A5049727461- Cosmology and Gravitation Theories
- Black Holes and Theoretical Physics
- Pulsars and Gravitational Waves Research
- Noncommutative and Quantum Gravity Theories
- Relativity and Gravitational Theory
- Astrophysical Phenomena and Observations
- Geophysics and Gravity Measurements
- Advanced Differential Geometry Research
- Gamma-ray bursts and supernovae
- Galaxies: Formation, Evolution, Phenomena
- Computational Physics and Python Applications
- Astrophysics and Cosmic Phenomena
- Solar and Space Plasma Dynamics
- Particle physics theoretical and experimental studies
- Experimental and Theoretical Physics Studies
- Quantum Electrodynamics and Casimir Effect
- Geophysics and Sensor Technology
- Dark Matter and Cosmic Phenomena
- Astronomy and Astrophysical Research
- History and Developments in Astronomy
- Physics of Superconductivity and Magnetism
- Quantum Mechanics and Applications
- Particle Accelerators and Free-Electron Lasers
- Distributed and Parallel Computing Systems
- Radio Astronomy Observations and Technology
University of Nottingham
2016-2025
Laboratoire des 2 Infinis Toulouse
2024
University of Tartu
2022
Laboratoire AstroParticule et Cosmologie
2022
Université Paris Cité
2022
Centre National de la Recherche Scientifique
2022
Instituto Politécnico de Lisboa
2022
University of Lisbon
2018-2022
University of Tübingen
2022
Bulgarian Academy of Sciences
2022
Modified gravity theories have received increased attention lately due to combined motivation coming from high-energy physics, cosmology and astrophysics. Among numerous alternatives Einstein's theory of gravity, which include higher order curvature invariants, specifically the particular class f(R) theories, a long history. In last five years there has been new stimulus for their study, leading number interesting results. We review here in an attempt comprehensively present most important...
We identify a class of scalar-tensor theories with coupling between the scalar and Gauss-Bonnet invariant that exhibit spontaneous scalarization for both black holes compact stars. In particular, these formally admit all stationary solutions general relativity, but are not dynamically preferred if certain conditions satisfied. Remarkably, their mass lies within one many narrow bands. find evidence can occur in neutron stars as well.
We show that in theories of generalized teleparallel gravity, whose Lagrangians are algebraic functions the usual Lagrangian, action and field equations not invariant under local Lorentz transformations. also argue these appear to have extra degrees freedom with respect general relativity. The which has been extensively studied leads a theory dynamically equivalent relativity, is an exception. Both facts overlooked recent literature on $f(T)$ but crucial for assessing viability as...
The most general action for a scalar field coupled to gravity that leads second order equations both the metric and --- Horndeski's theory is considered, with extra assumption satisfies shift symmetry. We show in such theories forced have nontrivial configuration black hole spacetimes, unless one carefully tunes away linear coupling Gauss--Bonnet invariant. Hence, holes generic this class will hair. This contradicts recent no-hair theorem, which seems overlooked presence of coupling.
In a recent paper we showed that in shift-symmetric Horndeski theory the scalar field is forced to obtain nontrivial configuration black hole spacetimes, unless linear coupling with Gauss--Bonnet invariant tuned away. As result, holes generically have hair this theory. companion paper, first review our argument and discuss it more detail. We then present actual solutions simplest case of scalar-Gauss--Bonnet coupling. generate exact numerically for wide range values also construct analytic...
Hawking has proven that black holes which are stationary as the end point of gravitational collapse in Brans-Dicke theory (without a potential) no different than general relativity. We extend this proof to much more class scalar-tensor and f(R) gravity theories, without assuming any symmetries apart from stationarity.
In the present paper we will investigate relation between scalar-tensor theory and $f(R)$ theories of gravity. Such studies have been performed in past for metric formalism gravity; here consider mainly Palatini formalism, where connections are treated as independent quantities. We try to under which circumstances gravity equivalent examine implications this equivalence, when it exists.
The grand challenges of contemporary fundamental physics---dark matter, dark energy, vacuum inflation and early universe cosmology, singularities the hierarchy problem---all involve gravity as a key component. And all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some most remarkable predictions General Relativity: event horizons, ergoregions. hitherto invisible landscape Universe is being unveiled before our eyes: historical direct detection...
Horava's "Lifschitz point gravity" has many desirable features, but in its original incarnation one is forced to accept a nonzero cosmological constant of the wrong sign be compatible with observation. We develop an extension model that abandons "detailed balance" and regains parity invariance, 3+1 dimensions exhibit all five marginal (renormalizable) four relevant (super-renormalizable) operators, as determined by power counting. also consider classical limit this theory, evaluate...
