Absence Of Ghosts Research Articles

Misalignment production of vector boson dark matter from axion-SU(2) inflation

We present a new mechanism to generate a coherently oscillating dark vector field from axion-SU(2) gauge field dynamics during inflation. The SU(2) gauge field acquires a nonzero background sourced by an axion during inflation, and it acquires a mass through spontaneous symmetry breaking after inflation. We find that the coherent oscillation of the dark vector field can account for dark matter in the mass range of 10-13 – 1 eV in a minimal setup. In a more involved scenario, the range can be wider down to the fuzzy dark matter region. One of the dark vector fields can be identified as the dark photon, in which case this mechanism evades the notorious constraints for isocurvature perturbation, statistical anisotropy, and the absence of ghosts that exist in the usual misalignment production scenarios. Phenomenological implications are discussed.

On the absence of ghosts in quadratic bigravity

An extension of the bimetric theory of gravity is considered that includes quadratic Ricci curvature terms associated with each metric. The issue of the Boulware-Deser ghost is analyzed. The Hamiltonian constraint is derived and the existence of a secondary constraint is shown, proving that the theory is ghost-free.

Reconciling absence of vDVZ discontinuity with absence of ghosts in nonlocal linearized gravity

The modern massive gravity theories resolve a historical tension between the absence of the so called vDVZ mass discontinuity and the absence of ghosts via a fine tuned gravitational potential and a sophisticated screening mechanism. Those theories have originated the modern covariant bimetric models which are local, ghost free and cosmologically viable apparently, they contain a massive plus a massless graviton in the spectrum. It seems hard to solve the mentioned tension if we do insist in a model with a minimal number of degrees of freedom, with only one massive spin-2 particle in the spectrum, even if we allow nonlocal theories. Here we show that this problem can be circumvented in linearized nonlocal theories by the introduction of exponential terms with infinite derivatives. The model admits non linear completions via nonlocal quadratic terms in curvatures. We also investigate the role of the exponential factors in linearized models where the graviton remains massless and a mass scale is introduced via nonlocal terms, they are also ghost free and approach the Einstein–Hilbert theory as we go much above the introduced mass scale.

Inflation in a scalar–vector–tensor theory

In this work, we study inflation in a particular scalar-vector-tensor theory of gravitation without the $U(1)$ gauge symmetry. The model is constructed from the more general action introduced in Heisenberg et al. (Phys Rev D 98:024038, 2018) using certain specific choices for the Lagrangians and the coupling functions. Also, for this model we build the explicit form for the action, and from it, we derive the general equations: the energy-momentum tensor and the equations of motion, and using the flat FLRW background, we have analyzed if it's possible to obtain an inflationary regime with it. Additionally, using particular choices for the potential, the coupling functions, suitable dimensionless coupling constants and initial conditions, it was possible verify numerically that this model of inflation is viable. In this sense, we could verify that the introduction of the coupling function $f(\phi)$ in our model of inflation, allows us to reach a suitable amount of $e$-foldings $N$ for sufficient inflation. This is a remarkable result, since without the coupling function contribution, the amount of $e$-foldings is smaller at the end of inflation, as has been demonstrated in Heisenberg et al. (2018). Also, the no-ghosts and stability conditions that the model during inflation must satisfy, i.e., absence of ghosts and Laplacian instabilities of linear cosmological perturbations were obtained, furthermore these conditions were verified numerically too.

Directional massive gravity

We propose a new model of Lorentz breaking massive gravity in which the mass of the graviton depends from the space-time directions. We explore its consistency: absence of ghosts, tachyons, gradient instabilities, VDVZ discontinuities. We demonstrate that the model is not plagued by any pathology in a large region of parameters space. Within this new theory, we find several interesting phenomena. First of all, gravitational waves propagate an-isotropically in space-time. Second, we find anisotropic corrections to the Newtonian potential, with new extra Yukawa-like phases. Such a scenario also carries intriguing consequences in context of large extra dimensions proposals, as a confinement mechanism of gravity in [Formula: see text] dimensions.

