**Testing the no-hair theorem with multiband black-hole binaries**

# Parallel Session: GRAVITY - TESTS OF GRAVITY (A)

## Location: Park 3.23

#### A1 - Monday 14:00-15:40 (Leonardo Gualtieri)

**14:00-14:25**

**Davide Gerosa (University of Birmingham)**

Merging black holes observed by ground-based gravitational-wave detectors LIGO and Virgo allow for new tests of Einstein’s General Relativity in its strong-field regime. In particular, the detection of multiple ringdown modes enables us to perform black-hole spectroscopy: experimental checks that astrophysical black holes indeed have no hair and are faithfully described by the Kerr metric. The early inspiral of some LIGO’s black-hole binaries will be detectable at low frequencies by the upcoming space mission LISA. LISA will predict, with years of forewarning, at what time and at what frequency binaries will be seen merging by LIGO. We will, therefore, find ourselves in the extraordinary position of knowing that a source is about to merge, with the unprecedented opportunity to optimize ground-based operations to increase their scientific payoff. We show how narrowband tunings can be used to boost the detectors’ sensitivity at frequencies corresponding to the first subdominant ringdown mode, thus vastly improving our prospects to perform black-hole spectroscopy. We define a new consistency parameter between the different modes and show that, in this metric, narrowband tunings have the potential to double the effectiveness of black-hole spectroscopy when compared to standard broadband setups. More on arXiv:1807.00075.

**Testing a disformal coupling between standard model matter and dark energy **

**14:25-14:50**

**Michaela Lawrence (University of Sussex)**

Current constraints on disformal couplings between standard model matter and dark energy are very week and difficult to obtain. Disformal couplings have, in the past, been best constrained though collider experiments and not cosmological probes because they are not sensitive to the fifth force tests used to constrain conformal couplings. We show that diformaly coupled photons propagating through the universe acquire a small effective mass. This effect can create small dispersions in arrival time between gravitational waves and gamma rays coming from a neutron star – neutron star binary mergers and dispersion between different wavelengths of light coming from supernovae. We use these methods to place the most complete constraints on Galileon disformal couplings to date.

**Dynamic Signatures of Black Hole Binaries with Superradiant Clouds**

**14:50-15:15**

**Huan Yang (Physics, Guelph University & Perimeter Institute)**

Superradiant cloud may develop around a rotating black hole. In this talk, we will discuss the effects of the cloud on the orbits of nearby compact objects. In particular, we consider the dynamical friction and the backreaction due to level mixing. Under these interactions, the probability of a black hole dynamically capturing other compact objects, such as stellar mass black holes and neutron stars, is generally enhanced with the presence of cloud. For extreme mass ratio inspirals and binary stellar mass binary black holes, the cloud-induced orbital modulation may be detected by observing the gravitational waveform using space-borne gravitational wave detectors, such as LISA. Interestingly within a certain range of boson Compton wavelength, the enhanced capture rate of stellar mass black holes could accelerate hierarchical mergers, with higher-generation merger product being more massive than the mass threshold predicted by supernova pair instability. These observational signatures provide promising ways of searching light bosons with gravitational waves.

**Flux-balance laws for Poincaré and BMS charges**

**15:15-15:40**

**Roberto Oliveri (CEICO – Institute of Physics, Czech Academy of Sciences)**

Asymptotically flat spacetimes admit both supertranslations and Lorentz transformations as asymptotic symmetries. Furthermore, they admit super-Lorentz transformations, namely superrotations and superboosts, as outer symmetries associated with super-angular momentum and super-center-of-mass charges. In this talk, we present comprehensively the flux-balance laws for all such BMS charges. We distinguish the Poincar’e flux-balance laws from the proper BMS flux-balance laws associated with the three relevant memory effects. In addition, we provide the exact form of all Poincar’e and proper BMS flux-balance laws in terms of radiative symmetric tracefree multipoles. Fluxes of energy, angular momentum and octupole super-angular momentum arise at 2.5PN, fluxes of quadrupole supermomentum arise at 3PN and fluxes of momentum, center-of-mass and octupole super-center-of-mass arise at 3.5PN. If time permits, we argue how each BMS flux-balance law can be thought of as a constraint on the source evolution.

[The talk is based on arXiv:1912.03164 [gr-qc] with G. Compère and A. Seraj]

#### A2 - Monday 16:10-17:50 (Leonardo Gualtieri)

**How can we solve the Hubble tension in the framework of Horndeski theory?**

**16:10-16:35**

**Shun Arai (Nagoya University)**

As an alternative to the conventional cosmological constant, modified gravity is a way of explanation for the cosmic acceleration observed at late-time. In the meantime, the discrepancy of the Hubble constant independently measured by the local distance ladders and the standard rulers at cosmological scale, has arisen with the inconsistent value of the Hubble constant in the flat Lambda CDM model. We investigate the Horndeski theory as a candidate that presumably solve the tension of the Hubble constant with the additional physical degree of freedoms. In the request of the local measurement of gravitational constant that passes the solar system or pulsar experiments, we find a parameter space that circumvents the Hubble tension while being still viable among the other observations. We discuss how to constrain the parameter space we found with upcoming observations.

