In May 2019, a ripple of gravitational waves passed through Earth after traveling across the cosmos for 7 billion years. The ripple came in four waves, each lasting just a fraction of a second. Although the ancient signal was faint, its source was cataclysmic: the biggest merger of two black holes ever observed.
It occurred when two mid-sized black holes — 66 and 85 times the mass of our Sun — drifted close together, began spinning around each other and merged into one black hole roughly 142 times the mass of our Sun.
"It's the biggest bang since the Big Bang observed by humanity," Caltech physicist Alan Weinstein, who was part of the discovery team, told The Associated Press.
A massive bang, sure. But a black hole of this size actually falls within the "intermediate-mass" category, which ranges from about 50 to 1,000 times the mass of our Sun.
Intermediate-mass black holes
Scientists know relatively little about these mid-sized black holes. They've catalogued small black holes only a few times more massive than the Sun, as well as supermassive black holes more than six billion times the mass of our star. But direct evidence of intermediate-mass black holes has remained elusive.
"Long have we searched for an intermediate-mass black hole to bridge the gap between stellar-mass and supermassive black holes," Christopher Berry, a professor at Northwestern University's Center for Interdisciplinary Exploration and Research in Astrophysics), told Northwestern Now. "Now, we have proof that intermediate-mass black holes do exist."
Still, how these middleweight black holes form is a mystery. Scientists know that smaller black holes form when stars explode in violent events called supernovas. But mid-sized black holes couldn't form this way, according to current physics, because stars of a certain mass range undergo a death process called pair instability, where they explode and leave nothing behind, not even a black hole.
This chart compares the mass of black-hole merger events observed by LIGO-Virgo.
Credit: LIGO/Caltech/MIT/R. Hurt (IPAC)
As for supermassive black holes? Scientists are pretty sure that these behemoths, which lie in the center of most galaxies, grow huge by gobbling up ancient dust, gas and other cosmic matter — including other black holes. Intermediate black holes may form in a similar way, by small-ish black holes repeatedly merging together.
In other words, an intermediate black hole might be on its way to becoming supermassive.
"We're talking here about a hierarchy of mergers, a possible pathway to make bigger and bigger black holes," Martin Hendry, a professor of gravitational astrophysics and cosmology at Glasgow University, told the BBC. "So, who knows? This 142-solar-mass black hole may have gone on to have merged with other very massive black holes — as part of a build-up process that goes all the way to those supermassive black holes we think are at the heart of galaxies."
Visualization of a black hole.
Credit: NASA
The recent discovery sheds light on how black holes form, but questions still remain. Scientists with the LIGO-Virgo collaboration hope to continue studying the newly discovered intermediate black hole — dubbed GW190521 — in 2021 when the facilities will be up and running again with improved instruments.
"Our ability to find a black hole a few hundred kilometers-wide from half-way across the Universe is one of the most striking realizations of this discovery," Karan Jani, an astrophysicist with LIGO told The Malaysian Reserve.
The discovery was described in two papers published in the Physical Review Letters and The Astrophysical Journal Letters.
Demonstrating again that space is a limitless reservoir of scientific wonders, a new study discovered why areas hosting black holes and neutron stars emit strange bright glows. Astrophysicists found that turbulence and reconnection of super-strong magnetic fields are behind the cosmic mystery.
The cause of the phenomenon, which illuminates these super-dense parts of space, has been attributed previously to high-energy electromagnetic radiation. Scientists speculated that it's created by electrons moving at just about the speed of light. The new study from researchers at Columbia University explained why these particles accelerate.
Astrophysicists Luca Comisso and Lorenzo Sironi carried out the research by running some of the largest super-computer simulations ever conducted in this area. They managed to calculate the trajectories of hundreds of billions of charged particles.
Comiso, a postdoctoral research scientist at Columbia, explained their conclusion:
"Turbulence and magnetic reconnection—a process in which magnetic field lines tear and rapidly reconnect—conspire together to accelerate particles, boosting them to velocities that approach the speed of light," said Comisso in a press release.
As Comiso further described, the space region that is home to black holes and neutron stars is also full of a super-hot gas of charged particles. Their chaotic motion affects magnetic field lines and results in "vigorous magnetic reconnection". This, in turn, creates an electric field which accelerates particles to energies that are "much higher than in the most powerful accelerators on Earth, like the Large Hadron Collider at CERN," added Comisso.
Amazing astronomy: How neutron stars create ripples in space-time
Interestingly, the simulations showed that the particles gathered most of their energy through the process of random bouncing at super-high speeds.
"This is indeed the radiation emitted around black holes and neutron stars that make them shine, a phenomenon we can observe on Earth," said Sironi, the study's principal investigator and an assistant professor of astronomy at Columbia.
