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Testing the Nature of Dark Compact Objects: a Status Report

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Testing the Nature of Dark Compact Objects: a Status Report Living Reviews in Relativity (2019) 22:4 https://doi.org/10.1007/s41114-019-0020-4 REVIEW ARTICLE Testing the nature of dark compact objects: a status report Vitor Cardoso1,2 · Paolo Pani3 Received: 8 April 2019 / Accepted: 22 June 2019 © The Author(s) 2019 Abstract Very compact objects probe extreme gravitational fields and may be the key to under- stand outstanding puzzles in fundamental physics. These include the nature of dark matter, the fate of spacetime singularities, or the loss of unitarity in Hawking evapora- tion. The standard astrophysical description of collapsing objects tells us that massive, dark and compact objects are black holes. Any observation suggesting otherwise would be an indication of beyond-the-standard-model physics. Null results strengthen and quantify the Kerr black hole paradigm. The advent of gravitational-wave astronomy and precise measurements with very long baseline interferometry allow one to finally probe into such foundational issues. We overview the physics of exotic dark compact objects and their observational status, including the observational evidence for black holes with current and future experiments. Keywords Black holes · Event horizon · Gravitational waves · Quantum gravity · Singularities Contents 1 Introduction ............................................... 1.1 Black holes: kings of the cosmos? ................................ 1.2 Problems on the horizon ..................................... 1.3 Quantifying the evidence for black holes ............................. 1.4 The dark matter connection .................................... 1.5 Taxonomy of compact objects: a lesson from particle physics .................. B Vitor Cardoso [email protected] Paolo Pani [email protected] 1 CENTRA, Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais 1, 1049 Lisbon, Portugal 2 CERN, 1 Esplanade des Particules, 1211 Geneva 23, Switzerland 3 Dipartimento di Fisica, “Sapienza” Università di Roma & Sezione INFN Roma1, Piazzale Aldo Moro 5, 00185 Rome, Italy 0123456789().: V,-vol 123 4 Page 2 of 104 V. Cardoso, P. Pani 1.6 The small -limit ......................................... 2 Structure of stationary compact objects ................................ 2.1 Anatomy of compact objects ................................... 2.1.1 Event horizons, trapped surfaces, apparent horizons ................... 2.1.2 Quantifying the shades of dark objects: the closeness parameter ............ 2.1.3 Quantifying the softness of dark objects: the curvature parameter ............ 2.1.4 Geodesic motion and associated scales .......................... 2.1.5 Photon spheres ....................................... 2.2 Escape trajectories and shadows ................................. 2.3 The role of the spin ........................................ 2.3.1 Ergoregion ......................................... 2.3.2 Multipolar structure .................................... 3 ECO taxonomy: from DM to quantum gravity ............................. 3.1 A compass to navigate the ECO atlas: Buchdahl’s theorem ................... 3.2 Self-gravitating fundamental fields ................................ 3.3 Perfect fluids ........................................... 3.4 Anisotropic stars ......................................... 3.5 Quasiblack holes ......................................... 3.6 Wormholes ............................................ 3.7 Dark stars ............................................. 3.8 Gravastars ............................................. 3.9 Fuzzballs and collapsed polymers ................................ 3.10 “Naked singularities” and superspinars .............................. 3.11 2–2 Holes and other geons .................................... 3.12 Firewalls, compact quantum objects and dirty BHs ....................... 4 Dynamics of compact objects ...................................... 4.1 Quasinormal modes ........................................ 4.2 Gravitational-wave echoes .................................... 4.2.1 Quasinormal modes, photon spheres, and echoes ..................... 4.2.2 A black-hole representation and the transfer function .................. 4.2.3 A Dyson-series representation ............................... 4.2.4 Echo modeling ....................................... 4.2.5 Echoes: a historical perspective .............................. 4.3 The role of the spin ........................................ 4.3.1 QNMs of spinning Kerr-like ECOs ............................ 4.3.2 Echoes from spinning ECOs ............................... 4.4 The stability problem ....................................... 