- 3488 - BLACKHOLES - the more we learn the more we don’t know. Binary blackholes can unlock another of Einstein’s predictions. The structure of a blackhole is pretty simple. All you need to know is its mass, electric charge, and rotation, and you know what the structure of space and time around the blackhole must be.
---- 3488 - BLACKHOLES - more we learn the more we don’t know.
- See Review 3487 to learn about the blackhole at the center of our Milky Way galaxy.
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- If you have two blackholes orbiting each other, then things get really complicated. Unlike a single blackhole, for which there is an exact solution to Einstein’s equations, there is no exact solution for two blackholes.
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- It’s similar to the “three-body problem” in Newtonian gravity. Although Einstein’s equations don’t have an exact solution for a binary blackhole system, there are aspects of binary blackholes that the equations predict. One of these is known as “spin-orbit resonance“.
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- When a blackhole rotates, the structure of space around it is twisted in the direction of rotation, known as “frame dragging“. When two blackholes orbit each other closely, the frame-dragging of each blackhole affects the rotation of the other. As a result, the two blackholes will tend to enter a resonance, where the rotations either align in the same way (parallel) or opposite (anti-parallel).
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- Astronomers looked at gravitational-wave data from known blackhole mergers, and found that their rotations tend to be parallel or anti-parallel. Given the small sample size, and the fact that blackhole binary rotations are never exactly aligned, there isn’t enough data to confirm the frame dragging effect.
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- One of the challenges to measuring blackhole spin is that the signal is weak. The gravitational waves we measure from distant blackhole mergers are so faint that it’s easy to get lost in the noise. Observatories such as LIGO and Virgo need to make extremely sensitive measurements, and their data must be filtered through computer models. Its the combination of data processing and computer simulation that makes the mergers detectable. Adding spin to the mix makes things even more difficult.
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- Astronomers found that the signal for spin resonance is strongest when they are just about ready to merge. That makes sense since that’s when they are closest together and when frame-dragging is strongest. But currently, the rotation information for binary blackholes is found by looking at gravitational waves while they are still orbiting each other.
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- Gravitational-wave astronomy is still a new field, and we’re still learning how to capture and analyze the data. As these new studies show, gravitational waves hold a great deal of information, and there’s plenty more we can uncover.
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- To extend the subject let’s look at a possible model to explain “dark matter“. Blackholes have long been proposed as the source of dark matter. In many ways, they are the perfect candidate because they only interact with light and matter gravitationally. But stellar-mass blackholes have been ruled out observationally. There simply aren’t enough of them to account for dark matter.
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- “Primordial blackholes” are a possible solution. Unlike “stellar blackholes” that would have a mass larger than the Sun, primordial black holes could have the mass of a mere planet or less. A planet-mass blackhole would be smaller than an apple, and an asteroid-mass blackhole could be smaller than a grain of sand.
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- They are known as primordial black holes because they are thought to have formed during the early moments of the universe. Primordial blackholes with a range of masses formed almost instantly after the big bang. Some of these blackholes could form the seeds of the first stars, and the largest primordial blackholes could have rapidly grown into supermassive blackholes by gobbling up surrounding hydrogen and helium.
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- This would explain how galaxies and their supermassive black holes seem to have formed so early in the universe. The smallest primordial black holes would be common enough to explain dark matter.
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- Being able to explain blackholes, galactic evolution, and dark matter all in one would be a tremendous theoretical boon. But the idea is useless unless the model can be proven. The James Webb telescope might be able to do just that.
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- One of the things about primordial blackholes is that they likely emit light via Hawking radiation. According to Hawking’s model, tiny black holes should cause an excess of infrared light in the early universe, which the Webb telescope should be able to pick up.
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- In the center of most galaxies lies a supermassive black hole. Some of these are actively feeding on the gas and dust around them, expelling excess energy as powerful jets that are seen as quasars across the entire observable Universe.
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- At the center of galaxies, supermassive blackholes are millions or even billion times more massive than our Sun. With their extreme gravitational pull, they are able to engulf vast amounts of gas, dust, and even stars that wander into their vicinity.
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- Physics tells us that this material tends to form a disk as it is drawn towards the blackhole in a phenomenon called "accretion." These accretion disks are some of the most uninviting, violent places in the known Universe, with velocities approaching the speed of light, and temperatures far in excess of the surface of our Sun.
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- This heat produces radiation which we see as light, but the conversion of heat to light is so efficient, 30 times more efficient than nuclear fusion,that physicists don't understand how.
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- The dietary patterns of blackholes have wide range. Some, like the one in our own Galaxy, aren't very hungry and don't seem to have accretion disks. But we see other galaxies with ravenous hunger whose supermassive black holes have grown extremely hot accretion disks so bright that they outshine all of the stars in their galaxy.
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- A new method for probing the size and structure of distant accretion disks seems promising: Although we cannot resolve the disks' various components, we can study how its intensity varies in time. By studying the variations in the disks' light astroomers can piece together a picture of the accretion disks of even the most distant galaxies.
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- Astronomers are looking into past observations of more than 9,000 galaxies with bright accretion disks, so-called quasars, from the "Sloan Digital Sky Survey." When the source is not resolved, the observed light from the accretion disk will be "contaminated" by light from the galaxy hosting the blackhole.
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- This unwanted light from the host galaxies has largely been ignored by previous studies. However, by using a new model for the variations in the quasar light astronomers were able to separate the light of the accretion disk from that of the host galaxy. This allowed them to directly see the light from the accretion disk around supermassive blackholes, even in galaxies billions of lightyears away.
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- Using several different models of cosmic dust to account for, and remove, its obscuring effects, they were able to determine how hot the accretion disk is, both near the blackhole and far from it at the edges of the disk. This difference in temperature between the hot inner disk and the cold outer disk has been theoretically predicted.
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- Astronomers found a very different picture of the temperature of the disk: the disks turned out to be even hotter near the blackhole than predicted. This suggests that our assumptions and theoretical models need to be revised with consequences for our understanding of supermassive blackholes altogether.
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- Not only do we have more to learn about supermassive blackholes, but the variations in their ravenous hunger are a marvelous demonstration that our Universe is a far more dynamic place than one would expect looking at the static night time sky.
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March 3, 2022 BLACKHOLES - the more we learn the more we don’t know. 3488
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