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Half of the ordinary baryonic matter has been tough to find but Fast Radio Bursts made it possible to detect the WHIM.
AgamemnonA Black Hole So Big It ‘Should Not Exist’
Black hole physicists have been excitedly discussing reports that the LIGO and Virgo gravitational-wave detectors recently picked up the signal of an unexpectedly enormous black hole, one with a mass that was thought to be physically impossible.
“The prediction is no black holes, not even a few” in this mass range, wrote Stan Woosley, an astrophysicist at the University of California, Santa Cruz, in an email. “But of course we know nature often finds a way.”
Seven experts contacted by Quanta said they’d heard that among the 22 flurries of gravitational waves detected by LIGO and Virgo since April, one of the signals came from a collision involving a black hole of unanticipated heft — purportedly as heavy as 100 suns. LIGO/Virgo team members would neither confirm nor deny the rumored detection.
[Update: On September 2, 2020, researchers confirmed that the colliding black holes had masses 65 and 85 times that of our sun. The resulting black hole was 150 times more massive than the sun.]
Chris Belczynski, an astrophysicist at Warsaw University, previously felt so sure that such a large specimen wouldn’t be seen that in 2017 he placed a bet with colleagues. “I think we are about to lose the bet,” Belczynski said, “and for the good of science!”
Belczynski’s former confidence came from the fact that such a big black hole can’t form in the usual way.
Black holes — dense, paradox-ridden spheres whose gravity traps everything, even light — form from the contracting cores of fuel-spent stars. But in 1967, three physicists at the Hebrew University in Jerusalem realized that when the core of a dying star is very heavy, it won’t gravitationally collapse into a black hole. Instead, the star will undergo a “pair-instability supernova,” an explosion that totally annihilates it in a matter of seconds, leaving nothing behind. “The star is completely dispersed into space,” the three physicists wrote.
A pair-instability supernova happens when the core grows so hot that light begins to spontaneously convert into electron-positron pairs. The light’s radiation pressure had kept the star’s core intact; when the light transforms into matter, the resulting pressure drop causes the core to rapidly shrink and become even hotter, further accelerating pair production and causing a runaway effect. Eventually the core gets so hot that oxygen ignites. This fully reverses the core’s implosion, so that it explodes instead. For cores with a mass between about 65 and 130 times that of our sun (according to current estimates), the star is completely obliterated. Cores between about 50 and 65 solar masses pulsate, shedding mass in a series of explosions until they drop below the range where pair instability occurs. Thus there should be no black holes with masses in the 50-to-130-solar-mass range.
“The prediction comes from straightforward calculations,” said Woosley, whose 2002 study of this “pair-instability mass gap” is considered definitive.
Black holes can exist on the other side of the mass gap, weighing in at more than 130 solar masses, because the runaway implosion of such heavy stellar cores can’t be stopped, even by oxygen fusion; instead, they continue to collapse and form black holes. But because stars shed mass throughout their lives, a star would need to be born weighing at least 300 suns in order to end up as a 130-solar-mass core, and such behemoths are rare. For this reason, most experts assumed black holes detected by LIGO and Virgo should top out at around 50 solar masses, the lower end of the mass gap. (The million- and billion-solar-mass supermassive black holes that anchor galaxies’ centers formed differently, and rather mysteriously, in the early universe. LIGO and Virgo are not mechanically capable of detecting the collisions of supermassive black holes.)
That said, a few experts did boldly predict that black holes in the mass gap would be seen — hence the 2017 bet.
At a meeting that February at the Aspen Center for Physics, Belczynski and Daniel Holz of the University of Chicago wagered that “black holes should not exist in the mass range between 55 and 130 solar masses because of pair instability,” and thus that none would be detected among LIGO/Virgo’s first 100 signals. Woosley later co-signed with Belczynski and Holz.
But Carl Rodriguez of the Massachusetts Institute of Technology and Sourav Chatterjee of the Tata Institute for Fundamental Research in Mumbai, India, later joined by Fred Rasio of Northwestern University, bet against them, wagering that a black hole would indeed be detected in the mass gap, because there’s a roundabout way for these plus-size black holes to form.
Whereas most of the colliding black holes that wiggle LIGO and Virgo’s instruments probably originated as pairs of isolated stars (binary star systems being common in the cosmos), Rodriguez and his co-signers argue that a fraction of the detected collisions occur in dense stellar environments such as globular clusters. The black holes swing around in one another’s gravity, and sometimes they catch each other and merge, like big fish swallowing smaller ones in a pond.
Inside a globular cluster, a 50-solar-mass black hole could merge with a 30-solar-mass one, for instance, and then the resulting giant could merge again. This second-generation merger is what LIGO/Virgo might have detected — a lucky catch of the big fish in the pond. “This can really only happen in clusters,” Rodriguez said. If the rumor is true, he, Chatterjee and Rasio will each receive a $100 bottle of wine from Belczynski, Holz and Woosley.
