arXiv subjects

Hamed Bouzari Nezhad

Publications and source records attributed to Hamed Bouzari Nezhad.

5 recordsLinked to original sources

arXiv2026

Removing Ostrogradsky modes in multi-field higher-order scalar-tensor theories

We study how the Ostrogradsky modes can be removed in multi-field higher-order scalar-tensor theories. For a general class of theories with an arbitrary number $\mathcal{N}$ of scalar fields and quadratic dependence on their second derivatives, we perform an ADM decomposition and carry out the Hamiltonian analysis in the branch where the metric kinetic block is invertible. The primary degeneracy condition is a matrix condition in field space. Previous studies of coupled higher-derivative systems have shown that primary degeneracy need not by itself be sufficient. In the generally covariant multi-scalar-tensor setting considered here, where the scalar and metric velocities mix in the ADM decomposition, preservation of the primary degeneracy constraints generates additional consistency conditions. We obtain these conditions explicitly in terms of the ADM coefficient blocks generated by the covariant action. Some of these conditions are antisymmetric in the field-space indices and vanish in the single-field limit. A rank condition on the constraint algebra is required in addition. Under the assumptions used in the Hamiltonian counting, in particular that the scalar second-class sector leaves the diffeomorphism constraints first class, the theory then propagates $2+\mathcal{N}$ degrees of freedom, the two tensor modes of gravity and one scalar mode per field, with no additional Ostrogradsky mode. We check the conditions in the single-field limit and in a multi-field quadratic Horndeski-type subclass. We also give a subclass with nonzero scalar-metric kinetic mixing, constructed directly at the level of the ADM coefficient blocks, showing that the Hamiltonian conditions admit nontrivial solutions at the ADM-block level.

gr-qc↗

arXiv2026

A principal-symbol obstruction in metric-affine Gauss--Bonnet gravity

The four-dimensional Riemannian Gauss--Bonnet combination admits more than one continuation to a geometry with an independent affine connection. Two natural candidates, the metric-contracted representative and the double-epsilon representative, agree in their Levi-Civita limit, and we ask whether they also give the independent connection the same leading differential structure. In a metric-affine model with a scalar coupling $f(ϕ)$ to the Gauss--Bonnet term, we compare their quadratic two-derivative connection principal forms about the Levi-Civita locus. For the metric-contracted continuation, the principal form, evaluated on a twenty-four-dimensional hook-free torsion-free witness subspace at a timelike principal covector, has rank $18$ and nullity $6$ and is indefinite; the full torsion-free form at the same covector is therefore indefinite with rank at least $18$. Locally where $f(ϕ)\neq0$, the two-derivative principal symbol of the linearized connection equations is nonzero, which obstructs purely algebraic local elimination of the connection in the activated directions. For the double-epsilon continuation, the corresponding quadratic two-derivative form vanishes identically for an arbitrary perturbation of the connection about that locus, so its principal symbol vanishes with it, and that particular obstruction is absent, though its absence does not by itself establish algebraic eliminability. Thus the leading differential structure of the independent connection depends on the affine continuation of the Riemannian Gauss--Bonnet combination.

gr-qc↗

arXiv2026

Degenerate higher-order scalar-tensor theories in metric-affine gravity

We construct the metric-affine analogue of the quadratic degenerate higher-order scalar-tensor (DHOST) theories. We begin with the metric-affine completion of the quadratic DHOST scalar-tensor action, which is linear in curvature and contains all operators that are at most quadratic in the covariant second derivatives of the scalar field, ensuring that the connection enters only through curvature and these second derivatives. Solving the connection equation by performing a full decomposition of the distortion tensor gives a closed-form effective metric theory. Imposing the standard metric DHOST degeneracy conditions then selects a Palatini Class Ia branch that is fully determined by two free functions in the original action. Analyzing the tensor sector shows that requiring gravitational waves to propagate at the speed of light further restricts the theory to a one-function family. These results provide a detailed and self-contained characterization of the quadratic metric-affine Class Ia sector within this operator basis and identify the theoretical conditions implied by gravitational wave observations.

gr-qc↗

arXiv2024

Scalar fields with derivative coupling to curvature in the Palatini and the metric formulation

We study models where a scalar field has derivative and non-derivative couplings to the Ricci tensor and the co-Ricci tensor with a view to inflation. We consider both the metric formulation and the Palatini formulation. In the Palatini case, the couplings to the Ricci tensor and the Ricci scalar give the same result regardless of whether the connection is unconstrained or the non-metricity or the torsion is assumed to vanish. When the co-Ricci tensor is included, the unconstrained case and the zero torsion case are physically different. We reduce all the actions to the Einstein frame with minimally coupled matter, and find the leading order differences between the metric case and the Palatini cases.

gr-qc↗

arXiv2017

On the gravitational instability in the Newtonian limit of MOG

We have found some analytical cosmological solutions to MOdified Gravity (MOG). These solutions describe different evolutionary epochs of an isotropic and homogeneous universe. During each epoch, the evolution of cosmological perturbation is studied in the Newtonian framework and compared with the corresponding results of GR.

gr-qc↗