Braided Magnetic Flux Ropes Are Found at Both Human and Light Year Scales

Four braided structures. (a) astrophysical jet M-87, 3000 light years long; (b) Double Helix Nebula, 70 light years long; (c) solar prominence, 3000 kilometers long; (d) solar loop manufactured in Bellan lab at Caltech, 3 centimeters long. Credit: (a) Passeto et al., Sophia Dagnello, NRAO/AUI/NSF; (b) NASA/JPL-Caltech/M. Morris (UCLA); (c) High Altitude Observatory Archives; (d) Yang Zhang, Caltech Bellan Plasma Lab

Four braided structures. (a) astrophysical jet M-87, 3000 light years long; (b) Double Helix Nebula, 70 light years long; (c) solar prominence, 3000 kilometers long; (d) solar loop manufactured in Bellan lab at Caltech, 3 centimeters long. Credit: (a) Passeto et al., Sophia Dagnello, NRAO/AUI/NSF; (b) NASA/JPL-Caltech/M. Morris (UCLA); (c) High Altitude Observatory Archives; (d) Yang Zhang, Caltech Bellan Plasma Lab

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Investigating solar corona structures has led Paul Bellan, Caltech professor of applied physics, and his former graduate student Yang Zhang (PhD '24) to discover a new equilibrium state of the magnetic field and its associated plasma. The solar corona, the outermost part of the Sun's atmosphere, is much less dense than the Sun's surface but is a million times hotter. The corona is composed of strong magnetic fields confining plasma, a gaseous soup of charged particles (electrons and ions). The new equilibrium, called a double helix, applies not only to the solar corona but also to much larger astrophysical configurations such as the Double Helix Nebula located near the center of the Milky Way galaxy.

Solar corona structures such as flares often have the form of magnetic flux ropes: twisted tubes of plasma-containing magnetic fields. Such a rope can be visualized as a plasma-filled garden hose with a stripe wrapped around it in a helical pattern. An electric current flows along the length of the hose, and the helical stripe corresponds to the twisted magnetic field. Because it is charged, plasma conducts electric currents and is attached, or "frozen," into magnetic fields.

Magnetic flux ropes occur in a variety of situations ranging from the human scale—say, a laboratory experiment—to the absolutely huge: solar flares that are few hundred thousand kilometers long. Astrophysical structures with magnetic flux ropes can also span hundreds or even thousands of light-years.

In a large laboratory vacuum chamber, Bellan and Zhang (now a NASA Jack Eddy postdoctoral fellow at Princeton) produced solar flare replicas measuring between 10 and 50 centimeters long. "We have two electrodes inside the vacuum chamber, which has coils producing a magnetic field spanning the electrodes. Then we apply high voltage across the electrodes to ionize initially neutral gas to form a plasma," Yang explains. "The resulting magnetized plasma configuration automatically forms a braided structure."

This braided structure consists of two flux ropes that wrap around one another to form a double helix structure. In the experiments, this double helix was observed to be in a stable equilibrium—in other words, it holds its structure without tending to twist tighter or untwist. In a new paper, Zhang and Bellan demonstrate that the stable equilibrium of these double-helix flux ropes can be understood, analyzed, and predicted accurately in mathematical terms.

Though the properties of single flux ropes are well known, braided flux ropes were not well understood—especially those configurations in which the electric currents flow in the same direction along both of the braided strands. Scientists have modeled the other possible situation—where currents flow in one direction in one flux rope and in the opposite direction in the other—but this scenario is thought to be unlikely in nature.

The same-current configuration is especially important because it would be susceptible to kinking and expansion driven by hoop forces—phenomena observed both in braided solar structures and in laboratory experiments. Such kinking and expansion should not occur when current flows in opposite directions in the braided strands (a "no-net-current" state).

Previously, scientists assumed that braided flux ropes where the strands have current flowing in the same direction would always merge, because parallel currents magnetically attract each another. However, in 2010, researchers at Los Alamos National Laboratory found that such flux ropes instead bounce off one another as they come closer together.

"There was clearly something more complicated going on when the flux ropes are braided, and now we have shown what that is," Bellan says. "If you have electrical currents flowing along two helical wires that wrap around each other to form a braided structure, as seen in our lab, the components of the two currents flowing along the length of the two wires are parallel and attract, but the components of the two currents flowing in the wrapping direction are anti-parallel and repel. This combination of both attractive and repulsive forces means there will be a critical helical angle at which these opposing forces balance, producing an equilibrium. If the helical flux ropes twist tighter, there will be too much magnetic repulsion; if they twist more loosely, there will be too much magnetic attraction. At the critical angle of twist, the helical structure arrives at its lowest energy state, or equilibrium."

The next task was to create a mathematical model of this behavior—something not previously done. Using what Bellan describes as "brute force mathematics," Zhang created a set of equations that could apply to multiple flux tubes in various configurations, including braided ropes, and showed there is indeed a state at which the attractive and repulsive forces balance each other, creating an equilibrium. "And as an unexpected bonus, Yang can calculate the magnetic fields inside and outside the flux ropes, and the current and pressure inside them," Bellan says, "giving us a full picture of the behavior of these braided structures."