But there are other possible origin stories for the putative big black hole. Perhaps it started out in an isolated binary star system. After the first star collapsed into a black hole, it might have grown by stripping matter from its companion star. Later, the second star would have collapsed as well, then eventually the two would have collided and merged, sending gravitational waves cascading through the fabric of space-time.
The LIGO/Virgo team quickly announces every potential gravitational-wave event and the region of sky from which it originated, so that other telescopes can swivel in that direction. But the tight-lipped team has yet to publish detailed information about any event from the current observing run that began in April, such as the inferred sizes of the colliding objects. The team plans to reveal all by the spring of 2020 at the latest. If the oversize black hole is among the results, the analysis should also reveal how fast the hole and its companion were spinning when they collided; this information will help favor one origin story or the other, or neither.
The rumor is “pushing us to alternative formation mechanisms,” said Chris Fryer, an astrophysicist at Los Alamos National Laboratory who has studied binary black hole formation and the mass gap. “In any event it will be an exciting event — if it’s true.”
As for Woosley, he still feels certain the mass gap exists, despite possible exceptions. “A likely outcome will be that when we have hundreds of black holes, we will indeed see a cliff at around 50,” he said, “but with a few events in the gap because nature abhors a vacuum.”
Very Large Telescope spots galaxies trapped in the web of a supermassive black hole
With the help of ESO’s Very Large Telescope (VLT), astronomers have found six galaxies lying around a supermassive black hole when the Universe was less than a billion years old. This is the first time such a close grouping has been seen so soon after the Big Bang and the finding helps us better understand how supermassive black holes, one of which exists at the centre of our Milky Way, formed and grew to their enormous sizes so quickly. It supports the theory that black holes can grow rapidly within large, web-like structures which contain plenty of gas to fuel them.
“This research was mainly driven by the desire to understand some of the most challenging astronomical objects—supermassive black holes in the early Universe. These are extreme systems and to date we have had no good explanation for their existence,” said Marco Mignoli, an astronomer at the National Institute for Astrophysics (INAF) in Bologna, Italy, and lead author of the new research published today in Astronomy & Astrophysics.
The new observations with ESO’s VLT revealed several galaxies surrounding a supermassive black hole, all lying in a cosmic “spider’s web” of gas extending to over 300 times the size of the Milky Way. “The cosmic web filaments are like spider’s web threads,” explains Mignoli. “The galaxies stand and grow where the filaments cross, and streams of gas—available to fuel both the galaxies and the central supermassive black hole—can flow along the filaments.”
The light from this large web-like structure, with its black hole of one billion solar masses, has travelled to us from a time when the Universe was only 0.9 billion years old. “Our work has placed an important piece in the largely incomplete puzzle that is the formation and growth of such extreme, yet relatively abundant, objects so quickly after the Big Bang,” says co-author Roberto Gilli, also an astronomer at INAF in Bologna, referring to supermassive black holes.
The very first black holes, thought to have formed from the collapse of the first stars, must have grown very fast to reach masses of a billion suns within the first 0.9 billion years of the Universe’s life. But astronomers have struggled to explain how sufficiently large amounts of “black hole fuel” could have been available to enable these objects to grow to such enormous sizes in such a short time. The new-found structure offers a likely explanation: the “spider’s web” and the galaxies within it contain enough gas to provide the fuel that the central black hole needs to quickly become a supermassive giant.
But how did such large web-like structures form in the first place? Astronomers think giant halos of mysterious dark matter are key. These large regions of invisible matter are thought to attract huge amounts of gas in the early Universe; together, the gas and the invisible dark matter form the web-like structures where galaxies and black holes can evolve.
“Our finding lends support to the idea that the most distant and massive black holes form and grow within massive dark matter halos in large-scale structures, and that the absence of earlier detections of such structures was likely due to observational limitations,” says Colin Norman of Johns Hopkins University in Baltimore, US, also a co-author on the study.
The galaxies now detected are some of the faintest that current telescopes can observe. This discovery required observations over several hours using the largest optical telescopes available, including ESO’s VLT. Using the MUSE and FORS2 instruments on the VLT at ESO’s Paranal Observatory in the Chilean Atacama Desert, the team confirmed the link between four of the six galaxies and the black hole. “We believe we have just seen the tip of the iceberg, and that the few galaxies discovered so far around this supermassive black hole are only the brightest ones,” said co-author Barbara Balmaverde, an astronomer at INAF in Torino, Italy.
