A powerful technique to calculate gravitational radiation from binary systems involves a perturbative expansion: if the masses of the two bodies are very different, the small body is treated as a point particle of mass $m_p$ moving in the gravitational field generated by the large mass $M$, and one keeps only linear terms in the small mass ratio $m_p/M$. This technique usually yields finite answers, which are often in good agreement with fully nonlinear numerical relativity results, even when extrapolated to nearly comparable mass ratios. Here we study two situations in which the point-particle approximation yields a divergent result: the instantaneous flux emitted by a small body as it orbits the light ring of a black hole, and the total energy absorbed by the horizon when a small body plunges into a black hole. By integrating the Teukolsky (or Zerilli/Regge-Wheeler) equations in the frequency and time domains we show that both of these quantities diverge. We find that these divergences are an artifact of the point-particle idealization, and are able to interpret and regularize this behavior by introducing a finite size for the point particle. These divergences do not play a role in black-hole imaging, e.g. by the Event Horizon Telescope.
Horava gravity breaks Lorentz symmetry by introducing a dynamical timelike scalar field (the khronon), which can be used as a preferred time coordinate (thus selecting a preferred space-time foliation). Adopting the khronon as the time coordinate, the theory is invariant only under time reparametrizations and spatial diffeomorphisms. In the infrared limit, this theory is sometimes referred to as khronometric theory. Here, we explicitly construct a generalization of khronometric theory, which avoids the propagation of Ostrogradski modes as a result of a suitable degeneracy condition (although stability of the latter under radiative corrections remains an open question). While this new theory does not have a general-relativistic limit and does not yield a Friedmann-Robertson-Walker-like cosmology on large scales, it still passes, for suitable choices of its coupling constants, local tests on Earth and in the solar system, as well as gravitational-wave tests. We also comment on the possible usefulness of this theory as a toy model of quantum gravity, as it could be completed in the ultraviolet into a degenerate Horava gravity theory that could be perturbatively renormalizable without imposing any projectability condition.
We consider the observation of stellar-mass black holes binaries with the Laser Interferometer Space Antenna (LISA). Preliminary results based on Fisher information matrix analyses have suggested that gravitational waves from those sources could be very sensitive to possible deviations from the theory of general relativity and from the strong equivalence principle during the low-frequency binary inspiral. We perform a full Markov Chain Monte Carlo Bayesian analysis to quantify the sensitivity of these signals to two phenomenological modifications of general relativity, namely a putative gravitational dipole emission and a non-zero mass for the graviton, properly accounting for the detectors response. Moreover, we consider a scenario where those sources could be observed also with Earth-based detectors, which should measure the coalescence time with precision better than $1 {rm ms}$. This constraint on the coalescence time further improves the bounds that we can set on those phenomenological deviations from general relativity. We show that tests of dipole radiation and the gravitons mass should improve respectively by seven and half an order(s) of magnitude over current bounds. Finally, we discuss under which conditions one may claim the detection of a modification to general relativity.
In this paper, which is of programmatic rather than quantitative nature, we aim to further delineate and sharpen the future potential of the LISA mission in the area of fundamental physics. Given the very broad range of topics that might be relevant to LISA, we present here a sample of what we view as particularly promising directions, based in part on the current research interests of the LISA scientific community in the area of fundamental physics. We organize these directions through a science-first approach that allows us to classify how LISA data can inform theoretical physics in a variety of areas. For each of these theoretical physics classes, we identify the sources that are currently expected to provide the principal contribution to our knowledge, and the areas that need further development. The classification presented here should not be thought of as cast in stone, but rather as a fluid framework that is amenable to change with the flow of new insights in theoretical physics.
Hov{r}ava gravity breaks boost invariance in the gravitational sector by introducing a preferred time foliation. The dynamics of this preferred slicing is governed, in the low-energy limit suitable for most astrophysical applications, by three dimensionless parameters $alpha$, $beta$ and $lambda$. The first two of these parameters are tightly bound by solar system and gravitational wave propagation experiments, but $lambda$ remains relatively unconstrained ($0leqlambdalesssim 0.01-0.1$). We restrict here to the parameter space region defined by $alpha=beta=0$ (with $lambda$ kept generic), which in a previous paper we showed to be the only one where black hole solutions are non-pathological at the universal horizon, and we focus on possible violations of the strong equivalence principle in systems involving neutron stars. We compute neutron star sensitivities, which parametrize violations of the strong equivalence principle at the leading post-Newtonian order, and find that they vanish identically, like in the black hole case, for $alpha=beta=0$ and generic $lambda eq0$. This implies that no violations of the strong equivalence principle (neither in the conservative sector nor in gravitational wave fluxes) can occur at the leading post-Newtonian order in binaries of compact objects, and that data from binary pulsars and gravitational interferometers are unlikely to further constrain $lambda$.
