Electromagnetic field and cylindrical compact objects in modified gravity

We study the cracking of compact object PSR J1614-2230 in quadratic regime with electromagnetic field. For this purpose, we develop a general formalism to determine the cracking of charged compact objects. We apply local density perturbations to hydrostatic equilibrium equation as well as physical variables involved in the model. We plot the force distribution function against radius of the star with different parametric values of model both with and without charge. It is found that PSR J1614-2230 remains stable (no cracking) corresponding to different values of parameters when charge is zero, while it exhibits cracking (unstable) when charge is introduced. We conclude that stability region increases as amount of charge increases.

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

Galaxies ◽

10.3390/galaxies9040075 ◽

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.

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Quasi static evolution of compact objects in modified gravity

General Relativity and Gravitation ◽

10.1007/s10714-021-02887-z ◽

2022 ◽

Vol 54 (1) ◽

Author(s):

Z. Yousaf ◽

Kazuharu Bamba ◽

M. Z. Bhatti ◽

U. Farwa

Keyword(s):

Modified Gravity ◽

Compact Objects

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Dark energy and modified gravity in degenerate higher-order scalar–tensor (DHOST) theories: A review

International Journal of Modern Physics D ◽

10.1142/s0218271819420069 ◽

2019 ◽

Vol 28 (05) ◽

pp. 1942006 ◽

Cited By ~ 48

Author(s):

David Langlois

Keyword(s):

Dark Energy ◽

Modified Gravity ◽

Higher Order ◽

Second Order ◽

Compact Objects ◽

Degree Of Freedom ◽

Scalar Mode

This paper reviews scalar–tensor theories characterized by a Lagrangian that, despite the presence of second-order derivatives, contains a single scalar degree of freedom. These theories, known as Degenerate Higher-Order Scalar–Tensor (DHOST) theories, include Horndeski and Beyond Horndeski theories. They propagate a single scalar mode as a consequence of the degeneracy of their Lagrangian and, therefore, are not plagued by an Ostrogradsky instability. They have been fully classified up to cubic order in second-order derivatives. The study of their phenomenological consequences restricts the subclass of DHOST theories that are compatible with observations. In cosmology, these theories can be described in the language of the unified effective approach to dark energy and modified gravity. Compact objects in the context of DHOST theories are also discussed.

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