784 research outputs found
Initial Conditions for Vector Inflation
Recently, a model of inflation using non-minimally coupled massive vector
fields has been proposed. For a particular choice of non-minimal coupling
parameter and for a flat FRW model, the model is reduced to the model of
chaotic inflation with massive scalar field. We study the effect of non-zero
curvature of the universe on the onset of vector inflation. We find that in a
curved universe the dynamics of vector inflation can be different from chaotic
inflation, and the fraction of the initial conditions leading to inflationary
solutions is reduced compared with the chaotic inflation case.Comment: 12 pages, 5 figures, version to be published in JCA
Speed of light in the extended gravity theories
We shall investigate the possibility of formulation of varying speed of light
(VSL) in the framework of Palatini non-linear Ricci scalar and Ricci squared
theories. Different speeds of light including the causal structure constant,
electromagnetic, and gravitational wave speeds are discussed. We shall see that
two local frames are distinguishable and discuss about the velocity of light in
these two frames. We shall investigate which one of these local frames is
inertial.Comment: 19 pages. to appear in Classical Quantum Gravit
On the Weyl - Eddington - Einstein affine gravity in the context of modern cosmology
We propose new models of an `affine' theory of gravity in -dimensional
space-times with symmetric connections. They are based on ideas of Weyl,
Eddington and Einstein and, in particular, on Einstein's proposal to specify
the space - time geometry by use of the Hamilton principle. More specifically,
the connection coefficients are derived by varying a `geometric' Lagrangian
that is supposed to be an arbitrary function of the generalized (non-symmetric)
Ricci curvature tensor (and, possibly, of other fundamental tensors) expressed
in terms of the connection coefficients regarded as independent variables. In
addition to the standard Einstein gravity, such a theory predicts dark energy
(the cosmological constant, in the first approximation), a neutral massive (or,
tachyonic) vector field, and massive (or, tachyonic) scalar fields. These
fields couple only to gravity and may generate dark matter and/or inflation.
The masses (real or imaginary) have geometric origin and one cannot avoid their
appearance in any concrete model. Further details of the theory - such as the
nature of the vector and scalar fields that can describe massive particles,
tachyons, or even `phantoms' - depend on the concrete choice of the geometric
Lagrangian. In `natural' geometric theories, which are discussed here, dark
energy is also unavoidable. Main parameters - mass, cosmological constant,
possible dimensionless constants - cannot be predicted, but, in the framework
of modern `multiverse' ideology, this is rather a virtue than a drawback of the
theory. To better understand possible applications of the theory we discuss
some further extensions of the affine models and analyze in more detail
approximate (`physical') Lagrangians that can be applied to cosmology of the
early Universe.Comment: 15 pages; a few misprints corrected, one footnote removed and two
added, the formulae and results unchanged but the text somewhat edited, esp.
in Sections 4,5; the reference to the RFBR grant corrected
Primordial statistical anisotropy generated at the end of inflation
We present a new mechanism for generating primordial statistical anisotropy
of curvature perturbations. We introduce a vector field which has a non-minimal
kinetic term and couples with a waterfall field in hybrid inflation model. In
such a system, the vector field gives fluctuations of the end of inflation and
hence induces a subcomponent of curvature perturbations. Since the vector has a
preferred direction, the statistical anisotropy could appear in the
fluctuations. We present the explicit formula for the statistical anisotropy in
the primordial power spectrum and the bispectrum of curvature perturbations.
Interestingly, there is the possibility that the statistical anisotropy does
not appear in the power spectrum but does appear in the bispectrum. We also
find that the statistical anisotropy provides the shape dependence to the
bispectrum.Comment: 9 pages, This version supersedes the JCAP version. Minor revision
Cosmological perturbations in the Palatini formulation of modified gravity
Cosmology in extended theories of gravity is considered assuming the Palatini
variational principle, for which the metric and connection are independent
variables. The field equations are derived to linear order in perturbations
about the homogeneous and isotropic but possibly spatially curved background.
The results are presented in a unified form applicable to a broad class of
gravity theories allowing arbitrary scalar-tensor couplings and nonlinear
dependence on the Ricci scalar in the gravitational action. The gauge-ready
formalism exploited here makes it possible to obtain the equations immediately
in any of the commonly used gauges. Of the three type of perturbations, the
main attention is on the scalar modes responsible for the cosmic large-scale
structure. Evolution equations are derived for perturbations in a late universe
filled with cold dark matter and accelerated by curvature corrections. Such
corrections are found to induce effective pressure gradients which are
problematical in the formation of large-scale structure. This is demonstrated
by analytic solutions in a particular case. A physical equivalence between
scalar-tensor theories in metric and in Palatini formalisms is pointed out.Comment: 14 pages; the published version (+ an appendix). Corrected typos in
eqs. 30,33 and B
Anisotropic Dark Energy and the Generalized Second Law of Thermodynamics
We consider a Bianchi type model in which anisotropic dark energy is
interacting with dark matter and anisotropic radiation. With this scenario, we
investigate the validity of the generalized second law of thermodynamics. It is
concluded that the validity of this law depends on different parameters like
shear, skewness and equation of state.Comment: 12 pages, accepted for publication in Phys. Scr. arXiv admin note:
text overlap with arXiv:1008.0692 and arXiv:1106.241
Modified gravity and its reconstruction from the universe expansion history
We develop the reconstruction program for the number of modified gravities:
scalar-tensor theory, , and string-inspired, scalar-Gauss-Bonnet
gravity. The known (classical) universe expansion history is used for the
explicit and successful reconstruction of some versions (of special form or
with specific potentials) from all above modified gravities. It is demonstrated
that cosmological sequence of matter dominance, decceleration-acceleration
transition and acceleration era may always emerge as cosmological solutions of
such theory. Moreover, the late-time dark energy FRW universe may have the
approximate or exact CDM form consistent with three years WMAP data.
The principal possibility to extend this reconstruction scheme to include the
radiation dominated era and inflation is briefly mentioned. Finally, it is
indicated how even modified gravity which does not describe the
matter-dominated epoch may have such a solution before acceleration era at the
price of the introduction of compensating dark energy.Comment: LaTeX file, 24 pages, no figure, prepared for the proceedings of ERE
2006, minor correction
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