We study the implications of the simplicity constraint in the spincube model of quantum gravity. By relating the edge lengths to the integer areas of triangles, the simplicity constraint imposes very strong restrictions between them, ultimately leading to a requirement that all 4-simplices in the triangulation must be almost mutually identical. As a surprising and unexpected consequence of this property, one can obtain the causal dynamical triangulations (CDT) state sum as a special case of the spincube state sum. This relationship brings new insight into the long-standing problem of the relationship between the spinfoam approach and the CDT approach to quantum gravity. In particular, it turns out that the spincube model contains properties of both approaches, providing a single unifying framework for their analysis and comparison. In addition, the spincube state sum also contains some other special cases, very similar but not equivalent to the CDT state sum.

We use the Legendre-invariant formalism of geometrothermodynamics to investigate the geometric properties of the equilibrium space of a spherically symmetric phantom black hole with electric charge and dilaton. We find that at certain points of the equilibrium space the thermodynamic curvature is characterized by the presence of singularities that are interpreted as phase transitions. We also investigate the phase transition structure by using the standard approach of black hole thermodynamics based upon the analysis of the heat capacity and response functions. We show compatibility between the two approaches. In addition, a new type of phase transition is found, which is due to the presence of phantom energy and corresponds to a transition between black hole states with different stability properties.

The glitch activity of young pulsars arises from the exchange of angular momentum between the crust and the interior of the star. Recently, it was inferred that the moment of inertia of the crust of a neutron star is not sufficient to explain the observed glitches. Such estimates are presumed in Einstein’s general relativity in describing the hydrostatic equilibrium of neutron stars. The crust of the neutron star has a spacetime curvature of 14 orders of magnitude larger than that probed in solar system tests. This makes gravity the weakest constrained physics input in the crust-related processes. We calculate the ratio of the crustal to the total moment of inertia of neutron stars in the scalar-tensor theory of gravity and the nonperturbative $f(\mathcal{R})=\mathcal{R}+a{\mathcal{R}}^2$ gravity. We find for the former that the crust-to-core ratio of the moment of inertia does not change significantly from what is inferred in general relativity. For the latter, we find that the ratio increases significantly from what is inferred in general relativity in the case of high mass objects. Our results suggest that the glitch activity of pulsars may be used to probe gravity models, although the gravity models explored in this work are not appropriate candidates.

We explore the possibility of finding pure Lovelock gravity as a particular limit of a Chern-Simons action for a specific expansion of the AdS algebra in odd dimensions. We derive in detail this relation at the level of the action in five and seven dimensions. We provide a general result for higher dimensions and discuss some issues arising from the obtained dynamics.

It is believed that curvature singularities are a creation of general relativity and, hence, in the absence of a quantum gravity, models of nonsingular black holes have received significant attention. We study the shadow (apparent shape), an optical appearance because of its strong gravitational field, cast by a nonsingular black hole which is characterized by three parameters, i.e., mass ($M$), spin ($a$), and a deviation parameter ($k$). The nonsingular black hole under consideration is a generalization of the Kerr black hole that can be recognized asymptotically ($r\gg k,k>0$) explicitly as the Kerr-Newman black hole, and in the limit $k\to 0$ as the Kerr black hole. It turns out that the shadow of a nonsingular black hole is a dark zone covered by a deformed circle. Interestingly, it is seen that the shadow of a black hole is affected due to the parameter $k$. Indeed, for a given $a$, the size of a shadow reduces as the parameter $k$ increases, and the shadow becomes more distorted as we increase the value of the parameter $k$ when compared with the analogous Kerr black hole shadow. We also investigate, in detail, how the ergoregion of a black hole is changed due to the deviation parameter $k$.

We present the first results on the saturation of the $f$-mode instability in neutron stars due to nonlinear mode coupling. Emission of gravitational waves drives the $f$-mode (fundamental mode) unstable in fast-rotating, newborn neutron stars. The initial growth phase of the mode is followed by its saturation, because of energy leaking to other modes of the star. The saturation point determines the strain of the generated gravitational-wave signal, which can then be used to extract information about the neutron star’s equation of state. The parent (unstable) mode couples via parametric resonances with pairs of daughter modes, with the triplets’ evolution exhibiting a rich variety of behaviors. We study both supernova- and merger-derived neutron stars, simply modeled as polytropes in a Newtonian context, and show that the parent may couple to many different daughter pairs during the star’s evolution through the instability window, with the saturation amplitude changing by orders of magnitude.

