Friday, January 27, 2023

3843 - STRANGE BLACKHOLE - Buchdahl star densest object.

 

     -  3843  -   STRANGE  BLACKHOLE  -   Buchdahl star densest object.   -    An elusive object in space has posed a riddle for scientists. It looks like a black hole. It acts like a black hole. It may even smell like a black hole. But it has one crucial difference: It has no event horizon, meaning that you can escape its gravitational clutches if you try hard enough.


            -----  3843   -   STRANGE  BLACKHOLE  -   Buchdahl star densest object.

            -    It's called a Buchdahl star, and it is the densest object that can exist in the universe without becoming a black hole itself. Evidence for blackholes is everywhere we look, including the release of gravitational waves when they collide and the dramatic shadows they carve out of surrounding materials.

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            -    Astronomers also understand how black holes form: They are the remnants of the catastrophic gravitational collapse of massive stars. When giant stars die, no force in nature is capable of sustaining the stars' own weight, so these doomed behemoths just keep crushing themselves to infinity.

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            -    Astronomers know of “white dwarfs”, which contain a sun's worth of mass in a volume equivalent to Earth, and we know of “neutron stars”, which compress all that down even further into the volume of a city. But we don't know if there's anything smaller still that avoids the fate of becoming a black hole.

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            -    In 1959, German-Australian physicist Hans Adolf Buchdahl explored how a highly idealized "star", represented as a perfectly spherical blob of material, might behave as it was compressed as much as possible. As the blob got smaller and smaller, its density rose, making its own gravitational pull even more intense. Using the tools of Einstein's general theory of relativity, Buchdahl found an absolute lower limit to the size of that blob.

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            -    That special radius is equal to 9/4 times the mass of the blob, multiplied byNewton's gravitational constant, all divided by the speed of light squared.

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            -   The Buchdahl limit is important because it defines the densest possible object that can still avoid becoming a black hole. Below that, the blob of material must always become a black hole, at least in the theory of relativity.

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            -    Finding exotic objects that come right to the edge of that limit,  “Buchdahl stars”, has become a popular pastime of theorists.   Buchdahl stars are "black hole mimics" because their observable properties would be nearly identical, studied what happens to the energy of a hypothetical star as it begins collapsing into a Buchdahl star.

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            -    As the star collapses, it picks up gravitational potential energy, which is negative because gravity is attractive. At the same time, the interior of the star gains kinetic energy as all the particles are forced to jostle against each other in a smaller volume.

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            -   By the time the star reaches the Buchdahl limit the total kinetic energy was equal to half the potential energy.

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            -    This relationship is known as the “virial theorem”, and it applies to numerous situations in astronomy where the force of gravity is in balance with other forces. This means that a Buchdahl star could theoretically exist as a stable object with known, well-understood properties.

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            -    This finding suggests that theoretical Buchdahl stars may really be out there, and could lead to insights about the inner workings of black holes.  We can interact with a Buchdahl star and study what it's made of, which may give us clues as to what black hole interiors are like.

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            -    To date,2023, there is no known arrangement of matter that can create a Buchdahl star. Further research will be needed to discover what other properties these exotic objects might have, and what they might tell us about black holes.

            -    Astronomers have uncovered more than 400 previously hidden black holes feeding on stars and dust in the center of galaxies. It appears that many of the new black holes, discovered using NASA's Chandra X-ray Observatory, remained unknown until now because they are buried beneath cocoons of dust.

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            -    Supermassive black holes, which can be millions of times heavier than the sun, live in the center of almost every galaxy in the universe. These colossal objects produce bright beams of energy as they feed on gas, dust, and stars in their immediate vicinity, creating what are known as Active Galactic Nuclei (AGN).

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            -    AGN are particularly bright in the X-ray portion of the electromagnetic spectrum.   But certain objects have been spotted giving off tons of X-rays without the specific optical signatures associated with AGN, "X-ray bright optically normal galaxies" or "XBONGs."

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            -   The researchers identified 820 XBONGs located between 550 million and 7.8 billion light-years from Earth, the largest such sample ever built.  One possibility is that Chandra is seeing extremely distant clusters of galaxies, which would shine bright in X-rays but lack the characteristic optical signature identifying them as AGN. This could explain around 20% of the remaining XBONGs.

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            -    The final 30% are galaxies whose optical light is particularly powerful, bright enough to wash out the optical AGN signature, which could happen when such galaxies are particularly far away.

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            -    The “singularity” at the center of a black hole is the ultimate no man's land: a place where matter is compressed down to an infinitely tiny point, and all conceptions of time and space completely break down. And it doesn't really exist. Something has to replace the singularity, but we're not exactly sure what.

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            -    It could be that deep inside a black hole, matter doesn't get squished down to an infinitely tiny point. Instead, there could be a smallest possible configuration of matter, the tiniest possible pocket of volume. This is called a “Planck star”, and it's a theoretical possibility envisioned by loop quantum gravity, which is itself a highly hypothetical proposal for creating a quantum version of gravity.

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            -     In the world of loop quantum gravity, space and time are quantized, the universe around us is composed of tiny discrete chunks, but at such an incredibly tiny scale that our movements appear smooth and continuous.

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            -    This theoretical chunkiness of space-time provides two benefits. One, it takes the dream of quantum mechanics to its ultimate conclusion, explaining gravity in a natural way. And two, it makes it impossible for singularities to form inside black holes.

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            -    As matter squishes down under the immense gravitational weight of a collapsing star, it meets resistance. The discreteness of space-time prevents matter from reaching anything smaller than the Planck length (around 1.68 times 10^-35 meters).  Perfectly microscopic, but definitely not infinitely tiny.

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            -    Another attempt to eradicate the singularity, one that doesn't rely on untested theories of quantum gravity, is known as the “gravastar”.   The difference between a black hole and a gravastar is that instead of a singularity, the gravastar is filled with dark energy.

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            -     Dark energy is a substance that permeates space-time, causing it to expand outward. Dark energy is currently in operation in the larger cosmos, causing our entire universe to accelerate in its expansion.   As matter falls onto a gravastar, it isn't able to actually penetrate the event horizon (due to all that dark energy on the inside) and therefore just hangs out on the surface. But outside that surface, gravastars look and act like normal black holes.

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            -     The idea of a single point of infinite density comes from our conception of stationary, non-rotating, uncharged, rather boring black holes. Real black holes are much more interesting characters, especially when they spin.

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            -    The spin of a rotating black hole stretches the singularity into a ring. And according to the math of Einstein's theory of general relativity, once you pass through the ring singularity, you enter a wormhole and pop out through a white hole (the polar opposite of a black hole, where nothing can enter and matter rushes out at the speed of light) into an entirely new and exciting patch of the universe.

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            -    The problem with rotating black holes is the singularity, stretched into a ring, is rotating at such a fantastic pace that it has incredible centrifugal force. And in general relativity, strong enough centrifugal forces act like antigravity: they push, not pull.

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            -    This creates a boundary inside the black hole, called the “inner horizon”. Outside this region, radiation is falling inward towards the singularity, compelled by the extreme gravitational pull. But radiation is pushed by the antigravity near the ring singularity, and the turning point is the inner horizon. If you were to encounter the inner horizon, you would face a wall of infinitely energetic radiation, the entire past history of the universe, blasted into your face in less than a blink of an eye.

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            -   What's really happening inside a black hole? We don't know — and the scary part is that we may never know.

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            January 27, 2022                   3843                                                                                                                             

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            --------------------- ---  Friday, January 27, 2023  ---------------------------

             

             

             

             

                     

             

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