A bouncing universe is a viable candidate to solve the initial singularity problem. Here we consider bouncing solutions in the context of f (R,G) gravity by using an order reduction technique which allows one to find solutions that are... more
A bouncing universe is a viable candidate to solve the initial singularity problem. Here we consider bouncing solutions in the context of f (R,G) gravity by using an order reduction technique which allows one to find solutions that are perturbatively close to General Relativity. This procedure also acts as a model selection approach. Indeed, several covariant gravitational actions leading to a bounce are directly selected by demanding that the Friedmann equation derived within such gravity theories coincides with the one emerging from Loop Quantum Cosmology.
Research Interests:
In this paper we consider spherically symmetric interior spacetimes filled by anisotropic fluids in the context of Hořava gravity and Einstein-aether theory. We assume a specific non-static configuration of the aether vector field and... more
In this paper we consider spherically symmetric interior spacetimes filled by anisotropic fluids in the context of Hořava gravity and Einstein-aether theory. We assume a specific non-static configuration of the aether vector field and show that the field equations admit a family of exact analytical solutions which can be obtained if one of the two metric coefficients is assigned. We study as an illustrative example the case in which the metric of the interior spacetime reproduces the Newtonian potential of a fluid sphere with constant density.
Research Interests:
Cyclic universes with bouncing solutions are candidates for solving the big bang initial singularity prob- lem. 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... more
Cyclic universes with bouncing solutions are candidates for solving the big bang initial singularity prob- lem. 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.
Research Interests:
In quartic-order degenerate higher-order scalar-tensor (DHOST) theories compatible with gravitational-wave constraints, we derive the most general Lagrangian allowing for tracker solutions characterized by ˙ φ/H p = constant, where ˙ φ is... more
In quartic-order degenerate higher-order scalar-tensor (DHOST) theories compatible with gravitational-wave constraints, we derive the most general Lagrangian allowing for tracker solutions characterized by ˙ φ/H p = constant, where ˙ φ is the time derivative of a scalar field φ, H is the Hubble expansion rate, and p is a constant. While the tracker is present up to the cubic-order Horndeski Lagrangian L = c2X − c3X (p−1)/(2p) φ, where c2, c3 are constants and X is the kinetic energy of φ, the DHOST interaction breaks this structure for p = 1. Even in the latter case, however , there exists an approximate tracker solution in the early cosmological epoch with the nearly constant field equation of state w φ = −1 − 2p ˙ H/(3H 2). The scaling solution, which corresponds to p = 1, is the unique case in which all the terms in the field density ρ φ and the pressure P φ obey the scaling relation ρ φ ∝ P φ ∝ H 2. Extending the analysis to the coupled DHOST theories with the field-dependent coupling Q(φ) between the scalar field and matter, we show that the scaling solution exists for Q(φ) = 1/(µ1φ + µ2), where µ1 and µ2 are constants. For the constant Q, i.e., µ1 = 0, we derive fixed points of the dynamical system by using the general Lagrangian with scaling solutions. This result can be applied to the model construction of late-time cosmic acceleration preceded by the scaling φ-matter-dominated epoch.
Research Interests:
The equations of state for a characteristic spacetime are studied in the context of the spherically symmetric interior exact and analytical solutions in Hořava gravity and Einstein-æther theory in which anisotropic fluids are considered.... more
The equations of state for a characteristic spacetime are studied in the context of the spherically symmetric interior exact and analytical solutions in Hořava gravity and Einstein-æther 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.
Research Interests:
The grand challenges of contemporary fundamental physics---dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem---all involve gravity as a key component. And of all... more
The grand challenges of contemporary fundamental physics---dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem---all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress.
