53 research outputs found
Galilean Genesis: an alternative to inflation
We propose a novel cosmological scenario, in which standard inflation is
replaced by an expanding phase with a drastic violation of the Null Energy
Condition (NEC): \dot H >> H^2. The model is based on the recently introduced
Galileon theories, that allow NEC violating solutions without instabilities.
The unperturbed solution describes a Universe that is asymptotically Minkowski
in the past, expands with increasing energy density until it exits the regime
of validity of the effective field theory and reheats. This solution is a
dynamical attractor and the Universe is driven to it, even if it is initially
contracting. The study of perturbations of the Galileon field reveals some
subtleties, related to the gross violation of the NEC and it shows that
adiabatic perturbations are cosmologically irrelevant. The model, however,
suggests a new way to produce a scale invariant spectrum of isocurvature
perturbations, which can later be converted to adiabatic: the Galileon is
forced by symmetry to couple to the other fields as a dilaton; the effective
metric it yields on the NEC violating solution is that of de Sitter space, so
that all light scalars will automatically acquire a nearly scale-invariant
spectrum of perturbations.Comment: 25 pages, 1 figure. v2: minor changes, JCAP published versio
Constraints on Single-Field Inflation
Many alternatives to canonical slow-roll inflation have been proposed over
the years, one of the main motivations being to have a model, capable of
generating observable values of non-Gaussianity. In this work, we (re-)explore
the physical implications of a great majority of such models within a single,
effective field theory framework (including novel models with large
non-Gaussianity discussed for the first time below.) The constraints we
apply---both theoretical and experimental---are found to be rather robust,
determined to a great extent by just three parameters: the coefficients of the
quadratic EFT operators and , and the
slow-roll parameter . This allows to significantly limit the
majority of single-field alternatives to canonical slow-roll inflation. While
the existing data still leaves some room for most of the considered models, the
situation would change dramatically if the current upper limit on the
tensor-to-scalar ratio decreased down to . Apart from inflationary
models driven by plateau-like potentials, the single-field model that would
have a chance of surviving this bound is the recently proposed slow-roll
inflation with weakly-broken galileon symmetry. In contrast to
\textit{canonical} slow-roll inflation, the latter model can support even if driven by a convex potential, as well as generate observable
values for the amplitude of non-Gaussianity.Comment: 19+10 pages, 6 figure
Weakly Broken Galileon Symmetry
Effective theories of a scalar invariant under the internal
\textit{galileon symmetry} have been extensively
studied due to their special theoretical and phenomenological properties. In
this paper, we introduce the notion of \textit{weakly broken galileon
invariance}, which characterizes the unique class of couplings of such theories
to gravity that maximally retain their defining symmetry. The curved-space
remnant of the galileon's quantum properties allows to construct (quasi) de
Sitter backgrounds largely insensitive to loop corrections. We exploit this
fact to build novel cosmological models with interesting phenomenology,
relevant for both inflation and late-time acceleration of the universe.Comment: 26+8 pages, 2 figures, 2 table
Interplay between Black Holes and Ultralight Dark Matter: Analytic Solutions
Dark matter (DM) can consist of a scalar field so light that DM particles in
the galactic halo are best described by classical waves. We investigate how
these classical solutions are influenced by the presence of a non-rotating
supermassive black hole at the center of the galaxy, using an analytical,
albeit approximate, approach.
Relying on this analytic control, we examine the consequences of imposing
causal boundary conditions at the horizon, which are typically overlooked.
First, we examine the scenario where the backreaction of dark matter can be
neglected. The scalar field decays like a power law at large distances, thus
endowing the black hole with "hair". We derive solutions for the field profile
over a wide range of parameters, including cases with rotating dark matter. As
a by-product, we extract the dynamical Love numbers for scalar perturbations.
Next, we determine the spectrum of bound states and their behaviour.
