Can environmental effects spoil precision gravitational-wave astrophysics?

Abstract

No, within a broad class of scenarios. Gravitational-wave (GW) astronomy will open a new window on compact objects such as neutron stars and black holes (BHs). It is often stated that large signal-to-noise detections of ringdown or inspiral waveforms can provide estimates of the masses and spins of compact objects to within fractions of a percent, as well as tests of General Relativity. These expectations usually neglect the realistic astrophysical environments in which compact objects live. With the advent of GW astronomy, environmental effects on the GW signal will eventually have to be quantified. Here we present a wide survey of the corrections due to these effects in two situations of great interest for GW astronomy: the BH ringdown emission and the inspiral of two compact objects (especially BH binaries). We mainly focus on future space-based detectors such as eLISA, but many of our results are also valid for ground-based detectors such as aLIGO, aVirgo and KAGRA. We take into account various effects such as: electric charges, magnetic fields, cosmological evolution, possible deviations from General Relativity, firewalls, and the effects related to various forms of matter such as accretion disks and dark matter halos.Our analysis predicts the existence of resonances dictated by the external mass distribution, which dominate the very late-time behavior of merger/ringdown waveforms. The mode structure can drastically differ from the vacuum case, yet the BH response to external perturbations is unchanged at the time scales relevant for detectors. This is because although the vacuum Schwarzschild resonances are no longer quasinormal modes of the system, they still dominate the response at intermediate times. Our results strongly suggest that both parametrized and ringdown searches should use at least two-mode templates.Our analysis of compact binaries shows that environmental effects are typically negligible for most eLISA sources, with the exception of very few special extreme mass ratio inspirals. We show in particular that accretion and hydrodynamic drag generically dominate over self-force effects for geometrically thin disks, whereas they can be safely neglected for geometrically thick disk environments, which are the most relevant for eLISA. Finally, we discuss how our ignorance of the matter surrounding compact objects implies intrinsic limits on the ability to constrain strong-field deviations from General Relativity.

Hamiltonian formulation of the conservative self-force dynamics in the Kerr geometry

“…Alternatively, it is also instructive, using Eqs. (2.24) and (2.25), to write this gauge transformation in the form 12) with similar expressions for the three other angle variables. Now, we will establish that the oscillating part of ω α =ẇ α can be freely changed by the gauge transformation.…”

Black holes in anti–de Sitter spacetime: Quasinormal modes, tails, and flat spacetime

“…We highlight that these stages are not captured by a frequency-domain analysis [19,20]. What frequency-domain analysis does capture are the late-time dynamics, when the fluctations have had time to bounce off the boundary several times and eventually decay in the Kerr-AdS quasinormal modes.…”

“…Perhaps the unexpected feature is that they play no role whatsoever in the spectral content of the wave operator in AdS, or in other words, the QNMs of asymptotically flat BHs do not show up as poles of the retarded Green's function in AdS (see also Ref. [19,20] for a related phenomena in connection with BHs in astrophysical environments). As is well known, a frequency-domain analysis for the QNMs of small BHs in AdS returns extremely long-lived modes lying close to the purely real, AdS normal modes…”

“…Thus, one expects the structure of the perturbation spectrum to change completely and to even find linear and nonlinear instabilities in these systems [14][15][16][17][18]. A further interesting aspect of the different boundary conditions is that, for small AdS BHs (i.e., for BHs whose areal radius at the horizon is much smaller than the AdS scale) one expects imprints of asymptotically flat spacetimes in the early signal [19,20]. This result, intuitive from a time evolution picture, seems to contradict frequencydomain analysis: the mode structure of small AdS BHs is completely different from that of say, asymptotically flat Schwarzschild or Kerr BHs.…”

Electrodynamics and spacetime geometry: Astrophysical applications

“…Such couplings, in particular, with the gravitomagnetic term, might be useful to deepen the understanding of astrophysical jets in radio (active) galaxies. Magnetic fields are particularly relevant, since they pervade the physical Cosmos at planetary, stellar, galactic and extragalactic scales with different magnitudes (see [26] and references therein). These fields also play a vital role in the complex interaction between protostars and the environment of the hosting molecular giant clouds, which drives the processes of star formation and constitute a fundamental ingredient for the understanding of (phenomenological) stellar formation rates in galaxies [27].…”