paper we generalize the equations of the method to the determination of
arbitrary Hermitian single-particle operators as applied to two-electron atomic
and ionic systems. The description is general and applicable to both ground and
excited states. A discussion on excited states in conjunction with the theorem
of Theophilou is provided.Comment: 26 pages, 4 figures, 5 table
The Evolution of Voids in the Adhesion Approximation
We apply the adhesion approximation to study the formation and evolution of
voids in the Universe. Our simulations -- carried out using 128 particles
in a cubical box with side 128 Mpc -- indicate that the void spectrum evolves
with time and that the mean void size in the standard COBE-normalised Cold Dark
Matter (hereafter CDM) model with scales approximately as where Mpc.
Interestingly, we find a strong correlation between the sizes of voids and the
value of the primordial gravitational potential at void centers. This
observation could in principle, pave the way towards reconstructing the form of
the primordial potential from a knowledge of the observed void spectrum.
Studying the void spectrum at different cosmological epochs, for spectra with a
built in -space cutoff we find that, the number of voids in a representative
volume evolves with time. The mean number of voids first increases until a
maximum value is reached (indicating that the formation of cellular structure
is complete), and then begins to decrease as clumps and filaments merge leading
to hierarchical clustering and the subsequent elimination of small voids. The
cosmological epoch characterizing the completion of cellular structure occurs
when the length scale going nonlinear approaches the mean distance between
peaks of the gravitational potential. A central result of this paper is thatComment: Plain TeX, 38 pages Plus 16 Figures (available on request from the
first author), IUCAA-28 To appear in The Astrophysical Journal, July 199
On the genera Clepsydropsis and Cladoxylon of Unger, and on a new genus Austroclepsis
This article does not have an abstract
Probing the Coupling between Dark Components of the Universe
We place observational constraints on a coupling between dark energy and dark
matter by using 71 Type Ia supernovae (SNe Ia) from the first year of the
five-year Supernova Legacy Survey (SNLS), the cosmic microwave background (CMB)
shift parameter from the three-year Wilkinson Microwave Anisotropy Probe
(WMAP), and the baryon acoustic oscillation (BAO) peak found in the Sloan
Digital Sky Survey (SDSS). The interactions we study are (i) constant coupling
delta and (ii) varying coupling delta(z) that depends on a redshift z, both of
which have simple parametrizations of the Hubble parameter to confront with
observational data. We find that the combination of the three databases
marginalized over a present dark energy density gives stringent constraints on
the coupling, -0.08 < delta < 0.03 (95% CL) in the constant coupling model and
-0.4 < delta_0 < 0.1 (95% CL) in the varying coupling model, where delta_0 is a
present value. The uncoupled LambdaCDM model (w_X = -1 and delta = 0) still
remains a good fit to the data, but the negative coupling (delta < 0) with the
equation of state of dark energy w_X < -1 is slightly favoured over the
LambdaCDM model.Comment: 9 pages, 7 figures, RevTeX, minor corrections, references added,
accepted for publication in Phys. Rev.
Evolution of perturbations in distinct classes of canonical scalar field models of dark energy
Dark energy must cluster in order to be consistent with the equivalence
principle. The background evolution can be effectively modelled by either a
scalar field or by a barotropic fluid.The fluid model can be used to emulate
perturbations in a scalar field model of dark energy, though this model breaks
down at large scales. In this paper we study evolution of dark energy
perturbations in canonical scalar field models: the classes of thawing and
freezing models.The dark energy equation of state evolves differently in these
classes.In freezing models, the equation of state deviates from that of a
cosmological constant at early times.For thawing models, the dark energy
equation of state remains near that of the cosmological constant at early times
and begins to deviate from it only at late times.Since the dark energy equation
of state evolves differently in these classes,the dark energy perturbations too
evolve differently. In freezing models, since the equation of state deviates
from that of a cosmological constant at early times, there is a significant
difference in evolution of matter perturbations from those in the cosmological
constant model.In comparison, matter perturbations in thawing models differ
from the cosmological constant only at late times. This difference provides an
additional handle to distinguish between these classes of models and this
difference should manifest itself in the ISW effect.Comment: 11 pages, 6 figures, accepted for publication in Phys. Rev.
