We find a model-independent upper bound on the strong coupling scale for a massive spin-2 particle coupled to Einstein gravity. Our approach is to directly construct tree-level scattering amplitudes for these degrees of freedom and use them to find the maximum scale of perturbative unitarity violation. The highest scale is ${\mathrm{\Lambda }}_3={(m^2{M}_P)}^{1/3}$, which is saturated by ghost-free bigravity. The strong coupling scale can be further raised to $M_P$ if the kinetic term for one particle has the wrong sign, which uniquely gives the amplitudes of quadratic curvature gravity. We also discuss the generalization to massive higher-spin particles coupled to gravity.

The presence of isotropic Lifshitz points for a $U(1)$ symmetric scalar theory is investigated with the help of the functional renormalization group at the conjectured lower critical dimension $d=4$. To this aim, a suitable truncation in the expansion of the effective action in powers of the field is considered and, consequently, the renormalization group flow is reduced to a set of ordinary differential equations for the parameters that define the effective action. Within this approximation, indications of a line of Lifshitz points are found that present evident similarities with the properties shown by the line of fixed points observed in the two-dimensional Berezinskii-Kosterlitz-Thouless phase. In particular, this line of Lifshitz points exhibits the vanishing of the expectation value of the field, together with a finite stiffness and, for specific combinations of the parameters that define the effective action, also the algebraic decay at a large distance of the order parameter correlations.

We present a regularization approach to the study of the quantum dynamics of the mixmaster universe that allows us to approximate the anisotropy potential with the explicitly integrable periodic three-particle Toda potential. This approach is based on a Gaussian weighted Weyl integral quantization. Such a procedure naturally amplifies the dynamical role of the underlying Toda system by smoothing out the three canyons of the anisotropy potential. Since the respective eigenfunctions can be explicitly constructed, our finding paves the way to a perturbative approach to the quantum mixmaster dynamics.

A supergravity extension of the $(R+R^2)$ gravity with the additional (Born-Infeld) structure of a massive vector multiplet gives rise to the specific $F(R)$ gravity, whose structure is investigated in detail. The massive vector multiplet has an inflaton (scalaron), goldstino, and massive vector field as its field components. The model describes Starobinsky inflation and allows us to extrapolate the $F(R)$ function beyond the inflationary scale (up to Planck scale). We observe some differences versus the $(R+R^2)$ gravity and several breaking patterns of the well-known correspondence between the $F(R)$ gravity and the scalar-tensor gravity.

We obtain the first cosmological constraints on interactions between dark matter and protons within the formalism of nonrelativistic effective field theory developed for direct detection. For each interaction operator in the effective theory, parametrized by different powers of the relative velocity of the incoming particles, we use the Planck 2015 cosmic microwave background (CMB) temperature, polarization, and lensing anisotropy to set upper limits on the scattering cross section for all dark matter masses above 15 keV. We find that for interactions associated with a stronger dependence on velocity dark matter and baryons stay thermally coupled for longer, but the interaction strengths are suppressed at the low temperatures relevant for Planck observations and are thus less constrained. At the same time, cross sections with stronger velocity dependencies are more constrained in the limit of small dark matter mass. In all cases, the effect of dark matter-proton scattering is most prominent on small scales in the CMB power spectra and in the matter power spectrum, and we thus expect substantial improvement over the current limits with data from ground-based CMB experiments and galaxy surveys.

Intermediate-mass black holes are the missing link that connects stellar-mass to supermassive black holes and are key to understanding galaxy evolution. Gravitational waves, like photons, can be lensed, leading to discernable effects such as diffraction or repeated signals. We investigate the detectability of intermediate-mass black hole deflectors in the LIGO-Virgo detector network. In particular, we simulate gravitational waves with variable source distributions lensed by an astrophysical population of intermediate-mass black holes, and use the standard LIGO tools to infer the properties of these lenses. We find detections of intermediate-mass black holes at 98% confidence level over a wide range of binary and lens parameters. Therefore, we conclude that intermediate-mass black holes can be detected through lensing of gravitational waves in the LIGO-Virgo detector network.

