We study superradiant scalar field configurations around Kerr black holes that possess a time-independent energy-momentum tensor. Motivated by the electromagnetic Blandford-Znajek (BZ) mechanism for black hole energy extraction, we explore whether scalar solutions could serve as a tractable proxy for the force-free magnetosphere in the BZ process. While stationary “scalar cloud” solutions, confined near the black hole by their own mass or a mirror at fixed radius, only exist at the threshold for energy extraction, we find that a stationary solution in the superradiant regime can be constructed when the reflecting mirror is replaced by a semipermeable surface. Tuning the boundary conditions on this surface allows some energy to be radiated to infinity while maintaining self-sustained superradiance. The time-independent radial energy flux displays the same behavior for rapidly rotating holes as magnetohydrodynamic simulations predict for the BZ mechanism.

The pseudo-Newtonian potential of Paczynski and Wiita for particles orbiting a Schwarzschild black hole is generalized to arbitrary static and spherically symmetric spacetimes, including black hole solutions of alternative theories of gravity. In addition to being more general, our prescription differs substantially from a previous one in the literature, showing that the association of a pseudo-Newtonian potential even with a simple black hole metric is not unique.

We consider generic linear perturbations of a nonbidiagonal class of static black-hole solutions in massive (bi-)gravity. We show that the quasinormal spectrum of these solutions coincides with that of a Schwarzschild black hole in general relativity, thus proving that these solutions are mode stable. This is in contrast to the case of bidiagonal black-hole solutions which are affected by a radial instability. On the other hand, the full set of perturbation equations is generically richer than that of a Schwarzschild black hole in general relativity, and this affects the linear response of the black hole to external perturbations. Finally, we argue that the generalization of these solutions to the spinning case does not suffer from the superradiant instability, despite the fact that the theory describes a massive graviton.

We develop a geometric criterion that unambiguously characterizes the residual symmetries of a gravitational Ansatz. It also provides a systematic and effective computational procedure for finding all the residual symmetries of any gravitational Ansatz. We apply the criterion to several examples starting with the Collinson Ansatz for circular stationary axisymmetric spacetimes. We reproduce the residual symmetries already known for this Ansatz including their conformal symmetry, for which we identify the corresponding infinite generators spanning the two related copies of the Witt algebra. We also consider the noncircular generalization of this Ansatz and show how the noncircular contributions on the one hand break the conformal invariance and on the other hand enhance the standard translation symmetries of the circular Killing vectors to supertranslations depending on the direction along which the circularity is lost. As another application of the method, the well-known relation defining conjugate gravitational potentials introduced by Chandrasekhar, which makes possible the derivation of the Kerr black hole from a trivial solution of the Ernst equations, is deduced as a special point of the general residual symmetry of the Papapetrou Ansatz. In this derivation we emphasize how the election of Weyl coordinates, which determines the Papapetrou Ansatz, breaks also the conformal freedom of the stationary axisymmetric spacetimes. Additionally, we study AdS waves for any dimension generalizing the residual symmetries already known for lower dimensions and exhibiting a very complex infinite-dimensional Lie algebra containing three families: two of them span the semidirect sum of the Witt algebra and scalar supertranslations and the third generates vector supertranslations. Independently of this complexity we manage to comprehend the true meaning of the infinite connected group as the precise diffeomorphisms subgroup allowing to locally deform the AdS background into AdS waves.

The unphysical spin-2 massive degrees of freedom in higher-derivative gravity may be either massive unphysical ghosts or tachyonic ghosts. In the last case there is no Planck-scale threshold protecting vacuum cosmological solutions from instabilities. Within the anomaly-induced action formalism the photon-driven IR running of the coefficient of the Weyl-squared term makes the ghost eventually become tachyonic, which should produce a gravitational explosion of vacuum. This effect is stable under higher-loop corrections and takes place also in known versions of perturbative quantum gravity. However, the contribution of massless fields in the far IR are not the same in flat and de Sitter spaces. In the asymptotically de Sitter case one can observe a kind of IR decoupling, which protects the cosmological solution from the future tachyonic instabilities.

