On Bouncing and Nonsingular Solutions in Einstein-scalar-Gauss-Bonnet Cosmology

It is generically believed that higher-order curvature corrections to the Einstein-Hilbert action might cure the curvature singularities that plague general relativity. Here we consider Einstein-scalar-Gauss-Bonnet gravity, the only four-dimensional, ghost-free theory with quadratic curvature terms. For any choice of the coupling function and of the scalar potential, we show that the theory does not allow for bouncing solutions in the flat and open Friedmann universe. For the case of a closed universe, using a reverse-engineering method, we explicitly provide a bouncing solution which is nevertheless linearly unstable in the scalar gravitational sector. Moreover, we show that the expanding, singularity-free, early-time cosmologies allowed in the theory are unstable. These results rely only on analyticity and finiteness of cosmological variables at early times.Comment: 9 pages, 4 figures; references and discussion added; further minor revision, accepted to PR

Frontiers of Gravity: Astrophysical Environments, Ringdown Nonlinearities and the Semiclassical Approximation

Einstein’s general relativity is based on a tensorial, nonlinear equation for the spacetime metric. The gravitational interaction is however so weak that, in most circumstances, the equations can be solved perturbatively. This is true in early-time cosmology, for the inspiral of binary systems, and even after black holes merge, releasing the equivalent of multiple solar masses in gravitational waves. In this thesis, we analyze a range of problems that can be addressed assuming a background gravitational field and small fluctuations over it. We will progress from problems where the perturbative hypothesis can be tested and holds, to ones that begin to show nonlinear effects, ending with an application of perturbation theory to quantum gravity, where it is only a working hypothesis. We first analyze the dynamics of black hole binaries immersed in a dense gas environment or interacting with a stellar companion. For binaries in a dense environment, we study the effect of accretion and dynamical friction on the gravitational wave emission. We derive the modification of the gravitational wave phase in the assumption of small accretion rates, and assess whether future gravitational wave observatories could detect this effect. For black holes in a binary with a white dwarf, we identify new evolutionary relations and propose a method to infer the black hole and white dwarf masses and their luminosity distance from the gravitational wave signal alone. Next, we study how isolated black holes react to perturbations, in the simplified setting of spherical symmetry and negative cosmological constant. We show that modes belonging to the linear spectrum can be excited nonlinearly. We further find that nonlinear effects can change the black hole mass at percent level, and that this effect can be explained by the flux of characteristic excitations through the black hole horizon. Finally, we propose a new definition of the semiclassical Einstein equations for cosmological spacetimes. We propose that the source on the right hand side of the Einstein equations could be the amount of stress-energy above the instantaneous ground state. In this more speculative application, the linear order semiclassical approximation is not guaranteed to hold. If our hypothesis were confirmed, however, the vacuum stress-energy above the instantaneous ground state would not renormalize the cosmological constant, hinting at a resolution of the longstanding problem connected to its observed value

Plasma-photon interaction in curved spacetime I: formalism and quasibound states around nonspinning black holes

We investigate the linear dynamics of an electromagnetic field propagating in curved spacetime in the presence of plasma. The dynamical equations are generically more involved and richer than the effective Proca equation adopted as a model in previous work. We discuss the general equations and focus on the case of a cold plasma in the background of a spherically-symmetric black hole, showing that the system admits plasma-driven, quasibound electromagnetic states that are prone to become superradiantly unstable when the black hole rotates. The quasibound states are different from those of the Proca equation and have some similarities with the case of a massive scalar field, suggesting that the linear instability can be strongly suppressed compared to previous estimates. Our framework provides the first step towards a full understanding of the plasma-photon interactions around astrophysical black holes.Comment: 11 pages, 4 figure

Conserved currents for Kerr and orthogonality of quasinormal modes

We introduce a bilinear form for Weyl scalar perturbations of Kerr. The form is symmetric and conserved, and we show that, when combined with a suitable renormalization prescription involving complex r integration contours, quasinormal modes are orthogonal in the bilinear form for different (l, m, n). These properties are apparently not evident consequences of standard properties for the radial and angular solutions to the decoupled Teukolsky relations and rely on the Petrov type D character of Kerr and its t-$\phi$ reflection isometry. We show that quasinormal mode excitation coefficients are given precisely by the projection with respect to our bilinear form. These properties can make our bilinear form useful to set up a framework for nonlinear quasinormal mode coupling in Kerr. We also provide a general discussion on conserved local currents and their associated local symmetry operators for metric and Weyl perturbations, identifying a collection containing an increasing number of derivatives.Comment: 14+11 pages, 2 figures. minor changes to match version accepted to PR

Constraints on millicharged dark matter and axion-like particles from timing of radio waves

We derive novel constraints on millicharged dark matter and ultralight axion-like particles using pulsar timing and fast radio burst observations. Millicharged dark matter affects the dispersion measure of the time of arrival of radio pulses in a way analogous to free electrons. Light pseudo-scalar dark matter, on the other hand, causes the polarization angle of radio signals to oscillate. We show that current and future data can set strong constraints in both cases. For dark matter particles of charge $\epsilon e$, these constraints are ${\epsilon}/{m_{\rm milli}} \lesssim 10^{-8}{\rm eV}^{-1}$, for masses $m_{\rm milli}\gtrsim 10^{-6}\,$eV. For axion-like particles, the analysis of signals from pulsars yields constraints in the axial coupling of the order of $g/m_a\lesssim 10^{-13} {\rm GeV}^{-1}/(10^{-22}{\rm eV})$. Both bounds scale as $(\rho/\rho_{\rm dm})^{1/2}$ if the energy density $\rho$ of the components is a fraction of the total dark matter energy density $\rho_{\rm dm}$. We do a detailed study of both effects using data from two samples of pulsars in the galaxy and in globular clusters, as well as data from FRB 121102 and PSR J0437$-$4715. We show that in both cases actual pulsar data constrain a new region of the parameter space for these models, and will improve with future pulsar-timing observations.Comment: 6 pages, 2 figures; v3: to appear on PR

Probing Accretion Physics with Gravitational Waves

Gravitational-wave observations of extreme mass ratio inspirals (EMRIs) offer the opportunity to probe the environments of active galactic nuclei (AGN) through the torques that accretion disks induce on the binary. Within a Bayesian framework, we study how well such environmental effects can be measured using gravitational wave observations from the Laser Interferometer Space Antenna (LISA). We focus on the torque induced by planetary-type migration on quasicircular inspirals, and use different prescriptions for geometrically thin and radiatively efficient disks. We find that LISA could detect migration for a wide range of disk viscosities and accretion rates, for both $\alpha$ and $\beta$ disk prescriptions. For a typical EMRI with masses $50M_\odot+10^6M_\odot$, we find that LISA could distinguish between migration in $\alpha$ and $\beta$ disks and measure the torque amplitude with $\sim 20\%$ relative precision. Provided an accurate torque model, we also show how to turn gravitational-wave measurements of the torque into constraints on the disk properties. Furthermore, we show that, if an electromagnetic counterpart is identified, the multimessenger observations of the AGN EMRI system will yield direct measurements of the disk viscosity. Finally, we investigate the impact of neglecting environmental effects in the analysis of the gravitational-wave signal, finding 3$\sigma$ biases in the primary mass and spin, and showing that ignoring such effects can lead to false detection of a deviation from general relativity. This work demonstrates the scientific potential of gravitational observations as probes of accretion-disk physics, accessible so far through electromagnetic observations only