Evidence
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DOF reappears, and the degeneracy conditions must be imposed to remove it.
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Card 1 · Effective description of higher-order scalar-tensor theories
Most existing theories of dark energy and/or modified gravity, involving a scalar degree of freedom, can be conveniently described within the framework of the Effective Theory of Dark Energy, based on the unitary gauge where the scalar field is uniform. We extend this effective approach by allowing…
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Most existing theories of dark energy and/or modified gravity, involving a scalar degree of freedom, can be conveniently described within the framework of the Effective Theory of Dark Energy, based on the unitary gauge where the scalar field is uniform. We extend this effective approach by allowing the Lagrangian in unitary gauge to depend on the time derivative of the lapse function. Although this dependence generically signals the presence of an extra scalar degree of freedom, theories that contain only one propagating scalar degree of freedom, in addition to the usual tensor modes, can be constructed by requiring the initial Lagrangian to be degenerate. Starting from a general quadratic action, we derive the dispersion relations for the linear perturbations around Minkowski and a cosmological background. Our analysis directly applies to the recently introduced Degenerate Higher-Order Scalar-Tensor (DHOST) theories. For these theories, we find that one cannot recover a Poisson-like e …
Card 2 · On the effective metric of axial black hole perturbations in DHOST gravity
Abstract We study axial (or odd-parity) perturbations about static and spherically symmetric hairy black hole (BH) solutions in shift-symmetric DHOST (Degenerate Higher-Order Scalar-Tensor) theories. We first extend to the family of DHOST theories the first-order formulation that we recently develop…
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Abstract We study axial (or odd-parity) perturbations about static and spherically symmetric hairy black hole (BH) solutions in shift-symmetric DHOST (Degenerate Higher-Order Scalar-Tensor) theories. We first extend to the family of DHOST theories the first-order formulation that we recently developed for Horndeski theories. Remarkably, we find that the dynamics of DHOST axial perturbations is equivalent to that of axial perturbations in general relativity (GR) evolving in a, distinct, effective metric. In the particular case of quadratic DHOST theories, this effective metric is derived from the background BH metric via a disformal transformation. We illustrate our general study with three examples of BH solutions. In some so-called stealth solutions, the effective metric is Schwarzschild with a shifted horizon. We also give an example of BH solution for which the effective metric is associated with a naked singularity.
Card 3 · Dark Energy and Modified Gravity in Degenerate Higher-Order Scalar-Tensor (DHOST) theories: a review
The property that the degeneracy of the Lagrangian implies the absence of any extra scalar degree of freedom has been shown explicity in the general case of quadratic DHOST theories by resorting to a Hamiltonian formulation that uses the variable Q ∗ Q_{*} and does not assume any particular gauge […
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The property that the degeneracy of the Lagrangian implies the absence of any extra scalar degree of freedom has been shown explicity in the general case of quadratic DHOST theories by resorting to a Hamiltonian formulation that uses the variable Q ∗ Q_{*} and does not assume any particular gauge [ 24 ] . Earlier Hamiltonian analyses were proposed for quadratic Beyond Horndeski theories described in the unitary gauge in [ 11 , 12 , 25 ] and for a particular Beyond Horndeski model in an arbitrary gauge in [ 26 ] . Note that there exists a special class of theories that appear degenerate in the unitary gauge but not in an arbitrary gauge (and therefore are not DHOST theories) and, in this case, the extra mode can be tamed via appropriate boundary conditions [ 27 ] .
Card 4 · Tracker and scaling solutions in DHOST theories
In quadratic-order degenerate higher-order scalar-tensor (DHOST) theories compatible with gravitationalwave constraints, we derive the most general Lagrangian allowing for tracker solutions characterized by φ/H p = constant, where φ is the time derivative of a scalar field φ, H is the Hubble expansi…
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In quadratic-order degenerate higher-order scalar-tensor (DHOST) theories compatible with gravitationalwave constraints, we derive the most general Lagrangian allowing for tracker solutions characterized by φ/H p = constant, where φ is the time derivative of a scalar field φ, H is the Hubble expansion rate, and p is a constant.While the tracker is present up to the cubic-order Horndeski Lagrangian 2p) φ, where c 2 , c 3 are constants and X is the kinetic energy of φ, the DHOST interaction breaks this structure for p = 1.Even in the latter case, however, there exists an approximate tracker solution in the early cosmological epoch with the nearly constant field equation of state w φ = -1 -2p Ḣ/(3H 2 ).The scaling solution, which corresponds to p = 1, is the unique case in which all the terms in the field density ρ φ and the pressure P φ obey the scaling relation ρ φ ∝ P φ ∝ H 2 .Extending the analysis to the coupled DHOST theories with the field-dependent coupling Q (φ) between the scala …
Card 5 · Dark Energy and Modified Gravity in Degenerate Higher-Order Scalar-Tensor (DHOST) theories: a review
The property that the degeneracy of the Lagrangian implies the absence of any extra scalar degree of freedom has been shown explicity in the general case of quadratic DHOST theories by resorting to a Hamiltonian formulation that uses the variable Q ∗ Q_{*} and does not assume any particular gauge […
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The property that the degeneracy of the Lagrangian implies the absence of any extra scalar degree of freedom has been shown explicity in the general case of quadratic DHOST theories by resorting to a Hamiltonian formulation that uses the variable Q ∗ Q_{*} and does not assume any particular gauge [ 24 ] . Earlier Hamiltonian analyses were proposed for quadratic Beyond Horndeski theories described in the unitary gauge in [ 11 , 12 , 25 ] and for a particular Beyond Horndeski model in an arbitrary gauge in [ 26 ] . Note that there exists a special class of theories that appear degenerate in the unitary gauge but not in an arbitrary gauge (and therefore are not DHOST theories) and, in this case, the extra mode can be tamed via appropriate boundary conditions [ 27 ] .
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