We study the relativistic accretion flow in a generic stationary axisymmetric space-time and obtain an effective potential (${\mathrm{\Phi }}^{\mathrm{eff}}$) that accurately mimics the general relativistic features of Kerr black hole having spin $0\le a_{\mathrm{k}}<1$. Considering the accretion disc to be confined around the equatorial plane of a rotating black hole and using the relativistic equation of state, we examine the properties of the relativistic accretion flow and compare them with the same obtained form semirelativistic as well as nonrelativistic accretion flows. Towards this, we first investigate the transonic properties of the accretion flow around the rotating black hole where good agreement is observed for relativistic and semirelativistic flows. Further, we study the nonlinearities such as shock waves in accretion flow. Here also we find that the shock properties are in agreement for both relativistic and semirelativistic flows irrespective of the black hole spin ($a_{\mathrm{k}}$), although it deviates significantly for nonrelativistic flow. In fact, when the particular shocked solutions are compared for flows with identical outer boundary conditions, the positions of shock transition in relativistic and semirelativistic flows agree well with the deviation of 6%–12% for $0\le a_{\mathrm{k}}\le 0.99$, but vast disagreement is observed for nonrelativistic flow. In addition, we compare the parameter space [in energy ($\mathcal{E}$) and angular momentum ($\lambda$) plane] for shock to establish the fact that relativistic as well as semirelativistic accretion flow dynamics do show close agreement irrespective of $a_{\mathrm{k}}$ values, whereas nonrelativistic flow fails to do so. With these findings, we point out that semirelativistic flow including ${\mathrm{\Phi }}^{\mathrm{eff}}$ satisfactorily mimics the relativistic accretion flows around the Kerr black hole. Finally, we discuss the possible implications of this work in the context of dissipative advective accretion flow around Kerr black holes.

We formulate the on-shell effective field theory (OSEFT) in an arbitrary frame and study its reparametrization invariance, which ensures that it respects Lorentz symmetry. In this formulation, the OSEFT Lagrangian looks formally equivalent to the sum over lightlike velocities of soft collinear effective field theory in the Abelian limit, but differences remain in the scale of the gauge fields involved in the two effective theories. We then use the OSEFT Lagrangian expanded in inverse powers of the on-shell energy to derive how the classical transport equations for charged massless fermions are corrected by quantum effects, as derived from quantum field theory. We provide a formulation in a full covariant way and explain how the consistent form of the chiral anomaly equation can be recovered from our results. We also show how the side-jump transformation of the distribution function associated with massless charged fermions can be derived from the reparametrization invariance transformation rules of the OSEFT quantum fields. Finally, we discuss the differences in our results with respect to others found in the literature.

We study the finite-temperature properties of the Randall-Sundrum model in the presence of brane-localized curvature. At high temperature, as dictated by $\mathrm{AdS}/\mathrm{CFT}$, the theory is in a confined phase dual to the planar anti-de Sitter black hole. When the radion is stabilized, á la Goldberger-Wise, a holographic first-order phase transition proceeds. The brane-localized curvature contributes to the radion kinetic energy, which allows us to substantially decrease the critical bubble energy. Contrary to previous results, the phase transition is completed at much larger values of N, the number of degrees of freedom in the conformal field theory. Moreover, the value of the bulk scalar on the TeV brane is allowed to become large, consistent with backreaction constraints. Assisted by this fact, we find that for a wide region in the parameter space tunneling happens rather quickly, i.e., $T_n/T_c\sim \mathcal{O}(0.1–1)$. At zero temperature, the most important signature of brane-localized curvature is the reduction of spin-2 Kaluza-Klein graviton masses and a heavier radion.

We investigate the tidal deformability of a superfluid neutron star. We calculate the equilibrium structure in the general relativistic two-fluid formalism with entrainment effect where we take neutron superfluid as one fluid and the other fluid is comprised of protons and electrons, making it a charge neutral fluid. We use a relativistic mean field model for the equation of state of matter where the interaction between baryons is mediated by the exchange $\sigma$, $\omega$, and $\rho$ mesons. Then, we study the linear, static $l=2$ perturbation on the star to compute the electric-type Love number following Hinderer’s prescription.

In this paper we construct a linearized metric solution for an electrically charged system in a ghost-free infinite derivative theory of gravity which is valid in the entire region of spacetime. We show that the gravitational potential for a point-charge with mass $m$ is nonsingular, the Kretschmann scalar is finite, and the metric approaches conformal flatness in the ultraviolet regime where the nonlocal gravitational interaction becomes important. We show that the metric potentials are bounded below one as long as two conditions involving the mass and the electric charge are satisfied. Furthermore, we argue that the cosmic censorship conjecture is not required in this case. Unlike in the case of the Reissner-Nordström metric in general relativity, where $|Q|\le m/M_p$ must always be satisfied, in ghost-free infinite derivative gravity $|Q|>m/M_p$ is also allowed, such as for an electron.

We study the gravitational radiation emitted by a massive point particle plunging from slightly below the innermost stable circular orbit into a Schwarzschild black hole. We consider both even- and odd-parity perturbations and describe them using the two gauge-invariant master functions of Cunningham, Price, and Moncrief. We obtain, for arbitrary directions of observation and, in particular, outside the orbital plane of the plunging particle, the regularized multipolar waveforms, i.e., the waveforms constructed by summing over of a large number of modes, and their unregularized counterparts constructed from the quasinormal-mode spectrum. They are in excellent agreement and our results permit us to especially emphasize the impact on the distortion of the waveforms of (i) the harmonics beyond the dominant ($\ell =2$, $m=\pm 2$) modes and (ii) the direction of observation, and therefore the necessity to take them into account in the analysis of the last phase of binary black hole coalescence.