We demonstrate that dark matter particles gravitationally bound to the Earth can induce a characteristic nuclear recoil signal at low energies in direct detection experiments. The new spectral feature that we predict can provide a complementary verification of dark matter discovery at experiments with positive signal but unclear background. The effect is generically expected, in that the ratio of bound over halo dark matter event rates at detectors is independent of the dark matter-nucleon cross section.

We perform a Bayesian analysis to study possible features in the primordial inflationary power spectrum of scalar perturbations. In particular, we analyze the possibility of detecting the imprint of these primordial features in the anisotropy temperature power spectrum of the cosmic microwave background (CMB) and also in the matter power spectrum $P(k)$. We use the most recent CMB data provided by the Planck Collaboration and $P(k)$ measurements from the 11th data release of the Sloan Digital Sky Survey. We focus our analysis on a class of potentials whose features are localized at different intervals of angular scales, corresponding to multipoles in the ranges $10<\ell <60$ (Oscill-1) and $150<\ell <300$ (Oscill-2). Our results show that one of the step potentials (Oscill-1) provides a better fit to the CMB data than does the featureless $\mathrm{\Lambda }\mathrm{CDM}$ scenario, with moderate Bayesian evidence in favor of the former. Adding the $P(k)$ data to the analysis weakens the evidence of the Oscill-1 potential relative to the standard model and strengthens the evidence of this latter scenario with respect to the Oscill-2 model.

In the cosmological scenario in $f(T)$ gravity, we find analytical solutions for an isotropic and homogeneous universe containing a dust fluid and radiation and for an empty anisotropic Bianchi I universe. The method that we apply is that of movable singularities of differential equations. For the isotropic universe, the solutions are expressed in terms of a Laurent expansion, while for the anisotropic universe we find a family of exact Kasner-like solutions in vacuum. Finally, we discuss when a nonlinear $f(T)$-gravity theory provides solutions for the teleparallel equivalence of general relativity and derive conditions for exact solutions of general relativity to solve the field equations of an $f(T)$ theory.

Modifications of gravity generated by a multiplicative coupling of a scalar field to the electromagnetic Lagrangian lead to a breaking of the Einstein equivalence principle (EEPB), as well as to variations of fundamental constants. In these theoretical frameworks, deviations of standard values of the fine structure constant, $\mathrm{\Delta }\alpha /\alpha =\varphi$, and of the cosmic distance duality relation, $D_L(1+z{)}^{-2}/D_A=\eta =1$, where $D_L$ and $D_A$ are the luminosity and angular diameter distances, respectively, are unequivocally linked. In this paper, we search for cosmological signatures of the EEPB by using angular diameter distance from galaxy clusters, obtained via their Sunyaev-Zel’dovich effect (SZE) and x-ray observations, and distance modulus of type Ia supernovae (SNe Ia). The crucial point here is that we take into account the dependence of the SZE/x-ray technique with $\varphi$ and $\eta$. Our new results show no indication of the EEPB.

We estimate the photon production rate from an SU(3) plasma at temperatures of about $1.1T_{\mathrm{c}}$ and $1.3T_{\mathrm{c}}$. Lattice results for the vector current correlator at spatial momenta $k\sim (2-6)T$ are extrapolated to the continuum limit and analyzed with the help of a polynomial interpolation for the corresponding spectral function, which vanishes at zero frequency and matches to high-precision perturbative results at large invariant masses. For small invariant masses the interpolation is compared with the next-to-leading-order (NLO) weak-coupling result, hydrodynamics, and a holographic model. At vanishing invariant mass we extract the photon rate which for $k\gtrsim 3T$ is found to be close to the NLO weak-coupling prediction. For $k\lesssim 2T$ uncertainties remain large but the photon rate is likely to fall below the NLO prediction, in accordance with the onset of a strongly interacting behavior characteristic of the hydrodynamic regime.

