Lorentz violation in gravity

The study of gravitational theories without Lorentz invariance plays an important role to understand different aspects of gravitation. In this short contribution we will describe the construction, main advantages and some phenomenological considerations associated with the presence of a preferred time direction.Comment: 4 pages. To appear in the proceedings of the 2015 Rencontres de Moriond, "Gravitation: 100 years after GR

Modified gravity and binary pulsars: the Lorentz violating case

The dynamics of binary pulsars can be used to test different aspects of gravitation. This is particularly important to constrain alternatives to general relativity in regimes which are not probed by other methods. In this short contribution, I will describe the case of theories of gravity without Lorentz invariance. The latter are important in the context of quantum gravity and modify the laws of gravity at basically all scales.Comment: 6 pages, 1 figure. Prepared for the proceedings of Marcel Grossman 1

Models of non-relativistic quantum gravity: the good, the bad and the healthy

Horava's proposal for non-relativistic quantum gravity introduces a preferred time foliation of space-time which violates the local Lorentz invariance. The foliation is encoded in a dynamical scalar field which we call `khronon'. The dynamics of the khronon field is sensitive to the symmetries and other details of the particular implementations of the proposal. In this paper we examine several consistency issues present in three non-relativistic gravity theories: Horava's projectable theory, the healthy non-projectable extension, and a new extension related to ghost condensation. We find that the only model which is free from instabilities and strong coupling is the non-projectable one. We elaborate on the phenomenology of the latter model including a discussion of the couplings of the khronon to matter. In particular, we obtain the parameters of the post-Newtonian expansion in this model and show that they are compatible with current observations.Comment: 50 pages, JHEP styl

Cosmological perturbation theory at three-loop order

We analyze the dark matter power spectrum at three-loop order in standard perturbation theory of large scale structure. We observe that at late times the loop expansion does not converge even for large scales (small momenta) well within the linear regime, but exhibits properties compatible with an asymptotic series. We propose a technique to restore the convergence in the limit of small momentum, and use it to obtain a perturbative expansion with improved convergence for momenta in the range where baryonic acoustic oscillations are present. Our results are compared with data from N-body simulations at different redshifts, and we find good agreement within this range.Comment: 29 pages, 8 figures, 1 table; v2 Typos corrected, references added. Matches published versio

Scale-invariant alternatives to general relativity

We study the general class of gravitational field theories constructed on the basis of scale invariance (and therefore absence of any mass parameters) and invariance under transverse diffeomorphisms (TDiff), which are the 4-volume conserving coordinate transformations. We show that these theories are equivalent to a specific type of scalar-tensor theories of gravity (invariant under all diffeomorphisms) with a number of properties, making them phenomenologically interesting. They contain, in addition to the dimensionless coupling constants of the original theory, an arbitrary dimensionful parameter $\Lambda_0$. This parameter is associated with an integration constant of the equations of motion, similar to the arbitrary cosmological constant appearing in unimodular gravity. We focus on the theories where Newton's constant and the electroweak scale emerge from the spontaneous breaking of scale invariance and are unrelated to $\Lambda_0$. The massless particle spectrum of these theories contains the graviton and a new particle -- dilaton. For $\Lambda_0=0$, the massless dilaton has only derivative couplings to matter fields and the bounds on the existence of a 5th force are easily satisfied. As for the matter fields, we determine the conditions leading to a renormalizable low-energy theory. If $\Lambda_0\neq 0$, scale invariance is broken. The arbitrary constant $\Lambda_0$ produces a "run-away" potential for the dilaton. As a consequence, the dilaton can act as a dynamical dark energy component. We elucidate the origin of the cosmological constant in the class of theories under consideration and formulate the condition leading to its absence. If this condition is satisfied, dark energy is purely dynamical and associated to the dilaton.Comment: 39 pages, no figure

Bounding the speed of gravity with gravitational wave observations

The time delay between gravitational wave signals arriving at widely separated detectors can be used to place upper and lower bounds on the speed of gravitational wave propagation. Using a Bayesian approach that combines the first three gravitational wave detections reported by the LIGO collaboration we constrain the gravitational waves propagation speed c_gw to the 90% credible interval 0.55 c < c_gw < 1.42 c, where c is the speed of light in vacuum. These bounds will improve as more detections are made and as more detectors join the worldwide network. Of order twenty detections by the two LIGO detectors will constrain the speed of gravity to within 20% of the speed of light, while just five detections by the LIGO-Virgo-Kagra network will constrain the speed of gravity to within 1% of the speed of light.Comment: Version published in PRL. 5 pages, 3 figure

Testing Lorentz invariance of dark matter with satellite galaxies

We develop the framework for testing Lorentz invariance in the dark matter sector using galactic dynamics. We consider a Lorentz violating (LV) vector field acting on the dark matter component of a satellite galaxy orbiting in a host halo. We introduce a numerical model for the dynamics of satellites in a galactic halo and for a galaxy in a rich cluster to explore observational consequences of such an LV field. The orbital motion of a satellite excites a time dependent LV force which greatly affects its internal dynamics. Our analysis points out key observational signatures which serve as probes of LV forces. These include modifications to the line of sight velocity dispersion, mass profiles and shapes of satellites. With future data and a more detailed modeling these signatures can be exploited to constrain a new region of the parameter space describing the LV in the dark matter sector.Comment: 27 pages, 11 figures, 2 tables, 1 appendix. Minor corrections in section 4.3.

Scattering of scalar, electromagnetic and gravitational waves from binary systems

The direct detection of gravitational waves crowns decades of efforts in the modelling of sources and of increasing detectors' sensitivity. With future third-generation Earth-based detectors or space-based observatories, gravitational-wave astronomy will be at its full bloom. Previously brushed-aside questions on environmental or other systematic effects in the generation and propagation of gravitational waves are now begging for a systematic treatment. Here, we study how electromagnetic and gravitational radiation is scattered by a binary system. Scattering cross-sections, resonances and the effect of an impinging wave on a gravitational-bound binary are worked out for the first time. The ratio between the scattered-wave amplitude and the incident wave can be of order $10^{-5}$ for known pulsars, bringing this into the realm of future gravitational-wave observatories. For currently realistic distribution of compact-object binaries, the interaction cross-section is too small to be of relevance.Comment: 19 pages, 3 figures, to appear in PR

Infrared modifications of gravity

[spa] El propósito de la Tesis es estudiar las posibles modificaciones de la relatividad general a distancias largas. La motivación de dicho estudio se debe tanto a motivos teóricos (la rigidez de la relatividad general en comparación con el resto de interacciones) como a experimentales (la explicación de la actual expansión del universo). En la tesis nos hemos centrado en dos puntos principales: la gravedad unimodular y la bigravedad. La primera parte de la tesis consiste en el estudio de las teorías linearizadas de gravitones donde encontramos las posibles teorías de gravedad masiva que no sufren de inestabilidades. Además, para el caso sin masa hemos encontrado un caso diferente al proveniente de la relatividad general, si bien con idénticas predicciones físicas. También hemos extendido este resultado a otras teorías de spin alto para fermiones. En la segunda parte nos centramos en aspectos no-lineales. En primer lugar, para el caso sin masa, encontramos una alternativa a la relatividad general con idénticas predicciones excepto para la constante cosmológica: la gravedad unimodular. Intentamos, sin éxito, demostrar su unicidad. Para el caso de gravedad masiva nos hemos centrado en la bigravedad como modelo no-lineal. Hemos estudiado tanto aspectos globales como locales. En lo que se refiere a las perturbaciones, hemos encontrado que la bigravedad puede dar lugar a modificaciones de relatividad general consistentes