These results contribute to our understanding of how supermassive black holes and large cosmic structures formed and evolved. ESO’s Extremely Large Telescope, currently under construction in Chile, will be able to build on this research by observing many more fainter galaxies around massive black holes in the early Universe using its powerful instruments.
Milestone’ Evidence for Anyons, a Third Kingdom of Particles
Anyons don’t fit into either of the two known particle kingdoms. To find them, physicists had to erase the third dimension.Every last particle in the universe — from a cosmic ray to a quark — is either a fermion or a boson. These categories divide the building blocks of nature into two distinct kingdoms. Now researchers have discovered the first examples of a third particle kingdom.
Anyons, as they’re known, don’t behave like either fermions or bosons; instead, their behavior is somewhere in the middle. In a recent paper published in Science, physicists have found the first experimental evidence that these particles don’t fit into either kingdom. “We had bosons and fermions, and now we’ve got this third kingdom,” said Frank Wilczek, a Nobel prize–winning physicist at the Massachusetts Institute of Technology. “It’s absolutely a milestone.”
What Is an Anyon?
To understand the quantum kingdoms, think of a drawing of loops. Imagine two indistinguishable particles, like electrons. Take one, then loop it around the other so that it ends up back where it started. Nothing seems to have changed. And indeed, in the mathematical language of quantum mechanics, the two wave functions describing the initial and final states must be either equal or off by a factor of −1. (In quantum mechanics, you calculate the probability of what you observe by squaring this wave function, so this factor of −1 washes out.)
If the wave functions are identical, your quantum particles are bosons. If they’re off by a factor of −1, you have fermions. And though the derivation may seem like a purely mathematical exercise, it has profound physical consequences.
Fermions are the antisocial members of the particle world. They never occupy the same quantum state. Because of this, electrons, which are fermions, get forced into the varied atomic shells around an atom. From this simple phenomenon arises most of the space in an atom, the astonishing variety of the periodic table, and all of chemistry.
Bosons, on the other hand, are gregarious particles, happy to bunch together and share the same quantum state. Thus photons, which are bosons, can pass through each other, allowing light rays to travel unimpeded rather than scattering about.
But what happens if, when you loop one quantum particle around another, you don’t get back to the same quantum state? To understand this possibility, we need to make a brief digression into topology, the mathematical study of shapes. Two shapes are topologically equivalent if one can be transformed into the other without any cutting or gluing. A doughnut and a coffee mug, the old saying goes, are topologically equivalent, because one can be gently and continuously shaped into the other.
Consider the loop that we made when we rotated one particle around the other. In three dimensions, you can shrink that loop all the way down to a point. Topologically speaking, it’s as if the particle hasn’t moved at all.
In two dimensions, however, the loop can’t shrink. It gets stuck on the other particle. You can’t shrink the loop without cutting it in the process. Because of this restriction — found only in two dimensions — looping one particle around another is not equivalent to leaving the particle in the same place.
We need a third particle possibility: anyons. Since their wave functions are not restricted to the two solutions that define fermions and bosons, these particles are free to be neither of the two, but anything in between. When Wilczek first coined the term anyon, it was a tongue-in-cheek suggestion that anything goes.
The Experiment
“The topological argument was the first indication that these anyons could exist,” said Gwendal Fève, a physicist at Sorbonne University in Paris who led the recent experiment. “What was left to find was physical systems.”
When electrons are restricted to motion in two dimensions, cooled nearly to absolute zero, and subjected to a strong magnetic field, very strange things begin to happen. In the early 1980s, physicists first used these conditions to observe the “fractional quantum Hall effect,” in which electrons come together to create so-called quasiparticles that have a fraction of the charge of a single electron. (If it seems strange to call the collective behavior of electrons a particle, think of the proton, which is itself made up of three quarks.)
In 1984, a seminal two-page paper by Wilczek, Daniel Arovas and John Robert Schrieffer showed that these quasiparticles had to be anyons. But scientists had never observed anyon-like behavior in these quasiparticles. That is, they had been unable to prove that anyons are unlike either fermions or bosons, neither bunching together nor totally repelling one another.
That’s what the new study does. In 2016, three physicists described an experimental setup that resembles a tiny particle collider in two dimensions. Fève and his colleagues built something similar and used it to smash anyons together. By measuring the fluctuations of the currents in the collider, they were able to show that the behavior of the anyons corresponds exactly with theoretical predictions.
“Everything fits with the theory so uniquely, there are no questions,” said Dmitri Feldman, a physicist at Brown University who was not involved in the recent work. “That’s very unusual for this field, in my experience.”
“There’s been a lot of evidence for a long time,” Wilczek said. “But if you ask: Is there a specific phenomenon you can point to and say the anyons are responsible for that phenomenon and you can’t explain it in any other way? I think this is pretty clearly at a different level.”
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