Black Hole 2021

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Spinning black holes could deform under an external and static gravitational field
This image depicts a rotating black hole with spin S and mass M being deformed by an external tidal field $\mathcal{E}_{ij}$. Credit: Le Tiec & Casals.
An open question among the physics community is whether black holes can be tidally deformed by an external gravitational field. If this were confirmed to be true, it could have important implications for many areas of physics, including fundamental physics, astrophysics and gravitational-wave astronomy.
Researchers at Observatoire de Paris- CNRS and Centro Brasileiro de Pesquisas Fisicas (CBPF) recently carried out a study investigating the tidal deformability of black holes under an external, static gravitational field. Their paper, published in Physical Review Letters, suggests that under such a field, spinning black holes could generally deform.
"The idea for this work partly arose from a couple of talks during the International Conference on General Relativity and Gravitation (GR22) in 2019," Marc Casals, one of the researchers who carried out the study, told Phys.org. "During these talks, the speakers discussed the deformability of neutron stars due to an external gravitational tidal field. They also mentioned that, contrarily to neutron stars, the (static) tidal deformability of non-rotating black holes is zero, as shown by several studies. This result immediately begged the question of whether the (static) tidal deformability of rotating black holes is also zero."
The deformability of rotating black holes under a static gravitational field had already been investigated by a team of researchers at Sapienza University of Rome. In a paper published in 2015, these researchers showed that when the static tidal field is symmetric with respect to a black hole's axis of rotation, the black hole's deformability is zero.
In their study, Casals and his colleague Alexandre Le Tiec wanted to investigate the deformability of rotating black holes when the tidal field applied to them is arbitrary (i.e., not necessarily axi-symmetric). This is a particularly important question, as all astrophysical black holes are believed to be rotating; thus, any external tidal fields would typically not be axi-symmetric.
"Past papers gave us some clues as to what methods to use," Casals explained. "One of them was a specific mathematical technique: Letting the so-called multipolar index temporarily take on real numbers, whereas its physical values are meant to be purely integer numbers (e.g., 2, 3, 4, ...)."
The mathematical technique used by Casals and Le Tiec can be used to disentangle the tidal deformation of a black hole from the external tidal field that caused it, in order to then set the multipolar index to be a physical integer number. Despite its advantages, however, this technique is probably difficult to use directly on equations that are satisfied by the gravitational field itself.
"Instead, we applied it first to another quantity, which involves derivatives of the gravitational field (it essentially measures the curvature of the spacetime) and, crucially, satisfies a simpler equation which was derived in a past paper by S. Teukolsky," Casals said. "From this quantity, we can then obtain the gravitational field."
The measurement of a gravitational field depends on who its 'observer' is, or, in mathematical terms, on the coordinate system. Therefore, as a final step, Casals and Le Tiec built quantities that are independent of the observer (or coordinates), so that they could identify the tidal deformability of rotating black holes in a way that was truly meaningful.
"These observer-independent quantities are the so-called Geroch-Hansen multipole moments, named after the authors who came up with them (namely, R. P. Geroch in 1970 and R.O. Hansen in 1974)," Casals said.
Overall, the calculations carried out by this team of researchers show that rotating black holes generically deform under an external and static gravitational field. This result is in stark contrast with past study findings related to non-rotating black holes or rotating black holes with an axi-symmetric tidal field.
"We calculated this deformation explicitly for the case of a weak tidal field with multipolar index equal to 2 and for small black hole rotation," Casals said. "Furthermore, we linked this tidal deformation to the previously known effect of tidal torquing; a change in the angular momentum of the black hole due to the tidal field."
The findings gathered by Casals and Le Tiec could pave the way for more studies investigating the deformability of spinning black holes under a static tidal field. In their paper, the researchers also speculate on the possibility that such a tidal deformation could be observed within the gravitational waves expected to be detected by the Laser Interferometer Space Antenna (LISA) mission, which is planned for 2034.
"Our research can naturally be extended in a number of directions," Alexandre Le Tiec told Phys.org. "We could, for instance, investigate the tidal deformability of spinning black holes: (i) for multipolar index higher than 2; (ii) for large black hole rotation; or (iii) for a strong tidal field. It would also be interesting to explore the precise link between tidal deformability, tidal heating and the nonzero viscosity of the event horizon of black holes within the so-called membrane paradigm."
More information: Spinning black holes fall in love. Physical Review Letters(2021). DOI: 10.1103/PhysRevLett.126.131102.
Relativistic tidal properties of neutron stars. Physical Review D(2009). DOI: 10.1103/PhysRevD.80.084035.
Tidal deformations of a spinning compact object. Physical Revew D(2015). DOI: 10.1103/PhysRevD.92.024010.
