Echoes from the Abyss: A Status Update

Gravitational wave echoes provide our most direct and surprising observational window into quantum nature of black holes. Three years ago, the first search for echoes from Planck-scale modifications of general relativity near black hole event horizons led to tentative evidence at false detection probability of 1\% arXiv:1612.00266 . The study introduced a naive phenomenological model and used the public data release by the Advanced LIGO gravitational wave observatory for the first observing run O1 (GW150914, GW151226, and LVT151012, now GW151012). Here, we provide a status update on various observational searches for echoes by independent groups, and argue that they can all be consistent if echoes are most prominent at lower frequencies and/or in binary mergers of more extreme mass ratio. We also point out that the only reported "detection" of echoes (with $>4\sigma$ confidence) at 1.0 second after the binary neutron star merger GW170817 arXiv:1803.10454 is coincident with the formation time of the black hole inferred from electromagnetic observations

GW190521: First Measurement of Stimulated Hawking Radiation from Black Holes

Being the most massive binary black hole merger event observed to date, GW190521 is in a class of its own. The exceptionally loud ringdown of this merger makes it an ideal candidate to search for gravitational wave echoes, a proposed smoking gun for the quantum structure of black hole horizons. We perform an unprecedented multi-pronged search for echoes via two well-established and independent pipelines: a template-based search for stimulated emission of Hawking radiation, or Boltzmann echoes, and the model-agnostic coherent WaveBurst (cWB) search. Stimulated Hawking radiation from the merger is expected to lead to post-merger echoes at horizon mode frequency of $\sim 50$ Hz (for quadrupolar gravitational radiation), repeating at intervals of $\sim 1$ second, due to partial reflection off Planckian quantum structure of the horizon. A careful analysis using dynamic nested sampling yields a Bayesian evidence of $7\pm 2$ (90% confidence level) for this signal following GW190521, carrying an excess of $10^{+9}_{-7}\%$ in gravitational wave energy, relative to the main event. Similarly, the reconstructed waveform of the first echo in cWB carries an energy excess of $13^{+16}_{-7}\%$. Accounting for the "look-elsewhere" effects, we estimate a p-value for false detection probability of $5.1 \times 10^{-3}$ (or 2.6$\sigma$) using cWB pipeline, although the verdict on the co-localization of the post-merger echo and the main event in the sky is inconclusive. While the current evidence for stimulated Hawking radiation does not reach the gold standard of $5\sigma$, our findings are in line with expectations for stimulated Hawking radiation at current detector sensitivities. The next generation of gravitational wave observatories can thus draw a definitive conclusion on the quantum nature of black hole horizons

Quantum Black Holes in the Sky

Black Holes are possibly the most enigmatic objects in our Universe. From their detection in gravitational waves upon their mergers, to their snapshot eating at the centres of galaxies, black hole astrophysics has undergone an observational renaissance in the past 4 years. Nevertheless, they remain active playgrounds for strong gravity and quantum effects, where novel aspects of the elusive theory of quantum gravity may be hard at work. In this review article, we provide an overview of the strong motivations for why "Quantum Black Holes" may be radically different from their classical counterparts in Einstein's General Relativity. We then discuss the observational signatures of quantum black holes, focusing on gravitational wave echoes as smoking guns for quantum horizons (or exotic compact objects), which have led to significant recent excitement and activity. We review the theoretical underpinning of gravitational wave echoes and critically examine the seemingly contradictory observational claims regarding their (non-)existence. Finally, we discuss the future theoretical and observational landscape for unraveling the "Quantum Black Holes in the Sky"

Primordial Black Holes as Dark Matter: The Power Spectrum and Evaporation of Early Structures

We consider the possibility that massive primordial black holes are the dominant form of dark matter. Black hole formation generates entropy fluctuations that adds a Poisson noise to the matter power spectrum. We use Lyman-alpha forest observations to constrain this Poisson term in matter power spectrum, then we constrain the mass of black holes to be less than few times 10^4 solar mass. We also find that structures with less than ~ 10^3 primordial black holes evaporate by now.Comment: Submitted to ApJL, 4 pages, 3 figure

Lower bound on the cosmological constant from the classicality of the early Universe

We use the quantum unimodular theory of gravity to relate the value of the cosmological constant, Λ , and the energy scale for the emergence of cosmological classicality. The fact that Λ and unimodular time are complementary quantum variables implies a perennially quantum Universe should Λ be zero (or, indeed, fixed at any value). Likewise, the smallness of Λ puts an upper bound on its uncertainty, and thus a lower bound on the unimodular clock’s uncertainty or the cosmic time for the emergence of classicality. Far from being the Planck scale, classicality arises at around 7 × 10 11     GeV for the observed Λ , and taking the region of classicality to be our Hubble volume. We confirm this argument with a direct evaluation of the wave function of the Universe in the connection representation for unimodular theory. Our argument is robust, with the only leeway being in the comoving volume of our cosmological classical patch, which should be bigger than that of the observed last scattering surface. Should it be taken to be the whole of a closed Universe, then the constraint depends weakly on Ω k : for − Ω k 4 × 10 12     GeV . If it is infinite, then this energy scale is infinite, and the Universe is always classical within the minisuperspace approximation. It is a remarkable coincidence that the only way to render the Universe classical just below the Planck scale is to define the size of the classical patch as the scale of nonlinearity for a red spectrum with the observed spectral index n s = 0.967 ( 4 ) (about 10 11 times the size of the current Hubble volume). In the context of holographic cosmology, we may interpret this size as the scale of confinement in the dual 3D quantum field theory, which may be probed (directly or indirectly) with future cosmological surveys