Research Interests:
We generalise the covariant Tolman-Oppenheimer-Volkoff equations proposed in arXiv:1709.02818 [gr-qc] to the case of static and spherically symmetric spacetimes with anisotropic sources. The extended equations allow a detailed analysis of... more
We generalise the covariant Tolman-Oppenheimer-Volkoff equations proposed in arXiv:1709.02818 [gr-qc] to the case of static and spherically symmetric spacetimes with anisotropic sources. The extended equations allow a detailed analysis of the role of the anisotropic terms in the interior solution of relativistic stars and lead to the generalisation of some well known solutions of this type. We show that, like in the isotropic case, one can define generating theorems for the anisotropic Tolman-Oppenheimer-Volkoff equations. We also find that it is possible to define a reconstruction algorithm able to generate a double infinity of interior solutions. Among these, we derive a class of solutions that can represent "quasi-isotropic" stars.
Research Interests:
We construct a covariant version of the Tolman-Oppenheimer-Volkoff equations in the case of isotropic sources. The new equations make evident the mathematical problems in the determination of interior solutions of relativistic stellar... more
We construct a covariant version of the Tolman-Oppenheimer-Volkoff equations in the case of isotropic sources. The new equations make evident the mathematical problems in the determination of interior solutions of relativistic stellar objects. Using a reconstruction algorithm we find two physically interesting generalisations of previously known stellar interior solutions. The variables that we use also allow an easier formulation of known generating theorems for solutions associated to relativistic stellar objects.
Research Interests:
We find a reconstruction algorithm able to generate all the static spherically symmetric interior solutions in the framework of Hořava gravity and Einstein-aether theory in presence of anisotropic fluids. We focus for simplicity on the... more
We find a reconstruction algorithm able to generate all the static spherically symmetric interior solutions in the framework of Hořava gravity and Einstein-aether theory in presence of anisotropic fluids. We focus for simplicity on the case of a static aether finding a large class of possible viable interior star solutions which present a very rich phenomenology. We study one illustrative example in more detail. Introduction. Hořava gravity has been proposed in 2009 as an UV complete theory for quantum gravity which breaks Lorentz invariance [1, 2] and it is expected to be renormalizable [3–5]. An interesting feature of this theory is that it admits a covariant formulation which coincides with Einstein-aether theory [6] once the aether has been chosen to be hypersurface orthogonal at the action level [7]. Hořava gravity has been proven to pass all the tests at Newtonian [2], post-Newtonian [8–10], astrophysical (binary pulsars) [11, 12] and cosmological scales [13, 14]. One big challenge in this endeavour concerns the phenomenology of the theory, such as the study of black holes [15–21] and interior solutions for relativistic stars [22, 23]. Indeed, because of the intrinsic highly non-linear structure of its field equations, even at low-energies, it is not trivial to find exact analytical solutions without reducing to very specific cases. In this letter, we focus on exact static spherically symmetric interior solutions of the low-energy limit of the covariantized version of Hořava gravity in presence of an anisotropic fluid. These solutions can be employed to model compact objects (see e.g. [24] and references therein) and derive observational constraints based on their structure. We will consider here the fully anisotropic case as it is well known that in many classes of astrophysical objects a number of processes can generate such anisotropies (e.g. viscosity, phase transitions, etc. [25]). In the following, we will show that in the context of the covariant formulation of Hořava gravity there exists a simple reconstruction algorithm able to generate a double infinity of such anisotropic solutions. Indeed, once the structure of the interior spacetime is chosen, it will be possible to solve algebraically in a trivial way the field equations for the density, the radial and transversal pressure. Such result can be considered as a first step in the study of the full structure and the properties of this type of system in Hořava gravity and the comprehension of its outstanding phenomenological implications. The theory. The action of the low-energy limit of Hořava gravity is:
Research Interests:
We consider Horava gravity within the framework of the EFT of dark energy and modified gravity. We work out a complete mapping of the theory into the EFT language for an action including all the operators which are relevant for linear... more