We present an original approach to compute the electromagnetic self-force acting on a static charge in Kerr spacetime. Our approach is based on an improved version of the Janis-Newman algorithm and extends its range of applicability. It leads to a closed expression which generalizes the existing one, and since it does not involve the electromagnetic potential, it simplifies the calculation of the self-force.

Motivated by the string theory corrections in the low-energy limit of both gauge and gravity sides, we consider three-dimensional black holes in the presence of dilatonic gravity and the Born-Infeld nonlinear electromagnetic field. We find that geometric behavior of the solutions is similar to the behavior of the hyperscaling violation metric, asymptotically. We also investigate thermodynamics of the solutions and show that the generalization to dilatonic gravity introduces novel properties into thermodynamics of the black holes which were absent in the Einstein gravity. Furthermore, we explore the possibility of tuning out part of the dilatonic effects using the Born-Infeld generalization.

We describe the effects of the cosmological background on the K-mouflage screening properties of an astrophysical structure. We show that the K-mouflage screening of the spatial gradients of the scalar field, i.e., the screening of the fifth force, happens inside a dynamically generated screening radius. This radius is smaller than the location where the quasistatic approximation, i.e., where the spatial gradients exceed the time derivative, holds. Even though this quasistatic radius is much smaller than the size of the matter overdensity, spatial gradients remain well described by the quasistatic approximation up to the horizon. However, cosmologically we find that the time derivatives can remain dominant at redshifts $z\gtrsim 2$, when the cosmic web shows a faster growth. Despite the existence of K-mouflage screening, we confirm that the values of the scalar field itself are still dominated by the cosmological background, down to the center of the matter overdensity, and that for instance the time drift of Newton’s constant due to the large-scale cosmological evolution highly constrains K-mouflage models.

We perform the detailed analysis of the primordial black hole (PBH) formation mechanism in an axionlike curvaton model with a coupling to inflaton. The phase direction of the complex scalar works as a curvaton and produces enough PBHs to explain the black hole binaries ($\sim 30M_{\odot }$) observed in the LIGO-Virgo Collaboration or PBHs as dark matter ($\sim {10}^{-12}{M}_{\odot }$). We examine whether our model satisfies the current constraints on the PBH mass spectrum, the curvature perturbation, and the secondarily produced gravitational waves. We also take into account ambiguity about the choice of the window functions and effect of the non-Gaussianity.

Although the cosmological perturbations with inverse-volume corrections from loop quantum cosmology have been studied using the anomaly-free algebra approach in much of the literature, there still remains an important issue that some counterterms in the perturbed constraints cannot be uniquely fixed on the spatially flat Friedmann-Robterson-Walker background, which causes ambiguities in the perturbation equations. In this paper, we show that this problem can be overcome by extending the anomaly-free algebra to a spatially closed Friedmann-Robterson-Walker background. We find that a consistent deformed algebra can be obtained in the spatially closed case, and each counterterm can be uniquely fixed in terms of the inverse-volume correction functions; then, by taking the large $r_o$ limit, we recover the anomaly-free Hamiltonian on the spatially flat background. Using this Hamiltonian, we obtain the gauge invariant cosmological perturbations for scalar, vector and tensor modes in the spatially flat case. Moreover, we also derive the quantum-corrected Mukhanov equations, from which the scalar and tensor spectral indices with inverse-volume corrections are given. Some key cosmological perturbation equations obtained in this paper are different from those in previous literature.

A shift-symmetric Galileon model in the presence of spacetime torsion has been constructed for the first time. This has been realized by localizing (or, gauging) the Galileon symmetry in flat spacetime in an appropriate manner. We have applied the above model to study the evolution of the universe at a cosmological scale. Interestingly, for a wide class of torsional structures we have shown that the model leads to late time cosmic acceleration. Furthermore, as torsion vanishes, our model reproduces the standard results.

The results of high energy simulated experiments where a given hadronic particle impacts on a given target are statistically analyzed. The energy range of the projectiles goes from below the LHC scale up to the highest cosmic ray energies. This study was carried out by using the pre- and post-LHC versions of the hadronic interaction models QGSJET, EPOS and SIBYLL. Our analysis indicates that the post-LHC models present smaller differences in various quantities that characterize the secondary particles produced after the hadronic collisions, in comparison with the corresponding differences that are found comparing the respective old (pre-LHC) versions of the hadronic models. However, it is also found that there exist some discrepancies among models that persist even at the LHC energy scale, that call for further theoretical investigation. An additional analysis of the impact that different modeling of hadronic collisions has on air shower development is also included. It consists of a detailed study of the impact of the different pre- and post-LHC versions of the hadronic models considered, for relevant observables like the muon production depth distribution.