**Chameleons in Streams: The Milky Way’s Stellar Streams as Tests of Screened MG Theories **

**16:35-17:00**

**Aneesh Naik (Institute of Astronomy, University of Cambridge)**

In recent years, a number of works have tested screened modified gravity theories on galactic scales. In this talk, I will discuss the possibility of using tidal streams from the Milky Way’s satellite galaxies as tests of chameleon gravity theories. The self-screening of stars in the satellites is an effective EP-violation which leads to an asymmetry between the leading and trailing streams. I will show results from simulations which demonstrate that this asymmetry is observable even when the satellite and Milky Way are both partially screened. I will then discuss the prospect of getting constraints from present and future observational data.

**Cosmological test of gravity using weak lensing voids**

**17:00-17:25**

**Chris Davies (Institute for computational cosmology, Durham)**

Modifications to General Relativity (GR) often incorporate screening mechanisms in order to remain compatible with existing tests of gravity. The screening is less efficient in underdense regions, which suggests that cosmic voids can be a useful cosmological probe for constraining modified gravity models. In particular, weak lensing by voids has been proposed as a promising test of such theories. Usually, voids are identified from galaxy distributions, making them biased tracers of the underlying matter field. An alternative approach is to study voids identified in weak lensing maps — weak lensing voids — which have been shown to better correspond to true underdense regions. In this paper, we study the ability of weak lensing voids to detect the signatures of modified gravity. Focusing on the void abundance and weak lensing profiles, we find that both statistics are sensitive probes of gravity. These are quantified in terms of the signal-to-noise ratios (SNR) with which an LSST-like survey will be able to distinguish between different gravity models. We find that the tangential shear profiles of weak lensing voids are considerably better than galaxy voids at this. The abundances of voids have SNR=40 for a popular class of modified gravity in an LSST-like survey.

**Reconstructing physical models from cosmological observables**

**17:25-17:50**

**Joe Kennedy (Theoretical Physics, University of Geneva)**

In order to use cosmological data-sets to constrain fundamental physics it is imperative to develop methods to connect generalised parameterizations of the theory space to underlying physical theories. In this talk I will present just such an approach which allows one to reconstruct Horndeski models, a class of theories which couple the metric to a scalar field, from general descriptions of deviations from General Relativity such as a modified Poisson equation. Constraints on nonlinear observables in particular will prove to be crucial in discriminating between different models. I will therefore discuss how the reconstruction can be used to connect nonlinear parameters to underlying theories before mentioning a novel class of theories which exhibit kinetic self-acceleration and can only be distinguished from concordance cosmology in the nonlinear regime.

#### A3 - Tuesday 14:00-15:40 (Leonardo Gualtieri)

**Testing the equivalence principle on cosmic scales**

**14:00-14:25**

**Pierre Fleury (Instituto de Física Teórica/Universidad Autónoma de Madrid)**

The equivalence principle is the main pillar of the general theory of relativity. Albeit exquisitely well constrained on Earth and in the Solar System, its validity remains to be proved on cosmic scales, especially when the unknown dark matter is concerned. In this talk, I will show that relativistic effects in galaxy surveys offer the possibility to directly test the equivalence principle. Order ten-percent constraints can be put with future extremely large surveys like the Square Kilometer Array.

**Current Result of GR Test at Galactic Central Massive Black Hole, using Subaru Telescope**

**14:25-14:50**

**Hiromi Saida (Daido University)**

A star, called S0-2 (American group nomenclature) or S2 (European group nomenclature), are orbiting around Galactic central massive black hole candidate Sgr A*, and passed the pericenter (the closest point to Sgr A* on its orbit) in May 2018. The gravity that S0-2 experiences at the pericenter is two orders of magnitude stronger than the gravity ever observed by photons (e.g. Hulse-Taylor pulsar). During the pericenter passage of S0-2, a deviation from the Newtonian prediction was observed in the motion of S0-2. The time evolution of the redshift of photons coming from S0-2, near the pericenter, shows the general relativistic (GR) evidence which is detectable by the large infra-red telescopes (e.g. VLT, Keck, Gemini and Subaru). The detectable GR evidence in the redshift is the 1st post-Newtonian (1PN) order, and the higher order PN evidences are lower than the present observational uncertainties.