Finally, we incorporate the self-gravity of the scalar field, with a focus on
the situation where dark matter forms a soliton (boson star) at the center of
the galaxy. We derive an analytical expression for the soliton at every
distance from the center. With a solution that remains applicable even at
horizon scales, we can reliably compute the accretion rate of the black hole.Comment: 29 pages + appendice
Stability of Geodesically Complete Cosmologies
We study the stability of spatially flat FRW solutions which are geodesically
complete, i.e. for which one can follow null (graviton) geodesics both in the
past and in the future without ever encountering singularities. This is the
case of NEC-violating cosmologies such as smooth bounces or solutions which
approach Minkowski in the past. We study the EFT of linear perturbations around
a solution of this kind, including the possibility of multiple fields and
fluids. One generally faces a gradient instability which can be avoided only if
the operator is present and its coefficient changes sign
along the evolution. This operator (typical of beyond-Horndeski theories) does
not lead to extra degrees of freedom, but cannot arise starting from any theory
with second-order equations of motion. The change of sign of this operator
prevents to set it to zero with a generalised disformal transformation.Comment: 18 pages, 2 figures. v2: minor changes; references added; version
published in JCA
Large Non-Gaussianity in Slow-Roll Inflation
Canonical models of single-field, slow-roll inflation do not lead to
appreciable non-Gaussianity, unless derivative interactions of the inflaton
become uncontrollably large. We propose a novel slow-roll scenario where scalar
perturbations propagate at a subluminal speed, leading to sizeable equilateral
non-Gaussianity, , largely insensitive
to the ultraviolet physics. The model is based on a low-energy effective theory
characterized by weakly broken invariance under internal galileon
transformations, , which protects the properties of
perturbations from large quantum corrections. This provides the unique
alternative to models such as DBI inflation in generating strongly
subluminal/non-Gaussian scalar perturbations.Comment: 5 page
Effective two-body approach to the hierarchical three-body problem : quadrupole to 1PN
Many binary systems of interest for gravitational-wave astronomy are orbited by a third distant body, which can considerably alter their relativistic dynamics. Precision computations are needed to understand the interplay between relativistic corrections and three-body interactions. We use an effective field theory approach to derive the effective action describing the long-timescale dynamics of hierarchical three-body systems up to 1PN quadrupole order. At this level of approximation, computations are complicated by the backreaction of small oscillations on orbital timescales as well as deviations from the adiabatic approximation. We address these difficulties by eliminating the fast modes through the method of near-identity transformations. This allows us to compute for the first time the complete expression of the 1PN quadrupole cross terms in generic configurations of three-body systems. We numerically integrate the resulting equations of motion and show that 1PN quadrupole terms can affect the long term dynamics of relativistic three-body systems
Subluminal Galilean Genesis
We put forward an improved version of the Galilean Genesis model that
addresses the problem of superluminality. We demote the full conformal group to
Poincare symmetry plus dilations, supplemented with approximate galilean shift
invariance in the UV and at small field values. In this way fluctuations around
the NEC-violating cosmological background are made substantially subluminal,
and superluminality cannot be reached by any small change of the solution, in
contrast with the original model. Dilation invariance still protects the
scale-invariance of correlation functions of a massless test scalar - which is
the source of the observed cosmological fluctuations - but the explicit
breaking of the conformal group can be potentially observed in higher-order
correlators. We also highlight a subtlety in matching the NEC-violating phase
with the standard cosmological evolution, and discuss the possible couplings of
the Galileon to gravity.Comment: 18 pages, 1 figur
The galileon as a local modification of gravity
In the DGP model, the ``self-accelerating'' solution is plagued by a ghost
instability, which makes the solution untenable. This fact as well as all
interesting departures from GR are fully captured by a four-dimensional
effective Lagrangian, valid at distances smaller than the present Hubble scale.
The 4D effective theory involves a relativistic scalar \pi, universally coupled
to matter and with peculiar derivative self-interactions. In this paper, we
study the connection between self-acceleration and the presence of ghosts for a
quite generic class of theories that modify gravity in the infrared. These
theories are defined as those that at distances shorter than cosmological,
reduce to a certain generalization of the DGP 4D effective theory. We argue
that for infrared modifications of GR locally due to a universally coupled
scalar, our generalization is the only one that allows for a robust
implementation of the Vainshtein effect--the decoupling of the scalar from
matter in gravitationally bound systems--necessary to recover agreement with
solar system tests. Our generalization involves an internal ``galilean''
invariance, under which \pi's gradient shifts by a constant. This symmetry
constrains the structure of the \pi Lagrangian so much so that in 4D there
exist only five terms that can yield sizable non-linearities without
introducing ghosts. We show that for such theories in fact there are
``self-accelerating'' deSitter solutions with no ghost-like instabilities. In
the presence of compact sources, these solutions can support spherically
symmetric, Vainshtein-like non-linear perturbations that are also stable
against small fluctuations. [Short version for arxiv]Comment: 35 pages; minor modifications, a typo corrected in eq. (114
Nonlinear Quasi-Normal Modes: Uniform Approximation
Recent works have suggested that nonlinear effects in black hole perturbation
theory may be important for describing a black hole ringdown. We show that the
technique of uniform approximations can be used to accurately compute 1)
nonlinear amplitudes at large distances in terms of the linear ones, 2) linear
(and nonlinear) quasi-normal mode frequencies, 3) the wavefunction for both
linear and nonlinear modes. Our method can be seen as a generalization of the
WKB approximation, with the advantages of not losing accuracy at large overtone
number and not requiring matching conditions. To illustrate the effectiveness
of this method we consider a simplified source for the second-order Zerilli
equation, which we use to numerically compute the amplitude of nonlinear modes
for a range of values of the angular momentum number.Comment: 17 pages + appendice
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