scholarly journals Bootstrapped Newtonian stars and black holes

2019 ◽  
Vol 79 (11) ◽  
Author(s):  
Roberto Casadio ◽  
Michele Lenzi ◽  
Octavian Micu
Keyword(s):  
Black Holes ◽  
Gravitational Potential ◽  
Strong Field ◽  
Compact Objects ◽  
Newtonian Gravity ◽  
Speed Of Light ◽  
Relativistic Case ◽  
Test Particles ◽  
Equilibrium Configurations ◽  
General Relativistic

Abstract We study equilibrium configurations of a homogenous ball of matter in a bootstrapped description of gravity which includes a gravitational self-interaction term beyond the Newtonian coupling. Both matter density and pressure are accounted for as sources of the gravitational potential for test particles. Unlike the general relativistic case, no Buchdahl limit is found and the pressure can in principle support a star of arbitrarily large compactness. By defining the horizon as the location where the escape velocity of test particles equals the speed of light, like in Newtonian gravity, we find a minimum value of the compactness for which this occurs. The solutions for the gravitational potential here found could effectively describe the interior of macroscopic black holes in the quantum theory, as well as predict consequent deviations from general relativity in the strong field regime of very compact objects.

scholarly journals Dynamics of Test Particles and Twin Peaks QPOs around Regular Black Holes in Modified Gravity

Galaxies ◽  
2021 ◽  
Vol 9 (4) ◽  
pp. 75
Author(s):  
Javlon Rayimbaev ◽  
Pulat Tadjimuratov ◽  
Ahmadjon Abdujabbarov ◽  
Bobomurat Ahmedov ◽  
Malika Khudoyberdieva
Keyword(s):  
Black Holes ◽  
Test Particle ◽  
Modified Gravity ◽  
Strong Field ◽  
Compact Objects ◽  
Precision Data ◽  
Regular Black Holes ◽  
Motion Study ◽  
Test Particles ◽  
Lower Frequencies

In this work, we have presented a detailed analysis of the event horizon of regular black holes (BHs) in modified gravity known as MOG, the so-called regular MOG BH. The motion of neutral particles around the BH has also been explored. The test particle motion study shows that the positive (negative) values of the MOG parameter mimic the spin of a rotating Kerr BH, providing the same values for the innermost stable pro-grade (retrograde) orbits of the particles in the range of the spin parameter a/M∈(−0.4125,0.6946). The efficiency of energy release from the accretion disk by the Novikov–Thorne model has been calculated, and the efficiency was shown to be linearly proportional to the increase of the MOG parameter α. Moreover, we have developed a new methodology to test gravity theories in strong-field regimes using precision data from twin-peaked quasiperiodic oscillations (QPOs) of objects calculating possible values of upper and lower frequencies. However, it is obtained that the positive MOG parameter can not mimic the spin of Kerr BHs in terms of the same QPO frequencies. We have provided possible ranges for upper and lower frequencies of twin-peak QPOs with the ratio of the upper and lower frequencies of 3:2 around regular MOG BHs in the different models. Moreover, as an example, we provide detailed numerical analysis of the QPO of GRS 1915+105 with the frequencies νU=168±5Hz and νL=113±3Hz. It is shown that the central BH of the QPO object can be a regular MOG BH when the value of the parameter is α=0.2844−0.1317+0.0074 and shines in the orbits located at the distance r/M=7.6322−0.0826+0.0768 from the central BH. It is also shown that the orbits where QPOs shine are located near the innermost stable circular orbit (ISCO) of the test particle. The correlation between the radii of ISCO and the QPO orbits is found, and it can be used as a new theoretical way to determine ISCO radius through observational data from the QPOs around various compact objects.

scholarly journals GENERAL RELATIVISTIC VELOCITY: THE ALTERNATIVE TO DARK MATTER

2008 ◽  
Vol 23 (21) ◽  
pp. 1745-1755 ◽  
Author(s):  
F. I. COOPERSTOCK ◽  
S. TIEU
Keyword(s):  
Dark Matter ◽  
Newtonian Gravity ◽  
Relativistic Velocity ◽  
Concentrated Mass ◽  
Cluster Of Galaxies ◽  
Spherical Collapse ◽  
Test Particles ◽  
Open Case ◽  
General Relativistic ◽  
Weak Gravity

We consider the gravitational collapse of a spherically symmetric ball of dust in the general relativistic weak gravity regime. The velocity of the matter as viewed by external observers is compared to the velocity gauged by local observers. While the comparison in the case of very strong gravity is seen to follow the pattern familiar to the studies of test particles falling towards a concentrated mass, the case of weak gravity is very different. The velocity of the dust that is witnessed by external observers is derived for the critically open case and is seen to differ markedly from the expectations based upon Newtonian gravity theory. Viewed as an idealized model for a cluster of galaxies, we find that with the general relativistic velocity expression, the higher-than-expected constituent velocities observed can be readily correlated with the solely baryonic measure of the mass, obviating the need to introduce extraneous dark matter. It would be particularly valuable if a laboratory or space-based realization of a spherical collapse could be implemented to compare the velocity expression as a new test of general relativity.