We present analytic force-free solutions modeling rotating stars and black holes immersed in the magnetic field of a thin disk that terminates at an inner radius. The solutions are exact in flat spacetime and approximate in Kerr spacetime. The compact object produces a conical jet whose properties carry information about its nature. For example, the jet from a star is surrounded by a current sheet, while that of a black hole is smooth. We compute an effective resistance in each case and compare to the canonical values used in circuit models of energy extraction. These solutions illustrate all of the basic features of the Blandford-Znajek process for energy extraction and jet formation in a clean setting.

The known static electrovacuum black holes in a globally ${\mathrm{AdS}}_4$ background have an event horizon which is geometrically a round sphere. In this work we argue that the situation is different in models with matter fields possessing an explicit dependence on the azimuthal angle $\phi$, which, however, does not manifest at the level of the energy-momentum tensor. As a result, the full solutions are axially symmetric only, possessing a single (timelike) Killing vector field. Explicit examples of such static black holes are constructed in Einstein–(complex) scalar field and Einstein–Yang-Mills theories. The basic properties of these solutions are discussed, looking for generic features. For example, we notice that the horizon has an oblate spheroidal shape for solutions with a scalar field and a prolate one for black holes with Yang-Mills fields. The deviation from sphericity of the horizon geometry manifests itself in the holographic stress tensor. Finally, based on the results obtained in the probe limit, we conjecture the existence in Einstein-Maxwell theory of static black holes with axial symmetry only.

We explore the potential use of the Radio Continuum (RC) survey conducted by the Square Kilometre Array (SKA) to remove (delens) the lensing-induced B-mode polarization and thus enhance future cosmic microwave background (CMB) searches for inflationary gravitational waves. Measurements of large-scale B-modes of the CMB are considered to be the best method for probing gravitational waves from the cosmic inflation. Future CMB experiments will, however, suffer from contamination by nonprimordial B modes, one source of which is the lensing B modes. Delensing, therefore, will be required for further improvement of the detection sensitivity for gravitational waves. Analyzing the use of the two-dimensional map of galaxy distribution provided by the SKA RC survey as a lensing mass tracer, we find that joint delensing using near-future CMB experiments and the SKA phase 1 will improve the constraints on the tensor-to-scalar ratio by more than a factor of $\sim 2$ compared to those without the delensing analysis. Compared to the use of CMB data alone, the inclusion of the SKA phase 1 data will increase the significance of the constraints on the tensor-to-scalar ratio by a factor 1.2–1.6. For LiteBIRD combined with a ground-based experiment such as Simons Array and Advanced ACT, the constraint on the tensor-to-scalar ratio when adding SKA phase 2 data is improved by a factor of 2.3–2.7, whereas delensing with CMB data alone improves the constraints by only a factor 1.3–1.7. We conclude that the use of SKA data is a promising method for delensing upcoming CMB experiments such as LiteBIRD.

The time renormalization group (TRG) is an effective method for accurate calculations of the matter power spectrum at the scale of the first baryonic acoustic oscillations. By using a particular variable transformation in the TRG formalism, we can reduce the 2D integral in the source term of the equations of motion for the power spectrum into a series of 1D integrals. The shape of the integrand allows us to precompute only 13 antiderivatives numerically, which can then be reused when evaluating the outer integral. While this introduces a few challenges to keep numerical noise under control, we find that the computation time for nonlinear corrections to the matter power spectrum decreases by a factor of 50. This opens up the possibility to use TRG for mass production as in Markov chain Monte Carlo methods. A fortran code demonstrating this new algorithm is publicly available.

