71 research outputs found
Tackling non-linearities with the effective field theory of dark energy and modified gravity
We present the extension of the effective field theory framework to the
mildly non-linear scales. The effective field theory approach has been
successfully applied to the late time cosmic acceleration phenomenon and it has
been shown to be a powerful method to obtain predictions about cosmological
observables on linear scales. However, mildly non-linear scales need to be
consistently considered when testing gravity theories because a large part of
the data comes from those scales. Thus, non-linear corrections to predictions
on observables coming from the linear analysis can help in discriminating among
different gravity theories. We proceed firstly by identifying the necessary
operators which need to be included in the effective field theory Lagrangian in
order to go beyond the linear order in perturbations and then we construct the
corresponding non-linear action. Moreover, we present the complete recipe to
map any single field dark energy and modified gravity models into the
non-linear effective field theory framework by considering a general action in
the Arnowitt-Deser-Misner formalism. In order to illustrate this recipe we
proceed to map the beyond-Horndeski theory and low-energy Horava gravity into
the effective field theory formalism. As a final step we derived the 4th order
action in term of the curvature perturbation. This allowed us to identify the
non-linear contributions coming from the linear order perturbations which at
the next order act like source terms. Moreover, we confirm that the stability
requirements, ensuring the positivity of the kinetic term and the speed of
propagation for scalar mode, are automatically satisfied once the viability of
the theory is demanded at linear level. The approach we present here will allow
to construct, in a model independent way, all the relevant predictions on
observables at mildly non-linear scales.Comment: 19 pages. Sec IV and Appendix B added. Matches JCAP versio
Deviations from General Relativity in Cosmology and Astrophysics
Two of the most prominent challenges of Modern Cosmology are the recent late-time accelerated expansion of the Universe and Dark Matter (DM).
DM is of fundamental importance in the process of structure formation at galactic, extragalactic and cosmic scales. It seems to dominate down to small galactocentric radii, as highlighted by the galaxies rotation curves and on cosmic scale, it is well known that the spatial distribution of galaxies is biased with respect to the underlying DM distribution. This relation is called ``bias''. Part of this thesis is devoted to the investigation of the DM issue. In particular, we study the DM density profile in the Orion dwarf galaxy. This galaxy is a good candidate to understand the physics of DM as in general, the kinematics of dwarf galaxies is dominated by this dark component. Moreover, due to the availability of high precision data, it becomes crucial to understand accurately the bias relation, so we elaborate on the Lagrangian bias, when the initial mass fluctuation field is considered Gaussian and the bias is local.
It is well known that the \Lambda CDM has been very successful in accounting for current cosmological data, although it suffers from some outstanding problems, such as the small value of \Lambda, DM problems on small scales, early Universe shortcomings and the lack of a correct scheme to quantize General Relativity. These lead people to propose and investigate alternative models, which are based on deviations from General Relativity at cosmic scales. The bulk of the present thesis is devoted to the study of gravity theories, which consider an extra scalar degree of freedom (DoF), in order to modify the gravitational interaction at large scales and account for the late time acceleration. In particular, we develop the gradient expansion formalism in order to explore the phenomenology associated with the non-linear derivative interactions of the most general scalar tensor theories that lead to second order field equations. This approach is very useful to probe on super horizon scale the Inflation scenario. Finally, in the quest of a model independent parametrization for gravity theories, the effective field theory formalism has been applied to the phenomenon of cosmic acceleration. It is developed using a perturbative approach in which an extra scalar DoF appears only at the level of perturbations. We investigate the viability of background functions by means of a thorough dynamical analysis. In conclusion, we present the implementation of this framework into CAMB/CosmoMC creating, what we dubbed, EFTCAMB/EFTCosmoMC. These codes will allow to test gravity theories with the most recent data releases
An Extended action for the effective field theory of dark energy: a stability analysis and a complete guide to the mapping at the basis of EFTCAMB
We present a generalization of the effective field theory (EFT) formalism for
dark energy and modified gravity models to include operators with higher order
spatial derivatives. This allows the extension of the EFT framework to a wider
class of gravity theories such as Horava gravity. We present the corresponding
extended action, both in the EFT and the Arnowitt-Deser-Misner (ADM) formalism,
and proceed to work out a convenient mapping between the two, providing a self
contained and general procedure to translate a given model of gravity into the
EFT language at the basis of the Einstein-Boltzmann solver EFTCAMB. Putting
this mapping at work, we illustrate, for several interesting models of dark
energy and modified gravity, how to express them in the ADM notation and then
map them into the EFT formalism. We also provide for the first time, the full
mapping of GLPV models into the EFT framework. We next perform a thorough
analysis of the physical stability of the generalized EFT action, in absence of
matter components. We work out viability conditions that correspond to the
