- 3066 - BLACKHOLES - mysteries left to be solved? Regardless of where or how they’re found, primordial blackholes could tell astronomers a lot about the universe we live in. Depending on their mass, they could serve as probes into galaxy evolution, high-energy physics, and even the earliest fractions of a second after the universe was birthed.
--------------------- 3066 - BLACKHOLES - mysteries left to be solved?
- On May 21, 2019, a ripple in spacetime alerted scientists to what they thought was an impossible event: a collision between two black holes that should not have existed.
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- The LIGO and Virgo gravitational wave observatories had witnessed over a dozen blackhole collisions, but this merger was different. Both blackholes were situated in the “mass gap,” a range of masses that, for blackholes, should be forbidden, according to our understanding of physics.
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- Blackholes form when stars collapse at the end of their lives. But, they must be big enough stars; the smallest ones become white dwarfs or neutron stars instead.
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- While a star lives, the nuclear reactions and radiation in its interior provide an outward pressure that balances the inward pull of its gravity. When that balance is lost, a core-collapse supernova can leave behind a blackhole with at most 50 times the mass of the sun.
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- That is what happens to medium-sized stars. In the cores of larger stars, high densities and temperatures trigger the creation of electron-positron pairs, resulting in a more powerful explosion called a “pair-instability supernova“.
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- These electron-positron pairs provide gravity but no pressure, so the star starts to collapse prematurely. The star becomes so hot that you can start to do nuclear reactions with the oxygen in the core. Then because the oxygen burns, you have this immediate explosion, and you’re left with nothing. No remnant, no black hole.
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- The most massive stars meet yet another end; they can bypass the explosion to collapse into a blackhole weighing at least 120 solar masses.
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- A blackhole can form with a mass less than about 50 or more than 120 times that of the sun, but no known mechanism allows a dying star to become a blackhole with a mass in the gap between 50 and 120 solar mass.
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- The “gravitational waves” spotted by LIGO and Virgo revealed blackholes weighing 66 and 85 solar masses. The calculations held up to much scrutiny and skepticism. Maybe the two blacholes that merged were in turn the children of prior mergers, or perhaps they were born below the mass gap and grew by gobbling up nearby objects.
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- Some scientists question the LIGO/Virgo analysis, proposing instead that the larger blackhole sits above the gap and the smaller below it.
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- Other scenarios explored look for an explanation at the tiniest scale, that is, particle physics beyond the Standard Model.
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- Particles that are candidates for dark matter, that mysterious substance that forms 85% of the universe’s matter, could also affect the inner workings of stars. Photons could occasionally transform into “hidden photons” that interact very weakly with ordinary matter and have a tiny but nonzero mass. While ordinary photons are continually absorbed and reemitted within a star, hidden photons would escape unscathed, carrying away some of the star’s energy.
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- That star would burn through its helium faster, which simulations suggest would give the star less oxygen in its old age. Having less oxygen, the star would need a larger mass to cross the threshold for a “pair-instability supernova“. Thus, blackholes heavier than 50 solar masses could form.
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- Other hypothesized particles called “axions” would have a similar effect. The presence of weakly interacting particles would affect more than just the final phase of a star’s life. As a result, scientists can use astrophysical observations to place limits on the properties of these theoretical particles.
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- New particle interactions could significantly shift blackhole masses while remaining compatible with observations of all types of stars. But if the particles have the right mass they could be created only in massive, hot stars. So, undiscovered particles cannot be ruled out as the reason we see these seemingly impossible blackholes.
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- Another proposal hinges not on extra particles, but on extra spatial dimensions. Physicists have long speculated that in addition to the three dimensions we see, more dimensions could lie curled up at the subatomic scale. If these dimensions are large enough, energy from the interiors of stars could leak into them.
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- In contrast to hidden particles carrying energy away from the star, the extra dimensions would hide energy within the star, but the result would be the same: Both the lower and upper bounds of the mass gap would increase.
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- A third possibility, “modified gravity“ MOND, , would overturn an assumption held by both Isaac Newton and Albert Einstein. The inherent strength of gravity, instead of being constant throughout the entire universe, could depend on the cosmic environment.
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- So different regions in space would have different mass gaps. In regions where gravity is stronger, both pair-instability supernovae and the shortcut taken by the largest stars would kick in at lower masses, putting the mysterious black holes above the local mass gap rather than within it.
