Violation of universal lower bound for the shear viscosity to entropy density ratio in dark energy dominated accretion

Dutta, Sandip; Biswas, Ritabrata

doi:10.1140/epjc/s10052-019-7050-7

Cited by 4 publications

(3 citation statements)

References 39 publications

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“…We have seen in the works [34][35][36] that the accretion disc is truncated as Chaplygin gas is accreted. The cause is better understood if we study the λ λ k curves.…”

Section: Brief Discussion and Conclusionmentioning

confidence: 99%

“…Physically, this interprets that matter is being thrown out with extreme speed and nothing is allowed to fall in. Viscosity is added to this model and studied in [34][35][36] and [37] to establish that viscosity will catalyze the whole process of accretion and as a result, the density of the accreting fluid drops drastically as we proceed towards the central black hole. Other physical quantities like Mach number, spin speed etc were not studied till date.…”

Section: Introductionmentioning

confidence: 99%

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Viscous dark energy accretion activities : sonic speed, angular momentum and Mach number studies

Dutta

Biswas

2021

Eur. Phys. J. C

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In this present article, we study different accretion properties regarding viscous accretion of dark energy. Modified Chaplygin gas is chosen as the dark energy candidate. Viscosity is encountered with the help of Shakura–Sunyaev viscosity parameter. We study sonic speed vs radial distance curves. We compare between adiabatic and dark energy dominated cases and follow that sonic speed falls as we go nearer to the central gravitating object. As viscosity is imposed, a threshold drop in accretion sonic speed is followed. Average rate of fall in accretion sonic speed is increased with black hole’s spin. This is signifying that this kind of accretion is weakening the overall matter/energy infall. Specific angular momentum to Keplerian angular momentum ratio is found to fall as we go far from the black hole. Accretion Mach number turns high as we go towards the inner region and high wind Mach number is not allowed as we are going out. Combining, we conclude that the system weakens the feeding process of accretion.

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“…We have seen in the works [34][35][36] that the accretion disc is truncated as Chaplygin gas is accreted. The cause is better understood if we study the λ λ k curves.…”

Section: Brief Discussion and Conclusionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Viscous dark energy accretion activities : sonic speed, angular momentum and Mach number studies

Dutta

Biswas

2021

Eur. Phys. J. C

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“…Dutta and Biswas have considered viscous MCG accretion in the reference [38]. Presently, the constraint of dissipation from the gravitational wave-event GW150914 predicted a nonzero value of the shear viscosity of DE through which the gravitational wave is propagating [39].…”

Section: Introductionmentioning

confidence: 99%

Search for missing links between two extreme wind speed profiles: dark energy accretion and adiabatic fluid accretion

Roy

Biswas

2020

Eur. Phys. J. C

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In the recent past, progress in accretion studies onto general relativistically gravitating central objects viz. a Schwarzschild singularity reveals that the accretion flow should be transonic. Regarding such cases, radial inward speed gradient might be written as a numerator over denominator form among which the denominator vanishes somewhere in between infinite distance to the event horizon of the attractor. For sustainability of a physical solution, the numerator should also have to be equal to zero at the same radial distance where the denominator does vanish. From this point, using L'Hospital's rule, we obtain a second degree first order differential equation of radial inward speed. Hence, using the initial conditions at the said radial distance, we obtain two branches of flow by the virtue of two first order differential equations. These branches are named as accretion and wind. For adiabatic accretion case, the slope of the wind curve in speed vs radial distance plane is formed to be more or less parallel to the radial distance axis as we move far from the central object. For dark energy accretion, alignment of this curve is parallel to the radial velocity axis. Here we face a question why there is no fluid speed profile in between these two extremities. While searching for the reasons, we follow that dark energy, if treated as an accreting object, should stay around the central compact star and hence will contaminate the metric which properties the compact star. In this research work, we have proposed a model with a rotating black hole embedded in quintessence where quintessence equation of state and spin parameter of the black hole are together working as the regulatory factors of the model. The resulting accretion and wind curves are studied. The Effect of negative pressure of dark energy is found to get catalyzed by the entry of the spin of the black hole. We tally our results with observations of accretion or outflow phenomenon near to different quasars.

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Concerns regarding the use of black hole shadows as standard rulers

Vagnozzi¹,

Bambi²,

Visinelli³

2020

Class. Quantum Grav.

135

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Recently, Tsupko et al. have put forward the very interesting proposal to use the shadows of highredshift supermassive black holes (SMBHs) as standard rulers. This would in principle allow us to probe the expansion history within a redshift range which would otherwise be challenging to access. In this note, we critically examine this proposal, and identify a number of important issues which had been previously overlooked. These include difficulties in obtaining reliable SMBH mass estimates and reaching the required angular resolution, and an insufficient knowledge of the accretion dynamics of high-redshift SMBHs. While these issues currently appear to prevent high-redshift SMBH shadows from being used as robust standard rulers, we hope that our flagging them early will help in making this probe theoretically mature by the time it will be experimentally feasible.

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Violation of universal lower bound for the shear viscosity to entropy density ratio in dark energy dominated accretion

Cited by 4 publications

References 39 publications

Viscous dark energy accretion activities : sonic speed, angular momentum and Mach number studies

Viscous dark energy accretion activities : sonic speed, angular momentum and Mach number studies

Search for missing links between two extreme wind speed profiles: dark energy accretion and adiabatic fluid accretion

Concerns regarding the use of black hole shadows as standard rulers

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