50 research outputs found
Dispersion in the growth of matter perturbations
We consider the linear growth of matter perturbations on low redshifts in
modified gravity Dark Energy (DE) models where G_eff(z,k) is explicitly
scale-dependent. Dispersion in the growth today will only appear for scales of
the order the critical scale ~ \lambda_{c,0}, the range of the fifth-force
today. We generalize the constraint equation satisfied by the parameters
\gamma_0(k) and \gamma'_0(k) \equiv \frac{d\gamma(z,k)}{dz}(z=0) to models with
G_{eff,0}(k) \ne G. Measurement of \gamma_0(k) and \gamma'_0(k) on several
scales can provide information about \lambda_{c,0}. In the absence of
dispersion when \lambda_{c,0} is large compared to the probed scales,
measurement of \gamma_0 and \gamma'_0 provides a consistency check independent
of \lambda_{c,0}. This applies in particular to results obtained earlier for a
viable f(R) model.Comment: 8 pages, 5 figure
Modified gravity a la Galileon: Late time cosmic acceleration and observational constraints
In this paper we examine the cosmological consequences of fourth order
Galileon gravity. We carry out detailed investigations of the underlying
dynamics and demonstrate the stability of one de Sitter phase. The stable de
Sitter phase contains a Galileon field which is an increasing function of
time (\dot{\pi}>0). Using the required suppression of the fifth force,
supernovae, BAO and CMB data, we constrain parameters of the model. We find
that the matter coupling parameter is constrained to small
numerical values such that <0.02. We also show that the parameters of
the third and fourth order in the action (c_3,c_4) are not independent and with
reasonable assumptions, we obtain constraints on them. We investigate the
growth history of the model and find that the sub-horizon approximation is not
allowed for this model. We demonstrate strong scale dependence of linear
perturbations in the fourth order Galileon gravity.Comment: 9 pages, 10 figures, references added, final version to appear in PR
On the consistency of the expansion with the perturbations
Assuming a simple form for the growth index gamma(z) depending on two
parameters gamma_0 = gamma(z=0) and gamma_1 = gamma'(z=0), we show that these
parameters can be constrained using background expansion data. We explore
systematically the preferred region in this parameter space. Inside General
Relativity we obtain that models with a quasi-static growth index and gamma_1 =
-0.02 are favoured. We find further the lower bounds gamma_0 > 0.53 and gamma_1
> -0.15 for models inside GR. Models outside GR having the same background
expansion as LCDM and arbitrary gamma(z) with gamma_0 = gamma_0^{LCDM}, satisfy
G_{eff,0}>G for gamma_1 > gamma_1^{LCDM}, and G_{eff,0}<G for gamma_1 <
gamma_1^{LCDM}. The first models will cross downwards the value G_{eff}=G on
very low redshifts z<0.3, while the second models will cross upwards G_{eff}=G
in the same redshift range. This makes the realization of such modified gravity
models even more problematic.Comment: 8 pages, 11 figures, v2 accepted for publication in PRD, updated
analysis, conclusions unchange
Q-balls in K-field theory
We study the existence and stability of Q-balls in noncanonical scalar field
theories, where is the complex scalar field and is
the kinetic term. We extend the Vakhitov-Kolokolov stability criterion to
K-field theories. We derive the condition for the perturbations to have a
well-posed Cauchy problem. We find that and are
necessary but not sufficient conditions. The perturbations define a strongly
hyperbolic system if . For all modifications studied, we found that perturbations propagate at a
speed different from light. Generically, the noncanonical scalar field can
lower the charge and energy of the Q-ball and therefore improves its stability.Comment: 10 pages, 8 figures, matches the published versio