74 research outputs found
Nonlocal Gravitational Models and Exact Solutions
A nonlocal gravity model with a function , where is
the d'Alembert operator, is considered. The algorithm, allowing to reconstruct
, corresponding to the given Hubble parameter and the state
parameter of the matter, is proposed. Using this algorithm, we find the
functions , corresponding to de Sitter solutions.Comment: 5 pages, v2: refs. added, to appear in the proceedings of the
International Workshop "Supersymmetries and Quantum Symmetries" (SQS'2011),
Dubna, Russia, July 18-23, 2011, http://theor.jinr.ru/sqs/2011
Jost-Lehmann-Dyson Representation, Analyticity in Angle Variable and Upper Bounds in Noncommutative Quantum Field Theory
The existence of Jost-Lehmann-Dyson representation analogue has been proved
in framework of space-space noncommutative quantum field theory. On the basis
of this representation it has been found that some class of elastic amplitudes
admits an analytical continuation into complex \cos\vartheta plane and
corresponding domain of analyticity is Martin ellipse. This analyticity
combined with unitarity leads to Froissart-Martin upper bound on total cross
section.Comment: LaTeX, 15 pages, improved version, misprints corrected, the
references added, to appear in Theor. Math. Phy
Non-minimally Coupled Cosmological Models with the Higgs-like Potentials and Negative Cosmological Constant
We study dynamics of non-minimally coupled scalar field cosmological models
with Higgs-like potentials and a negative cosmological constant. In these
models the inflationary stage of the Universe evolution changes into a
quasi-cyclic stage of the Universe evolution with oscillation behaviour of the
Hubble parameter from positive to negative values. Depending on the initial
conditions the Hubble parameter can perform either one or several cycles before
to become negative forever.Comment: 22 pages, 6 figures, v4:Section 2 expanded, references added,
accepted for publication in Class. Quant. Gra
Statefinder Parameters for Interacting Phantom Energy with Dark Matter
We apply in this paper the statefinder parameters to the interacting phantom
energy with dark matter. There are two kinds of scaling solutions in this
model. It is found that the evolving trajectories of these two scaling
solutions in the statefinder parameter plane are quite different, and that are
also different from the statefinder diagnostic of other dark energy models.Comment: 9 pages, 12 figures, some references are added, some words are
modifie
Construction of Special Solutions for Nonintegrable Systems
The Painleve test is very useful to construct not only the Laurent series
solutions of systems of nonlinear ordinary differential equations but also the
elliptic and trigonometric ones. The standard methods for constructing the
elliptic solutions consist of two independent steps: transformation of a
nonlinear polynomial differential equation into a nonlinear algebraic system
and a search for solutions of the obtained system. It has been demonstrated by
the example of the generalized Henon-Heiles system that the use of the Laurent
series solutions of the initial differential equation assists to solve the
obtained algebraic system. This procedure has been automatized and generalized
on some type of multivalued solutions. To find solutions of the initial
differential equation in the form of the Laurent or Puiseux series we use the
Painleve test. This test can also assist to solve the inverse problem: to find
the form of a polynomial potential, which corresponds to the required type of
solutions. We consider the five-dimensional gravitational model with a scalar
field to demonstrate this.Comment: LaTeX, 14 pages, the paper has been published in the Journal of
Nonlinear Mathematical Physics (http://www.sm.luth.se/math/JNMP/
Bouncing and Accelerating Solutions in Nonlocal Stringy Models
A general class of cosmological models driven by a non-local scalar field
inspired by string field theories is studied. In particular cases the scalar
field is a string dilaton or a string tachyon. A distinguished feature of these
models is a crossing of the phantom divide. We reveal the nature of this
phenomena showing that it is caused by an equivalence of the initial non-local
model to a model with an infinite number of local fields some of which are
ghosts. Deformations of the model that admit exact solutions are constructed.
These deformations contain locking potentials that stabilize solutions.
