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A black hole is a region of spacetime where gravity is so strong that nothing – no particles or even electromagnetic radiation such as light – can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of no escape is called the event horizon. Although it has a great effect on the fate and circumstances of an object crossing it, it has no locally detectable features according to general relativity. In many ways, a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is of the order of billionths of a kelvin for stellar black holes, making it essentially impossible to observe directly.
In general relativity, a naked singularity is a hypothetical gravitational singularity without an event horizon. In a black hole, the singularity is completely enclosed by a boundary known as the event horizon, inside which the curvature of spacetime caused by the singularity is so strong that light cannot escape. Hence, objects inside the event horizon—including the singularity itself—cannot be directly observed. A naked singularity, by contrast, would be observable from the outside.
Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics; it deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the vicinity of black holes or similar compact astrophysical objects, such as neutron stars.
A wormhole is a speculative structure connecting disparate points in spacetime, and is based on a special solution of the Einstein field equations.
A gravitational singularity, spacetime singularity or simply singularity is a condition in which gravity is so intense that spacetime itself breaks down catastrophically. As such, a singularity is by definition no longer part of the regular spacetime and cannot be determined by "where" or "when". Gravitational singularities exist at a junction between general relativity and quantum mechanics; therefore, the properties of the singularity cannot be described without an established theory of quantum gravity. Trying to find a complete and precise definition of singularities in the theory of general relativity, the current best theory of gravity, remains a difficult problem. A singularity in general relativity can be defined by the scalar invariant curvature becoming infinite or, better, by a geodesic being incomplete.
Hawking radiation is black body radiation that is theorized to be released outside a black hole's event horizon because of relativistic quantum effects. It is named after the physicist Stephen Hawking, who developed a theoretical argument for its existence in 1974. Hawking radiation is a purely kinematic effect that is generic to Lorentzian geometries containing event horizons or local apparent horizons.
Jorge Pullin is an American theoretical physicist known for his work on black hole collisions and quantum gravity. He is the Horace Hearne Chair in theoretical Physics at the Louisiana State University.
A gravastar is an object hypothesized in astrophysics by Pawel O. Mazur and Emil Mottola as an alternative to the black hole theory. It has usual black hole metric outside of the horizon, but de Sitter metric inside. On the horizon there is a thin shell of matter. The term "gravastar" is a portmanteau of the words "gravitational vacuum star".
The Immirzi parameter is a numerical coefficient appearing in loop quantum gravity (LQG), a nonperturbative theory of quantum gravity. The Immirzi parameter measures the size of the quantum of area in Planck units. As a result, its value is currently fixed by matching the semiclassical black hole entropy, as calculated by Stephen Hawking, and the counting of microstates in loop quantum gravity.
The Unruh effect is a kinematic prediction of quantum field theory that an accelerating observer will observe a thermal bath, like blackbody radiation, whereas an inertial observer would observe none. In other words, the background appears to be warm from an accelerating reference frame; in layman's terms, an accelerating thermometer in empty space, removing any other contribution to its temperature, will record a non-zero temperature, just from its acceleration. Heuristically, for a uniformly accelerating observer, the ground state of an inertial observer is seen as a mixed state in thermodynamic equilibrium with a non-zero temperature bath.
Micro black holes, also called mini black holes or quantum mechanical black holes, are hypothetical tiny black holes, for which quantum mechanical effects play an important role. The concept that black holes may exist that are smaller than stellar mass was introduced in 1971 by Stephen Hawking.
The black hole information paradox is a puzzle that appears when the predictions of quantum mechanics and general relativity are combined. The theory of general relativity predicts the existence of black holes that are regions of spacetime from which nothing — not even light — can escape. In the 1970s, Stephen Hawking applied the rules of quantum mechanics to such systems and found that an isolated black hole would emit a form of radiation called Hawking radiation. Hawking also argued that the detailed form of the radiation would be independent of the initial state of the black hole and would depend only on its mass, electric charge and angular momentum. The information paradox appears when one considers a process in which a black hole is formed through a physical process and then evaporates away entirely through Hawking radiation. Hawking's calculation suggests that the final state of radiation would retain information only about the total mass, electric charge and angular momentum of the initial state. Since many different states can have the same mass, charge and angular momentum this suggests that many initial physical states could evolve into the same final state. Therefore, information about the details of the initial state would be permanently lost. However, this violates a core precept of both classical and quantum physics—that, in principle, the state of a system at one point in time should determine its value at any other time. Specifically, in quantum mechanics the state of the system is encoded by its wave function. The evolution of the wave function is determined by a unitary operator, and unitarity implies that the wave function at any instant of time can be used to determine the wave function either in the past or the future.
