50 Years of Horndeski Gravity: Past, Present and Future

Horndeski, Gregory W.; Silvestri, Alessandra

doi:10.1007/s10773-024-05558-2

Int J Theor Phys

2024

DOI: 10.1007/s10773-024-05558-2

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50 Years of Horndeski Gravity: Past, Present and Future

Gregory W. Horndeski,

Alessandra Silvestri

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2024

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Cited by 8 publications

References 128 publications

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Recent Developments in Degenerate Higher Order Scalar Tensor Theories

Lazanu

2024

Annalen der Physik

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Degenerate Higher Order Scalar Tensor (DHOST) theories are the most general scalar‐tensor theories whose Lagrangian depends on the metric tensor and a single scalar field and its derivatives up to second order. They propagate only one scalar degree of freedom, without being plagued by Ostrogradsky instabilities. This is achieved through certain degeneracies of the functions forming their Lagrangian. They generalize the Horndeski and beyond‐Horndeski theories. Originally proposed to describe the late‐time acceleration of the expansion of the universe, generalizing the cosmological constant, they can also be used to build models of the early universe, to describe inflation or alternatives to standard inflation. In the late universe, they modify the standard Vainstein screening mechanism from Horndeski theories (which can have observable consequences) and are suited to build black hole models, featuring non‐stealth Kerr black hole solutions. In this work, their phenomenology is reviewed, looking at their basic properties, their parameterizations and classifications, focusing on solutions in the early and the late universe and at cosmological and astrophysical constraints.

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Recent Developments in Degenerate Higher Order Scalar Tensor Theories

Lazanu

2024

Annalen der Physik

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Spatial dependence of the growth factor in scalar-tensor cosmology

Caruana,

Farrugia,

Said

et al. 2024

J. Cosmol. Astropart. Phys.

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Scalar-tensor theories have taken on a key role in attempts to confront the growing open questions in standard cosmology. It is important to understand entirely their dynamics at perturbative level including any possible spatial dependence in their growth of large scale structures. In this work, we investigate the spatial dependence of the growth rate of scalar-tensor theories through the Mészáros equation. We confirm that at subhorizon level this dependence does not play a major role for viable models. However, we establish conditions on which this criterion is met which may be important for developing new models. In our work, we consider three specific models that exhibit spatial dependence of the growth rate at subhorizon modes, which may also be important for early Universe models.

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Phenomenology of Horndeski gravity under positivity bounds

de Boe,

Ye,

Renzi

et al. 2024

J. Cosmol. Astropart. Phys.

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A set of conditions that any effective field theory needs to satisfy in order to allow for the existence of a viable UV completion, has recently gained attention in the cosmological context under the name of positivity bounds. In this paper we revisit the derivation of such bounds for Horndeski gravity, highlighting the limitations that come from applying the traditional methodology to a theory of gravity on a cosmological background. We then translate these bounds into a complete set of viability conditions in the language of effective field theory of dark energy. We implement the latter into EFTCAMB and explore the large scale structure phenomenology of Horndeski gravity under positivity bounds. We build a statistically significant sample of viable Horndeski models, and derive the corresponding predictions for the background evolution, in terms of w DE, and the dynamics of linear perturbations, in terms of the phenomenological functions μ and Σ, associated to clustering and weak lensing, respectively. We find that the addition of positivity bounds to the traditional no-ghost and no-gradient conditions considerably tightens the theoretical constraints on all these functions. The most significant feature is a strengthening of the correlation μ ≃ Σ, and a related tight constraint on the luminal speed of gravitational waves c 2 T ≃ 1. In this work we demonstrate the strong potential of positivity bounds in shaping the viable parameter space of scalar-tensor theories. This is certainly promising, but it also highlights the importance of overcoming all issues that still plague a rigorous formulation of the positivity bounds in the cosmological context.

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50 Years of Horndeski Gravity: Past, Present and Future

Cited by 8 publications

References 128 publications

Recent Developments in Degenerate Higher Order Scalar Tensor Theories

Recent Developments in Degenerate Higher Order Scalar Tensor Theories

Spatial dependence of the growth factor in scalar-tensor cosmology

Phenomenology of Horndeski gravity under positivity bounds

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