89 research outputs found
Modified-Source Gravity and Cosmological Structure Formation
One way to account for the acceleration of the universe is to modify general
relativity, rather than introducing dark energy. Typically, such modifications
introduce new degrees of freedom. It is interesting to consider models with no
new degrees of freedom, but with a modified dependence on the conventional
energy-momentum tensor; the Palatini formulation of theories is one
example. Such theories offer an interesting testing ground for investigations
of cosmological modified gravity. In this paper we study the evolution of
structure in these ``modified-source gravity'' theories. In the linear regime,
density perturbations exhibit scale dependent runaway growth at late times and,
in particular, a mode of a given wavenumber goes nonlinear at a higher redshift
than in the standard CDM model. We discuss the implications of this
behavior and why there are reasons to expect that the growth will be cut off in
the nonlinear regime. Assuming that this holds in a full nonlinear analysis, we
briefly describe how upcoming measurements may probe the differences between
the modified theory and the standard CDM model.Comment: 22 pages, 6 figures, uses iopart styl
Mass Screening in Modified Gravity
Models of modified gravity introduce extra degrees of freedom, which for
consistency with the data, should be suppressed at observable scales. In the
models that share properties of massive gravity such a suppression is due to
nonlinear interactions: An isolated massive astrophysical object creates a halo
of a nonzero curvature around it, shielding its vicinity from the influence of
the extra degrees of freedom. We emphasize that the very same halo leads to a
screening of the gravitational mass of the object, as seen by an observer
beyond the halo. We discuss the case when the screening could be very
significant and may rule out, or render the models observationally interesting.Comment: 16 pages, 4 figures, A contribution to the Proceedings of the
International Workshop on Cosmology and Gravitation, Peyresq 12, June 16-22,
2007, Peyresq, Franc
Ellipsoidal universe in the brane world
We study a scenario of the ellipsoidal universe in the brane world cosmology
with a cosmological constant in the bulk . From the five-dimensional Einstein
equations we derive the evolution equations for the eccentricity and the scale
factor of the universe, which are coupled to each other. It is found that if
the anisotropy of our universe is originated from a uniform magnetic field
inside the brane, the eccentricity decays faster in the bulk in comparison with
a four-dimensional ellipsoidal universe. We also investigate the ellipsoidal
universe in the brane-induced gravity and find the evolution equation for the
eccentricity which has a contribution determined by the four- and
five-dimensional Newton's constants. The role of the eccentricity is discussed
in explaining the quadrupole problem of the cosmic microwave background.Comment: 15 pages, 1 figure, Version 3, references added, contents expande
Reconstruction of the Scalar-Tensor Lagrangian from a LCDM Background and Noether Symmetry
We consider scalar-tensor theories and reconstruct their potential U(\Phi)
and coupling F(\Phi) by demanding a background LCDM cosmology. In particular we
impose a background cosmic history H(z) provided by the usual flat LCDM
parameterization through the radiation (w_{eff}=1/3), matter (w_{eff}=0) and
deSitter (w_{eff}=-1) eras. The cosmological dynamical system which is
constrained to obey the LCDM cosmic history presents five critical points in
each era, one of which corresponding to the standard General Relativity (GR).
In the cases that differ from GR, the reconstructed coupling and potential are
of the form F(\Phi)\sim \Phi^2 and U(\Phi)\sim F(\Phi)^m where m is a constant.