We analyze the relation between teleparallelism and local Lorentz invariance. show that generic modifications of teleparallel equivalent to general relativity will not respect symmetry. clarify reasons for this explain why situation is different in relativity. give a prescription constructing equivalents known theories. also explicitly consider recently proposed class generalized theories, called f(T) theories gravity, restoring symmetry such cannot lead sensible dynamics, even if one gives...
In this work we study the cosmology of general f(T) gravity theory. We express modified Einstein equations using covariant quantities, and derive gauge-invariant perturbation in form. consider a specific choice f(T), designed to explain observed late-time accelerating cosmic expansion without including an exotic dark energy component. Our numerical solution shows that extra degree freedom such models generally decays as one goes smaller scales, consequently its effects on scales galaxies...
We study spherical black-hole solutions in Einstein-aether theory, a Lorentz-violating gravitational theory consisting of General Relativity with dynamical unit timelike vector (the "aether") that defines preferred direction. These are also to the infrared limit Horava-Lifshitz gravity. explore parameter values two theories where all presently known experimental constraints satisfied, and find type expected form by collapse exist for those parameters. Outside metric horizon, deviations away...
The Laser Interferometer Space Antenna (LISA) has the potential to reveal wonders about fundamental theory of nature at play in extreme gravity regime, where gravitational interaction is both strong and dynamical. In this white paper, Fundamental Physics Working Group LISA Consortium summarizes current topics physics observations GWs can be expected provide key input. We briefest reviews then delineate avenues for future research directions discuss connections between working group, other...
It was recently shown that a scalar field suitably coupled to the Gauss-Bonnet invariant $\mathcal{G}$ can undergo spin-induced linear tachyonic instability near Kerr black hole. This appears only once dimensionless spin $j$ is sufficiently large, is, $j \gtrsim 0.5$. A hallmark of spontaneous scalarization. Focusing, for illustrative purposes, on class theories do exhibit this instability, we show stationary, rotating hole solutions indeed have hair threshold exceeded, while holes lie below...
We study scalar fields in a black hole background and show that, when the is suitably coupled to curvature, rapid rotation can induce tachyonic instability. This instability, which hallmark of spontaneous scalarization linearized regime, expected be quenched by nonlinearities endow with hair. Hence, our results demonstrate existence broad class theories that share same stationary solutions general relativity at low spins, but exhibit hair sufficiently high spins...
We study the nonlinear dynamics of black holes that carry scalar hair and binaries composed such holes. The is due to a linear or exponential coupling between Gauss-Bonnet invariant. work perturbatively in constant interaction but nonperturbatively fields. first consider dynamical formation for isolated arbitrary spin determine final state. This also allows us compute time quasinormal modes rotating presence this coupling. then evolution nonspinning hole with various mass ratios produce...
Scalar-tensor theories of gravity where a new scalar degree freedom couples to the Gauss-Bonnet invariant can exhibit phenomenon spontaneous black hole scalarization. These admit both classic solutions predicted by general relativity as well novel hairy solutions. The stability holes is strongly dependent on precise form scalar-gravity coupling. A radial investigation revealed that all scalarized are unstable when coupling between field and quadratic in scalar, whereas stable exist for...
Recent observations of compact astrophysical objects have opened the possibility to probe nature gravity in its strong-field regime. Such could reveal deviations from general relativity or standard model. Spontaneous scalarization, which is controlled by scalar-field couplings gravity, leads a behavior that resembles phase transition: scalar induces measurable effects regime while remaining undetectable weak-field gravitational experiments. This review presents spontaneous scalarization...
It has long been known that a maximally spinning black hole cannot be overspun by tossing in test body. Here we show if instead the starts out with below maximal spin, then indeed overspinning can achieved. We find requirements on size and internal structure of body met carries orbital but not spin angular momentum. Our analysis neglects radiative self-force effects, which may prevent overspinning.
It has recently been pointed out that particles falling freely from rest at infinity outside a Kerr black hole can in principle collide with an arbitrarily high center of mass energy the limiting case maximal spin. Here we aim to elucidate mechanism for this fascinating result, and point its practical limitations, which imply ultraenergetic collisions cannot occur near holes nature.
This is intended to be a brief introduction and overview of Hořava-Lifshitz gravity. The motivation all the various versions theory (to date) are presented. dynamics discussed in some detail, with focus on low energy viability consistency, as these have been issues that attracted most attention literature so far. Other properties developments within its framework also covered, such as: relation Einstein-aether theory, cosmology, future perspectives.