Parity singlets and doublets of massive spin-3 particles in D = 2 + 1 via Noether gauge embedding

Here we demonstrate that the sixth-order (in derivatives) spin-3 self-dual model can be obtained from the fifth-order self-dual model via a Noether Gauge Embedding (NGE) of longitudinal Weyl transformations $ \eta_{(\mu\nu} \partial_{\alpha)} \Phi$ . In the case of doublet models we can show that the massive spin-3 Singh-Hagen theory is dual to a fourth- and to a sixth-order theory, via a double round of the NGE procedure by imposing traceless longitudinal (reparametrization-like) symmetries $ \partial_{(\mu} \tilde{\xi}_{\nu\alpha)}$ in the first round and transverse Weyl transformations $ \eta_{(\mu\nu}\psi^T_{\alpha)}$ in the second one. Our procedure automatically furnishes the dual maps between the corresponding fields. Contrary to the spin-2 case where an extra (Weyl) symmetry shows up in the highest-order term, in the spin-3 case only the required symmetries by the NGE procedure appear in the sixth-order doublet model. Consequently, the absence of ghosts still demands an auxiliary scalar field.

Temporally, spatially, or lightlike modulated vacua in Lorentz invariant theories

We study the vacuum structure of a class of Lorentz invariant field theories where the vacuum expectation values are not constant but are (phase) modulated. The vacua are classified into spatial, temporal, and light-like modulation types according to the patterns of spontaneous breaking of translational symmetry. The conditions for having temporal or light-like modulated vacua imply severe constraints on the models. We utilize the notion of generalized Nambu-Goldstone modes which appear in the modulated vacua. Finally, we examine fluctuation modes around these vacua and discuss their dynamics and the absence of ghosts.

Spatially modulated vacua in a Lorentz-invariant scalar field theory

Spatial modulation has been studied for a long time in condensed matter, nuclear matter and quark matter, where the manifest Lorentz invariance is lost due to the finite density/temperature effects and so on. In this paper, spatially modulated vacua at zero temperature and zero density are studied in Lorentz invariant field theories. We first propose an adaptation of the Nambu–Goldstone theorem to higher derivative theories under the assumption of the absence of ghosts: when a global symmetry is spontaneously broken due to vacuum expectation values of space-time derivatives of fields, a Nambu–Goldstone (NG) boson appears without a canonical kinetic (quadratic derivative) term with a quartic derivative term in the modulated direction while a Higgs boson appears with a canonical kinetic term. We demonstrate this in a simple model allowing (meta)stable modulated vacuum of a phase modulation (Fulde–Ferrell state), where an NG mode associated with spontaneously broken translational and U(1) symmetries appears.

Non-linear partially massless symmetry in an SO(1,5) continuation of conformal gravity

We construct a non-linear theory of interacting spin-2 fields that is invariant under the partially massless (PM) symmetry to all orders. This theory is based on the SO(1, 5) group, in analogy with the SO(2, 4) formulation of conformal gravity, but has a quadratic spectrum free of ghost instabilities. The action contains a vector field associated with a local SO(2) symmetry which is manifest in the vielbein formulation of the theory. We show that, in a perturbative expansion, the SO(2) symmetry transmutes into the PM transformations of a massive spin-2 field. In this context, the vector field is crucial to circumvent earlier obstructions to an order-by-order construction of PM symmetry. Although the non-linear theory lacks enough first class constraints to remove all helicity-0 modes from the spectrum, the PM transformations survive to all orders. The absence of ghosts and strong coupling effects at the non-linear level are not addressed here.

Hyperecho PROPELLER-MRI: Application to rapid high-resolution motion-insensitiveT2-weighted black-blood imaging of the carotid arterial vessel wall and plaque