Through the development of a parallel code called tmswift, an extensive light-front quantization study of the nonperturbative spectrum of the bound state $({\mu }^{+}{\mu }^{-})$, true muonium, has been performed. Using Padé approximants, it has been possible to extract continuum and infinite-cutoff limits for the singlet and triplet states for a range of values of the coupling constant $\alpha$. This data set allows for an investigation of the $\alpha$ dependence of the light-front spectra, the results of which are compared to standard calculations. Decay constants have also been obtained. Improved calculations have been undertaken for the energy shifts due to the presence of a second, lighter flavor ($e$). Finally, initial results for three-flavor ($e$, $\mu$, $\tau$) calculations are presented.

We study in detail the little-Higgs model proposed by Low, Skiba and Smith with an $SU(6)/Sp(6)$ group structure. The effective theory at the TeV scale is a two-Higgs doublet model (2HDM) with additional heavy vectorlike fermions and vector-bosons. We identify a set of independent input parameters and develop expressions for masses and couplings in terms of these. We perform a random scan of the parameter space and find points that satisfy constraints, including the recent 8 TeV LHC Higgs measurements, namely, the Higgs mass, Higgs couplings to the top, bottom, $\tau$, $W^{\pm }$ and $Z$, top-quark mass, and collider bounds on colored vector-like fermions ($t^{\prime }$ and $b^{\prime }$), and also precision electroweak constraints. The LHC constraints on the $hWW$ and $hZZ$ couplings are satisfied by being close to the “alignment limit.” We find how fine-tuned the model is after including these constraints. For the points that satisfy the constraints, we present the 1-loop effective couplings of the $CP$-even and $CP$-odd neutral scalars to two gluons including contributions of standard model and heavy vector-like quarks. We also present the branching ratios of the heavy neutral scalars into the $\gamma \gamma$, $\tau \overline{\tau }$, $b\overline{b}$, $t\overline{t}$, $WW$, $ZZ$, $Zh$, $hh$ modes, and the heavy charged scalar into $tb$, $\tau \nu$, $cs$, $Wh$ modes. These will aid searches of the heavy scalars at the LHC and other future colliders.

A new scalar particle, coupled to photons and gluons via loops of vectorlike quarks, provides a simple theoretical interpretation of the 750 GeV diphoton excess reported by the experiments at the Large Hadron Collider (LHC). In this paper, we show that this model contains a large, phenomenologically viable parameter space region in which the electroweak phase transition (EWPT) is strongly first order, opening the possibility that the electroweak baryogenesis mechanism can be realized in this context. A large coupling between the Higgs doublet and the heavy scalar, required for a strongly first-order EWPT, can arise naturally in composite Higgs models. The scenario makes robust predictions that will be tested in near-future experiments. The cross section of resonant di-Higgs production at the 13 TeV LHC is predicted to be at least 20 fb, while the Higgs cubic self-coupling is enhanced by 40% or more with respect to its Standard Model (SM) value.

We present a local setup for the recently introduced BRST-invariant formulation of Yang-Mills theories for linear covariant gauges that takes into account the existence of gauge copies à la Gribov and Zwanziger. Through the convenient use of auxiliary fields, including one of the Stueckelberg type, it is shown that both the action and the associated nilpotent BRST operator can be put in local form. Direct consequences of this fully local and BRST-symmetric framework are drawn from its Ward identities: (i) an exact prediction for the longitudinal part of the gluon propagator in linear covariant gauges that is compatible with recent lattice results and (ii) a proof of the gauge-parameter itxndependence of all correlation functions of local BRST-invariant operators.

The low-energy dynamics of relativistic continuous media is given by a shift-symmetric effective theory of four scalar fields. These scalars describe the embedding in spacetime of the medium and play the role of Stückelberg fields for spontaneously broken spatial and time translations. Perfect fluids are selected imposing a stronger symmetry group or reducing the field content to a single scalar. We explore the relation between the field theory description of perfect fluids to thermodynamics. By drawing the correspondence between the allowed operators at leading order in derivatives and the thermodynamic variables, we find that a complete thermodynamic picture requires the four Stückelberg fields. We show that thermodynamic stability plus the null-energy condition imply dynamical stability. We also argue that a consistent thermodynamic interpretation is not possible if any of the shift symmetries is explicitly broken.

