International Journal of Modern Physics D

Evidence has accumulated that there are SuperMassive Black Holes (SMBHs) in the centers of most galaxies, and that these were formed in the very early universe by some as yet unknown process. In particular, there is evidence [J. Silk, M. C. Begelman, C. Norman, A. Nusser and R. F. G. Wyse, Astrophys. J. Lett. 961 (2024) L39, https://doi.org/10.3847/2041-8213/ad1bf0, arXiv:2401.02482 [astroph. GA]; A. Bogdan et al., Nature Astron. 8 (2024) 126, https://doi.org/10.1038/s41550-023-02111-9, arXiv:2305.15458 [astro-ph.GA]; S. Y. Guo, M. Khlopov, X. Liu, L. Wu, Y. Wu and B. Zhu, arXiv:2306.17022 [hep-ph]] that at least some galaxies formed as early as $10^8-10^9$ years after the big bang host SMBHs. We suggest that the holographic model of inflation, whose dark matter candidates are primordial black holes carrying a discrete gauge charge, which originated as a small subset of the inflationary horizon volumes in the very early universe, can provide the seeds for this early structure formation. Aspects of the model pointed out long ago suggested an early era of structure formation, with structures dominated by dark matter. The additional assumption that the dark matter consists of discretely charged black holes implies black hole dominance of early structures, which seems to be implied by JWST data.

We argue that the vacuum of quantum gravity must contain a hierarchical structure of correlations spanning all length scales. These correlated domains (called “vecros”) correspond to virtual fluctuations of black hole microstates. Larger fluctuations are suppressed by their larger action, but this suppression is offset by a correspondingly larger phase space of possible configurations. We give an explicit lattice model of these vecro fluctuations, noting how their distribution changes as the gravitational pull of a star becomes stronger. At the threshold of formation of a closed trapped surface, these virtual fluctuations transition into on-shell black hole microstates (fuzzballs). Fuzzballs radiate from their surface like normal bodies, resolving the information paradox. We also argue that any model without vecro-type extended vacuum correlations cannot resolve the paradox.

Observations of velocity dispersions of galactic structures over a wide range of scales point to the existence of a universal acceleration scale $a_0\sim 10^{-10}$m/s². Focusing on the fuzzy dark matter paradigm, which proposes ultralight dark matter with mass around $10^{-22}$eV and de Broglie wavelength $\lambda \sim \mathrm{\text{few}}\times 10^2$ parsecs, we highlight the emergence of the observed acceleration scale from quantum effects in a fluid-like description of the dark matter dynamics. We then suggest the possibility of a natural connection between the acceleration scale and dark energy within the same paradigm.

Many galaxies contain SuperMassive Black Holes (SMBHs), whose formation and history raise many puzzles. Pulsar timing arrays have recently discovered a low-frequency cosmological “hum” of gravitational waves that may be emitted by SMBH binary systems, and the JWST and other telescopes have discovered an unexpectedly large population of high-redshift SMBHs. We argue that these two discoveries may be linked, and that they may enhance the prospects for measuring gravitational waves emitted during the mergers of massive black holes, thereby opening the way towards resolving many puzzles about SMBHs as well as providing new opportunities to probe general relativity.

While the standard point of view is that the ultimate endpoint of black hole evolution is determined by Hawking evaporation, there is a growing evidence that classical and semi-classical instabilities affect both black holes with inner horizons as well as their ultra-compact counterparts. In this paper we start from this evidence pointing towards extremal black holes as stable endpoints of the gravitational collapse, and develop a general class of spherical and axisymmetric solutions with multiple extremal horizons. Excluding more exotic possibilities, entailing regular cores supporting wormhole throats, we argue that these configurations could be the asymptotic graveyard, the end-point, of dynamical black hole evolution — albeit the timescale of such evolution are still unclear and possibly long and compatible with current astrophysical observations.

We expect the final theory of gravity to have more symmetries than we suspect and our research points in this direction. To start with, standard general coordinate invariance can be extended to complex holomorphic general coordinate transformations. This is possible by introducing a non-Riemannian measure of integration (NRMI) and where we avoid the nonholomorphic standard $\sqrt{-g}$ measure of integration. Second, locally signed coordinate transformations where the Jacobian changes sign locally but the Jacobian approaches one asymptotically should be symmetries of Nature. This is unlike globally signed transformations that produce a change of boundary conditions, like in the cases of global parity and global time reversal, which are not symmetries of Nature. The holomorphic extension can regularize the regions of spacetime where the Jacobian changes sign. Consequences for Quantum Gravity are discussed.

