Papers by Daniele Vernieri
The 2nd Electronic Conference on Universe
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Monthly Notices of the Royal Astronomical Society
We have explored a completely new and alternative way to restrict the parameter space of Horndesk... more We have explored a completely new and alternative way to restrict the parameter space of Horndeski theory of gravity. Using its Newtonian limit, it is possible to test the theory at a regime where, given its complexity and the small magnitude of the expected effects, it is poorly probed. At Newtonian level, it gives rise to a generalized Yukawa-like Newtonian potential which we have tested using S2 star orbit data. Our model adds five parameters to the General Relativity model, and the analysis constrains two of them with unprecedented precision to these energy scales, while it only gives an exclusion region for the remaining parameters. We have shown the potential of weak-field tests to constrain Horndeski gravity opening, as a matter of fact, which is a new avenue that deserves to be further, and deeply, explored in the near future.
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Physical Review D
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MNRAS 518, 3372–3385, 2023
We investigate the capability of Einstein Telescope to constrain the cosmological parameters of t... more We investigate the capability of Einstein Telescope to constrain the cosmological parameters of the non-flat CDM cosmological model. Two types of mock data sets are considered depending on whether or not a short gamma-ray burst is detected, and associated with the gravitational wave emitted by binary neutron stars merger, using the THESEUS satellite. Depending on the mock data set, two statistical estimators are applied: one assumes that the redshift is known, while the other marginalizes o v er it assuming a specific redshift prior distribution. We demonstrate that (i) using mock catalogues collecting gravitational wave signals emitted by binary neutron stars systems to which a short gamma-ray burst has been associated, Einstein Telescope may achieve an accuracy on the cosmological parameters of σ H 0 ≈ 0. 40 km s −1 Mpc −1 , σ k, 0 ≈ 0. 09, and σ , 0 ≈ 0. 07; while (ii) using mock catalogues collecting all gra vitational wa ve signals emitted by binary neutron stars systems for which an electromagnetic counterpart has not been detected, Einstein Telescope may achieve an accuracy on the cosmological parameters of σ H 0 ≈ 0. 04 km s −1 Mpc −1 , σ k, 0 ≈ 0. 01, and σ , 0 ≈ 0. 01, once the redshift probability distribution of GW events is known from from population synthesis simulations and/or the measure of the tidal deformability parameter. These results show an impro v ement of a factor 2-75 with respect to earlier results using complementary data sets.
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MNRAS 519, 1981–1988 , 2023
We have explored a completely new and alternative way to restrict the parameter space of Horndesk... more We have explored a completely new and alternative way to restrict the parameter space of Horndeski theory of gravity. Using its Newtonian limit, it is possible to test the theory at a regime where, given its complexity and the small magnitude of the expected effects, it is poorly probed. At Newtonian level, it gives rise to a generalized Yukaw a-lik e Newtonian potential which we have tested using S2 star orbit data. Our model adds five parameters to the General Relativity model, and the analysis constrains two of them with unprecedented precision to these energy scales, while it only gives an e xclusion re gion for the remaining parameters. We have shown the potential of weak-field tests to constrain Horndeski gravity opening, as a matter of fact, which is a new avenue that deserves to be further, and deeply, explored in the near future.
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Monthly Notices of the Royal Astronomical Society
We investigate the capability of Einstein Telescope to constrain the cosmological parameters of t... more We investigate the capability of Einstein Telescope to constrain the cosmological parameters of the non-flat ΛCDM cosmological model. Two types of mock datasets are considered depending on whether or not a short Gamma-Ray Burst is detected and associated with the gravitational wave emitted by binary neutron stars merger using the THESEUS satellite. Depending on the mock dataset, two statistical estimators are applied: one assumes that the redshift is known, while the other marginalizes over it assuming a specific redshift prior distribution. We demonstrate that (i) using mock catalogs collecting gravitational wave signals emitted by binary neutron stars systems to which a short Gamma-Ray Burst has been associated, Einstein Telescope may achieve an accuracy on the cosmological parameters of $\sigma _{H_0}\approx 0.40$ km s−1 Mpc−1, $\sigma _{\Omega _{k,0}}\approx 0.09$, and $\sigma _{\Omega _{\Lambda ,0}}\approx 0.07$; while (ii) using mock catalogs collecting all gravitational wave ...