This class of scalar tensor theories is also theoretically motivated by a
completely independent approach: imposing maximal Noether symmetry on the
scalar-tensor Lagrangian. This approach provides independently: i) the form of
the coupling and the potential as F(\Phi)\sim \Phi^2 and U(\Phi)\sim F(\Phi)^m,
ii) a conserved charge related to the potential and the coupling and iii)
allows the derivation of exact solutions by first integrals of motion.Comment: Added comments, discussion, references. 15 revtex pages, 5 fugure
Structural instability of Friedmann-Robertson-Walker cosmological models
Cosmological singularity and asymptotic behaviour of scale factor of
generalized cosmological models are analyzed in respect of their structural
stability. It is shown, that cosmological singularity is structurally unstable
for the majority of models with barotropic perfect fluid with strong energy
condition. Inclusion of Lambda-term extends the set of structurally stable
cosmological models.Comment: 14 pages, 4 figures in TeXCad, developed version of talk, presented
at XIII Russian Gravitational Conference, June 2008, to be published in GRG,
minor changes concerning added reference
Modified gravity in a viscous and non-isotropic background
We study the dynamical evolution of an model of gravity in a viscous
and anisotropic background which is given by a Bianchi type-I model of the
Universe. We find viable forms of gravity in which one is exactly the
Einsteinian model of gravity with a cosmological constant and other two are
power law models. We show that these two power law models are stable
with a suitable choice of parameters. We also examine three potentials which
exhibit the potential effect of models in the context of scalar tensor
theory. By solving different aspects of the model and finding the physical
quantities in the Jordan frame, we show that the equation of state parameter
satisfy the dominant energy condition. At last we show that the two power law
models behave like quintessence model at late times and also the shear
coefficient viscosity tends to zero at late times.Comment: 7 pages, 2 figure
Gravitational instability on the brane: the role of boundary conditions
An outstanding issue in braneworld theory concerns the setting up of proper
boundary conditions for the brane-bulk system. Boundary conditions (BC's)
employing regulatory branes or demanding that the bulk metric be nonsingular
have yet to be implemented in full generality. In this paper, we take a
different route and specify boundary conditions directly on the brane thereby
arriving at a local and closed system of equations (on the brane). We consider
a one-parameter family of boundary conditions involving the anisotropic stress
of the projection of the bulk Weyl tensor on the brane and derive an exact
system of equations describing scalar cosmological perturbations on a generic
braneworld with induced gravity. Depending upon our choice of boundary
conditions, perturbations on the brane either grow moderately (region of
stability) or rapidly (instability). In the instability region, the evolution
of perturbations usually depends upon the scale: small scale perturbations grow
much more rapidly than those on larger scales. This instability is caused by a
peculiar gravitational interaction between dark radiation and matter on the
brane. Generalizing the boundary conditions obtained by Koyama and Maartens, we
find for the Dvali-Gabadadze-Porrati model an instability, which leads to a
dramatic scale-dependence of the evolution of density perturbations in matter
and dark radiation. A different set of BC's, however, leads to a more moderate
and scale-independent growth of perturbations. For the mimicry braneworld,
which expands like LCDM, this class of BC's can lead to an earlier epoch of
structure formation.Comment: 35 pages, 9 figures, an appendix and references added, version to be
published in Classical and Quantum Gravit
The Adiabatic Instability on Cosmology's Dark Side
We consider theories with a nontrivial coupling between the matter and dark
energy sectors. We describe a small scale instability that can occur in such
models when the coupling is strong compared to gravity, generalizing and
correcting earlier treatments. The instability is characterized by a negative
sound speed squared of an effective coupled dark matter/dark energy fluid. Our
results are general, and applicable to a wide class of coupled models and
provide a powerful, redshift-dependent tool, complementary to other
constraints, with which to rule many of them out. A detailed analysis and
applications to a range of models are presented in a longer companion paper.Comment: 4 pages, 1 figur
Perturbations of Self-Accelerated Universe
We discuss small perturbations on the self-accelerated solution of the DGP
model, and argue that claims of instability of the solution that are based on
linearized calculations are unwarranted because of the following: (1) Small
perturbations of an empty self-accelerated background can be quantized
consistently without yielding ghosts. (2) Conformal sources, such as radiation,
do not give rise to instabilities either. (3) A typical non-conformal source
could introduce ghosts in the linearized approximation and become unstable,
however, it also invalidates the approximation itself. Such a source creates a
halo of variable curvature that locally dominates over the self-accelerated
background and extends over a domain in which the linearization breaks down.
Perturbations that are valid outside the halo may not continue inside, as it is
suggested by some non-perturbative solutions. (4) In the Euclidean continuation
of the theory, with arbitrary sources, we derive certain constraints imposed by
the second order equations on first order perturbations, thus restricting the
linearized solutions that could be continued into the full nonlinear theory.
Naive linearized solutions fail to satisfy the above constraints. (5) Finally,
we clarify in detail subtleties associated with the boundary conditions and
analytic properties of the Green's functions.Comment: 39 LaTex page
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
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