I discuss how five reasonably sounding assumptions lead to a dilemma — the Page-time paradox — which appears to challenge a conventional statistical mechanical underpinning of black hole thermodynamics. By inspecting the conceptual subtleties behind each hypothesis, I list questions that require clarification before the puzzle can be deemed paradoxical. I devote particular attention to using thermodynamic arguments for a system that never reaches equilibrium. As a proof of concept, I show that the paradox is absent in a modified setting that admits an equilibrium thermodynamics formulation.

As famously observed by Ooguri and Vafa nearly 20 years ago, scalar field moduli spaces in quantum gravity appear to exhibit various universal features. For instance, they seem to be infinite in diameter, have trivial fundamental group, and feature towers of massive particles that become light in their asymptotic limits. In this paper, we explain how these features can be reformulated in more modern language using generalized notions of global symmetries. Such symmetries are ubiquitous in nongravitational quantum field theories, but it is widely believed that they must be either gauged or broken in quantum gravity. In what follows, we will see that the observations of Ooguri and Vafa can be understood heuristically as consequences of such gauging or breaking.

Classical and quantum mechanical descriptions of motion are fundamentally different. The Universality of Free Fall (UFF) is a distinguishing feature of the classical motion (which has been verified with astonishing precision), while quantum theory tell us only about probabilities and uncertainties thus breaking the UFF. There are strong reasons to believe that the classical description must emerge, under plausible hypothesis, from quantum mechanics. In this paper, we show that the UFF is an emergent phenomenon: the coarse-grained quantum distribution for high-energy levels leads to the classical distribution as the lowest order plus quantum corrections. We estimate the size of these corrections on the Eötvös parameter and discuss the physical implications.

In this essay, it is proven that, in a self-consistent semiclassical theory of gravity, the asymptotically measured orbital periods of test particles around central compact objects are fundamentally bounded from below by the compact universal relation $T_{\infty }\ge \frac{2\pi e\hslash }{\sqrt{G}{c}^2{m}_e^2}$ [here $\{m_e,e\}$ are, respectively, the proper mass and the electric charge of the electron, the lightest charged particle]. The explicit dependence of the lower bound on the fundamental constants $\{G,c,\hslash \}$ of gravity, special relativity, and quantum theory suggests that it provides a rare glimpse into the yet unknown quantum theory of gravity.

Recent experimental progresses in controlling classical and quantum fluids have made it possible to realize acoustic analogs of gravitational black holes, where a flowing fluid provides an effective spacetime on which sound waves propagate, demonstrating Hawking-like radiation and superradiance. We propose the exciting possibility that new hydrodynamic systems might provide insights to help resolve mysteries associated with quantum gravity, including the black hole information-loss paradox and the removal of spacetime singularities.

Renormalization of quantum loop effects generated from large fluctuations is a hugely debatable topic of research these days which rules out the Primordial Black Hole (PBH) formation within the framework of single-field inflation. In this paper, we briefly discuss that the correct implementation of regularization, renormalization, and resummation techniques in a setup described by an ultra-slow-roll phase sandwiched between two slow-roll phases in the presence of smooth or sharp transitions can lead to a stringent constraint on the PBH mass (i.e. $\mathcal{𝒪}(10^2\mathrm{\text{gm}})$), which we advertise as a new No-go theorem. Finally, we will give some of the possible way-outs using which one can evade this proposed No-go theorem and produce solar/sub-solar mass PBHs.

General relativity is highly successful in explaining a wide range of gravitational phenomena including the gravitational waves emitted by binary systems and the shadows cast by supermassive black holes. From a modern perspective the theory is not fundamental though, but constitutes the lowest order term in an effective field theory description of the gravitational force. As a consequence, the gravitational dynamics should receive corrections by higher-derivative terms. This essay discusses structural aspects associated with these corrections and summarizes their imprint on static, spherically symmetric geometries. Along these lines, we critically reassess the common practice of using local field redefinitions in order to simplify the dynamics at the danger of shifting physics effects into sectors which are beyond the approximation under consideration.

Cosmological inflation is a popular paradigm for understanding Cosmic Microwave Background Radiation (CMBR); however, it faces many conceptual challenges. An alternative mechanism to inflation for generating an almost scale-invariant spectrum of perturbations is a bouncing cosmology with an initial matter-dominated contraction phase, during which the modes corresponding to currently observed scales exited the Hubble radius. Bouncing cosmology avoids the initial singularity but has fine-tuning problems. Taking an agnostic view of the two early-universe paradigms, we propose a quantum measure — Dynamical Fidelity Susceptibility (DFS) of CMBR — that distinguishes the two scenarios. Taking two simple models with the same power-spectrum, we explicitly show that DFS behaves differently for the two scenarios. We discuss the possibility of using DFS as a distinguisher in the upcoming space missions.