Bypass to Turbulence in Hydrodynamic Accretion: Lagrangian Analysis of Energy Growth

Despite observational evidence for cold neutral astrophysical accretion disks, the viscous process which may drive the accretion in such systems is not yet understood. While molecular viscosity is too small to explain the observed accretion efficiencies by more than ten orders of magnitude, the absence of any linear instability in Keplerian accretion flows is often used to rule out the possibility of turbulent viscosity. Recently, the fact that some fine tuned disturbances of any inviscid shear flow can reach arbitrarily large transient growth has been proposed as an alternative route to turbulence in these systems. We present an analytic study of this process for 3D plane wave disturbances of a general rotating shear flow in Lagrangian coordinates, and demonstrate that large transient growth is the generic feature of non-axisymmetric disturbances with near radial leading wave vectors. The maximum energy growth is slower than quadratic, but faster than linear in time. The fastest growth occurs for two dimensional perturbations, and is only limited by viscosity, and ultimately by the disk vertical thickness. After including viscosity and vertical structure, we find that, as a function of the Reynolds number, R, the maximum energy growth is approximately 0.4 (R/log R)^{2/3}, and put forth a heuristic argument for why R > 10^4 is required to sustain turbulence in Keplerian disks. Therefore, assuming that there exists a non-linear feedback process to replenish the seeds for transient growth, astrophysical accretion disks must be well within the turbulent regime. However, large 3D numerical simulations running for many orbital times, and/or with fine tuned initial conditions, are required to confirm Keplerian hydrodynamic turbulence on the computer.Comment: 25 preprint pages, 2 figures, some modifications mainly to the Discussions section, Accepted for publication in Ap

Bypass to Turbulence in Hydrodynamic Accretion Disks: An Eigenvalue Approach

Cold accretion disks such as those in star-forming systems, quiescent cataclysmic variables, and some active galactic nuclei, are expected to have neutral gas which does not couple well to magnetic fields. The turbulent viscosity in such disks must be hydrodynamic in origin, not magnetohydrodynamic. We investigate the growth of hydrodynamic perturbations in a linear shear flow sandwiched between two parallel walls. The unperturbed flow is similar to plane Couette flow but with a Coriolis force included. Although there are no exponentially growing eigenmodes in this system, nevertheless, because of the non-normal nature of the eigenmodes, it is possible to have a large transient growth in the energy of perturbations. For a constant angular momentum disk, we find that the perturbation with maximum growth has a wave-vector in the vertical direction. The energy grows by more than a factor of 100 for a Reynolds number R=300 and more than a factor of 1000 for R=1000. Turbulence can be easily excited in such a disk, as found in previous numerical simulations. For a Keplerian disk, on the other hand, similar vertical perturbations grow by no more than a factor of 4, explaining why the same simulations did not find turbulence in this system. However, certain other two-dimensional perturbations with no vertical structure do exhibit modest growth. For the optimum two-dimensional perturbation, the energy grows by a factor of ~100 for R~10^4.5 and by a factor of 1000 for R~10^6. It is conceivable that these two-dimensional disturbances might lead to self-sustained turbulence. The Reynolds numbers of cold astrophysical disks are much larger even than 10^6, therefore, hydrodynamic turbulence may be possible in disks.Comment: 39 pages including 9 figures; Final version to appear in The Astrophysical Journa

The holographic fluid dual to vacuum Einstein gravity

We present an algorithm for systematically reconstructing a solution of the (d+2)-dimensional vacuum Einstein equations from a (d+1)-dimensional fluid, extending the non-relativistic hydrodynamic expansion of Bredberg et al in arXiv:1101.2451 to arbitrary order. The fluid satisfies equations of motion which are the incompressible Navier-Stokes equations, corrected by specific higher derivative terms. The uniqueness and regularity of this solution is established to all orders and explicit results are given for the bulk metric and the stress tensor of the dual fluid through fifth order in the hydrodynamic expansion. We establish the validity of a relativistic hydrodynamic description for the dual fluid, which has the unusual property of having a vanishing equilibrium energy density. The gravitational results are used to identify transport coefficients of the dual fluid, which also obeys an interesting and exact constraint on its stress tensor. We propose novel Lagrangian models which realise key properties of the holographic fluid.Comment: 31 pages; v2: references added and minor improvements, published versio