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- In October, 2020, the LIGO/Virgo collaboration published its latest batch of data, bringing the running total to 47 blackhole mergers, including two more that seem to feature at least one blackhole in the mass gap. And a new gravitational-wave observatory in Japan, KAGRA, ran for two months earlier this year.
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- Astronomers don't understand the origins of the biggest blackholes in the universe. These blackholes appear so early in the cosmological record that we might have to invoke new physics to explain their appearance.
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- New research proposes an intriguing origin story: the first black holes didn't come from stars but from clumps of super-exotic, super-hypothetical particles known as “gravitinos” that managed to survive the first chaotic years of the Big Bang.
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- The largest blackholes in the universe, appropriately named "supermassive blackholes", sit at the centers of almost every galaxy in the cosmos. Even the Milky Way has one, a monster at 4,000,000 solar masses, designated as Sagittarius A*.
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- In the past decade astronomers have revealed the existence of supermassive blackholes at the very dawn of stars and galaxies, when the universe wasn't even a billion years old.
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- The only way to form blackholes is through the deaths of massive stars. When they die, they leave behind a blackhole a few times more massive than the sun. To get to supergiant status, they have to merge with other blackholes and/or consume as much gas as possible, bulking up all those millions of solar masses. That takes time. A lot of time.
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- In the early universe, stars themselves took hundreds of millions of years to first appear. And as far as we can tell, right alongside that first generation of stars and galaxies were supermassive blackholes. There doesn't appear to have been enough time for those giant blackholes to form through the usual and customary stellar death route, so something else is going on.
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- Either we don't understand something fundamental about the astrophysics of blackhole growth, or, the first, giant blackholes actually formed in a much earlier, much more primordial epoch. But in order for that to happen, the physics that created those possible first blackholes has to be something different.
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- This goes far, far beyond the current boundaries of known physics. One such example is called “super symmetry“, and it's an attempt by physicists to both explain some of the inner workings of the particle world and to predict the existence of brand-new particles.
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- In supersymmetry, every particle of the “Standard Model of Particle Physics” is paired with a partner. The reason for this pairing is a fundamental symmetry found deep in the mathematics that might describe nature. But this symmetry is broken , so the super-symmetry partner particles don't simply float around in the world or make grand entrances in our particle colliders.
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- Instead, because of the broken symmetry, the partner particles are forced to have incredible masses, so high that they can only appear in the highest-energy reactions in the universe.
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- So far, we haven't found any evidence for supersymmetry partner particles in our collider experiments, but we're still looking.
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- Within the various models and possibilities of supersymmetry there is a particle known as the “gravitino“. The gravitino is the supersymmetry partner particle of the “graviton“, which itself is the hypothetical particle that carries the force of gravity.
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- The existence of the gravitino is highly speculative and not based on any existing evidence. If you want to make some blackholes in the early universe, you have to pass a few challenges. Well before the first stars and galaxies appeared, our universe was dominated by radiation, this high-energy light flooded the cosmos.
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- If you want to create some random blackholes in that radiation-dominated epoch, you have to do it fast, because that era in our universe was extremely chaotic. And once you form the blackholes, you have to keep them alive.
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- Blackholes evaporate through a quantum mechanical process known as “Hawking radiation“, and small blackholes can quickly disappear before they get a chance at greatness, let alone supermassiveness.
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- The high-energy early universe could have had just the right conditions to populate the universe with gravitinos. Because of their unique properties, they could quickly form microscopic blackholes.
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- As time goes on in the early universe, the blackholes could grow large enough that they could feast on the surrounding radiation before succumbing to Hawking evaporation. Once the radiation cleared away, they could be big enough to continue collecting matter through normal astrophysical processes, providing the seeds for the first giant blackholes.
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- All the blackholes that astronomers have seen fall into one of three categories: stellar-mass black holes, intermediate-mass black holes, and supermassive black holes. Each is more massive than our Sun and formed at least hundreds of thousands of years after the Big Bang, as our universe grew and evolved.
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But there is another type of blackhole astronomers haven’t yet seen, but think could exist. These are “primordial blackholes“. Primordial black holes were born very early in the life of the universe, a mere fraction of a second after the Big Bang.