The gravitational interplay between a supermassive black hole and dark matter has set up an exotic environment at the center of a galaxy. In this paper, I present an analytic framework modeling the gravitational influence on the stars orbiting a galactic supermassive black hole. In particular, we discuss two intriguing features, stellar orbital precession and orbital shrinking, which demonstrate an extraordinary gravitational environment near a galactic supermassive black hole. Moreover, I show that these features can be analytically determined by the supermassive black hole mass.

Recent observations from the James Webb Space Telescope have identified a population of massive galaxy sources ($>10^{10}{M}_{\odot }$) at $z>7$–10, formed less than 700Myr after the Big Bang. Such massive galaxies do not have enough time to form within the standard cosmological model, and hence these observations pose a significant challenge. A number of possible solutions to this problem have been put forward. In this essay, we discuss two more theoretical and speculative possibilities.

It is well known that gravitational waves (GWs) undergo no absorption or dissipation when traversing through a perfect fluid. However, in the presence of a viscous fluid, GWs transfer energy to the fluid medium. In this paper, we present a review of our recent series of results regarding the interaction between GWs and surrounding matter. Additionally, we examine the impact of a viscous fluid shell on GW propagation, focusing particularly on GW damping and GW heating. Furthermore, we explore the significance of these effects in various astrophysical scenarios such as core-collapse supernovae and primordial GWs.

In the Emergent scenario, the Universe should evolve from a nonsingular state replacing the typical singularity of General Relativity, for any initial condition. For the scalar field model by Ellis and Maartens [Class. Quantum Grav. 21 (2003) 223] we show that only a set of measure zero of trajectories leads to emergence, either from a static state (an Einstein model), or from a de Sitter state. Assuming a scenario based on CDM interacting with a Dark Energy fluid, we show that in general flat and open models expand from a nonsingular unstable de Sitter state at high energies; for some closed models this state is a transition phase with a bounce, other closed models are cyclic. A subset of these models are qualitatively in agreement with the observable Universe, accelerating at high energies, going through a matter-dominated decelerated era, then accelerating toward a de Sitter phase.

In light of NANOGrav data, we provide for the first time possible observational signatures of Superstring theory. First, we work with inflection-point inflationary potentials naturally realized within Wess–Zumino-type no-scale Supergravity, which give rise to the formation of microscopic primordial black holes (PBHs) triggering an early matter-dominated era (eMD) and evaporating before big bang nucleosynthesis (BBN). Remarkably, we obtain an abundant production of primordial gravitational waves (PGW) at the frequency ranges of nHz, Hz and kHz and in strong agreement with pulsar time array (PTA) gravitational wave (GW) data. This PGW background could serve as a compelling observational signature for the presence of quantum gravity via no-scale Supergravity.

Chae’s analyses on GAIA observations of wide binary stars have fortified the paradigm of extended gravity with particular attention to MOND-like theories. We recall that, starting from the origin of Einstein’s general relativity, the request of Mach on the structure of the theory has been the core of the foundational debate. This issue is strictly connected with the nature of the mass–energy equivalence. This was exactly the key point that Einstein used to derive the same general relativity. On the other hand, the current requirements of particle physics and the open questions within extended gravity theories, which have recently been further strengthened by the analyses of GAIA observations, request a better understanding of the Equivalence Principle. By considering a direct coupling between the Ricci curvature scalar and the matter Lagrangian, a nongeodesic ratio between the inertial and gravitational masses can be fixed and MOND-like theories are retrieved at low energies.

Through use of the Pauli–Villars regulator procedure, we construct a second- plus fourth-order derivative theory of gravity that serves as an ultraviolet completion of standard second-order derivative quantum Einstein gravity that is ghost-free, unitary and power-counting renormalizable.

In this essay, we argue that certain aspects of the measurement require revision in Quantum Gravity. Using entropic arguments, we propose that the number of measurement outcomes and the accuracy (or the range) of the measurement are limited by the entropy of the black hole associated with the observer’s scale. This also implies the necessity of modifying the algebra of commutation relationships to ensure a finite representation of observables, changing the Heisenberg Uncertainty Principle in this manner.