We consider Horava gravity within the framework of the EFT of dark energy and modified gravity. We work out a complete mapping of the theory into the EFT language for an action including all the operators which are relevant for linear perturbations with up to sixth order spatial derivatives. We then employ an updated version of the EFTCAMB/EFTCosmoMC package to study the cosmology of the low-energy limit of Horava gravity and place constraints on its parameters using several cosmological data sets. In particular we consider two cases: the first in which the three parameters of the low-energy theory are all varied and a second case that is tuned to evade PPN constraints, reducing the number of free parameters to two. We employ data sets which include the CMB TT and lensing power spectra by Planck 2013, WMAP low-l polarization spectra, the WiggleZ galaxy power spectrum, the local Hubble measurements, Supernovae data from SNLS, SDSS and HST and the BAO measurements from BOSS, SDSS and 6dFGS. For both cases we estimate the deviation of the cosmological gravitational constant from the local Newtonian one, getting improved upper bounds with respect to the previous ones from BBN data. At the level of the background, we find a relevant rescaling of the Hubble rate at all epoch, which has a strong impact on the cosmological observables; at the level of perturbations, we discuss all the relevant effects that the modifications of gravity induce, ranging from modifications of the late time ISW effect, the growth of matter perturbations, gravitational lensing and differences in the B-modes of the CMB. In general the quasi-static approximation is not safe to describe the evolution of perturbations in Horava gravity. Overall we find that the effects of the modifications induced by the low-energy Horava gravity action are quite dramatic and current data place tight bounds on the theory parameters.
Research Interests:
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... more
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.
Research Interests:
We consider the version of Hořava 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... more
We consider the version of Hořava 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.
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.
Research Interests:
We study black hole solutions in the infrared sector of three-dimensional Hořava gravity. It is shown that black solutions with anti-de Sitter asymptotics are admissible only in the sector of the theory in which the scalar degree of... more
We study black hole solutions in the infrared sector of three-dimensional Hořava gravity. It is shown that black 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.
Research Interests:
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... more
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 demonstrate using the Newtonian limit as an example.
Research Interests:
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... more
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.
Research Interests:
Dwarf galaxies are good candidates to investigate the nature of Dark Matter, because their kinematics are dominated by this component down to small galactocentric radii. We present here the results of detailed kinematic analysis and mass... more
Dwarf galaxies are good candidates to investigate the nature of Dark Matter, because their kinematics are dominated by this component down to small galactocentric radii. We present here the results of detailed kinematic analysis and mass modelling of the Orion dwarf galaxy, for which we derive a high quality and high resolution rotation curve that contains negligible non-circular motions and we correct it for the asymmetric drift. Moreover, we leverage the proximity (D = 5.4 kpc) and convenient inclination (47{\deg}) to produce reliable mass models of this system. We find that the Universal Rotation Curve mass model (Freeman disk + Burkert halo + gas disk) fits the observational data accurately. In contrast, the NFW halo + Freeman disk + gas disk mass model is unable to reproduce the observed Rotation Curve, a common outcome in dwarf galaxies. Finally, we attempt to fit the data with a MOdified Newtonian Dynamics (MOND) prescription. With the present data and with the present assumptions on distance, stellar mass, constant inclination and reliability of the gaseous mass, the MOND "amplification" of the baryonic component appears to be too small to mimic the required "dark component". The Orion dwarf reveals a cored DM density distribution and a possible tension between observations and the canonical MOND formalism.
Research Interests:
Spherical symmetry for f(R)-gravity is discussed by searching for Noether symmetries. The method consists in selecting conserved quantities in form of currents that reduce dynamics of f(R)-models compatible with symmetries. In this way we... more
Spherical symmetry for f(R)-gravity is discussed by searching for Noether symmetries. The method consists in selecting conserved quantities in form of currents that reduce dynamics of f(R)-models compatible with symmetries. In this way we get a general method to obtain constants of motion without setting a priori the form of f(R). In this sense, the Noether symmetry results a physical criterium. Relevant cases are discussed.
