* Astronomy

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RE: Black Holes

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Title: How fast can a black hole rotate?
Author: Carlos A. R. Herdeiro, Eugen Radu

Kerr black holes have their angular momentum, J, bounded by their mass, M: Jc \leqslant GM². There are, however, known black hole solutions violating this Kerr bound. We propose a very simple universal bound on the rotation, rather than on the angular momentum, of four-dimensional, stationary and axisymmetric, asymptotically flat black holes, given in terms of an appropriately defined horizon linear velocity, vH. The vH bound is simply that vH cannot exceed the velocity of light. We verify the vH bound for known black hole solutions, including some that violate the Kerr bound, and conjecture that only extremal Kerr black holes saturate the vH bound.

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NASA's Chandra Suggests Black Holes Gorging at Excessive Rates

Astronomers have studied 51 quasars with NASA's Chandra X-ray Observatory and found they may represent an unusual population of black holes that consume excessive amounts of matter, as described in our latest press release. Quasars are objects that have supermassive black holes that also shine very brightly in different types of light. By examining the X-ray properties with Chandra, and combining them with data from ultraviolet and visible light observations, scientists are trying to determine exactly how these large black holes grow so quickly in the early Universe.
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Title: Giant black hole ringings induced by massive gravity
Author: Yves Decanini, Antoine Folacci, Mohamed Ould El Hadj

A distorted black hole radiates gravitational waves in order to settle down in one of the geometries permitted by the no-hair theorem. During that relaxation phase, a characteristic damped ringing is generated. It can be theoretically constructed from the black hole quasinormal frequencies (which govern its oscillating behavior and its decay) and from the associated excitation factors (which determine intrinsically its amplitude) by carefully taking into account the source of the distortion. Here, by considering the Schwarzschild black hole in the framework of massive gravity, we show that the excitation factors have an unexpected strong resonant behavior leading to giant ringings which are, moreover, slowly decaying. Such extraordinary black hole ringings could be observed by the next generations of gravitational wave detectors and allow us to test the various massive gravity theories or their absence could be used to impose strong constraints on the graviton mass.

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Obese black holes outshone stars in earliest galaxies

Obese black holes, not stars, may have lit up the first galaxies and could have grown into the earliest supermassive black holes. A new study suggests that these fatties were numerous and bright enough that we should be able to detect them now, billions of years after they shone.
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NASA Chandra, Spitzer Study Suggests Black Holes Abundant Among The Earliest Stars

By comparing infrared and X-ray background signals across the same stretch of sky, an international team of astronomers has discovered evidence of a significant number of black holes that accompanied the first stars in the universe.
Using data from NASA's Chandra X-ray Observatory and NASA's Spitzer Space Telescope, which observes in the infrared, researchers have concluded one of every five sources contributing to the infrared signal is a black hole.
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Quantum gravity takes singularity out of black holes

Falling into a black hole may not be as final as it seems. Apply a quantum theory of gravity to these bizarre objects and the all-crushing singularity at their core disappears.
In its place is something that looks a lot like an entry point to another universe. Most immediately, that could help resolve the nagging information loss paradox that dogs black holes.
Though no human is likely to fall into a black hole anytime soon, imagining what would happen if they did is a great way to probe some of the biggest mysteries in the universe. Most recently this has led to something known as the black hole firewall paradox - but black holes have long been a source of cosmic puzzles.
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Black Hole inner-disk winds

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RE: Black Holes

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Explaining black holes

University researchers have discovered a new property of black holes: their dying tones could reveal the cosmic crash that produced them.
Black holes are regions of space where gravity is so strong that not even light can escape and so isolated black holes are truly dark objects and don't emit any form of radiation.
However, black holes that get deformed, because of other black holes or stars crashing into them, are known to emit a new sort of radiation, called gravitational waves, which Einstein predicted nearly a hundred years ago.
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