It is shown that the basic observed properties of the gamma-ray bursts (GRBs) are accounted for if one assumes that the GRBs arise by blueshifting the emission radiation of hydrogen and helium generated during the last scattering epoch. The blueshift generator for a single GRB is a region with a nonconstant bang-time function $t_B(r)$ (described by a Lemaître–Tolman (L–T) exact solution of Einstein’s equations) matched into a homogeneous and isotropic (Friedmann) background. Blueshift visible to the present observer arises only on those rays that are emitted radially in an L–T region. The paper presents three L–T models with different Big Bang profiles, adapted for the highest and the lowest end of the GRB frequency range. The models account for (1) the observed frequency range of the GRBs; (2) their limited duration; (3) the afterglows; (4) their hypothetical collimation into narrow jets; (5) the large distances to their sources; (6) the multitude of the observed GRBs. Properties (2), (3) and (6) are accounted for only qualitatively. With a small correction of the parameters of the model, the implied perturbations of the CMB radiation will be consistent with those actually caused by the GRBs. A complete model of the Universe would consist of many L–T regions with different $t_B(r)$ profiles, matched into the same Friedmann background. This paper is meant to be an initial exploration of the possibilities offered by models of this kind; the actual fitting of all parameters to observational results requires fine-tuning of several interconnected variables and is left for a separate study.

We describe a big brake singularity in terms of a modified Chaplygin gas equation of state $p=({\gamma }_m-1)\rho +\alpha {\gamma }_m{\rho }^{-n}$, accommodate this late-time event as an exotic quintessence model obtained from an energy-momentum tensor, and focus on the cosmological behavior of the exotic field, its kinetic energy, and the potential energy. At the background level the exotic field does not blow up, whereas its kinetic energy and potential both grow without limit near the future singularity. We evaluate the classical stability of this background solution by examining the scalar perturbations of the metric along with the inclusion of entropy perturbation in the perturbed pressure. Within the Newtonian gauge, the gravitational field approaches a constant near the singularity plus additional regular terms. When the perturbed exotic field is associated with $\alpha >0$ the perturbed pressure and contrast density both diverge, whereas the perturbed exotic field and the divergence of the exotic field’s velocity go to zero exponentially. When the perturbed exotic field is associated with $\alpha <0$ the contrast density always blows up, but the perturbed pressure can remain bounded. In addition, the perturbed exotic field and the divergence of the exotic field’s velocity vanish near the big brake singularity. We also briefly look at the behavior of the intrinsic entropy perturbation near the singular event.

We investigate the potential of using cosmic voids as a probe to constrain cosmological parameters through the gravitational lensing effect of the cosmic microwave background (CMB) and make predictions for the next generation surveys. By assuming the detection of a series of $\approx 5–10$ voids along a line of sight within a square-degree patch of the sky, we found that they can be used to break the degeneracy direction of some of the cosmological parameter constraints (for example ${\omega }_b$ and ${\mathrm{\Omega }}_{\mathrm{\Lambda }}$) in comparison with the constraints from random CMB skies with the same size area for a survey with extensive integration time. This analysis is based on our current knowledge of the average void profile and analytical estimates of the void number function. We also provide combined cosmological parameter constraints between a sky patch where series of voids are detected and a patch without voids (a randomly selected patch). The full potential of this technique relies on an accurate determination of the void profile to $\approx 10\%$ level. For a small-area CMB observation with extensive integration time and a high signal-to-noise ratio, CMB lensing with such series of voids will provide a complementary route to cosmological parameter constraints to the CMB observations. Example of parameter constraints with a series of five voids on a $1.0°\times 1.0°$ patch of the sky are $100{\omega }_b=2.20\pm 0.27$, ${\omega }_c=0.120\pm 0.022$, ${\mathrm{\Omega }}_{\mathrm{\Lambda }}=0.682\pm 0.078$, ${\mathrm{\Delta }}_{\mathcal{R}}^2=(2.22\pm 7.79)\times {10}^{-9}$, $n_s=0.962\pm 0.097$ and $\tau =0.925\pm 1.747$ at 68% C.L.

