arXiv subjects
Publications and source records attributed to Alex Kehagias.
We provide a simple proof of why the Love number vanishes for extremal Reissner-Nordstrom and Kerr-Newman black holes. The argument is based on a conformal inversion isometry of the spacetime connecting the horizon with large distances.
We show why the threshold for primordial black hole formation is universal (independent from the shape of the perturbation) when expressed in terms of the volume averaged compaction function. The proof is rooted in the self-similarity of the gravitational collapse phenomenon at criticality.
We revisit the genuine Fayet-Iliopoulos terms of 4D N=1 supergravity. Such terms are commonly believed to preserve a global symmetry, and therefore they are in conflict with the principles of quantum gravity. However, we find that generically there do exist supersymmetric terms that break explicitly the specific global symmetry, while preserving gauge invariance. We illustrate this, by providing a series of examples, including superpotentials and superspace higher order terms, along with a general prescription for their construction.
Due to the nature of gravity, non-linear effects are left imprinted in the quasi-normal modes generated in the ringdown phase of the merger of two black holes. We offer an analytical treatment of the quasi-normal modes at second-order in black hole perturbation theory which takes advantage from the fact that the non-linear sources are peaked around the light ring. As a byproduct, we describe why the amplitude of the second-order mode relative to the square of the first-order amplitude depends only weakly on the initial condition of the problem.
The Hellings and Downs correlation curve describes the correlation of the timing residuals from pairs of pulsars as a function of their angular separation on the sky and is a smoking-gun signature for the detection of an isotropic stochastic background of gravitational waves. We show that it can be easily obtained from realizing that Lorentz transformations are conformal transformations on the celestial sphere and from the conformal properties of the two-point correlation of the timing residuals. This result allows several generalizations, e.g. the calculation of the three-point correlator of the time residuals and the inclusion of additional polarization modes (vector and/or scalar) arising in alternative theories of gravity.
The formation of primordial black holes in the early universe may happen through the collapse of large curvature perturbations generated during a non-attractor phase of inflation or through a curvaton-like dynamics after inflation. The fact that such small-scale curvature perturbation is typically non-Gaussian leads to the renormalization of composite operators built up from the smoothed density contrast and entering in the calculation of the primordial black abundance. Such renormalization causes the phenomenon of operator mixing and the appearance of an infinite tower of local, non-local and higher-derivative operators as well as to a sizable shift in the threshold for primordial black hole formation. This hints that the calculation of the primordial black hole abundance is more involved than what generally assumed.
An extended search for anomaly free matter coupled $N=(1,0)$ supergravity in six dimension is carried out by two different methods which we refer to as the graphical and rank methods. In the graphical method the anomaly free models are built from single gauge group models, called nodes, which can only have gravitational anomalies. We search for anomaly free theories with gauge groups $G_1\times...\times G_n$ with $n=1,2,...$ (any number of factors) and $G_1\times...\times G_n \times U(1)_R$ where $n=1,2,3$ and $U(1)_R$ is the $R$-symmetry group. While we primarily consider models with the tensor multiplet number $n_T=1$, we also provide some results for $n_T\ne 1$ with an unconstrained number of charged hypermultiplets. We find a large number of ungauged anomaly free theories. However, in the case of $R$-symmetry gauged models with $n_T=1$, in addition to the three known anomaly free theories with $G_1\times G_2 \times U(1)_R$ type symmetry, we find only six new remarkably anomaly free models with symmetry groups of the form $G_1\times G_2\times G_3 \times U(1)_R$. In the case of $n_T=1$ and ungauged models, excluding low rank group factors and considering only low lying representations, we find all anomaly free theories. Remarkably, the number of group factors does not exceed four in this class. The proof of completeness in this case relies on a bound which we establish for a parameter characterizing the difference between the number of non-singlet hypermultiplets and the dimension of the gauge group.
We discuss the non-linear corrections entering in the calculation of the primordial black hole abundance from the non-linear radiation transfer function and the determination of the true physical horizon crossing. We show that the current standard techniques to calculate the abundance of primordial black holes suffer from uncertainties and argue that the primordial black hole abundance may be much smaller than what routinely considered. This would imply, among other consequences, that the interpretation of the recent pulsar timing arrays data from scalar-induced gravitational waves may not be ruled out because of an overproduction of primordial black holes.
We note that the typical gauge mediation of supersymmetry breaking is in tension with the global limit of the festina lente swampland bound. The alternatives are mediation/breaking schemes that decouple together with gravity, as for example gravity mediation, for which we highlight some basic phenomenological properties. Gauge mediation remains instead a viable mechanism only in models where supersymmetry is restored in the global limit, as for example in no-scale supergravity.