Research Interests:
In spiral galaxies, we explain their non-Keplerian rotation curves (RCs) by means of a non-luminous component embedding their stellar-gaseous disks. Understanding the detailed properties of this component (labelled Dark Matter, DM) is one... more
In spiral galaxies, we explain their non-Keplerian rotation curves (RCs) by means of a non-luminous component embedding their stellar-gaseous disks. Understanding the detailed properties of this component (labelled Dark Matter, DM) is one of the most pressing issues of Cosmology. We investigate the recent relationship (claimed by Walker et al. 2010, hereafter W+10) between $r$, the galaxy radial coordinate, and $V_h(r)$, the dark halo contribution to the circular velocity at $r$, {\it a}) in the framework of the Universal Rotation Curve (URC) paradigm and directly {\it b}) by means of the kinematics of a large sample of DM dominated spirals. We find a general agreement between the W+10 claim, the distribution of DM emerging from the URC and that inferred in the (low luminosity) objects of our sample. We show that such a phenomenology, linking the spiral's luminosity, radii and circular velocities, implies an evident inconsistency with (naive) predictions in the $\Lambda$ Cold Dark Matter ($\Lambda$CDM) scenario.
Research Interests:
We attempt a critical reconsideration of "detailed balance" as a principle that can be used to restrict the proliferation of couplings in Horava-Lifshitz gravity. We re-examine the shortcomings that have been usually associated with it in... more
We attempt a critical reconsideration of "detailed balance" as a principle that can be used to restrict the proliferation of couplings in Horava-Lifshitz gravity. We re-examine the shortcomings that have been usually associated with it in the literature and we argue that easy remedies can be found for all of them within the framework of detailed balance, and that the most persistent of them are actually related to projectability. We show that, once projectability is abandoned, detailed balance reduces the number of independent couplings by roughly an order of magnitude and imposes only one restriction that constitutes a phenomenological concern: the size of the (bare) cosmological constant is unacceptably large. Remarkably, this restriction (which is present in the projectable version as well) has been so far under-appreciated in the literature. Optimists might prefer to interpret it as a potential blessing in disguise, as it allows one to entertain the idea of a miraculous cancelation between the bare cosmological constant and the (still poorly understood) vacuum energy contribution.
Research Interests:
We discuss the possibility to obtain an electromagnetic emission accompanying the gravitational waves emitted in the coalescence of a compact binary system. Motivated by the existence of black hole configurations with open magnetic field... more
We discuss the possibility to obtain an electromagnetic emission accompanying the gravitational waves emitted in the coalescence of a compact binary system. Motivated by the existence of black hole configurations with open magnetic field lines along the rotation axis, we consider a magnetic dipole in the system, the evolution of which leads to (i) electromagnetic radiation, and (ii) a contribution to the gravitational radiation, the luminosity of both being evaluated. Starting from the observations on magnetars, we impose upper limits for both the electromagnetic emission and the contribution of the magnetic dipole to the gravitational wave emission. Adopting this model for the evolution of neutron star binaries leading to short gamma ray bursts, we compare the correction originated by the electromagnetic field to the gravitational waves emission, finding that they are comparable for particular values of the magnetic field and of the orbital radius of the binary system. Finally we calculate the electromagnetic and gravitational wave energy outputs which result comparable for some values of magnetic field and radius.
Research Interests:
We discuss the conformal symmetry between Jordan and Einstein frames considering their relations with the metric and Palatini formalisms for modified gravity. Appropriate conformal transformations are taken into account leading to the... more
We discuss the conformal symmetry between Jordan and Einstein frames considering their relations with the metric and Palatini formalisms for modified gravity. Appropriate conformal transformations are taken into account leading to the evident connection between the gravitational actions in the two mentioned frames and the Hilbert-Einstein action with a cosmological constant. We show that the apparent differences between Palatini and metric formalisms strictly depend on the representation while the number of degrees of freedom is preserved. This means that the dynamical content of both formalism is identical.
Research Interests:
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... more
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.