In quantum cosmology, it is expected that the Big Bang singularity is resolved and the universe undergoes a bounce. We find that for Gaussian initial states, matter-gravity entanglement entropy rises rapidly during the bounce, declines, and then approaches a steady-state value following the bounce. These observations suggest that matter-gravity entanglement is a feature of the macroscopic universe and that there is no Second Law of entanglement entropy.

If entanglement builds spacetime, then conversely, disentanglement ought to destroy spacetime. From the quantum null energy condition and quantum focusing conjecture, we obtain disentanglement criteria which necessitate infinite energies and strong spacetime singularities. We apply our results to the strong cosmic censorship proposal, where singularities at the Cauchy horizons in black holes are desirable. Using our disentanglement criteria and without resorting to any detailed calculations, we provide an exceedingly general and physically transparent discussion of strong cosmic censorship in semiclassical black holes. We argue that strong cosmic censorship is enforced in asymptotically flat and de Sitter black holes by disentanglement and describe how similar disentanglement might be avoided in some anti-de Sitter cases.

In this paper, we consider the single-parameter $\mathcal{ℛ}+c{\mathcal{ℛ}}^2$ gravitational action and use constraints from astrophysics and the laboratory to derive a natural relation between the coefficient c and the value of the cosmological constant. We find that the renormalization of c from the energy of the inflationary phase to the infrared, where the acceleration of the expansion of the universe takes place, is correlated with the evolution of the vacuum energy. Our results suggest that the coefficient of the ${\mathcal{ℛ}}^2$ term may provide an unexpected bridge between high-energy physics and cosmological phenomena such as inflation and dark energy.

One of the classical tests of general relativity is the precision measurements by Pound and Rebka of red-shift/blue-shift of photons in a gravitational field. In this essay, we lay out a temporal version of the Pound–Rebka experiment. The emission and absorption of photons occurs at different times, rather than at different spatial locations as in the original Pound–Rebka experiment. This temporal Pound–Rebka experiment is equivalent to a gravitational Aharonov–Bohm Effect and is testable via current or near future satellite experiments.

We argue that gravitational interactions between particles require a departure from the conventional picture of the quantum state of a multiparticle system in terms of tensor products of one-particle states. This modification is essential in order to accommodate the existence of a new boost-like relativistic angular momentum charge which pairs of particles must carry asymptotically due to long-range effects of gravity. These findings challenge conventional assumptions, prompting a re-evaluation of the constraints on quantum entanglement between particle subsystems in a black hole geometry.

Honorable Mention in the 2024 Gravity Research Foundation Essay Competition

Horizons and bound photon orbits are defining features of black holes that translate into key features of black hole images. We review a purely geometric proof that spherically symmetric, isolated objects with horizons in gravity theories with null-geodesic propagation of light must display bound photon orbits forming a photon sphere. Identifying the key elements of the proof, we articulate a simpler argument that carries over to more general situations with modified light propagation and implies the existence of equatorial spherical photon orbits in axisymmetric spacetimes with reflection symmetry. We conclude that the non-observation of photon rings with very-large-baseline interferometry would be a very strong indication against a horizon, irrespective of whether or not the image shows a central brightness depression.

We consider interstellar power transmission facilitated by gravitational lensing. Our study explores realistic scenarios for signal transmission and relevant detection strategies. We determine that optimal link efficiency is achieved in lensing geometries where the transmitter, lens, and receiver are closely aligned. Our findings confirm the feasibility of interstellar power transmission via gravitational lensing. Additionally, we demonstrate that laser signals, amplified by nearby stellar gravitational lenses, can be detected using established photonics and optical engineering technologies. The search for these transmitted laser signals can be conducted by networks of collaborative astronomical facilities, utilizing both ground-based and space-based instruments, thereby directly contributing to ongoing optical Search for ExtraTerrestrial Intelligence (SETI) efforts.

The detectors of astrophysical gravitational waves are based on optical interferometers. There is not yet an analogue of a quantum mechanical atomic detector of photons for the detection of gravitational waves. I point out the remarkable fact that the already observed pico-electron volt quantization of energy levels of ultra-cold neutrons, gravitationally bound to the Earth in a bouncing mode, fortuitously coincide with the range of energies of the hypothetical quanta of astrophysical gravitational waves. Thus, it is feasible to use a cold-neutron bouncing reservoir as a novel quantum sensor for directly detecting astrophysical gravitational waves, in a wide frequency band. These considerations also lead us to the discovery that many detected signals in interferometric detectors like LIGO correspond to the energy of a single gravitational quantum or less. This surprising finding enables a decisive experimental test of quantum gravity.