1,013 research outputs found

### On power-counting renormalizability of Ho\v{r}ava gravity with detailed balance

We consider the version of Ho\v{r}ava gravity where "detailed balance" is
consistently implemented, so as to limit the huge proliferation of couplings in
the full theory and obtain healthy dynamics at low energy. Since a
superpotential which is third-order in spatial derivatives is not sufficient to
guarantee the power-counting renormalizability of the spin-0 graviton, one
needs to go an order beyond in derivatives, building a superpotential up to
fourth-order spatial derivatives. Here we perturb the action to quadratic order
around flat space and show that the power-counting renormalizability of the
spin-0 graviton is achieved only by setting to zero a specific coupling of the
theory, while the spin-2 graviton is always power-counting renormalizable for
any choice of the couplings. This result raises serious doubts about the use of
detailed balance.Comment: v1: 4 pages; v2: references added, published versio

### On the Anisotropic Interior Solutions in Ho\v{r}ava Gravity and Einstein-{\ae}ther Theory

We find a reconstruction algorithm able to generate all the static
spherically symmetric interior solutions in the framework of Ho\v{r}ava gravity
and Einstein-\ae ther theory in presence of anisotropic fluids. We focus for
simplicity on the case of a static \ae ther finding a large class of possible
viable interior star solutions which present a very rich phenomenology. We
study one illustrative example in more detail.Comment: v1: 5 pages, 1 figure; v2: 6 pages, 1 figure. Matches published
version in EP

### Horava-Lifshitz gravity with detailed balance

Horava-Lifshitz gravity with "detailed balance" but without the
projectability assumption is discussed. It is shown that detailed balance is
quite efficient in limiting the proliferation of couplings in Horava-Lifshitz
gravity, and that its implementation without the projectability assumption
leads to a theory with sensible dynamics. However, the (bare) cosmological
constant is restricted to be large and negative.Comment: Contribution to the proceedings of NEB 15 conference, Chania, 20-23
June 2012 (talk given by D.V.

### Equivalence between Palatini and metric formalisms of f(R)-gravity by divergence free current

The equivalence between metric and Palatini formalisms in f(R)-gravity can be
achieved in the general context of theories with divergence free current. This
equivalence is a necessary result of a symmetry which is included in a
particular conservation equation of the current. In fact the conservation
equation, by an appropriate redefinition of the introduced auxiliary field, may
be encoded in a massless scalar field equation.Comment: 6 page

### Gravity beyond General Relativity: New Proposals and their Phenomenology

This Thesis is devoted to the study of phenomenologically viable gravitational theories,
in order to address the most pressing open issues both at very small and very large energy scales. Lovelock\u2019s theorem singles out General Relativity as the only theory with secondorder field equations for the metric tensor. So, two possible ways to circumvent it and modify the gravitational sector are taken into account. The first route consists in giving
up diffeomorphism invariance, which generically leads to extra propagating degrees of
freedom. In this framework Horava gravity is discussed, presenting two restrictions, called
respectively "projectability" and "detailed balance", which are imposed in order to reduce
the number of terms in the full theory. We introduce a new version of the theory assuming
detailed balance but not projectability, and we show that such theory is dynamically
consistent as both the spin-0 and spin-2 gravitons have a well behaved dynamics at low-energy. Moreover three-dimensional rotating black hole solutions are found and fully
studied in the context of Horava gravity, shedding light on its causal structure. A new
concept of black hole horizon, dubbed "universal horizon", arises besides the usual event
horizon one, since in Lorentz-violating gravity theories there can be modes propagating
even at infinite speed. The second route which is considered, consists in adding extra
fields to the gravitational action while diffeomorphism invariance is preserved. In this
respect we consider the less explored option that such fields are auxiliary fields, so they
do not satisfy dynamical equations but can be instead algebraically eliminated. A very
general parametrization for these theories is constructed, rendering also possible to put
on them very tight, theory-independent constraints. Some insight about the cosmological
implications of such theories is also given. Finally in the conclusions we discuss about
the future challenges that the aforementioned gravity theories have to face