Research Interests:
The gravitational phase shift of neutrino oscillation can be discussed in the framework of f(R)-gravity. We show that the shift of quantum mechanical phase can depend on the given f(R)-theory that we choose. This fact is general and could... more
The gravitational phase shift of neutrino oscillation can be discussed in the framework of f(R)-gravity. We show that the shift of quantum mechanical phase can depend on the given f(R)-theory that we choose. This fact is general and could constitute a fundamental test to discriminate among the various alternative relativistic theories of gravity. Estimations of ratio between the gravitational phase shift and the standard phase are carried out for the electronic Solar neutrinos.
Research Interests:
Research Interests:
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’s theorem singles out General... more
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’s theorem singles out General Relativity as the only theory with second-order 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 lowenergy. 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.
scales. Lovelock’s theorem singles out General Relativity as the only theory with second-order 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 lowenergy. 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.
Research Interests:
I will consider the possibility of modifying gravity by adding auxiliary fields, so as to avoid adding extra degrees of freedom. I will argue that theories of this type can generically be rewritten as general relativity with an effective... more
I will consider the possibility of modifying gravity by adding auxiliary fields, so as to avoid adding extra degrees of freedom. I will argue that theories of this type can generically be rewritten as general relativity with an effective stress-energy tensor that contains higher order derivatives of the matter fields and vanishes in vacuum.
This provides a very efficient parametrization of such theories that allows to study phenomenology and obtain constraints without knowing the exact field content. Known theories with auxiliary fields fit in this parametrization.
Finally I will discuss how the presence of higher order derivatives of the matter fields leads to very tight viability constraints.
This provides a very efficient parametrization of such theories that allows to study phenomenology and obtain constraints without knowing the exact field content. Known theories with auxiliary fields fit in this parametrization.
Finally I will discuss how the presence of higher order derivatives of the matter fields leads to very tight viability constraints.
Research Interests:
I will consider the possibility of modifying gravity by adding auxiliary fields, so as to avoid adding extra degrees of freedom. I will argue that theories of this type can generically be rewritten as general relativity with an effective... more
I will consider the possibility of modifying gravity by adding auxiliary fields, so as to avoid adding extra degrees of freedom. I will argue that theories of this type can generically be rewritten as general relativity with an effective stress-energy tensor that contains higher order derivatives of the matter fields and vanishes in vacuum.
This provides a very efficient parametrization of such theories that allows to study phenomenology and obtain constraints without knowing the exact field content. Known theories with auxiliary fields fit in this parametrization.
Finally I will discuss how the presence of higher order derivatives of the matter fields leads to very tight viability constraints.
This provides a very efficient parametrization of such theories that allows to study phenomenology and obtain constraints without knowing the exact field content. Known theories with auxiliary fields fit in this parametrization.
Finally I will discuss how the presence of higher order derivatives of the matter fields leads to very tight viability constraints.
Research Interests:
I will attempt a critical reconsideration of “detailed balance” as a principle that can be used to restrict the proliferation of couplings in Horava–Lifshitz gravity. I will discuss the shortcomings that have been usually associated with... more
I will attempt a critical reconsideration of “detailed balance” as a principle that can be used to restrict the proliferation of couplings in Horava–Lifshitz gravity.
I will discuss the shortcomings that have been usually associated with it in the literature and I will show that, once projectability is abandoned, easy remedies can be found for all of them within the framework of detailed balance.
In this way the number of independent couplings is reduced by roughly an order of magnitude even if the theory restricts the size of the (bare) cosmological constant to be unacceptably large.
This fact allows one to entertain the idea of a miraculous cancellation between the bare cosmological constant and the still poorly understood vacuum energy contribution.
I will discuss the shortcomings that have been usually associated with it in the literature and I will show that, once projectability is abandoned, easy remedies can be found for all of them within the framework of detailed balance.
In this way the number of independent couplings is reduced by roughly an order of magnitude even if the theory restricts the size of the (bare) cosmological constant to be unacceptably large.
This fact allows one to entertain the idea of a miraculous cancellation between the bare cosmological constant and the still poorly understood vacuum energy contribution.