To demonstrate the usefulness of hyperecho and PROPELLER (HEP) for carotid arterial vessel wall imaging by using a quantitative comparison with conventional methods. PROPELLER is a motion-insensitive turbo spin-echo (TSE) sequence and has recently been utilized in magnetic resonance (MR) plaque imaging instead of double inversion recovery TSE (DIR-TSE). Wider blade-width, higher k-space density, and an improved blood suppression effect result in better image quality. In this study we introduce a new combination of HEP. A total of 17 subjects were examined on a 3.0T system. We conducted quantitative comparisons for signal-to-noise ratio (SNR), contrast-to-noise-ratio, and image sharpness among HEP, DIR-TSE, and conventional PROPELLER (c-PROPELLER). Subsequently, images obtained with DIR-TSE, c-PROPELLER, and HEP were visually evaluated using a three-point scale by two board-certified radiologists. HEP showed high SNR similar to c-PROPELLER, good T2 contrast approximating DIR-TSE, and better blood suppression compared with the other two methods (P < 0.05). The image sharpness of HEP (2.55 ± 0.53) was higher than that of DIR-TSE (1.89 ± 0.33) and the absence of ghost or streak artifacts in HEP (2.89 ± 0.33) was better than that in both other methods (2.22 ± 0.83 for DIR-TSE and 2.00 ± 0.50 for c-PROPELLER) (P < 0.05). Furthermore, the degree of blood suppression, particularly in cases of slow or turbulent flow close to the atherosclerotic plaque, was identical for HEP (2.80 ± 0.45) and DIR-TSE (2.80 ± 0.45) but was significantly better than for c-PROPELLER (1.60 ± 0.55) (P < 0.05). This study demonstrates the usefulness of HEP in the carotid arteries. HEP can provide higher-resolution T2 -weighted black-blood imaging without flow- and/or motion-related artifacts, compared to conventional techniques. 3 J. Magn. Reson. Imaging 2017;45:515-524.

Can Galileons support Lorentzian wormholes?

We discuss the possibility of constructing stable, static, spherically symmetric, asymptotically flat Lorentzian wormhole solutions in General Relativity coupled to a generalized Galileon field $\pi$. Assuming that Minkowski space-time is obtained at $\partial \pi =0$, we find that there is tension between the properties of the energy-momentum tensor required to support a wormhole (violation of average null energy conditions) and stability of the Galileon perturbations about the putative solution (absence of ghosts and gradient instabilities). In 3-dimensional space-time, this tension is strong enough to rule out wormholes with above properties. In higher dimensions, including the most physically interesting case of 4-dimensional space-time, wormholes, if any, must have fairly contrived shapes.

Superluminality in dilatationally invariant generalized Galileon theories

We consider small perturbations about homogeneous backgrounds in dilatationally-invariant Galileon models. The issues we address are stability (absence of ghosts and gradient instabilities) and superluminality. We show that in Minkowski background, it is possible to construct the Lagrangian in such a way that any homogeneous Galileon background solution is stable and small perturbations about it are subluminal. On the other hand, in the case of FLRW backgrounds, for any Lagrangian functions there exist homogeneous background solutions to the Galileon equation of motion and time-dependence of the scale factor, such that the stability conditions are satisfied, but the Galileon perturbations propagate with superluminal speed. Thus, a popular class of the generalized Galileon models is plagued by superluminality.

Cosmology in bimetric theory with an effective composite coupling to matter

We study the cosmology of bimetric theory with a composite matter coupling. We find two possible branches of background evolution. We investigate the question of stability of cosmological perturbations. For the tensor and vector perturbations, we derive conditions on the absence of ghost and gradient instabilities. For the scalar modes, we obtain conditions for avoiding ghost degrees. In the first branch, we find that one of the scalar modes becomes a ghost at the late stages of the evolution. Conversely, this problem can be avoided in the second branch. However, we also find that the constraint for the second branch prevents the doubly coupled matter fields from being the standard ingredients of cosmology. We thus conclude that a realistic and stable cosmological model requires additional minimally coupled matter fields.

The coupling to matter in massive, bi- and multi-gravity

In this paper we construct a family of ways in which matter can couple to one or more `metrics'/spin-2 fields in the vielbein formulation. We do so subject to requiring the weak equivalence principle and the absence of ghosts from pure spin-2 interactions generated by the matter action. Results are presented for Massive, Bi- and Multi-Gravity theories and we give explicit expressions for the effective matter metric in all of these cases.

Ghosts and matter couplings in massive gravity, bigravity and multigravity

Recently, several works have investigated the coupling to matter in ghost-free massive (bi-&multi-)gravity and a new effective coupling to matter has been proposed. In this note we clarify some confusion on the existence and the implications of a ghost above the strong coupling scale. We confirm that the standard constraint which is otherwise typically present in this type of theories disappears on generic backgrounds as soon as this new coupling is considered. This implies the re-emergence of the Boulware-Deser ghost. Nevertheless the absence of ghost in the decoupling limit implies that the cut-off scale (if identified with the scale at which the ghost enters) is higher than the strong coupling scale. Therefore there is a valid interesting region of applicability for these couplings at scales below the cut-off.