The Weak Gravity Conjecture predicts that in quantum gravity there should exist overcharged states, that is states with charge larger than their mass. Extending this to large masses and charges, we are expecting similar overcharged classical solutions. This has been demonstrated in higher-derivative extensions of General Relativity. In this paper we investigate the existence of overcharged solutions in General Relativity. We study the dynamics of a thin shell of mass $m$ and charge $Q$ under the action of its own gravitational and $U(1)$ fields. We show that shells with surface energy $σ$ and pressure $P$ obeying $P=wσ$ with $0\leq w\leq 1$ are necessarily undercharged $m\geq |Q|$ and always collapse to form Reissner-Nordström black holes. Nevertheless, if $-1\leq w<0$, we find that overcharged $m\leq |Q|$ shells exist, which however, are inevitably stabilized at finite radial distance.
We show that vanishing of the sigma-model beta-functions for the metric $g_{μν}$, antisymmetric tensor $B_{μν}$ and dilaton $Φ$ of any string-theory model in $D$ dimensions at tree level can be obtained from a universal effective action, provided that the field combination $\sqrt{-g}e^{-2Φ}$ is non-dynamical. The central-charge deficit term is omitted from the action and appears as an integration constant at the level of the field equations. The analysis is done at leading order in $α'$, i.e. at the two-derivative level in the effective description. It can be extended to incorporate higher order string-loop corrections, Ramond-Ramond forms and moduli fields. In all cases, the non-dynamical field combination turns out to be $\sqrt{-g}\, U$, where $U$ is the effective potential. When certain fluxes are fixed and the moduli fields are stabilized, no cosmological term will arise in the action. Instead, the cosmological constant will appear as an integration constant as in unimodular modifications of general relativity.
It has been recently shown that nonlinear effects emerging at the time of the generation of the quasi-normal modes are necessary to model ringdowns from black hole mergers. In this note, we describe how nonlinerarities also arise when defining gauge-invariant tensor modes and in the calculation of the observable measured in the interferometers beyond linear order.
It has been recently pointed out that nonlinear effects are necessary to model the ringdown stage of the gravitational waveform produced by the merger of two black holes giving rise to a remnant Kerr black hole. We show that this nonlinear behaviour is explained, both on the qualitative and quantitative level, by near-horizon symmetries of the Kerr black hole within the Kerr/CFT correspondence.
We show that the linear perturbations of any spin field in the near-zone limit of the Kerr black hole are identical to those of an AdS$_2$ black hole which enjoys the same basic properties of the Kerr black hole. Thanks to this identification, we calculate the spectrum of the quasinormal modes and the Love numbers of Kerr black holes using an AdS$_2$/CFT$_1$ correspondence and a group theoretical approach.
The Standard Model Higgs potential becomes unstable at large Higgs field values where its quartic coupling becomes negative. While the tunneling lifetime of our current electroweak vacuum is comfortably longer than the age of the universe, quantum fluctuations during inflation might push the Higgs over the barrier, forming patches which might be lethal for our universe. We study the cosmological evolution of such regions and find that, at least in the thin wall approximation, they may be harmless as they collapse due to the backreaction of the Higgs itself. The presence of the Standard Model Higgs instability can provide a novel mechanism to end inflation and to reheat the universe through the evaporation of the black holes left over by the collapse of the Higgs bubbles. The bound on the Hubble rate during inflation may be therefore relaxed.
We study supersymmetry breaking from a complex linear superfield coupled to 4D N=1 supergravity. The theory has two classically decoupled vacua, one supersymmetric and one with broken and intrinsically non-linear supersymmetry. Depending on the values of the parameters the scalar potential can lead to no-scale Minkwoski or a stable de Sitter or anti-de Sitter vacuum. We also provide a dual description of the system in terms of a nilpotent chiral superfield and a standard chiral coupled to supergravity.
We determine the inner product on the Hilbert space of wavefunctions of the universe by imposing the Hermiticity of the quantum Hamiltonian in the context of the minisuperspace model. The corresponding quantum probability density reproduces successfully the classical probability distribution in the $\hbar \to 0$ limit, for closed universes filled with a perfect fluid of index $w$. When $-1/3<w\le 1$, the wavefunction is normalizable and the quantum probability density becomes vanishingly small at the big bang/big crunch singularities, at least at the semi-classical level. Quantum expectation values of physical geometrical quantities, which diverge classically at the singularities, are shown to be finite.
We consider ${\cal N}=4$ conformal supergravity with an arbitrary holomorphic function of the complex scalar $S$ which parametrizes the $SU(1,1)/U(1)$ coset. Assuming non-vanishings vevs for $S$ and the scalars in a symmetric matrix $E_{ij}$ of the $\overline{\bf 10}$ of $SU(4)$ R-symmetry group, we determine the vacuum structure of the theory. We find that the possible vacua are classified by the number of zero eigenvalues of the scalar matrix and the spacetime is either Minkowski, de Sitter or anti-de Sitter. We determine the spectrum of the scalar fluctuations and we find that it contains tachyonic states which however can be removed by appropriate choice of the unspecified at the supergravity level holomorphic function. Finally, we also establish that $S$-supersymmetry is always broken whereas $Q$-supersymmetry exists only on flat Minkowski spacetime.