Postdoc, UC Berkeley


Gravitational waves can help understand Dark Energy, the mechanism for our universe's acceleration

Our universe is known to be in a phase of accelerated expansion, meaning that distance galaxies move further away from each other faster and faster as time goes on. This is strange because gravity tends to decelerate things that move away from each other: distant galaxies should slow down just as apples falls from trees. The mysterious mechanism that causes cosmic acceleration is known as Dark Energy, and identifying its nature has become a major goal of physics.

A possible explanation for cosmic acceleration is that gravity behaves different than we think on cosmological scales. However, tweaking gravity so that the universe accelerates also causes side effects that can be used to test the theory and ultimately understand what is the right mechanism for cosmic acceleration. One of the goals in my research is to predict these secondary effects and using them to characterize Dark Energy.

A peculiar side effect of cosmic acceleration is that it can alter the speed at which gravitational waves travel through space. Gravitational waves are minute distortions in the fabric of space-time travelling through the universe, similarly to sound in the air or waves on the surface of water. Gravitational waves from black hole pairs have been detected only recently, but they are likely to change drastically the way in which we observe the universe.

It is believed that gravitational waves propagate at the speed of light, but in some of the most promising dark energy models gravitational waves travel at a different speed. Because gravitational wave sources are millions of light-years away, even a tiny difference in the propagation speed will produce a huge delay with respect to a light signal emitted by the same event. If the light and gravity signals are observed simultaneously, all the dark energy models predicting a different speed would be spectacularly eliminated. Conversely, establishing that the speed of gravitational waves is not the speed of light will constitute very strong evidence of exotic gravitational physics.

My work explores what we can learn about cosmic acceleration using gravitational waves, as well as other data such as galaxy surveys, which are sensitive to a range of secondary effects introduced by different dark energy models. The lessons to be extracted will allow us to better understand gravity, the most obvious, yet mysterious force in our universe.


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: 09 Aug '17

Galileon Gravity in Light of ISW, CMB, BAO and $H_0$ data

Abstract: Cosmological models with Galileon gravity are an alternative to the standard $\Lambda {\rm CDM}$ paradigm with testable predictions at the level of its self-accelerating solutions for the expansion history, as well as large-scale structure formation. Here, we place constraints on the full parameter space of these models using data from the cosmic microwave background (CMB) (including lensing), baryonic acoustic oscillations (BAO) and the Integrated Sachs-Wolfe (ISW) effect. We pay special attention to the ISW effect for which we use the cross-spectra, $C_\ell^{\rm T g}$, of CMB temperature maps and foreground galaxies from the WISE survey. The sign of $C_\ell^{\rm T g}$ is set by the time evolution of the lensing potential in the redshift range of the galaxy sample: it is positive if the potential decays (like in $\Lambda {\rm CDM}$), negative if it deepens. We constrain three subsets of Galileon gravity separately known as the Cubic, Quartic and Quintic Galileons. The cubic Galileon model predicts a negative $C_\ell^{\rm T g}$ and exhibits a $7.8\sigma$ tension with the data, which effectively rules it out. For the quartic and quintic models the ISW data also rule out a significant region of the parameter space but permit regions where the goodness-of-fit is comparable to $\Lambda {\rm CDM}$. The data prefers a non zero sum of the neutrino masses ($\sum m_\nu\approx 0.5$eV) with $ \sim 5\sigma$ significance in these models. The best-fitting models have values of $H_0$ consistent with local determinations, thereby avoiding the tension that exists in $\Lambda {\rm CDM}$. We also identify and discuss a $\sim 2\sigma$ tensions that Galileon gravity exhibits with recent BAO measurements. Our analysis shows overall that Galileon cosmologies cannot be ruled out by current data but future lensing, BAO and ISW data hold strong potential to do so.

Pub.: 07 Jul '17, Pinned: 09 Aug '17

Gravity at the horizon: on relativistic effects, CMB-LSS correlations and ultra-large scales in Horndeski's theory

Abstract: We address the impact of consistent modifications of gravity on the largest observable scales, focusing on relativistic effects in galaxy number counts and the cross-correlation between the matter large scale structure (LSS) distribution and the cosmic microwave background (CMB). Our analysis applies to a very broad class of general scalar-tensor theories encoded in the Horndeski Lagrangian and is fully consistent on linear scales, retaining the full dynamics of the scalar field and not assuming quasi-static evolution. As particular examples we consider self-accelerating Covariant Galileons, Brans-Dicke theory and parameterizations based on the effective field theory of dark energy, using the \hiclass\, code to address the impact of these models on relativistic corrections to LSS observables. We find that especially effects which involve integrals along the line of sight (lensing convergence, time delay and the integrated Sachs-Wolfe effect -- ISW) can be considerably modified, and even lead to $\mathcal{O}(1000\%)$ deviations from General Relativity in the case of the ISW effect for Galileon models, for which standard probes such as the growth function only vary by $\mathcal{O}(10\%)$. These effects become dominant when correlating galaxy number counts at different redshifts and can lead to $\sim 50\%$ deviations in the total signal that might be observable by future LSS surveys. Because of their integrated nature, these deep-redshift cross-correlations are sensitive to modifications of gravity even when probing eras much before dark energy domination. We further isolate the ISW effect using the cross-correlation between LSS and CMB temperature anisotropies and use current data to further constrain Horndeski models (abridged).

Pub.: 17 Jul '16, Pinned: 17 Aug '17