A pinboard by
Jose Maria Ezquiaga

PhD student, IFT UAM-CSIC


Testing Dark Energy and Gravity with Gravitational Waves observations

We are living the dawn of a new era in fundamental physics. Gravitational Wave astronomy opens a new window to the universe that will revolutionize our understanding of the cosmos. My research focuses on exploiting this novel tool to probe the nature of gravity and Dark Energy.

One of the biggest mysteries in physics is why the universe undergoes a present epoch of accelerated expansion. Recalling that gravity is an attractive force only and that the universe started from a phase of rapid expansion (Big Bang), our intuition would tell us that gravity would try to retain everything together and thus slow down the original expansion. However, we observe exactly the opposite. The universe keeps expanding and every second it expands faster. The explanation for this unexpected observation is that there is an additional component of the universe, known as Dark Energy, that acts as a repulsive force at cosmic scales.

Since Dark Energy affects the universe at large scales, any information coming from distant objects would feel its presence and could be a potential way to measure it. In particular, gravitational waves are perfect candidates. These waves are tiny ripples of space-time that propagate throughout the universe carrying information about the gravitational field of its source. They are the analogue of the electromagnetic waves, which carry information of the electromagnetic field generated by our smartphone for instance.

We have already detected gravitational waves from the merging of black-holes and neutron stars. Specially interesting was the latter case because the same event was also observed with electromagnetic waves by different telescopes and satellites. This complementary information served to determine the propagation speed of gravitational waves which turned out to be equal to the speed of light to high level of precision.

The implications of this measurement of the speed of gravitational waves is that Dark Energy cannot alter the velocity of these waves to be different from the speed of light. This has strong implications for many Dark Energy models in which the same mechanism that triggers the accelerated expansion produces an anomalous gravitational wave speed.

Future gravitational waves detections will continue contributing to unveil the nature of Dark Energy since other effects may affect as well their propagation. A new era in Dark Energy research has already begun.


Gravitational Waves and the Fate of Scalar-Tensor Gravity

Abstract: We investigate the propagation speed of gravitational waves (GWs) in generic scalar-tensor gravity. A difference in the speed of gravity relative to the speed of light can be caused by the emergence of a disformal geometry in the gravitational sector. This requires the background scalar configuration to both spontaneously break Lorentz symmetry and couple to second derivatives of the metric perturbations through the Weyl tensor or higher derivatives of the scalar. The latter requirement allows a division of gravitational theories into two families: those that predict that GWs propagate exactly at the speed of light and those that allow for anomalous speed. Neutron star binary mergers and other GW events with an associated electromagnetic counterpart can place extremely tight constraints on the speed of GWs relative to the speed of light. However, such observations become impossible if the speed is modified too much, as predicted by some models of cosmic acceleration. Complementary measurements of the speed of gravity may be possible by monitoring nearby periodic sources, such as the binary white dwarf system WDS J0651+2844 and other eLISA verification binaries, and looking for a phase difference between the gravitational wave signal and an electromagnetic signal. Future multi-messenger GW astronomy thus has the potential to detect an anomalous speed, thereby ruling out GR and significantly changing our understanding of gravity. A negative detection will rule out or severely constrain any solution in any theory which allows for anomalous propagation of GWs.

Pub.: 05 Aug '16, Pinned: 31 Dec '17

Field redefinitions in theories beyond Einstein gravity using the language of differential forms

Abstract: We study the role of field redefinitions in general scalar-tensor theories. In particular, we first focus on the class of field redefinitions linear in the spin-2 field and involving derivatives of the spin-0 mode, generically known as disformal transformations. We start by defining the action of a disformal transformation in the tangent space. Then, we take advantage of the great economy of means of the language of differential forms to compute the full transformation of Horndeski's theory under general disformal transformations. We obtain that Horndeski's action maps onto itself modulo a reduced set of non-Horndeski Lagrangians. These new Lagrangians are found to be invariant under disformal transformation that depend only in the first derivatives of the scalar. Moreover, these combinations of Lagrangians precisely appear when expressing in our basis the constraints of the recently proposed Extended Scalar-Tensor (EST) theories. These results allow us to classify the different orbits of scalar-tensor theories invariant under particular disformal transformations, namely the special disformal, kinetic disformal and disformal Horndeski orbits. In addition, we consider generalizations of this framework. We find that there are possible well-defined extended disformal transformations that have not been considered in the literature. However, they generically cannot link Horndeski theory with EST theories. Finally, we study further generalizations in which extra fields with different spin are included. These field redefinitions can be used to connect different gravity theories such as multi-scalar-tensor theories, generalized Proca theories and bi-gravity. We discuss how the formalism of differential forms could be useful for future developments in these lines.

Pub.: 19 Jan '17, Pinned: 31 Dec '17