Perturbations of a rotating black hole. I. Fundamental equations for gravitational, electromagnetic, and neutrino-field perturbations. Astrophysical Journal(1973). DOI: 10.1086/152444.
Absorption of mass and angular momentum by a black hole: time-domain formalisms for gravitational perturbations, and the small-hole or slow-motion approximation. Physical Review D(2004). DOI: 10.1103/PhysRevD.70.084044.
Black holes: the membrane paradigm. Yale University Press(1986). ui.adsabs.harvard.edu/abs/1986 … .book.....T/abstract
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Spinning black holes could deform under an external and static gravitational field
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Updated 2024 GMT (0424 HKT) March 30, 2021
(CNN)Astronomers have a new, more complete picture of the supermassive black hole at the center of a galaxy 55 million light-years from Earth -- the first black hole ever to be imaged.
While the first image of this black hole and its shadow was released in 2019, the new image released Wednesday shows the cosmic body in polarized light.
Think about your polarized sunglasses, which help reduce glare and reflections of brightness. Light can also be polarized when it's emitted in hot regions of space near magnetic fields.
In this case, analyzing how the light around this black hole at the center of the M87 galaxy is polarized allowed astronomers a sharper view and the ability to map magnetic field lines near its inner edge. The scientists also discovered that a significant amount of light around the black hole is polarized.
The Event Horizon Telescope collaboration used a global network of telescopes in April 2017 to capture the first-ever picture of a black hole, which the team shared in 2019. It was the first direct visual evidence that black holes exist, the researchers said.
In the new image, astronomers have been able to learn more about how the black hole launches energetic jets of material moving near the speed of light.
These bright jets of energy and matter extend about 5,000 light-years from the center of the galaxy. While most matter near the edge of a black hole falls inside it, some of the matter is able to escape just before and is blasted out in the jets.
They were able to learn about the gas that actually produces the light in the image, as well as how the black hole grows, said study coauthor Jason Dexter, a coordinator of the EHT theory working group and assistant professor at the University of Colorado Boulder.
Multiple studies about the black hole published Wednesday in The Astrophysical Journal. More than 300 scientists around the world contributed to the research.
"Polarized light tells us about magnetic fields near the black hole, how strong they are and how they connect the black hole's accretion (eating habits) and the jet of plasma it's able to eject out of the entire galaxy," said study coauthor Sara Issaoun, a doctoral student in astrophysics at Radboud University in the Netherlands.
"Magnetic fields are a key element to understanding gas processes and feeding habits of black holes, and this is the very first time we're able to see them at play so close to a black hole event horizon."
The event horizon is defined as the boundary marking the limits of a black hole, which means nothing can cross that threshold and then escape it -- including light.
The gas around a black hole is moving incredibly fast due to the pull of gravity. Speed heats the gas to billions of degrees, causing atoms to separate and forming plasma of loose electrons and protons. These charged particles moving extremely fast create electromagnetic forces, Issaoun explained.
"Magnetic fields created play a role in how the gas moves, how turbulent it is, and how much gas can make it to the black hole and how much gets flung out at nearly the speed of light in an outflow or jet," she said. "The production of these jets of plasma is the most powerful and energetic process in the entire universe and still quite a puzzle to unravel, and we believe magnetic fields play a key role in launching and maintaining this process."
In the polarized image, astronomers realized that the magnetic field is actively pushing back and resisting the motions of the gas that is dragged around the black hole. This means that the magnetic fields at the edge of the black hole are strong enough to help the hot gas resist the pull of gravity.
"Such 'strong' magnetic fields are capable of launching the most powerful jets, and their presence would have important implications for how black holes grow," Dexter said.
8 radio telescopes capture black hole
In the new image, streaks can be seen showing light oscillating in a specific direction that indicate the strength of the magnetic field.
Imaging this activity is no easy feat.
In their attempt to capture an image of a black hole, scientists combined the power of eight radio telescopes around the world using a technique called Very-Long-Baseline-Interferometry, according to the European Southern Observatory, which is part of the EHT. This effectively creates a virtual telescope around the same size as the Earth itself.
The resolution created by this could measure the length of a credit card on the surface of the moon, the researchers said.
The EHT collaboration could also shed more light on the evolution of the black hole over the course of a year, based on the data the team already has collected, Issaoun said. The collaboration will observe the M87 black hole again to gather more information about the black hole and its jet.
"We are now seeing the next crucial piece of evidence to understand how magnetic fields behave around black holes, and how activity in this very compact region of space can drive powerful jets that extend far beyond the galaxy," said study author Monika Mościbrodzka, a coordinator of the EHT polarimetry working group and assistant professor at Radboud University, in a statement.
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