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General Relativity and Gravitation, 2020
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Physical Review D
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Lovelock gravity in D-dimensional space-times is considered adopting Cartan's structure equations... more Lovelock gravity in D-dimensional space-times is considered adopting Cartan's structure equations. In this context, we find out exact solutions in cosmological and spherically symmetric backgrounds. In the latter case, we also derive horizons and the corresponding Bekenstein--Hawking entropies. Moreover, we focus on the topological Chern--Simons theory, providing exact solutions in 5 dimensions. Specifically, it is possible to show that Anti-de Sitter invariant Chern--Simons gravity can be framed within Lovelock--Zumino gravity in 5 dimensions, for particular choices of Lovelock parameters.
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General Relativity is an extremely successful theory, at least for weak gravitational fields, how... more General Relativity is an extremely successful theory, at least for weak gravitational fields, however, it breaks down at very high energies, such as in correspondence of the initial singularity. Quantum Gravity is expected to provide more physical insights concerning this open question. Indeed, one alternative scenario to the Big Bang, that manages to completely avoid the singularity, is offered by Loop Quantum Cosmology (LQC), which predicts that the Universe undergoes a collapse to an expansion through a bounce. In this work, we use metric f(R) gravity to reproduce the modified Friedmann equations which have been obtained in the context of modified loop quantum cosmologies. To achieve this, we apply an order reduction method to the f(R) field equations, and obtain covariant effective actions that lead to a bounce, for specific models of modified LQC, considering matter as a scalar field.
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The European Physical Journal C
Loop quantum cosmology (LQC) is a theory which renders the Big Bang initial singularity into a qu... more Loop quantum cosmology (LQC) is a theory which renders the Big Bang initial singularity into a quantum bounce, by means of short-range repulsive quantum effects at the Planck scale. In this work, we are interested in reproducing the effective Friedmann equation of LQC, by considering a generic f(R, P, Q) theory of gravity, where $$R=g^{\mu \nu }R_{\mu \nu }$$ R = g μ ν R μ ν is the Ricci scalar, $$P=R_{\mu \nu }R^{\mu \nu }$$ P = R μ ν R μ ν , and $$Q=R_{\alpha \beta \mu \nu }R^{\alpha \beta \mu \nu }$$ Q = R α β μ ν R α β μ ν is the Kretschmann scalar. An order reduction technique allows us to work in f(R, P, Q) theories which are perturbatively close to General Relativity, and to deduce a modified Friedmann equation in the reduced theory. Requiring that the modified Friedmann equation mimics the effective Friedmann equation of LQC, we are able to derive several functional forms of f(R, P, Q). We discuss the necessary conditions to obtain viable bouncing cosmologies for the propose...
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Non-local gravity cosmologies are considered under the standard of Noether Symmetry Approach. In ... more Non-local gravity cosmologies are considered under the standard of Noether Symmetry Approach. In particular , we focus on non-local theories whose gravitational actions depend on curvature and Gauss-Bonnet scalar invariants. Specific functional forms of the related point-like Lagrangians are selected by Noether symmetries and we solve the corresponding field equations finding out exact cosmological solutions.
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We consider f(Q) extended symmetric teleparallel cosmologies, where Q is the non-metricity scalar... more We consider f(Q) extended symmetric teleparallel cosmologies, where Q is the non-metricity scalar, and constrain its functional form through the order reduction method. By using this technique, we are able to reduce and integrate the field equations and thus to select the corresponding models giving rise to bouncing cosmology. The selected Lagrangian is then used to develop the Hamiltonian formalism and to obtain the Wave Function of the Universe which suggests that classical observable universes can be recovered according to the Hartle Criterion.
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We study non-rotating and isotropic strange quark stars in Lorentz-violating theories of gravity,... more We study non-rotating and isotropic strange quark stars in Lorentz-violating theories of gravity, and in particular in Hořava gravity and Einstein-aether theory. For quark matter we adopt both linear and non-linear equations of state, corresponding to the MIT bag model and color flavor locked state, respectively. The new structure equations describing hydrostatic equilibrium generalize the usual Tolman-Oppenheimer-Volkoff (TOV) equations of Einstein's general relativity. A dimension-less parameter ν measures the deviation from the standard TOV equations, which are recovered in the limit ν → 0. We compute the mass, the radius as well as the compactness of the stars, and we show graphically the impact of the parameter ν on the mass-to-radius profiles for different equations of state describing quark matter. The energy conditions and stability criteria are also considered, and they are all found to be fulfilled.
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A bouncing universe is a viable candidate to solve the initial singularity problem. Here we consi... 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.
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In this paper we consider spherically symmetric interior spacetimes filled by anisotropic fluids ... 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.
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Cyclic universes with bouncing solutions are candidates for solving the big bang initial singular... 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.
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In quartic-order degenerate higher-order scalar-tensor (DHOST) theories compatible with gravitati... 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.
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Papers by Daniele Vernieri
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.
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.
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.
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.
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.