absence of ghosts and modes that propagate with a negative speed of sound in
the scalar and tensor sector, as well as the absence of tachyonic modes in the
scalar sector. Finally, we extend and generalize the phenomenological basis in
terms of -functions introduced to parametrize Horndeski models, to
cover all theories with higher order spatial derivatives included in our
extended action. We elaborate on the impact of the additional functions on
physical quantities, such as the kinetic term and the speeds of propagation for
scalar and tensor modes.Comment: 36 pages, matches published version, typos correcte
A de Sitter limit analysis for dark energy and modified gravity models
The effective field theory of dark energy and modified gravity is supposed to
well describe, at low energies, the behaviour of the gravity modifications due
to one extra scalar degree of freedom. The usual curvature perturbation is very
useful when studying the conditions for the avoidance of ghost instabilities as
well as the positivity of the squared speeds of propagation for both the scalar
and tensor modes, or the St\"uckelberg field performs perfectly when
investigating the evolution of linear perturbations. We show that the viable
parameters space identified by requiring no-ghost instabilities and positive
squared speeds of propagation does not change by performing a field
redefinition, while the requirement of the avoidance of tachyonic instability
might instead be different. Therefore, we find interesting to associate to the
general modified gravity theory described in the effective field theory
framework, a perturbation field which will inherit the whole properties of the
theory. In the present paper we address the following questions: 1) how can we
define such a field? and 2) what is the mass of such a field as the background
approaches a final de Sitter state? We define a gauge invariant quantity which
identifies the density of the dark energy perturbation field valid for any
background. We derive the mass associated to the gauge invariant dark energy
field on a de Sitter background, which we retain to be still a good
approximation also at very low redshift (). On this background we
also investigate the value of the speed of propagation and we find that there
exist classes of theories which admit a non-vanishing speed of propagation,
even among the Horndeski model, for which in literature it has previously been
found a zero speed. We finally apply our results to specific well known models.Comment: 22 page
The role of the tachyonic instability in Horndeski gravity
The tachyonic instability is associated with the unboundedness of the
Hamiltonian from below and results in an unstable low- regime. In the
cosmological exploration of modified gravity, it is seldom taken into account,
with more focus given to the popular no-ghost and no-gradient conditions. The
latter though are intrinsically high- statements. Here we combine all three
conditions into a full set of requirements that we show to guarantee stability
on the whole range of cosmological scales. We then explore the impact of the
different conditions on the parameter space of scalar-tensor gravity, with
particular emphasis on the no-tachyon one. We focus on Horndeski gravity and
also consider separately the two subclasses of and Generalized Brans
Dicke theories. We identify several interesting features, for instance in the
parameter space of designer on a CDM background, shedding light on
previous findings. When looking at the phenomenological functions and
, associated to the weak lensing and clustering potential respectively, we
find that in the case of Generalized Brans Dicke the no-tachyon condition
clearly cuts models with . This effect is less prevalent in
the Horndeski case due to the larger amount of free functions in the theory.Comment: 9 pages, 4 figures - accepted version by JCA
Growth of non-linear structures and spherical collapse in the Galileon Ghost Condensate model
We present a detailed study of the collapse of a spherical matter overdensity
and the non-linear growth of large scale structures in the Galileon ghost
condensate (GGC) model. This model is an extension of the cubic covariant
Galileon (G3) which includes a field derivative of type
in the Lagrangian. We find that the cubic
term activates the modifications in the main physical quantities whose time
evolution is then strongly affected by the additional term. Indeed, the GGC
model shows largely mitigated effects in the linearised critical density
contrast, non-linear effective gravitational coupling and the virial
overdensity with respect to G3 but still preserves peculiar features with
respect to the standard CDM cosmological model, e.g. both the linear
critical density contrast and the virial overdensity are larger than those in
CDM. The results of the spherical collapse model are then used to
predict the evolution of the halo mass function, non-linear matter and lensing
power spectra. While at low masses the GGC model presents about 10% fewer
objects with respect to CDM, at higher masses for it predicts
10% ()-20% () more objects per comoving volume. Using a
phenomenological approach to include the screening effect in the matter power
spectrum, we show that the difference induced by the modifications of gravity
are strongly dependent on the screening scale and that differences can be up to
20% with respect to CDM. These differences translate to the lensing
power spectrum where qualitatively the largest differences with respect to the
standard cosmological model are for . Depending on the screening
scale, they can be up to 25% on larger angular scales and then decrease for
growing . These results are obtained for the best fit parameters from
linear cosmological data for each model.Comment: 15 pages, 11 figures, 2 tables. Figures 8, 9 and 10 updated; new
figure 11 added. Extended discussion in Sections 6 and 7. Matches the PDU
journal versio
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