Numerical relativity is one of the branches of general relativity that uses numerical methods and algorithms to solve and analyze problems. To this end, supercomputers are often employed to study black holes, gravitational waves, neutron stars and many other phenomena governed by Einstein's theory of general relativity. A currently active field of research in numerical relativity is the simulation of relativistic binaries and their associated gravitational waves.
A ring singularity or ringularity is the gravitational singularity of a rotating black hole, or a Kerr black hole, that is shaped like a ring.
Gravitational-wave astronomy is an emerging branch of observational astronomy which aims to use gravitational waves to collect observational data about objects such as neutron stars and black holes, events such as supernovae, and processes including those of the early universe shortly after the Big Bang.
A nonsingular black hole model is a mathematical theory of black holes that avoids certain theoretical problems with the standard black hole model, including information loss and the unobservable nature of the black hole event horizon.
Modern searches for Lorentz violation are scientific studies that look for deviations from Lorentz invariance or symmetry, a set of fundamental frameworks that underpin modern science and fundamental physics in particular. These studies try to determine whether violations or exceptions might exist for well-known physical laws such as special relativity and CPT symmetry, as predicted by some variations of quantum gravity, string theory, and some alternatives to general relativity.
In theoretical physics, a dynamical horizon (DH) is a local description of evolving black-hole horizons. In the literature there exist two different mathematical formulations of DHs—the 2+2 formulation developed by Sean Hayward and the 3+1 formulation developed by Abhay Ashtekar and others. It provides a description of a black hole that is evolving. A related formalism, for black holes with zero influx, is an isolated horizon.
Manuela Campanelli is a distinguished professor of astrophysics and mathematical sciences of the Rochester Institute of Technology, and the director of their Center for Computational Relativity and Gravitation and Astrophysics and Space Sciences Institute for Research Excellence. Her work focuses on the astrophysics of merging black holes and neutron stars, which are powerful sources of gravitational waves, electromagnetic radiation and relativistic jets. This research is central to the new field of multi-messenger astronomy.
Infinite derivative gravity is a theory of gravity which attempts to remove cosmological and black hole singularities by adding extra terms to the Einstein–Hilbert action, which weaken gravity at short distances.
References
- ↑ Hayward, Sean A. (26 January 2006). "Formation and evaporation of non-singular black holes". Physical Review Letters . 96 (3): 031103. arXiv: gr-qc/0506126 . Bibcode:2006PhRvL..96c1103H. doi:10.1103/PhysRevLett.96.031103. PMID 16486679. S2CID 15851759.
- ↑ De Lorenzo, Tommaso; Pacilio, Costantino; Rovelli, Carlo; Speziale, Simone (1 April 2015). "On the Effective Metric of a Planck Star". General Relativity and Gravitation . 47 (4): 41. arXiv: 1412.6015 . Bibcode:2015GReGr..47...41D. doi:10.1007/s10714-015-1882-8. S2CID 118431674.
- ↑ Chiba, Takeshi; Kimura, Masashi (1 April 2017). "A Note on Geodesics in Hayward Metric". Progress of Theoretical and Experimental Physics . 2017 (4). arXiv: 1701.04910 . doi:10.1093/ptep/ptx037.
- ↑ Contreras, E.; Bargueño, P. (20 October 2018). "Scale--dependent Hayward black hole and the generalized uncertainty principle". Modern Physics Letters A . 33 (32): 1850184–1850228. arXiv: 1809.00785 . Bibcode:2018MPLA...3350184C. doi:10.1142/S0217732318501845. S2CID 59946026.
- ↑ Frolov, Valeri P. (28 November 2016). "Notes on non-singular models of black holes". Physical Review D . 94 (10): 104056. arXiv: 1609.01758 . Bibcode:2016PhRvD..94j4056F. doi:10.1103/PhysRevD.94.104056. S2CID 119309868.
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