Black Hole, Neutron Star and Numerical Relativity

2021 ◽  
Vol 30 (6) ◽  
pp. 7-13
Author(s):  
Jinho KIM
Keyword(s):  
Black Holes ◽  
General Relativity ◽  
Black Hole ◽  
Neutron Star ◽  
Neutron Stars ◽  
Numerical Method ◽  
High Velocity ◽  
Compact Objects ◽  
Compact Stars ◽  
Speed Of Light

Compact stars, e.g., black holes and neutron stars, are the most energetic objects in astrophysics. These objects are accompanied by extremely strong gravity and a high velocity, which approaches the speed of light. Therefore, compact objects should be dealt with in Einstein’s relativity. This article will briefly introduce a numerical method that will allow us to obtain general solutions in general relativity. Several applications using numerical relativistic simulations will also be presented.

scholarly journals Exotic orbits due to spin–spin coupling around Kerr black holes

2017 ◽  
Vol 27 (01) ◽  
pp. 1750179 ◽  
Author(s):  
Wen-Biao Han ◽  
Shu-Cheng Yang
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Equatorial Plane ◽  
Strong Field ◽  
Kerr Black Hole ◽  
Spin Coupling ◽  
Spinning Particles ◽  
Test Particles ◽  
Kerr Black Holes ◽  
Large Spins

We report exotic orbital phenomena of spinning test particles orbiting around a Kerr black hole, i.e. some orbits of spinning particles are asymmetrical about the equatorial plane. When a nonspinning test particle orbits around a Kerr black hole in a strong field region, due to relativistic orbital precessions, the pattern of trajectories is symmetrical about the equatorial plane of the Kerr black hole. However, the patterns of the spinning particles’ orbit are no longer symmetrical about the equatorial plane for some orbital configurations and large spins. We argue that these asymmetrical patterns come from the spin–spin interactions between spinning particles and Kerr black holes, because the directions of spin–spin forces can be arbitrary, and distribute asymmetrically about the equatorial plane.

scholarly journals A pseudo-Newtonian description of any stationary space-time

2016 ◽  
Vol 12 (S324) ◽  
pp. 45-46
Author(s):  
Vojtěch Witzany ◽  
Claus Lämmerzahl
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Strong Field ◽  
Kerr Black Hole ◽  
Space Time ◽  
Schwarzschild Black Hole ◽  
Test Particles ◽  
Kerr Field ◽  
Alternative Gravity Theories ◽  
Static Space

AbstractSince the first investigations into accretion onto black holes, astrophysicists have proposed effective Newtonian-like potentials to mimic the strong-field behavior of matter near a Schwarzschild or Kerr black hole. On the other hand, the fields of neutron stars or black holes in many of the alternative gravity theories differ from the idealized Schwarzschild or Kerr field which would require a number of new potentials. To resolve this, we give a Newtonian-like Hamiltonian which almost perfectly mimics the behavior of test particles in any given stationary space-time. The properties of the Hamiltonian are excellent in static space-times such as the Schwarzschild black hole, but become worse for space-times with gravito-magnetic or dragging effects such as near the Kerr black hole.

IMPULSIVE LIGHT-LIKE SIGNALS

2002 ◽  
Vol 17 (20) ◽  
pp. 2746-2746
Author(s):  
C. BARRABÈS ◽  
P. A. HOGAN
Keyword(s):  
Black Holes ◽  
Neutron Star ◽  
Neutron Stars ◽  
Gravitational Wave ◽  
Gravitational Radiation ◽  
High Speed ◽  
Compact Objects ◽  
Speed Of Light ◽  
Gravitational Fields ◽  
Multipole Source

A general characterisation of an impulsive light–like signal was given1,2. The signal may consist of a shell of null matter and/or an impulsive gravitational wave. Both parts of the signal can be unambiguously identified3,4. The signals can be used to model bursts of gravitational radiation and light– like matter accompanying cataclysmic events such as supernovae and neutron star collisions. Also in high speed collisions of compact objects such as black–holes or neutron stars the gravitational fields of these objects resemble those of impulsive light–like signals when the objects are boosted to the speed of light. Several examples of impulsive light–like signals were presented, in particular those produced by recoil effects5 and by the Aichelburg–Sexl boost of an isolated gravitating multipole source6. The detection of these signals was also discussed7.

scholarly journals Returning radiation in strong gravity around black holes: reverberation from the accretion disc

2020 ◽  
Vol 498 (3) ◽  
pp. 3302-3319
Author(s):  
D R Wilkins ◽  
J A García ◽  
T Dauser ◽  
A C Fabian
Keyword(s):  
Black Holes ◽  
Strong Field ◽  
Time Lag ◽  
Accretion Disc ◽  
X Rays ◽  
X Ray ◽  
Gravitational Fields ◽  
Strong Gravity ◽  
General Relativistic ◽  
Strong Gravitational Fields

