10 research outputs found
Reviving Horndeski Theory using Teleparallel Gravity after GW170817
Horndeski gravity was highly constrained from the recent gravitational wave
observations by the LIGO Collaboration down to .
In this Letter we study the tensorial perturbations in a flat cosmological
background for an analogue version of Horndenki gravity which is based in
Teleparallel Gravity constructed from a flat manifold with a nonvanishing
torsion tensor. It is found that in this approach, one can construct a more
general Horndeski theory satisfying without eliminating the
coupling functions and that were highly constrained
in standard Horndeski theory. Hence, in the Teleparallel approach one is able
to restore these terms, creating an interesting way to revive Horndeski
gravity.Comment: 12 pages, 0 figure
Cosmological perturbations in modified teleparallel gravity models: Boundary term extension
Teleparallel gravity offers a new avenue in which to construct gravitational
models beyond general relativity. While teleparallel gravity can be framed in a
way to be dynamically equivalent to general relativity, its modifications are
mostly not equivalent to the traditional route to modified gravity.
gravity is one such gravitational theory where the second and fourth order
contributions to the field equations are decoupled. In this work, we explore
the all important cosmological perturbations of this new framework of gravity.
We derive the gravitational propagation equation, its vector perturbation
stability conditions, and its scalar perturbations. Together with the matter
perturbations, we derive the effective gravitational constant in this
framework, and find an interesting branching behaviour that depends on the
particular gravitational models being probed. We close with a discussion on the
relation of these results with other gravitational theories.Comment: 21 page
Teleparallel Gravity: From Theory to Cosmology
Teleparallel gravity has significantly increased in popularity in recent
decades, bringing attention to Einstein's other theory of gravity. In this
Review, we relate this form of geometry to the broader metric-affine approach
to forming gravitational theories where we describe a systematic way of
constructing consistent teleparallel theories that respect certain physical
conditions such as local Lorentz invariance. We first use teleparallel gravity
to formulate a teleparallel equivalent of general relativity which is
dynamically equivalent to general relativity but which may have different
behaviors for other scenarios, such as quantum gravity. After setting this
foundation, we describe the plethora of modified teleparallel theories of
gravity that have been proposed in the literature. In the second part of the
Review, we first survey works in teleparallel astrophysics literature where we
focus on the open questions in this regime of physics. We then discuss the
cosmological consequences for the various formulations of teleparallel gravity.
We do this at background level by exploring works using various approaches
ranging from dynamical systems to Noether symmetries, and more. Naturally, we
then discuss perturbation theory, firstly by giving a concise approach in which
this can be applied in teleparallel gravity theories and then apply it to a
number of important theories in the literature. Finally, we examine works in
observational and precision cosmology across the plethora of proposal theories.
This is done using some of the latest observations and is used to tackle
cosmological tensions which may be alleviated in teleparallel cosmology. We
also introduce a number of recent works in the application of machine learning
to gravity, we do this through deep learning and Gaussian processes, together
with discussions about other approaches in the literature.Comment: 387 pages, 55 figures; comments and suggestions for references are
welcome, please note Section 1.4 `Note to the teleparallel gravity community
New models and big bang nucleosynthesis constraints in
The f(Q) theories of modified gravity arise from the consideration of non-metricity as the basic geometric quantity, and have been proven to be very efficient in describing the late-time Universe. We use the Big Bang Nucleosynthesis (BBN) formalism and observations in order to extract constraints on various classes of f(Q) models. In particular, we calculate the deviations that f(Q) terms bring on the freeze-out temperature in comparison to that of the standard CDM evolution, and then we impose the observational bound on to extract constraints on the involved parameters of the considered models. Concerning the polynomial model, we show that the exponent parameter should be negative, while for the power-exponential model and the new hyperbolic tangent-power model we find that they pass the BBN constraints trivially. Finally, we examine two DGP-like f(Q) models, and we extract the bounds on their model parameters. Since many gravitational modifications, although able to describe the late-time evolution of the Universe, produce too-much modification at early times and thus fall to pass the BBN confrontation, the fact that f(Q) gravity can safely pass the BBN constraints is an important advantage of this modified gravity class
Polarization of gravitational waves in symmetric teleparallel theories of gravity and their modifications
Soudi I, Farrugia G, Said JL, Gakis V, Saridakis EN. Polarization of gravitational waves in symmetric teleparallel theories of gravity and their modifications. Physical Review D. 2019;100(4): 044008.Symmetric teleparallel gravity (STG) offers an interesting avenue to formulate a theory of gravitation that relies neither on curvature nor torsion but only on nonmetricity Q. Given the growing number of confirmed observations of gravitational waves (GWs) and their use to explore gravitational theories, in this work we investigate the GWs in various extensions of STG, focusing on their speed and polarization. We explore the plethora of theories that this new framework opens up, that is, as general relativity can be modified, so can the symmetric teleparallel equivalent of general relativity (STEGR). In this work, we investigate the fate of GWs in the generalized irreducible decomposition of STEGR, generalizations of the STEGR Lagrangian, f(Q), a scalar field nonminimally coupled to the STEGR Lagrangian, and the general setup of f(Q,B) theory where B is the boundary term difference between the Ricci scalar and the STEGR Lagrangian. Coincidentally, f(Q,B) forms a more general theory than f(R) gravity since Q embodies the second-order elements of the Ricci scalar while B takes on its fourth-order boundary terms. Our work deals mainly with the resulting scalar-vector-tensor polarization modes of the plethora of STG theories, and how they effect their respective speeds of propagation
Teleparallel gravity: from theory to cosmology
Teleparallel gravity has significantly increased in popularity in recent decades, bringing attention to Einstein's other theory of gravity. In this Review, we give a comprehensive introduction to how teleparallel geometry is developed as a gauge theory of translations together with all the other properties of gauge field theory. We also related this form of geometry to the broader metric-affine approach to forming gravitational theories where we describe a systematic way of constructing consistent teleparallel theories that respect certain physical conditions such as local Lorentz invariance. We first use teleparallel gravity to formulate a teleparallel equivalent of general relativity which is dynamically equivalent to general relativity but which may have different behaviors for other scenarios, such as quantum gravity. After setting this foundation, we describe the plethora of modified teleparallel theories of gravity that have been proposed in the literature. We attempt to connect them together into general classes of covariant gravitational theories. Of particular interest, we highlight the recent proposal of a teleparallel analogue of Horndeski gravity which offers the possibility of reviving all of the regular Horndeski contributions. In the second part of the Review, we first survey works in teleparallel astrophysics literature where we focus on the open questions in this regime of physics. We then discuss the cosmological consequences for the various formulations of teleparallel gravity. We do this at background level by exploring works using various approaches ranging from dynamical systems to Noether symmetries, and more. Naturally, we then discuss perturbation theory, firstly by giving a concise approach in which this can be applied in teleparallel gravity theories and then apply it to a number of important theories in the literature. Finally, we examine works in observational and precision cosmology across the plethora of proposal theories. This is done using some of the latest observations and is used to tackle cosmological tensions which may be alleviated in teleparallel cosmology. We also introduce a number of recent works in the application of machine learning to gravity, we do this through deep learning and Gaussian processes, together with discussions about other approaches in the literature