We use a complete and rigorous statistical indicator to measure the level of concordance between cosmological data sets, without relying on the inspection of the marginal posterior distribution of some selected parameters. We apply this test to state of the art cosmological data sets, to assess their agreement within the $\mathrm{\Lambda }$ cold dark matter model. We find that there is a good level of concordance between all the experiments with one noticeable exception. There is substantial evidence of tension between the cosmic microwave background temperature and polarization measurements of the Planck satellite and the data from the CFHTLenS weak lensing survey even when applying ultraconservative cuts. These results robustly point toward the possibility of having unaccounted systematic effects in the data, an incomplete modeling of the cosmological predictions or hints toward new physical phenomena.

In this paper we consider the single patch pseudospectral scheme for tensorial and spinorial evolution problems on the 2-sphere presented by Beyer et al. [Classical Quantum Gravity 32, 175013 (2015); Classical Quantum Gravity31, 075019 (2014)], which is based on the spin-weighted spherical harmonics transform. We apply and extend this method to Einstein’s equations and certain classes of spherical cosmological spacetimes. More specifically, we use the hyperbolic reductions of Einstein’s equations obtained in the generalized wave map gauge formalism combined with Geroch’s symmetry reduction, and focus on cosmological spacetimes with spatial ${\mathbb{S}}^3$-topologies and symmetry groups U(1) or $\mathrm{U}(1)\times \mathrm{U}(1)$. We discuss analytical and numerical issues related to our implementation. We test our code by reproducing the exact inhomogeneous cosmological solutions of the vacuum Einstein field equations obtained by Beyer and Hennig [Classical Quantum Gravity 31, 095010 (2014)].

In the standard inflationary scenario, primordial perturbations are adiabatic. The amplitudes of most types of isocurvature perturbations are generally constrained by current data to be small. If, however, there is a baryon-density perturbation that is compensated by a dark-matter perturbation in such a way that the total matter density is unperturbed, then this compensated isocurvature perturbation (CIP) has no observable consequence in the cosmic microwave background (CMB) at linear order in the CIP amplitude. Here, we search for the effects of CIPs on CMB power spectra to quadratic order in the CIP amplitude. An analysis of the Planck temperature data leads to an upper bound ${\mathrm{\Delta }}_{\mathrm{rms}}^2\le 7.1\times {10}^{-3}$, at the 68% confidence level, to the variance ${\mathrm{\Delta }}_{\mathrm{rms}}^2$ of the CIP amplitude. This is then strengthened to ${\mathrm{\Delta }}_{\mathrm{rms}}^2\le 5.0\times {10}^{-3}$ if Planck small-angle polarization data are included. A cosmic-variance-limited CMB experiment could improve the $1\sigma$ sensitivity to CIPs to ${\mathrm{\Delta }}_{\mathrm{rms}}^2\lesssim 9\times {10}^{-4}$. It is also found that adding CIPs to the standard $\mathrm{\Lambda }\mathrm{CDM}$ model can improve the fit of the observed smoothing of CMB acoustic peaks just as much as adding a nonstandard lensing amplitude.

The inert doublet model relies on a discrete symmetry to prevent couplings of the new scalars to Standard Model fermions. This stabilizes the lightest inert state, which can then contribute to the observed dark matter density. In the presence of additional approximate symmetries, the resulting spectrum of exotic scalars can be compressed. Here, we study the phenomenological and cosmological implications of this scenario. We derive new limits on the compressed inert doublet model from LEP, and outline the prospects for exclusion and discovery of this model at dark matter experiments, the LHC, and future colliders.

Accommodating both a 126 GeV mass and standard model (SM)-like couplings for the Higgs has a fine-tuning price in supersymmetric models. Examples are the minimal supersymmetric standard model, in which SM-like couplings are natural, but raising the Higgs mass to 126 GeV requires a considerable tuning, and the nonminimal supersymmetric standard model, in which the situation is reversed: the Higgs is naturally heavier, but being SM-like requires some tuning. We show that models with nondecoupling $D$ terms alleviate this tension—a 126 GeV SM-like Higgs comes out basically with no fine-tuning cost. In addition, the analysis of the fine-tuning of the extended gauge sector shows that naturalness requires the heavy gauge bosons to likely be within the reach of LHC run II.

