Andrea Maselli

Slowly rotating black hole solutions in Horndeski gravity

We study black hole solutions at first order in the Hartle-Thorne slow-rotation approximation in Horndeski gravity theories. We derive the equations of motion including also cases where the scalar depends linearly on time. In the Hartle-Thorne formalism, all first-order rotational corrections are described by a single frame-dragging function. We show that the frame-dragging function is exactly the same as in general relativity for all known black hole solutions in shift symmetric Horndeski theories, with the exception of theories with a linear coupling to the Gauss-Bonnet invariant. Our results extend previous no-hair theorems for a broad class of Horndeski gravity theories.

Rotating black holes in Einstein-Dilaton-Gauss-Bonnet gravity with finite coupling

Among various strong-curvature extensions to General Relativity, Einstein-Dilaton-Gauss-Bonnet gravity stands out as the only nontrivial theory containing quadratic curvature corrections while being free from the Ostrogradsky instability to any order in the coupling parameter. We derive an approximate stationary and axisymmetric black-hole solution of this gravitational theory in closed form, which is quadratic in the black-hole spin angular momentum and of seventh order in the coupling parameter of the theory. This extends previous work that obtained the corrections to the metric only at the leading order in the coupling parameter, and allows us to consider values of the coupling parameter close to the maximum permitted by theoretical constraints. We compute some geometrical properties of this solution, such as the dilaton charge, the moment of inertia and the quadrupole moment, and its geodesic structure, including the innermost-stable circular orbit and the epicyclic frequencies for massive particles. The latter represent a valuable tool to test General Relativity against strong-curvature corrections through observations of the electromagnetic spectrum of accreting black holes.

Equation-of-state-independent relations in neutron stars

Neutron stars are extremely relativistic objects which abound in our universe and yet are poorly understood, due to the high uncertainty on how matter behaves in the extreme conditions which prevail in the stellar core. It has recently been pointed out that the moment of inertia I, the Love number lambda and the spin-induced quadrupole moment Q of an isolated neutron star, are related through functions which are practically independent of the equation of state. These surprising universal I-lambda-Q relations pave the way for a better understanding of neutron stars, most notably via gravitational-wave emission. Gravitational-wave observations will probe highly-dynamical binaries and it is important to understand whether the universality of the I-lambda-Q relations survives strong-field and finite-size effects. We apply a Post-Newtonian-Affine approach to model tidal deformations in compact binaries and show that the I-lambda relation depends on the inspiral frequency, but is insensitive to the equation of state. We provide a fit for the universal relation, which is valid up to a gravitational wave frequency of ~900 Hz and accurate to within a few percent. Our results strengthen the universality of I-lambda-Q relations, and are relevant for gravitational-wave observations with advanced ground-based interferometers. We also discuss the possibility of using the Love-compactness relation to measure the neutron-star radius with an uncertainty of about 10% or smaller from gravitational-wave observations.

Testing Gravity with Quasi Periodic Oscillations from accreting Black Holes: the Case of the Einstein-Dilaton-Gauss-Bonnet Theory

Quasi-Periodic Oscillations (QPOs) observed in the X-ray flux emitted by accreting black holes, are associated to phenomena occurring near the horizon. Future very large area X-ray instruments will be able to measure QPO frequencies with very high precision, thus probing this strong-field region. By using the relativistic precession model, we show the way in which QPO frequencies could be used to test general relativity against those alternative theories of gravity which predict deviations from the classical theory in the strong-field regime. We consider one of the best motivated strong-curvature corrections to general relativity, namely the Einstein-Dilaton-Gauss-Bonnet theory, and show that a detection of QPOs with the expected sensitivity of the proposed ESA M-class mission LOFT would set the most stringent constraints on the parameter space of this theory.

Missing link: A nonlinear post-Friedmann framework for small and large scales

We present a nonlinear post-Friedmann framework for structure formation, generalizing to cosmology the weak-field (post-Minkowskian) approximation, unifying the treatment of small and large scales. We consider a universe filled with a pressureless fluid and a cosmological constant

Parameter estimation of gravitational wave echoes from exotic compact objects

\mathrm{\ensuremath{\Lambda}}

Compact objects in Horndeski gravity

, the theory of gravity is Einsteins general relativity and the background is the standard flat

Rotating protoneutron stars: Spin evolution, maximum mass, and I-Love-Q relations

\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}

Constraining the equation of state of nuclear matter with gravitational wave observations: Tidal deformability and tidal disruption

cosmological model. We expand the metric and the energy-momentum tensor in powers of

Constraining Modified Theories of Gravity with Gravitational-Wave Stochastic Backgrounds

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