4.4.1 The ergoregion instability ................................. 4.4.2 Nonlinear instabilities I: long-lived modes and their backreaction ............ 4.4.3 Nonlinear instabilities II: causality, hoop conjecture, and BH formation ......... 4.5 Binary systems .......................................... 4.5.1 Multipolar structure .................................... 4.5.2 Tidal heating ........................................ 4.5.3 Tidal deformability and Love numbers .......................... 4.5.4 Accretion and drag in inspirals around and inside DM objects .............. 4.5.5 GW emission from ECOs orbiting or within neutron stars ................ 4.6 Formation and evolution ..................................... 5 Observational evidence for horizons .................................. 5.1 Tidal disruption events and EM counterparts ........................... 5.2 Equilibrium between ECOs and their environment: Sgr A* ................... 5.3 Bounds with shadows: Sgr A* and M87 ............................. 5.4 Tests with accretion disks ..................................... 5.5 Signatures in the mass-spin distribution of dark compact objects ................ 5.6 Multipole moments and tests of the no-hair theorem ....................... 5.6.1 Constraints with comparable-mass binaries ........................ 5.6.2 Projected constraints with EMRIs ............................. 5.7 Tidal heating ........................................... 123 Testing the nature of dark compact objects: a status report Page 3 of 104 4 5.8 Tidal deformability ........................................ 5.9 Resonance excitation ....................................... 5.10 QNM tests ............................................. 5.11 Inspiral-merger-ringdown consistency .............................. 5.12 Tests with GW echoes ....................................... 5.13 Stochastic background ...................................... 5.14 Motion within ECOs ....................................... 6 Discussion and observational bounds .................................. References .................................................. The crushing of matter to infinite density by infinite tidal gravitation forces is a phenomenon with which one cannot live comfortably. From a purely philo- sophical standpoint it is difficult to believe that physical singularities are a fundamental and unavoidable feature of our universe [...] one is inclined to dis- card or modify that theory rather than accept the suggestion that the singularity actually occurs in nature. Kip Thorne, Relativistic Stellar Structure and Dynamics (1966) No testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous than the fact which it endeav- ors to establish. David Hume, An Enquiry concerning Human Understanding (1748) 1 Introduction The discovery of the electron and the known neutrality of matter led in 1904 to J. J. Thomson’s “plum-pudding” atomic model. Data from new scattering experiments was soon found to be in tension with this model, which was eventually superseeded by Rutherford’s, featuring an atomic nucleus. The point-like character of elementary particles opened up new questions. How to explain the apparent stability of the atom? How to handle the singular behavior of the electric field close to the source? What is the structure of elementary particles? Some of these questions were elucidated with quantum mechanics and quantum field theory. Invariably, the path to the answer led to the discovery of hitherto unknown phenomena and to a deeper understanding of the fundamental laws of Nature. The history of elementary particles is a timeline of the understanding of the electromagnetic (EM) interaction, and is pegged to its characteristic 1/r 2 behavior (which necessarily implies that other structure has to exist on small scales within any sound theory). Arguably, the elementary particle of the gravitational interaction are black holes (BHs). Within General Relativity (GR), BHs are indivisible and the simplest macroscopic objects that one can conceive. The uniqueness results—establishing that the two-parameter Kerr family of BHs describes any vacuum, stationary and asymp- totically flat, regular solution to GR—have turned BHs into somewhat of a miracle elementary particle.1 1 Quoting Subrahmanyan Chandrasekhar: “In my entire scientific life, extending over forty-five years, the most shattering experience has been the realization that an exact solution of Einstein’s equations of general 123 4 Page 4 of 104 V. Cardoso, P. Pani Even though the first nontrivial regular, asymptotically flat, vacuum solution to the field equations describing BHs were written already in 1916 (Schwarzschild 1916; Droste 1917), several decades would elapse until such solutions became accepted and understood. The dissension between Eddington and Chandrasekhar
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