Bulletin of the American Physical Society
APS April Meeting 2021
Volume 66, Number 5
Saturday–Tuesday, April 17–20, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B17: Modified Theories of GravityLive

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Sponsoring Units: DGRAV Chair: Felipe LlanesEstrada, U. Complutense de Madrid 
Saturday, April 17, 2021 10:45AM  10:57AM Live 
B17.00001: Scalarized Neutron Stars in Scalartensor Theories: Analytic Charges and Universal Relations Kent Yagi, Michael Stepniczka Neutron stars in scalartensor theories may undergo spontaneous scalarization, which is important for probing the theories with binary pulsar and gravitational wave observations. Since the effect is nonlinear, most studies of spontaneous scalarization were carried out numerically. In this talk, I explain how one can understand and compute the effect of scalarization analytically based on a perturbative analysis and analytic modeling of neutron stars through the Tolman VII solution. I show that the analytic calculations match accurately with numerical ones. We also discovered a universal relation between the scalar charge and the stellar binding energy that is insensitive to the underlying equation of state. These findings improve our understanding of spontaneous scalarization and provide us quick and readytouse expressions of scalar charges. [Preview Abstract] 
Saturday, April 17, 2021 10:57AM  11:09AM Live 
B17.00002: Relativistic Beaming of Gravity and the Missing Mass Problem Bradford C Blake My research is based on the principle that gravity emanating from a relativistically moving source should be beamed in the same manner as light. Further, that an array of compact objects in the galaxy resulting from supernovae, primarily stellar mass black holes undergoing relativistic rotation, should thereby concentrate their gravitational force into the stellar plane of the galactic disk. This would make possible speeds in excess of Keplerian limits for stars and gas clouds, especially in the outer portions of the Galaxy. I model the galactic disk as a cylinder, and use Gauss's law to derive a 1/r formula for the resulting gravitational force, which I call ``directed gravity.'' This 1/r force could explain the flat rotation curve of the Galaxy. [Preview Abstract] 
Saturday, April 17, 2021 11:09AM  11:21AM Live 
B17.00003: Principal Null Directions and Symmetries of Black holes in Quadratic Gravity Caroline Owen, Nicolas Yunes, Helvi Witek The gravitational waves emitted when a small compact object falls into a supermassive black hole depend sensitively on the orbital trajectory of the small object, which in turn depends on the properties of the spacetime generated by the supermassive black hole. The ability to test general relativity in this extreme gravity regime using gravitational wave observations from future spacebased detectors such as the Laser Interferometer Space Antenna motivates the mathematical study of the symmetries of black holes in modified theories of gravity. In this talk, I will focus on the quadratic gravity modified theories dynamical ChernSimons gravity and scalar GaussBonnet gravity. While the Kerr metric for a spinning black hole in general relativity possesses two distinct principal null directions, the still unknown analogous exact solutions in both modified theories are each thought to possess four. I will present all principal null directions and Weyl scalars of such modified black holes in the slow rotation approximation. Additionally, for each modified theory I will investigate the existence of a Killing tensor that would generate a fourth constant of the motion. [Preview Abstract] 
Saturday, April 17, 2021 11:21AM  11:33AM Live 
B17.00004: Constraints from Nuclear Physics on NonMinimally Coupled Gravity Sarah Fisher, Eric Carlson Nonminimally coupled gravity, where the gravitational action takes the form $\int d^4 x \sqrt{g} \left\{f_1(R) + f_2(R){\cal L}_m\right\}$, where $f_1$ and $f_2$ are arbitrary functions of the Ricci scalar $R$, has been considered as an alternative to dark matter and dark energy. We argue that the best constraints on many types of theories come not from cosmology or even astrophysics, but from systems with large gradients of the stressenergy tensor. Nuclei, particularly the ${}^4$He nucleus, can produce especially strong constraints. In the case $f_1(R)=\frac{R}{16\pi G}$ and $f_2(R) = 1 + \lambda R$ we find constraints on $\lambda$ that are thirty orders of magnitude stronger than astrophysical constraints. We argue that such nuclear constraints on modified gravity should be applied to a wide variety of nonminimally coupled gravity theories, as such constraints might invalidate the choice of parameters required to produce the desired cosmological effects. [Preview Abstract] 
Saturday, April 17, 2021 11:33AM  11:45AM Live 