Bouncing and accelerating solutions are presented.Comment: Minor corrections, references added, published in JHE
New mechanism to cross the phantom divide
Recently, type Ia supernovae data appear to support a dark energy whose
equation of state crosses -1, which is a much more amazing problem than the
acceleration of the universe. We show that it is possible for the equation of
state to cross the phantom divide by a scalar field in the gravity with an
additional inverse power-law term of Ricci scalar in the Lagrangian. The
necessary and sufficient condition for a universe in which the dark energy can
cross the phantom divide is obtained. Some analytical solutions with or
are obtained. A minimal coupled scalar with different potentials,
including quadratic, cubic, quantic, exponential and logarithmic potentials are
investigated via numerical methods, respectively. All these potentials lead to
the crossing behavior. We show that it is a robust result which is hardly
dependent on the concrete form of the potential of the scalar.Comment: 11 pages, 5 figs, v3: several references added, to match the
published versio
Crossing the Phantom Divide: Theoretical Implications and Observational Status
If the dark energy equation of state parameter w(z) crosses the phantom
divide line w=-1 (or equivalently if the expression d(H^2(z))/dz - 3\Omega_m
H_0^2 (1+z)^2 changes sign) at recent redshifts, then there are two possible
cosmological implications: Either the dark energy consists of multiple
components with at least one non-canonical phantom component or general
relativity needs to be extended to a more general theory on cosmological
scales. The former possibility requires the existence of a phantom component
which has been shown to suffer from serious theoretical problems and
instabilities. Therefore, the later possibility is the simplest realistic
theoretical framework in which such a crossing can be realized. After providing
a pedagogical description of various dark energy observational probes, we use a
set of such probes (including the Gold SnIa sample, the first year SNLS
dataset, the 3-year WMAP CMB shift parameter, the SDSS baryon acoustic
oscillations peak (BAO), the X-ray gas mass fraction in clusters and the linear
growth rate of perturbations at z=0.15 as obtained from the 2dF galaxy redshift
survey) to investigate the priors required for cosmological observations to
favor crossing of the phantom divide. We find that a low \Omega_m prior
(0.2<\Omega_m <0.25) leads, for most observational probes (except of the SNLS
data), to an increased probability (mild trend) for phantom divide crossing. An
interesting degeneracy of the ISW effect in the CMB perturbation spectrum is
also pointed out.Comment: Accepted in JCAP (to appear). Comments added, typos corrected. 19
pages (revtex), 8 figures. The numerical analysis files (Mathematica +
Fortran) with instructions are available at
http://leandros.physics.uoi.gr/pdl-cross/pdl-cross.htm . The ppt file of a
relevant talk may be downloaded from
http://leandros.physics.uoi.gr/pdl-cross/pdl2006.pp
Isometric Embeddings and Noncommutative Branes in Homogeneous Gravitational Waves
We characterize the worldvolume theories on symmetric D-branes in a
six-dimensional Cahen-Wallach pp-wave supported by a constant Neveu-Schwarz
three-form flux. We find a class of flat noncommutative euclidean D3-branes
analogous to branes in a constant magnetic field, as well as curved
noncommutative lorentzian D3-branes analogous to branes in an electric
background. In the former case the noncommutative field theory on the branes is
constructed from first principles, related to dynamics of fuzzy spheres in the
worldvolumes, and used to analyse the flat space limits of the string theory.
The worldvolume theories on all other symmetric branes in the background are
local field theories. The physical origins of all these theories are described
through the interplay between isometric embeddings of branes in the spacetime
and the Penrose-Gueven limit of AdS3 x S3 with Neveu-Schwarz three-form flux.
The noncommutative field theory of a non-symmetric spacetime-filling D-brane is
also constructed, giving a spatially varying but time-independent
noncommutativity analogous to that of the Dolan-Nappi model.Comment: 52 pages; v2: References adde
Large Nongaussianity from Nonlocal Inflation
We study the possibility of obtaining large nongaussian signatures in the
Cosmic Microwave Background in a general class of single-field nonlocal
hill-top inflation models. We estimate the nonlinearity parameter f_{NL} which
characterizes nongaussianity in such models and show that large nongaussianity
is possible. For the recently proposed p-adic inflation model we find that
f_{NL} ~ 120 when the string coupling is order unity. We show that large
nongaussianity is also possible in a toy model with an action similar to those
which arise in string field theory.Comment: 27 pages, no figures. Added references and some clarifying remark
- …