Low-frequency gravitational-wave astronomy can perform precision tests of general relativity and probe fundamental physics in a regime previously inaccessible. A space-based detector will be a formidable tool to explore gravitys role in the cosmos, potentially telling us if and where Einsteins theory fails and providing clues about some of the greatest mysteries in physics and astronomy, such as dark matter and the origin of the Universe.
The recent detections of gravitational waves from binary systems of black holes are in remarkable agreement with the predictions of General Relativity. In this pedagogical mini-review, I will go through the physics of the different phases of the evolution of black hole binary systems, providing a qualitative physical interpretation of each one of them. I will also briefly describe how these phases would be modified if gravitation were described by a theory extending or deforming General Relativity, or if the binary components turned out to be more exotic compact objects than black holes.
The upcoming LISA mission offers the unique opportunity to study the Milky Way through gravitational wave radiation from Galactic binaries. Among the variety of Galactic gravitational wave sources, LISA is expected to individually resolve signals from $sim 10^5$ ultra-compact double white dwarf (DWD) binaries. DWDs detected by LISA will be distributed across the Galaxy, including regions that are hardly accessible to electromagnetic observations such as the inner part of the Galactic disc, the bulge and beyond. We quantitatively show that the large number of DWD detections will allow us to use these systems as tracers of the Milky Way potential. We demonstrate that density profiles of DWDs detected by LISA may provide constraints on the scale length parameters of the baryonic components that are both accurate and precise, with statistical errors of a few percent to $10$ percent level. Furthermore, the LISA sample is found to be sufficient to disentangle between different (commonly used) disc profiles, by well covering the disc out to sufficiently large radii. Finally, up to $sim 80$ DWDs can be detected through both electromagnetic and gravitational wave radiation. This enables multi-messenger astronomy with DWD binaries and allows one to extract their physical properties using both probes. We show that fitting the Galactic rotation curve constructed using distances inferred from gravitational waves {it and} proper motions from optical observations yield a unique and competitive estimate of the bulge mass. Instead robust results for the stellar disc mass are contingent upon knowledge of the Dark Matter content.
Gravitational-wave astronomy has the potential to explore one of the deepest and most puzzling aspects of Einsteins theory: the existence of black holes. A plethora of ultracompact, horizonless objects have been proposed to arise in models inspired by quantum gravity. These objects may solve Hawkings information-loss paradox and the singularity problem associated with black holes, while mimicking almost all of their classical properties. They are, however, generically unstable on relatively short timescales. Here, we show that this ergoregion instability leads to a strong stochastic background of gravitational waves, at a level detectable by current and future gravitational-wave detectors. The absence of such background in the first observation run of Advanced LIGO already imposes the most stringent limits to date on black-hole alternatives, showing that certain models of quantum-dressed stellar black holes can be at most a small percentage of the total population. The future LISA mission will allow for similar constraints on supermassive black-hole mimickers.
Ultralight bosons can induce superradiant instabilities in spinning black holes, tapping their rotational energy to trigger the growth of a bosonic condensate. Possible observational imprints of these boson clouds include (i) direct detection of the nearly monochromatic (resolvable or stochastic) gravitational waves emitted by the condensate, and (ii) statistically significant evidence for the formation of holes at large spins in the spin versus mass plane (sometimes also referred to as Regge plane) of astrophysical black holes. In this work, we focus on the prospects of LISA and LIGO detecting or constraining scalars with mass in the range $m_sin [10^{-19},,10^{-15}]$ eV and $m_sin [10^{-14},,10^{-11}]$ eV, respectively. Using astrophysical models of black-hole populations calibrated to observations and black-hole perturbation theory calculations of the gravitational emission, we find that, in optimistic scenarios, LIGO could observe a stochastic background of gravitational radiation in the range $m_sin [2times 10^{-13}, 10^{-12}]$ eV, and up to $10^4$ resolvable events in a $4$-year search if $m_ssim 3times 10^{-13},{rm eV}$. LISA could observe a stochastic background for boson masses in the range $m_sin [5times 10^{-19}, 5times 10^{-16}]$, and up to $sim 10^3$ resolvable events in a $4$-year search if $m_ssim 10^{-17},{rm eV}$. LISA could further measure spins for black-hole binaries with component masses in the range $[10^3, 10^7]~M_odot$, which is not probed by traditional spin-measurement techniques. A statistical analysis of the spin distribution of these binaries could either rule out scalar fields in the mass range $sim [4 times 10^{-18}, 10^{-14}]$ eV, or measure $m_s$ with ten percent accuracy if light scalars in the mass range $sim [10^{-17}, 10^{-13}]$ eV exist.