### Relativistic polytropic equations of state in Ho\v{r}ava gravity and Einstein-\ae ther theory

The equations of state for a characteristic spacetime are studied in the
context of the spherically symmetric interior exact and analytical solutions in
Horava gravity and Einstein-aether theory in which anisotropic fluids are
considered. In particular, for a given anisotropic interior solution, the
equations of state relating the density to the radial and tangential pressure
are derived, by means of a polynomial best-fit. Moreover, the well-known
relativistic polytropic equations of state are used in order to obtain the
profile of the thermodynamical quantities inside the stellar object as provided
by the specific exact solution considered. It is then shown that these
equations of state need to be modified in order to account for the profiles of
density and pressures.Comment: 9 pages, 6 figures. Matches published version in PR

### Rotating black holes in three-dimensional Ho\v{r}ava gravity

We study black holes in the infrared sector of three-dimensional Ho\v{r}ava
gravity. It is shown that black hole solutions with anti-de Sitter asymptotics
are admissible only in the sector of the theory in which the scalar degree of
freedom propagates infinitely fast. We derive the most general class of
stationary, circularly symmetric, asymptotically anti-de Sitter black hole
solutions. We also show that the theory admits black hole solutions with de
Sitter and flat asymptotics, unlike three-dimensional general relativity. For
all these cases, universal horizons may or may not exist depending on the
choice of parameters. Solutions with de Sitter asymptotics can have universal
horizons that lie beyond the de Sitter horizon.Comment: 16 pages, 9 figures, final published versio

### Gravity with Auxiliary Fields

Modifications of General Relativity usually include extra dynamical degrees
of freedom, which to date remain undetected. Here we explore the possibility of
modifying Einstein's theory by adding solely nondynamical fields. With the
minimal requirement that the theory satisfies the weak equivalence principle
and admits a covariant Lagrangian formulation, we show that the field equations
generically have to include higher-order derivatives of the matter fields. This
has profound consequences for the viability of these theories. We develop a
parametrization based on a derivative expansion and show that - to next to
leading order - all theories are described by just two parameters. Our approach
can be used to put stringent, theory-independent constraints on such theories,
as we demonstrates using the Newtonian limit as an example.Comment: 5 pages, no figures; v2: clarifications and minor improvements,
matches published versio

### Covariant action for bouncing cosmologies in modified Gauss-Bonnet gravity

Cyclic universes with bouncing solutions are candidates for solving the big
bang initial singularity problem. Here we seek bouncing solutions in a modified
Gauss-Bonnet gravity theory, of the type $R+f(G)$, where $R$ is the Ricci
scalar, $G$ is the Gauss-Bonnet term, and $f$ some function of it. In finding
such a bouncing solution we resort to a technique that reduces the order of the
differential equations of the $R+f(G)$ theory to second order equations. As
general relativity is a theory whose equations are of second order, this order
reduction technique enables one to find solutions which are perturbatively
close to general relativity. We also build the covariant action of the order
reduced theory.Comment: 8 page

### String duality transformations in $f(R)$ gravity from Noether symmetry approach

We select $f(R)$ gravity models that undergo scale factor duality
transformations. As a starting point, we consider the tree-level effective
gravitational action of bosonic String Theory coupled with the dilaton field.
This theory inherits the Busher's duality of its parent String Theory. Using
conformal transformations of the metric tensor, it is possible to map the
tree-level dilaton-graviton string effective action into $f(R)$ gravity,
relating the dilaton field to the Ricci scalar curvature. Furthermore, the
duality can be framed under the standard of Noether symmetries and exact
cosmological solutions are derived. Using suitable changes of variables, the
string-based $f(R)$ Lagrangians are shown in cases where the duality
transformation becomes a parity inversion.Comment: v1: 13 pages; v2: minor rephrasings, published versio

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