We use the recent observations of cosmic microwave background (CMB) temperature and polarization anisotropies provided by the Planck satellite experiment to place constraints on the running ${\alpha }_{\mathrm{s}}=\mathrm{d}{n}_{\mathrm{s}}/\mathrm{d}\log k$ and the running of the running ${\beta }_{\mathrm{s}}=\mathrm{d}{\alpha }_{\mathrm{s}}/\mathrm{d}\log k$ of the spectral index $n_{\mathrm{s}}$ of primordial scalar fluctuations. We find ${\alpha }_{\mathrm{s}}=0.011\pm 0.010$ and ${\beta }_{\mathrm{s}}=0.027\pm 0.013$ at 68% C.L., suggesting the presence of a running of the running at the level of two standard deviations. We find no significant correlation between ${\beta }_{\mathrm{s}}$ and foregrounds parameters, with the exception of the point sources amplitude at 143 GHz, $A_{143}^{\mathrm{PS}}$, which shifts by a half-sigma when the running of the running is considered. We further study the cosmological implications of such a preference for ${\alpha }_{\mathrm{s}},{\beta }_{\mathrm{s}}\sim 0.01$ by including in the analysis the lensing amplitude $A_L$, the curvature parameter ${\mathrm{\Omega }}_k$, and the sum of neutrino masses $\sum m_{\nu }$. We find that when the running of the running is considered Planck data are more compatible with the standard expectations of $A_L=1$ and ${\mathrm{\Omega }}_k=0$ but still hint at possible deviations. The indication for ${\beta }_{\mathrm{s}}>0$ survives at two standard deviations when external data sets such as baryon acoustic oscillation surverys and CFHTLenS are included in the analysis and persists at $\sim 1.7$ standard deviations when CMB lensing is considered. We discuss the possibility of constraining ${\beta }_{\mathrm{s}}$ with current and future measurements of CMB spectral distortions, showing that an experiment like PIXIE could provide strong constraints on ${\alpha }_{\mathrm{s}}$ and ${\beta }_{\mathrm{s}}$.

We find new higher derivative models describing a parity doublet of massive spin-3 modes in $D=2+1$ dimensions. One of them is of fourth order in derivatives while the other one is of sixth order. They are complete, in the sense that they contain the auxiliary scalar field required to remove spurious degrees of freedom. Both of them are obtained through the master action technique starting with the usual (second-order) spin-3 Singh-Hagen model, which guarantees that they are ghost free. The fourth- and sixth-order terms are both invariant under (transverse) Weyl transformations, quite similarly to the fourth-order $K$-term of the “new massive gravity.” The sixth-order term slightly differs from the product of the Schouten by the Einstein tensor, both of third order in derivatives. It is also possible to write down the fourth-order term as a product of a Schouten-like by an Einstein-like tensor (both of second order in derivatives) in close analogy with the $K$-term.

The $\mathbb{C}{P}^N$ extended Skyrme-Faddeev model possesses planar soliton solutions. We consider quantum aspects of the solutions applying collective coordinate quantization in regime of rigid body approximation. In order to discuss statistical properties of the solutions we include an Abelian Chern-Simons term (the Hopf term) in the Lagrangian. Since ${\mathrm{\Pi }}_3(\mathbb{C}{P}^1)=\mathbb{Z}$ then for $N=1$ the term becomes an integer. On the other hand for $N>1$ it became perturbative because ${\mathrm{\Pi }}_3(\mathbb{C}{P}^N)$ is trivial. The prefactor of the Hopf term (anyon angle) $\mathrm{\Theta }$ is not quantized and its value depends on the physical system. The corresponding fermionic models can fix value of the angle $\mathrm{\Theta }$ for all $N$ in a way that the soliton with $N=1$ is not an anyon type whereas for $N>1$ it is always an anyon even for $\mathrm{\Theta }=n\pi$, $n\in \mathbb{Z}$. We quantize the solutions and calculate several mass spectra for $N=2$. Finally we discuss generalization for $N\geqq 3$.