Session B10: Alternative Gravity Theories

Neutron star structure in alternative theories of gravity

We study the structure of neutron stars in a broad class of alternative theories of gravity. In particular, we focus on scalar-tensor theories and $f(R)$ theories of gravity. We construct static and slowly rotating stars numerically for a set of equations of state, including a polytropic model and more realistic equations of state motivated by nuclear physics. Observable quantities such as masses, radii, etc are calculated for a set of parameters of the theories. We also calculate the sensitivity of the mass of the models to variations in the asymptotic value of the scalar field at infinity. These quantities enter post-Newtonian equations of motion and gravitational waveforms of two body systems that are used for gravitational-wave parameter estimation, in order to test these theories against observations.   

Spinning Black Holes and Neutron Stars in Dynamical Chern-Simons Gravity

In the near future, gravitational waves emitted during black hole and neutron star binary coalescence will allow unprecedented tests in the dynamical, non-linear, strong-field regime. For such tests to be possible we must first construct the waveform observable in theories that deviate from General Relativity, which, in turn, require finding and understanding stationary and axisymmetric black hole and neutron star solutions in these theories. In this talk, I will describe these solutions in dynamical Chern-Simons gravity, the only quadratic curvature theory that intrinsically violates parity with a single axion-like field. I will show how, although the Schwarzschild metric is a solution in this theory, spinning black holes must deviate from the Kerr metric. Similarly, neutron star solutions must deviate from Hartle's solutions for slowly-rotating neutron stars. When accounting for second-order spin corrections in a slow-rotation approximation, these deviations induce a quadrupolar deformation much larger than those previously found in the gravitomagnetic sector to leading-order in the spin. Such solutions are essential to test dynamical Chern-Simons gravity with binary pulsars and with gravitational waves, which could lead to much stronger constraints than Solar system ones.   

Probing Dynamical Chern-Simons Gravity with Gravitational Waves from Black Hole Binaries

The study of how well gravitational waves can test General Relativity in the strong, non-linear and dynamical regime is essential. As an example, we focus on tests of dynamical Chern-Simons gravity, which is the only parity-violating, quadratic-curvature theory with a single dynamical axion, and is well-motivated by several fundamental theories, such as heterotic superstring theory. With the new, quadratic-in-spin black hole solutions we recently found, we construct a self-consistent gravitational waveform from black hole binaries in the inspiral phase. We find that both dissipative and conservative corrections enter at 2nd post-Newtonian order in the waveform and couple to spin. The network of second-generation ground-based interferometers will be able to place constraints that will be 6 orders of magnitude stronger than current Solar System ones. We find that gravitational wave observations might be the only ways to constrain this theory to such accuracy.   

Eccentric binary effects in dynamical Chern-Simons gravity

One of the most promising natural laboratories for testing corrections to general relativity is an eccentric pulsar binary such as J0737-3039. The correction on which we focus is dynamical Chern-Simons (DCS) gravity, a theory containing a parity-odd scalar, motivated by fundamental physics. Because of parity violation, DCS exhibits corrections distinct from well-studied even-parity theories (Brans-Dicke). We compute the leading conservative and dissipative corrections to the orbit, most importantly the rate of pericenter advance, change in inclination, and the ascending node, and less prominent effects such as the correction to the orbital decay and the precession of the bodies' spins. Given how (non-)relativistic the presently-known systems are, we comment on the difficulty of constraining DCS with presently available data.   

Detection of a Boson Star through Gravitational Lensing

Observations of the Sgr A* region in the galactic center have implied a large amount of matter in a small volume, leading to the assumption of a black hole there. However, dynamical observations cannot rule out the presence of a boson star, a compact object made up of scalar particles, as both objects are far more compact than current observational resolutions. While a boson star in the galactic center is disfavored for a number of theoretical considerations, we outline the first test that can directly observe a boson star. We accomplish this by studying the strong gravitational lensing of S stars resulting from the assumption of a boson star in the Galactic Center. Boson stars have an extended mass distribution and are transparent to electromagnetic radiation, giving rise to a radial caustic curve. We calculate the brightness of images formed by stars crossing these radial caustics and show that a boson star would give rise to much brighter images than a black hole with a similar mass and that those images would be easily bright enough to be detected with upcoming instruments.   

Spherical Shell Cosmological Model and Uniformity of CMB

The paradigm of~$\Lambda $CDM cosmology works impressively well and with concept of inflation it explains universe after the time of decoupling. However there are still a few concerns, namely after all efforts there is no detection of dark matter and there are significant problems in theoretical description of dark energy. We will consider a variant of cosmological spherical shell model, within FRW formalism and will compare it with the standard $\Lambda $CDM model. We will show that our new topological model satisfies cosmological principles and is consistent with observed data, the SNe Ia and CMB, but that it may require new interpretation for some data. Considered will be constrains imposed on the model by the data, as for instance the range for the size and allowed thickness of the shell. Dynamics of the shell model will be discussed and its impact on the interpretation of the comoving radius of the visible universe and interpretation of the CMB data. One prediction of this model is interpretation of the uniformity of the CMB without inflation scenario.   

FLRW Cosmology from Yang-Mills Gravity

We extend to basic cosmology the subject of Yang-Mills gravity - a theory of gravity based on local translational gauge invariance in flat spacetime. It has been shown that this particular gauge invariance leads to tensor factors in the macroscopic limit of the equations of motion of particles which plays the same role as the metric tensor of General Relativity. The assumption that this ``effective metric" tensor takes on the standard FLRW form is our starting point. Equations analogous to the Friedman equations are derived and then solved in closed form for the three special cases of a universe dominated by 1) matter, 2) radiation, and 3) dark energy. We find that the solutions for the scale factor are similar to, but distinct from, those found in the corresponding GR based treatment.   

Conformal Gravity Rotation Curves in Tidal Dwarf Galaxies

We extend the application of the conformal gravity theory to tidal dwarf galaxies (TDGs). These dwarf galaxies are formed in the tidal tails of collisions of disk galaxies, and are thought to be predominantly composed of material expelled from the galactic disk of a parent galaxy. With any dark matter present in the parent galaxies expected to predominantly be in spherical haloes, tidal galaxies should thus have a very low dark matter content, and thus should not themselves be expected to possess the substantial spherical dark matter haloes that are ordinarily required to accompany and stabilize disk galaxies in standard gravity. In consequence, in the standard dark matter picture TDG rotation curves should not be expected to display any substantial mass discrepancies. Tidal dwarf galaxies thus provide a quite unusual laboratory for exploring the missing mass problem. Rotation curve data have become available for three TDGs associated with the parent galaxy NGC 5291, and it has been shown that there are in fact mass discrepancies, and that a good accounting of the data can be provided by MOND. Here we show that conformal gravity can also provide a good accounting of the data.