- 3270 - - BLACKHOLES - merging boson stars? The biggest and heaviest blackhole collision ever observed, produced by the gravitational-wave. This collision might actually be something even more mysterious: the merger of two “boson stars“.
---------------------- 3270 - BLACKHOLES - merging boson stars?
- This collision,“GW190521“, would be the first evidence of the existence of bosons . Bosons are hypothetical objects that constitute one of the main candidates to form dark matter, and dark matter makes up 27% of the Universe.
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- Gravitational waves are ripples in the fabric of spacetime that travel at the speed of light. These waves originate in the most violent events of our Universe, carrying information about their sources.
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- Since 2015, we can detect and interpret gravitational waves thanks to the two LIGO detectors (Livingston and Hanford, USA) and Virgo (Cascina, Italy) detectors. To date in 2021, these detectors have already observed around 50 gravitational-wave signals.
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- All of these were originated in the collision and merger of two of the most mysterious entities in the Universe, blackholes and neutron stars.
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- On September, 2020, the LIGO and Virgo collaborations (LVC) announced to the world the gravitational-wave signal “GW190521“. The signal was consistent with the collision of two heavy blackholes, of 85 and 66 times the mass of the sun, which produced a final blackhole with 142 solar masses. The difference was released energy.
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- This blackhole was the first of a new, previously unobserved, blackhole family: intermediate-mass blackholes. This discovery is of paramount importance, as such intermediate-mass blackholes were the missing link between two well-known blackhole families: the stellar-mass blackholes that form from the collapse of stars, and the supermassive black holes that hide in the center of almost every galaxy, including the Milky Way.
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- The heaviest of the colliding black holes (85 solar masses) could not form from the collapse of a star at the end of its life, which opens up a range of doubts and possibilities about its origin.
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- The proposed alternative explanation for the origin of this signal is the collision of two exotic objects known as “boson stars“, which are one of the most solid candidates to form what we know as dark matter and make up 27% of the Universe.
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- The estimate mass of a new particle constituent of these stars, an ultra-light boson with a mass billionth of times smaller than that of the electron.
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- The computer simulations of the event implies that the source would have different properties other than colliding blackholes . Because boson star mergers are much weaker, we infer a much closer distance than the one estimated by LIGO and Virgo. This leads to a much larger mass for the final blackhole, of about 250 solar masses.
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- Boson stars are objects almost as compact as blackholes but, unlike them, do not have a “no-return” surface. When they collide, they form a boson star that can become unstable, eventually collapsing to a blackhole, and producing a signal consistent with what LIGO and Virgo observed.
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- Unlike regular stars, which are made of what we commonly know as matter, boson stars are made up of what we know as ultralight bosons. These bosons are candidates for constituting what we know as “dark matter“.
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- This result would not only involve the first observation of boson stars, but also that of their building block, a new particle known as ultra-light boson. Such ultra-light bosons have been proposed as the constituents of what we know as dark matter, which makes up around 27% of the observable Universe.
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- One of the most fascinating results is that we can actually measure the mass of this putative new dark-matter particle, and that a value of zero is discarded with high confidence. If confirmed by subsequent analysis of this and other gravitational-wave observations, our result would provide the first observational evidence for a long-sought dark matter candidate
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- Black holes are one the most fascinating objects in the Universe. At their surface, known as the ‘event horizon’, gravity is so strong that not even light can escape from them. Usually, blackholes are quiet creatures that swallow anything getting too close to them.
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- When two blackholes collide and merge together, they produce one of the most catastrophic events in Universe: in a fraction of a second, a highly-deformed blackhole is born and releases tremendous amounts of energy as it settles to its final form. This phenomenon gives astronomers a unique chance to observe rapidly changing blackholes and explore gravity in its most extreme form.
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- Although colliding blackholes do not produce light, astronomers can observe the detected gravitational waves, ripples in the fabric of space and time, that bounce off them. After a collision, the behavior of the remnant blackhole is key to understanding gravity and should be encoded in the emitted gravitational waves.
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- Gravitational waves encode the shape of merging blackholes as they settle to their final form. Simulations of blackhole collisions using supercomputers compare the rapidly changing shape of the remnant blackhole to the gravitational waves it emits. These signals are far richer and more complex than commonly thought, allowing scientists to learn more about the vastly changing shape of the final blackhole.
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- The gravitational waves from colliding black holes are signals known as ‘chirps’. As the two blackholes approach each other, they emit a signal of increasing frequency and amplitude that indicates the speed and radius of the orbit.
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- The pitch and amplitude of the chirp increases as the two blackholes approach faster and faster. After the collision, the final remnant blackhole emits a signal with a constant pitch and decaying amplitude, like the sound of a bell being struck.
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- There are the stages to this blackhole merger:
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- First, both blackholes orbit each other, slowly approaching, during the inspiral stage.
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- Second, the two blackholes merge, forming a distorted blackhole.
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- Next, the blackhole reaches its final form.
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- When blackholes are observed from their equator, they found that the final blackhole emits a more complex signal, with a pitch that goes up and down a few times before it dies. In other words, the blackhole actually chirps several times.
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- The scientist discovered that this is related to the shape of the final blackhole, which acts like a kind of gravitational-wave lighthouse. When the two original, ‘parent’ blackholes are of different sizes, the final blackhole initially looks like a chestnut, with a cusp on one side and a wider, smoother back on the other.
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It turns out that the blackhole emits more intense gravitational waves through its most curved regions, which are those surrounding its cusp. This is because the remnant blackhole is also spinning and its cusp and back repeatedly point to all observers, producing multiple chirps.
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- Last year, 2020, the Event Horizon Telescope obtained the first image of a blackhole, known as M87. However, it is a blackhole that is rather at rest and does not undergo significant changes.
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- In the future, gravitational waves detectors could allow us to understand how newborn blackholes behave while undergoing rapid and violent changes.
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- September 6, 2021 - BLACKHOLES - merging boson stars? 3269
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