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- It was a time long before stars or galaxies and other types of blackholes could exist. These theories predict that primordial blackholes should have popped onto the scene anyway. That’s because in that fraction of a second after the universe itself began, space was not completely homogenous, the same at every point. Instead, some areas were denser and hotter than others, and these dense regions could have collapsed into blackholes.
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- There was only a small period of time, about 1 second, following the Big Bang when primordial black holes could have formed. But in the extreme world of our expanding early universe, a lot can happen in just one second.
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- The later in this window of time that primordial blackholes formed, the more massive they would be. Depending on when exactly they formed, primordial blackholes could have masses as low as 10-7 ounces (10-5 grams), or 100,000 times less than a paperclip, up to about 100,000 times greater than the Sun.
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- The idea of such tiny blackholes intrigued astrophysicist Stephen Hawking, who explored their quantum mechanical properties. That work led to his 1974 discovery that blackholes can evaporate over time.
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- Hawking ultimately realized a large blackhole would evaporate away in more time than the universe has been around so far, small black holes could have indeed evaporated away or currently be doing so, depending on their mass.
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- Hawking calculated that any primordial blackhole with a mass greater than 10^12 pounds; that’s far less than the mass of any planet, dwarf planet, and most named asteroids and comets in our solar system could still be around today, while those less massive would have already disappeared.
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- And depending on their mass any primordial blackholes left today could neatly explain some of the outstanding problems in astronomy.
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- Although dark matter makes up about 30 percent of our universe, astronomers remain stumped as to what exactly dark matter is. Primordial blackholes could be the answer.
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- Primordial blackholes could be a type of dark matter called MACHOs, which stands for “massive compact halo objects“, because astronomers think they’re found in the halos, or outskirts, of galaxies.
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- Such blackholes would be difficult to see if they’re simply floating quietly in space and keeping to themselves. One way to spot MACHOs is by looking for events called “microlensing“, which occur when a massive object such as a blackhole passes in front of a more distant object, like a star or galaxy.
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- The blackhole bends the light from the distant source around it, brightening and magnifying the image. These events are infrequent and short lived, but catching enough of them could allow astronomers to determine what the objects doing the microlensing are and whether they could be primordial blackholes.
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- However, several recent studies have determined that even if primordial blackholes of this type exist, they probably can’t explain all or even most of the dark matter effects we see.
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- Another way to search for large primordial black holes is through mergers. Gravitational-wave observatories like LIGO and VIRGO have already seen several black hole mergers, and future projects like LISA will be detect mergers of different masses than the ones we can currently spot.
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- Because astronomers can trace back the masses of the merging blackholes, they could find that future events were caused by blackholes with the right masses to make them primordial blackholes.
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- Primordial blackholes could be tiny. Some theories hold that although blackholes evaporate, there may be a size limit. When an evaporating blackhole reaches a certain mass, it stops evaporating and simply stays very small. If this is the case, primordial black holes could still account for dark matter, albeit in a different way, and searching them out would be more challenging.
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- Perhaps astronomers could spot blackholes that are still evaporating, which would give off energetic particles, which in turn give off gamma rays. If blackholes do eventually pop out of existence without stopping, they could die in intense blasts of energy, equivalent to about one million 1-megaton hydrogen bombs, which we might also spot as bursts of gamma rays.
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- Even if they don’t account for dark matter, there is a second problem in astrophysics that primordial blackholes could answer. Primordial blackholes of a larger size than those needed to explain dark matter might instead explain the supermassive blackholes astronomers see in the centers of massive galaxies.
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- These blackholes, millions or billions of times the mass of the Sun, can’t be created by one or even several exploding stars. Astronomers don’t know how these blackholes got there or what created them; perhaps they are built from primordial blackholes that have been around since the first second of our universe, serving as seeds out of which supermassive blackholes could grow.
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- This possibility may also not be likely, because primordial blackholes had to form by the time the universe was just 1 second old. Even primordial blackholes that formed at the last possible instant possible would be, only about 100,000 times as massive as the Sun, which is not really in the supermassive blackhole weight class.
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- To get the even larger blackholes we see today, they’d have to pull in a lot of material and grow very quickly. This isn’t impossible, but it may be less likely to explain the sheer number of supermassive blackholes that exist today.
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- Although primordial blackholes could exist, they have yet to be seen, and currently remain one of astronomy’s great questions. You like a mystery, solve this one.
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March 1, 2021 BLACKHOLES - mysteries left to be solved? 3066
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