Accelerating Universes with Scaling Dark Matter
Friedmann-Robertson-Walker universes with a presently large fraction of the
energy density stored in an -component with , are considered. We
find all the critical points of the system for constant equations of state in
that range. We consider further several background quantities that can
distinguish the models with different values. Using a simple toy model
with a varying equation of state, we show that even a large variation of
at small redshifts is very difficult to observe with measurements up
to . Therefore, it will require accurate measurements in the range
and independent accurate knowledge of (and/or
) in order to resolve a variable from a constant .Comment: submitted to IJMPD (uses Latex, 12 pages, 6 Figures) Minor
corrections, Figures 4, 6 revised. Conclusions unchange
Quantum effects, soft singularities and the fate of the universe in a braneworld cosmology
We examine a class of braneworld models in which the expanding universe
encounters a "quiescent" future singularity. At a quiescent singularity, the
energy density and pressure of the cosmic fluid as well as the Hubble parameter
remain finite while all derivatives of the Hubble parameter diverge (i.e.,
, , etc. ). Since the Kretschmann invariant
diverges () at the singularity, one expects
quantum effects to play an important role as the quiescent singularity is
approached. We explore the effects of vacuum polarization due to massless
conformally coupled fields near the singularity and show that these can either
cause the universe to recollapse or, else, lead to a softer singularity at
which , , and remain finite while {\dddot H} and
higher derivatives of the Hubble parameter diverge. An important aspect of the
quiescent singularity is that it is encountered in regions of low density,
which has obvious implications for a universe consisting of a cosmic web of
high and low density regions -- superclusters and voids. In addition to vacuum
polarization, the effects of quantum particle production of non-conformal
fields are also likely to be important. A preliminary examination shows that
intense particle production can lead to an accelerating universe whose Hubble
parameter shows oscillations about a constant value.Comment: 19 pages, 3 figures, text slightly improved and references added.
Accepted for publication in Classical and Quantum Gravit
Statefinder diagnostic in a torsion cosmology
We apply the statefinder diagnostic to the torsion cosmology, in which an
accounting for the accelerated universe is considered in term of a
Riemann-Cartan geometry: dynamic scalar torsion. We find that there are some
typical characteristic of the evolution of statefinder parameters for the
torsion cosmology that can be distinguished from the other cosmological models.
Furthermore, we also show that statefinder diagnostic has a direct bearing on
the critical points. The statefinder diagnostic divides the torsion parameter
into differential ranges, which is in keeping with the requirement of
dynamical analysis. In addition, we fit the scalar torsion model to ESSENCE
supernovae data and give the best fit values of the model parameters.Comment: 18 pages, 15 figures, accepted paper in JCA
Dark energy models toward observational tests and data
A huge amount of good quality astrophysical data converges towards the
picture of a spatially flat universe undergoing the today observed phase of
accelerated expansion. This new observational trend is commonly addressed as
Precision Cosmology. Despite of the quality of astrophysical surveys, the
nature of dark energy dominating the matter-energy content of the universe is
still unknown and a lot of different scenarios are viable candidates to explain
cosmic acceleration. Methods to test these cosmological models are based on
distance measurements and lookback time toward astronomical objects used as
standard candles. I discuss the characterizing parameters and constraints of
three different classes of dark energy models pointing out the related
degeneracy problem which is the signal that more data at low (z= 0- 1), medium
(1<z<10) and high (10 <z< 1000) redshift are needed to definitively select
realistic models.Comment: 17 pages, 9 figures, Lectures for 42nd Karpacz Winter School of
Theoretical Physics: Current Mathematical Topics in Gravitation and
Cosmology, Ladek, Poland, 6-11 Feb 200
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