Stable FLRW solutions in generalized massive gravity

We present exact Friedmann Lemaítre Robertson Walkers (FLRW) solutions in generalized massive gravity where the mass parameters are naturally promoted to Lorentz-invariant functions of the Stückelberg fields. This new dependence relaxes the constraint that would otherwise prevent massive gravity from possessing exact FLRW solutions. It does so without the need to introduce additional degrees of freedom. We find self-accelerating cosmological solutions and show that, with a mild restriction on the region of phase space, these cosmological solutions exhibit full stability, i.e. absence of ghosts and gradient instabilities for all the tensor, vector and scalar modes, for all cosmic time. We perform the full decoupling limit analysis, including vector degrees of freedom, which can be used to confirm the existence of an active Vainshtein mechanism about these solutions.

Gothic: New Directions in Media and Popular Culture

Gothic: New Directions in Media and Popular Culture

Effective field theory of cosmic acceleration: An implementation in CAMB

We implement the effective field theory (EFT) approach to dark energy and modified gravity in the public Einstein-Boltzmann solver CAMB. The resulting code, which we dub EFTCAMB, is a powerful and versatile tool that can be used for several objectives. It can be employed to evolve the full dynamics of linear scalar perturbations in any given single field dark energy or modified gravity model, once the latter is mapped into the EFT formalism. It offers a numerical implementation of EFT as a model-independent framework to test gravity on cosmological scales. EFTCAMB has a built-in check for the fulfillment of general stability conditions such as the absence of ghost and superluminal propagation of perturbations. It handles phantom-divide crossing models and does not contain any quasi-static approximation, but rather evolves the full dynamics of perturbations on linear scales. As we will show, the latter is an important feature in view of the accuracy and scale range of upcoming surveys. We show the reliability and applicability of our code by evolving the dynamics of linear perturbations and extracting predictions for power spectra in several models. In particular we perform a thorough analysis of f(R) theories, comparing our outputs with those of an existing code for LCDM backgrounds, and finding an agreement that can reach 0.1% for models with a Compton wavelength consistent with current cosmological data. We then showcase the flexibility of our code studying two different scenarios. First we produce new results for designer f(R) models with a time-varying dark energy equation of state. Second, we extract predictions for linear observables in some parametrized EFT models with a phantom-divide crossing equation of state for dark energy.

Generalized interaction in multigravity

We consider a general approach to describing the interaction in multigravity models in a D-dimensional space-time. We present various possibilities for generalizing the invariant volume. We derive the most general form of the interaction potential, which becomes a Pauli-Fierz-type model in the bigravity case. Analyzing this model in detail in the (3+1)-expansion formalism and also requiring the absence of ghosts leads to this bigravity model being completely equivalent to the Pauli-Fierz model. We thus in a concrete example show that introducing an interaction between metrics is equivalent to introducing the graviton mass.

MODIFIED SUPERGRAVITY AND EARLY UNIVERSE: THE MEETING POINT OF COSMOLOGY AND HIGH-ENERGY PHYSICS

We review the new theory of modified supergravity, dubbed the [Formula: see text] supergravity, and some of its recent applications to inflation and reheating in the early universe cosmology. The [Formula: see text] supergravity is the N = 1 locally supersymmetric extension of the f(R) gravity in four space–time dimensions. A manifestly supersymmetric formulation of the [Formula: see text] supergravity exist in terms of N = 1 superfields, by using the (old) minimal Poincaré supergravity in curved superspace. We find the conditions for stability, the absence of ghosts and tachyons. Three models of the [Formula: see text] supergravity are studied. The first example is devoted to a recovery of the standard (pure) N = 1 supergravity with a negative cosmological constant from the [Formula: see text] supergravity. As the second example, a generic [Formula: see text] supergravity is investigated, and the existence of the AdS bound on the scalar curvature is found. As the third (and most important) example, a simple viable realization of chaotic inflation in supergravity is found. Our approach is minimalistic since it does not introduce new exotic fields or new interactions, beyond those already present in (super)gravity. The universal reheating mechanism is automatic. We establish the consistency of our approach and also apply it to preheating and reheating after inflation. The Higgs inflation and its correspondence to the Starobinsky inflation are established in the context of supergravity. We briefly review other relevant issues such as non-Gaussianity, CP-violation, origin of baryonic asymmetry, lepto- and baryo-genesis. The [Formula: see text] supergravity has promise for possible solutions to those outstanding problems too.