We, Japanese group, have been monitoring S0-2 using the Subaru telescope, and performed our own analysis for GR test at 1PN order. European and American groups have also been monitoring S0-2 using their accessible telescopes, and reported their results of GR test at 1PN order. Our theoretical method for GR test is different from the other groups’ method. Our method takes into account not only the relativistic redshift formula but also the difference of the time evolution of S0-2’s motion between the Newtonian gravity and GR, while the other groups’ method focuses on the relativistic redshift formula. If GR is favored by two different methods (our method and the other groups’ one), then GR can be favored more definitely than the case favored by only one method.

In this talk, the result of GR test by the method of Japanese group is reported.

** Testing the Milky Way rotational curve with Gaia DR2**

**14:50-15:15**

**Mariateresa Crosta (INAF- OATo)**

Gaia mission is offering the unique possibility of being a multi laboratory for extensively testing weak gravitational fields at local (Solar System) and more distant (MIlky Way) scales.

In particular, Gaia can probe the structure of our Galaxy as a product of the cosmological evolution shaped by gravity (Local Cosmology), namely the relations among baryonic structures (and their evolution) and the dark components of the Universe.

In this respect, we present the first attempt to apply the relativistic kinematics delivered by Gaia to trace the Milky Way rotation curves within a general relativistic scenario.

The study of the rotation curve profile of our Galaxy required the selection of the most suitable stellar tracers of the bulk circular velocity around the galactic center. We selected Gaia-only DR2 sources according to the requirements for a proper 6-dimensional reconstruction of the phase-space location occupied by each individual star.

We fitted to the observed data (independent from any model) two theoretical models, i.e. to both a “classical”, i.e. with a DM halo, rotation curve as well as to a relativistic one, derived from the tailored physical solution exploiting the hypothesis of a GR dust in a stationary and axisymmetric space-time.

The metric terms invoked in the relativistic model are the g_{0,i} terms. Unlike the g_{0,0} term, that tends to Newtonian in the weak field, those off-diagonal terms have no Newtonian counterparts, i.e., they are purely relativistic. This is the simple argument that has generated the relativistic velocity profile we successfully put to test on real data for the first time.

Our likelihood analysis in fact shows that both models appear equally consistent with the data and confirms, a posteriori, the hypothesis of validity of a relativistic model. As for the local matter density, derived from the time-time component of the Einstein Equation for a pressure-less fluid in a stationary and axisymmetric metric, we obtain the value 0.093 solar masses per cubic parsec that is in line with current estimates of the total baryonic matter density at the Sun.

What we obtain with the DM model is also really in line with very recent results fitting different data for different samples. Of course we put emphasis on the GR-only model as it has received much less attention over the years, in the hope that it will catch the interest and the attention of the relevant communities.

In brief, our findings tell that, the gravitomagnetic term of the axisymmetric and stationary Einstein field solution (the Ansatz) could mimic a “DM” effect for the observed flatness of the Galactic rotational curve, no matter other than baryonic needs to be present.

**Hearing the strength of gravity (with the Sun)**

**15:15-15:40**

**Ippocratis Saltas (CEICO – Czech Academy of Sciences)**

Generic extensions of General Relativity aiming to explain dark energy typically introduce fifth forces of gravitational origin. In this talk, I will explain how helioseismic observations can provide a powerful and novel tool towards precision constraints of fifth forces, as predicted by general theories for dark energy, and I will discuss the implications for cosmology.

#### A4 - Tuesday 16:10-17:50 (Leonardo Gualtieri)

**Casimir force tests of symmetron modified gravity**

**16:10-16:35**

**Benjamin Elder (Physics, University of Nottingham)**

Many theories of dark energy hypothesize the presence of an additional scalar field that modifies gravity by mediating a fifth force between matter particles. Such forces may be dynamically suppressed in certain environments, i.e. screened, which allows the theory to evade traditional tests of gravity. This situation has spurred the development of new tests of gravity that are designed to be sensitive to screened fifth forces. Casimir force tests are one such example. Traditionally described as a two-plate system, Casimir tests are in practice often constructed of a sphere and a plate to avoid alignment issues. Taking the symmetron as a prototypical model of screening, I have computed the symmetron force between the sphere and plate, allowing the symmetron’s fundamental parameters to be constrained. This calculation involves the use of a variety of techniques, both analytic and numerical, to study the symmetron’s non-linear equation of motion.

**The Wheeler-DeWitt Equation and the Cosmological Constant**

**16:35-17:00**

**Remo Garattini (Engineering and Aplllied Science, Università di Bergamo)**

The Wheeler-DeWitt Equation represents a tool to study Quantum Gravity and Quantum Cosmology. Its solution in a very general context is, of course, impossible. To this purpose we consider some distortions of General Relativity like Gravity’s Rainbow, Varying Speed of Light Cosmology, Generalized Uncertainty Principle deformations and Hořava-Lifshitz gravity which could allow the calculation of some observables like the cosmological constant.