We investigate the sensitivity of the next generation of flavor-based low-energy experiments to probe the supersymmetric parameter space in the context of the phenomenological minimal supersymmetric Standard Model and examine the complementarity with direct searches for supersymmetry at the 13 TeV LHC in a quantitative manner. To this end, we enlarge the previously studied phenomenological minimal supersymmetric Standard Model parameter space to include all physical nonzero $CP$-violating phases, namely those associated with the gaugino mass parameters; Higgsino mass parameter and the trilinear couplings of the top quark, bottom quark, and tau lepton. We find that future electric dipole moment and flavor measurements can have a strong impact on the viability of these models even if the sparticle spectrum is out of reach of the 13 TeV LHC. In particular, the lack of positive signals in future low-energy probes would exclude values of the phases between $\mathcal{O}(10^{-2})$ and $\mathcal{O}(10^{-1})$. We also find regions of parameter space where large phases remain allowed due to cancellations. Most interestingly, in some rare processes, such as $\mathrm{BR}(B_s\to {\mu }^{+}{\mu }^{-})$, we find that contributions arising from $CP$-violating phases can bring the potentially large supersymmetry contributions into better agreement with experiment and Standard Model predictions.

While it is often stated that the notion of electroweak (EW) naturalness in supersymmetric models is subjective, fuzzy and model dependent, here we argue the contrary: electroweak naturalness can be elevated to a principle which is both objective and predictive. We demonstrate visually when too much fine-tuning sets in at the electroweak scale which corresponds numerically to the measure ${\mathrm{\Delta }}_{\mathrm{BG}}\sim {\mathrm{\Delta }}_{\mathrm{EW}}\gtrsim 30$. While many constrained supersymmetry models are already excluded by this value, we derive updated upper bounds on sparticle masses within the two-extra parameter nonuniversal Higgs model (NUHM2). We confirm the classic Barbieri-Giudice (BG) result that ${\mathrm{\Delta }}_{\mathrm{BG}}<30$ implies $\mu <350\mathrm{GeV}$. However, by combining dependent soft terms which appear as multiples of $m_{3/2}$ in supergravity models, then we obtain $m_{\tilde{g}}\lesssim 4\mathrm{TeV}$ as opposed to the BG result that $m_{\tilde{g}}\lesssim 350\mathrm{GeV}$. We compare the NUHM2 results to a similar scan in the phenomenological minimal supersymmetric standard model (pMSSM) with 19 weak scale parameters. In the pMSSM with complete one-loop scalar potential plus dominant two-loop terms, then a $m_{\tilde{g}}<7\mathrm{TeV}$ bound is found. Our tabulation of upper bounds provides a target for experimenters seeking to discover or else falsify the existence of weak scale supersymmetry. In an Appendix, we show contributions to the naturalness measure from one-loop contributions to the weak scale scalar potential.

Using extended mean field theory (EMFT) on the lattice, we study properties of the Higgs-Yukawa model as an approximation of the standard model Higgs sector, and the effect of higher dimension operators. We remark, as has been noted before, that the discussion of vacuum stability is completely modified in the presence of a ${\varphi }^6$ term, and that the Higgs mass no longer appears fine tuned. We also study the finite temperature transition. Without higher dimension operators the transition is found to be second order (crossover with gauge fields) for the experimental value of the Higgs mass $M_h=125\mathrm{GeV}$. By taking a ${\varphi }^6$ interaction in the Higgs potential as a proxy for a UV completion of the standard model, the transition becomes stronger and turns first order if the scale of new physics, i.e., the mass of the lightest mediator particle, is around 1.5 TeV. This implies that electroweak baryogenesis may be viable in models which introduce new particles around that scale.