Next, the scientists plan to confirm their findings by comparing them to the electromagnetic spectrum from the Crab Nebula, a bright remnant of a supernova.
You can check out the study published in the December issue of The Astrophysical Journal.
A massive super-computer simulation demonstrates the strong particle density fluctuations that happen in the extreme turbulent environments home to black holes and neutron stars. The dark blue regions are low particle density regions, and the yellow regions are over-dense regions. Particles are accelerated to extremely high speeds from interacting with turbulence fluctuations.
Black holes are perhaps the most mysterious objects in the universe. They are the consequence of gravity crushing a dying star without limit, leading to the formation of a true singularity – which happens when an entire star gets compressed down to a single point yielding an object with infinite density. This dense and hot singularity punches a hole in the fabric of spacetime itself, possibly opening up an opportunity for hyperspace travel. That is, a short cut through spacetime allowing for travel over cosmic scale distances in a short period.
Researchers previously thought that any spacecraft attempting to use a black hole as a portal of this type would have to reckon with nature at its worst. The hot and dense singularity would cause the spacecraft to endure a sequence of increasingly uncomfortable tidal stretching and squeezing before being completely vaporized.
Flying through a black hole
My team at the University of Massachusetts Dartmouth and a colleague at Georgia Gwinnett College have shown that all black holes are not created equal. If the black hole like Sagittarius A*, located at the center of our own galaxy, is large and rotating, then the outlook for a spacecraft changes dramatically. That's because the singularity that a spacecraft would have to contend with is very gentle and could allow for a very peaceful passage.
The reason that this is possible is that the relevant singularity inside a rotating black hole is technically “weak," and thus does not damage objects that interact with it. At first, this fact may seem counter intuitive. But one can think of it as analogous to the common experience of quickly passing one's finger through a candle's near 2,000-degree flame, without getting burned.
My colleague Lior Burko and I have been investigating the physics of black holes for over two decades. In 2016, my Ph.D. student, Caroline Mallary, inspired by Christopher Nolan's blockbuster film “Interstellar," set out to test if Cooper (Matthew McConaughey's character), could survive his fall deep into Gargantua – a fictional, supermassive, rapidly rotating black hole some 100 million times the mass of our sun. “Interstellar" was based on a book written by Nobel Prize-winning astrophysicist Kip Thorne and Gargantua's physical properties are central to the plot of this Hollywood movie.
Building on work done by physicist Amos Ori two decades prior, and armed with her strong computational skills, Mallary built a computer model that would capture most of the essential physical effects on a spacecraft, or any large object, falling into a large, rotating black hole like Sagittarius A*.
The fictional Miller's planet orbiting the black hole Gargantua, in the movie 'Interstellar.' interstellarfilm.wikia.com
Not even a bumpy ride?
What she discovered is that under all conditions an object falling into a rotating black hole would not experience infinitely large effects upon passage through the hole's so-called inner horizon singularity. This is the singularity that an object entering a rotating black hole cannot maneuver around or avoid. Not only that, under the right circumstances, these effects may be negligibly small, allowing for a rather comfortable passage through the singularity. In fact, there may no noticeable effects on the falling object at all. This increases the feasibility of using large, rotating black holes as portals for hyperspace travel.
Mallary also discovered a feature that was not fully appreciated before: the fact that the effects of the singularity in the context of a rotating black hole would result in rapidly increasing cycles of stretching and squeezing on the spacecraft. But for very large black holes like Gargantua, the strength of this effect would be very small. So, the spacecraft and any individuals on board would not detect it.
This graph depicts the physical strain on the spacecraft's steel frame as it plummets into a rotating black hole. The inset shows a detailed zoom-in for very late times. The important thing to note is that the strain increases dramatically close to the black hole, but does not grow indefinitely. Therefore, the spacecraft and its inhabitants may survive the journey. Khanna/UMassD
The crucial point is that these effects do not increase without bound; in fact, they stay finite, even though the stresses on the spacecraft tend to grow indefinitely as it approaches the black hole.
There are a few important simplifying assumptions and resulting caveats in the context of Mallary's model. The main assumption is that the black hole under consideration is completely isolated and thus not subject to constant disturbances by a source such as another star in its vicinity or even any falling radiation. While this assumption allows important simplifications, it is worth noting that most black holes are surrounded by cosmic material – dust, gas, radiation.
Therefore, a natural extension of Mallary's work would be to perform a similar study in the context of a more realistic astrophysical black hole.
Mallary's approach of using a computer simulation to examine the effects of a black hole on an object is very common in the field of black hole physics. Needless to say, we do not have the capability of performing real experiments in or near black holes yet, so scientists resort to theory and simulations to develop an understanding, by making predictions and new discoveries.
Gaurav Khanna, Professor of Physics, University of Massachusetts Dartmouth
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