Research Interests:
I will speak about Horava-Lifshitz gravity, which has been proposed as an ultraviolet completion to general relativity. Adding to the gravitational action higher order spatial derivatives without adding higher-order time derivatives leads... more
I will speak about Horava-Lifshitz gravity, which has been proposed as an ultraviolet completion to general relativity. Adding to the gravitational action higher order spatial derivatives without adding higher-order time derivatives leads to a modification of the graviton propagator, which renders the theory power counting renormalizable at the expense of violating Lorentz symmetry. Perhaps surprisingly, the theory appears to be phenomenologically viable in its more general form. However, it has the unappealing feature of having a very large number of higher order terms in the action. I will consider the possibility of limiting the proliferation of independent couplings by imposing various restrictions to the theory and I will critically discuss the shortcomings of the various restrictions one can use.
Research Interests:
A critical reconsideration of “detailed balance” as a principle that can be used to restrict the proliferation of couplings in Horava–Lifshitz gravity will be attempted. The shortcomings that have been usually associated with it in the... more
A critical reconsideration of “detailed balance” as a principle that can be used to restrict the proliferation of couplings in Horava–Lifshitz gravity will be attempted.
The shortcomings that have been usually associated with it in the literature will be discussed, and it will be shown that, once projectability is abandoned, easy remedies can be found for all of them within the framework of detailed balance.
In this way the number of independent couplings is reduced by roughly an order of magnitude even if the theory restricts the size of the (bare) cosmological constant to be unacceptably large.
This fact allows one to entertain the idea of a miraculous cancellation between the bare cosmological constant and the still poorly understood vacuum energy contribution.
The shortcomings that have been usually associated with it in the literature will be discussed, and it will be shown that, once projectability is abandoned, easy remedies can be found for all of them within the framework of detailed balance.
In this way the number of independent couplings is reduced by roughly an order of magnitude even if the theory restricts the size of the (bare) cosmological constant to be unacceptably large.
This fact allows one to entertain the idea of a miraculous cancellation between the bare cosmological constant and the still poorly understood vacuum energy contribution.
Research Interests:
Dwarf galaxies are good candidates to investigate the nature of Dark Matter, because their kinematics are dominated by this component down to small galactocentric radii. We present here the results of detailed kinematic analysis and mass... more
Dwarf galaxies are good candidates to investigate the nature of Dark Matter, because their kinematics are dominated by this component down to small galactocentric radii. We present here the results of detailed kinematic analysis and mass modelling of the Orion dwarf galaxy, for which we derive a high quality and high resolution rotation curve that contains negligible non-circular motions and we correct it for the asymmetric drift. Moreover, we leverage the proximity (D = 5.4 \pm 1 Mpc) and convenient inclination (47 \pm 3) to produce reliable mass models of this system. We find that the Universal Rotation Curve mass model (Freeman disk + Burkert halo + gas disk) fits the observational data accurately. In contrast, the NFW halo + Freeman disk + gas disk mass model is unable to reproduce the observed Rotation Curve, a common outcome in dwarf galaxies. Finally, we attempt to t the data with a MOdi ed Newtonian Dynamics (MOND) prescription. With the present data and with the present assumptions on distance, stellar mass, constant inclination and reliability of the gaseous mass, the MOND "ampli fication" of the baryonic component appears to be too small to mimic the required dark component. The Orion dwarf reveals a cored DM density distribution and a possible tension between observations and the canonical MOND formalism.
Research Interests:
Modifications of GR 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 extra fields. With the minimal... more
Modifications of GR 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 extra fields. With the minimal requirement that the theory satisfies the Weak Equivalence Principle (WEP) 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 demonstrate by using the Newtonian limit as an example.
Research Interests:
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’s theorem singles out General Relativity... more
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’s theorem singles out General Relativity as the only theory with second-order 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 lowenergy. 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.
scales. Lovelock’s theorem singles out General Relativity as the only theory with second-order 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 lowenergy. 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.