B17.00005: A Modern Comparison of MOND and Conformal Gravity James O'Brien, Thomas Chiarelli, Will Kerin In recent years, there have been numerous papers in alternative gravitational theories attempting to give resolution to some problems in standard gravity. With more studies of the Radial Acceleration Rule (RAR) since 2017, and the extension of the Baryonic Tully Fisher (BTF) relation, comparison of theories can be more well established outside of pure comparison of rotation curves. In this presentation, we show how MOND and Conformal gravity accommodate some recent galactic surveys through the use of RAR and BTF and revisit some older surveys where alternative gravity has been challenged. [Preview Abstract] 
Saturday, April 17, 2021 11:45AM  11:57AM Live 
B17.00006: Testing Ho\v ravaLifshitz Gravity With ILoveQ Siddarth Ajith, Kent Yagi, Nicolas Yunes Relations between the neutron star moment of inertia, tidal Love number and quadrupole moment are known to be insensitive to the nuclear equation of state (the socalled ILoveQ relations). Such universal relations are powerful for testing general relativity and beyond in the strongfield regime with neutron star observations. Ho\v ravaLifshitz gravity is one such alternative theory of gravity which has interesting properties such as ultraviolet completion of gravity while also inducing a preferred time direction. This theory is characterized by three coupling constants; two of them have been constrained stringently from existing neutron star observations, such as GW170817, while the remaining parameter is only weakly constrained. We thus studied how the ILoveQ relations depend on this third parameter. We found that this sole parameter disappears from the field equations in Ho\v ravaLifshitz gravity. Therefore, the ILoveQ relations are universal against not only the nuclear physics uncertainty but also the gravitational physics uncertainty within Ho\v ravaLifshitz gravity. [Preview Abstract] 
Saturday, April 17, 2021 11:57AM  12:09PM Live 
B17.00007: nEoS: neutron star equations of state specifically tailored to modified gravity studies Eva LopeOter, Felipe J. LlanesEstrada Full testing of General Relativity and its extensions requires a dense medium as found only in neutron stars and their mergers, where the field equations have a significant matter source. The Equation of State of neutron star matter for these studies needs to avoid inducing contradictory reasoning, as can happen because many if not all modern EoS sets have been constrained to some extent by astrophysical observables (whose calculation assumes standard GR) or by model assumptions on the nuclear side. Our nEoS sets, available at http://teorica.fis.ucm.es/nEoS/, provide an alternative where only first principles of hadron physics have been used (Chiral Perturbation Theory at low density, perturbative Quantum Chromodynamics at asymptotically high density, and in the intermediate nonperturbative region, only causality and monotony). Thus, they are ideally suited to test gravity with neutron star matter without logical pitfalls. They span a band in the $P(\rho)$ plane that incorporates all solid knowledge from hadron physics to date. We also newly discuss finitetemperature modifications of the nEoS sets and a bound on the maximum latent heat that, unlike the Seidov limit of GR, follows from hadron physics alone, thus being valid in modified theories of gravity. [Preview Abstract] 
Saturday, April 17, 2021 12:09PM  12:21PM Live 
B17.00008: Energy carried by gravitational waves in bimetric gravity Leo Stein, Alexander Saffer The shrinking of a compact binary's orbit is controlled by its energy flux. The gravitationalwave energy functional differs between theories of gravity, so must be rederived when investigating a new theory. We investigate the energy carried by gravitational waves in the HassanRosen bimetric gravity theory (bigravity), in two different ways. First, we start by following the approach of Isaacson, performing second order perturbation theory, to find effective stressenergy tensors for gravitational waves. This approach suffers a drawback in that it does not clearly identify the energy. Second, we pursue a Noether approach, following Wald and Zoupas. Along the way we discuss the multiple physical length scales in the problem, and the limitations of the two approaches. [Preview Abstract] 
Saturday, April 17, 2021 12:21PM  12:33PM On Demand 
B17.00009: Newtonian FractionalDimension Gravity and Galactic Dynamics Gabriele Varieschi I will present an alternative model of gravity based on the theory of fractionaldimension spaces applied to Newton's law of gravitation. In this Newtonian FractionalDimension Gravity (NFDG), Gauss's law as well as other fundamental classical gravitational laws are extended to a Ddimensional metric space, where D is a fractional (i.e., noninteger) dimension. NFDG can also be related to Modified Newtonian Dynamics (MOND) and used to explain the correlation (RAR) between the radial acceleration traced by galactic rotation curves and the baryonic acceleration obtained from the galactic mass distribution, without any additional dark matter contribution. In particular, NFDG can be applied to the galactic dynamics of sphericallysymmetric and axiallysymmetric structures and three examples of rotationally supported galaxies (NGC7814, NGC6503, NGC3741) will also be presented. [Preview Abstract] 
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