ABSTRACT We study reflected X-ray emission that returns to the accretion disc in the strong gravitational fields around black holes using General Relativistic ray-tracing and radiative transfer calculations. Reflected X-rays that are produced when the inner regions of the disc are illuminated by the corona are subject to strong gravitational light bending, causing up to 47 per cent of the reflected emission to be returned to the disc around a rapidly spinning black hole, depending upon the scale height of the corona. The iron Kα line is enhanced relative to the continuum by 25 per cent, and the Compton hump by up to a factor of 3. Additional light traveltime between primary and secondary reflections increases the reverberation time lag measured in the iron K band by 49 per cent, while the soft X-ray lag is increased by 25 per cent and the Compton hump response time is increased by 60 per cent. Measured samples of X-ray reverberation lags are shown to be consistent with X-rays returning to the accretion disc in strong gravity. Understanding the effects of returning radiation is important in interpreting reverberation observations to probe black holes. Reflected X-rays returning to the disc can be uniquely identified by blueshifted returning iron K line photons that are Compton scattered from the inner disc, producing excess, delayed emission in the 3.5–4.5 keV energy range that will be detectable with forthcoming X-ray observatories, representing a unique test of General Relativity in the strong field limit.

Dynamic spherical collapses towards growing black holes in relativistically degenerate or hot host mass reservoirs

2019 ◽  
Author(s):  
Yu-Qing Lou ◽  
Wei Shen
Keyword(s):  
Black Holes ◽  
Compact Objects ◽  
Effective Pressure ◽  
Solution Type ◽  
Hot Matter ◽  
Important Solution ◽  
Supernova Explosions ◽  
General Relativistic ◽  
Self Similar ◽  
Similar Dynamic

Abstract We study the self-similar spherical hydrodynamics of γ = 4/3, with or without shocks, of general polytropic (GP) relativistically degenerate or hot matter in global mass reservoirs under the Paczynski-Wiita (PW) gravity that retains essential general relativistic (GR) effects for dynamic accretions onto growing Schwarzschild black holes (BHs). Various self-similar dynamic solutions can be derived and constructed. Among others, one important solution type is the expansion-wave collapse solution (EWCS) with a central BH event horizon in expansion. Such GP dynamic collapses are highly efficient for the rapid dynamic growth of supermassive and hypermassive BHs (SMBHs and HMBHs) in the Universe including the early Universe, in contrast to much slower disk mass accretion and BH merger processes. Particularly, the extra degree of freedom for the effective pressure of those relativistically degenerate or hot mass reservoirs makes it possible to construct for EWCSs with even higher efficiency for BH growths. Pertinent astrophysical applications of our model also include dynamic core collapses of massive or very massive stars or supermassive stars and compact objects as well as supernova explosions. Under joint actions of effective pressure and the PW gravity, self-similar dynamic solutions with central expanding spherical voids or cavities with either sharp or smooth density transitions across their rims can emerge. Dynamic expansion shocks are also constructed and examined.

scholarly journals PSEUDO-NEWTONIAN GRAVITATIONAL POTENTIAL FOR SCHWARZSCHILD–DE SITTER SPACE–TIMES

2008 ◽  
Vol 17 (11) ◽  
pp. 2089-2105 ◽  
Author(s):  
ZDENĚK STUCHLÍK ◽  
JIŘÍ KOVÁŘ
Keyword(s):  
Black Holes ◽  
Gravitational Field ◽  
Accretion Disks ◽  
Gravitational Potential ◽  
Test Particle ◽  
De Sitter Space ◽  
Spherically Symmetric ◽  
De Sitter ◽  
General Relativistic ◽  
The Universe

Pseudo-Newtonian gravitational potential describing the gravitational field of static and spherically symmetric black holes in the universe with a repulsive cosmological constant is introduced. In order to demonstrate the accuracy of the pseudo-Newtonian approach, the related effective potential for test particle motion is constructed and compared with its general-relativistic counterpart given by the Schwarzschild–de Sitter geometry. The results indicate that such an approach could be useful in applications of developed Newtonian theories of accretion disks in astrophysically interesting situations in large galactic structures for the Schwarzschild–de Sitter space–times with the cosmological parameter y = Λ M2/3 ≤ 10-6.

scholarly journals From fundamental physics to tests with compact objects in metric-affine theories of gravity

2020 ◽  
Vol 29 (11) ◽  
pp. 2041007
Author(s):  
Diego Rubiera-Garcia
Keyword(s):  
Black Holes ◽  
General Relativity ◽  
Strong Field ◽  
Compact Objects ◽  
Fundamental Physics ◽  
Theories Of Gravity

This paper provides a short but comprehensible overview of some relevant aspects of metric-affine theories of gravity in relation to the physics and astrophysics of compact objects. We shall highlight the pertinence of this approach to supersede General Relativity on its strong-field regime, as well as its advantages and some of its difficulties. Moreover, we shall reflect on the present and future opportunities to test its predictions with relativistic and nonrelativistic stars, black holes, and other exotic horizonless compact objects.

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