Zhang tested his mathematical model against the Double Helix Nebula, an astrophysical plasma formation located 25,000 light-years from Earth that covers a 70 light-year swath of space, to see if the equations could describe a large model as well as it did the structures he and Bellan created in the lab. "What was rather amazing about this calculation is that Yang didn't really need to know much about the nebula," Bellan says. "Just knowing the diameter of the strands and the periodicity of the twist, numbers that can be observed astronomically, Yang was able to predict the angle of twist that yielded an equilibrium structure, and that was consistent with observations of this nebula. One of the most exciting aspects of this research is that magnetohydrodynamics, the theory of magnetized plasmas, turns out to be fantastically scalable. When I first started looking into this, I thought the phenomena of magnetic structures at different scales were qualitatively similar, but because their sizes are so different, they couldn't be described by the same equations. It turns out that this is not so. What we see in lab experiments and in solar and astrophysical observations are governed by the same equations."

The paper, titled "Magnetic Double Helix," was published in Physical Review Letters. The work was funded by the National Science Foundation.

Source: Caltech/News

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Caltech Media Relations
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"Third Wheel" Star Brings Companions Closer Together

This artwork depicts a triple-star system in which two of the stars are locked in a tight gravitational orbit. The bright star in the foreground on the right is a white dwarf, which is stealing mass from its stellar companion. Eventually, this building up of mass on the white dwarf will trigger periodic explosions. Together, the two stars form an object called cataclysmic variable. New Caltech-led research has shown that a third star in triple-star systems (like the star depicted in the background here), can gravitationally influence its neighboring stars. and lead to the formation of cataclysmic variables. Credit: Caltech/R. Hurt (IPAC)

Cheyanne Shariat and Kareem El-Badry
Credit: Lance Hayashida/Caltech

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New findings show that certain explosive star systems may form with help of third star

When white dwarfs—the hot remnants of stars like our Sun—are orbited closely by another star, they sometimes steal mass away from their companion. The stolen matter builds up on the surface of the white dwarf, triggering eruptions called novae.

Theorists have long predicted how these volatile partnerships, called cataclysmic variables (CVs), form, but now a new Caltech-led study reveals a surprising twist: In some cases, a third star, circling farther away from the primary pair, may in fact be the reason the star couple got together in the first place.

"Our results are revealing another formation channel for CVs," says Kareem El-Badry, assistant professor of astronomy at Caltech and a co-author of a new paper appearing in the Publications of the Astronomical Society of the Pacific. "Sometimes, a lurking third star is key," he says. The lead author of the study is Caltech graduate student Cheyanne Shariat.

Before now, scientists believed that CVs formed from a process called common envelope evolution, in which the partner stars are brought closer together via an envelope of gas that cocoons them. An aging star destined to become a white dwarf expands into a red giant that encompasses both stars, creating a shared envelope. The envelope corrals the two stars, causing them to spiral inward. Eventually, the envelope is ejected, leaving a tight pair that have become close enough for the white dwarf to steal its companion's mass.

Although a third star was not mentioned in these descriptions, the team wondered if one could be involved. After all, they reasoned, triple-star dynamics do play a role in other types of star systems.

To further investigate the matter, the researchers turned to data from the European Space Agency's Gaia mission, now retired. Sorting through these observations, they identified 50 CVs in hierarchical triple-star systems, or triples, as the researchers call them. A hierarchical triple is one in which two stars are located fairly close together, while the third is much farther out and orbits the primary pair. The results suggested that at least 10 percent of all known CVs are part of triple systems.

That 10 percent number was higher than what would be expected if triples had no role in CV formation, so the researchers decided to learn more by running computer simulations. They performed so-called three-body simulations on 2,000 hypothetical triples; these simulations sped up the gravitational interactions of the trio of stars, evolving them over time.

In 20 percent of the triple-star simulations, CVs formed without the traditional mechanism of common envelope evolution. In these cases, the researchers say, the third star torqued the main binary.

"The gravity of the third star causes the binary stars to have a super eccentric orbit, and this forces the companion star closer to the white dwarf. Tidal forces dissipate energy and shrink and circularize the orbit," Shariat says. "The star doesn't have to spiral in through the common envelope."

In 60 percent of the simulations, the triple star helped initiate the process of common envelope evolution, bringing the two primary stars close enough to one another to be encased in the same envelope. In the remaining 20 percent of the simulations, the CVs formed via the traditional common envelope evolution route that requires just two stars.

When the researchers accounted for a realistic population of stars in our galaxy, including CVs known to have formed from just two stars, their theoretical models predicted around 40 percent of all CVs form in triple systems. This is higher than the 10 percent they observed using Gaia because, in many cases, the third stars can be either hard to see or have become unbound from the CV.

Finally, the simulation results enabled predictions about the types of triple-star systems that would be more likely to form CVs. Specifically, the triple systems would be expected to start out in wider configurations, such that the tight-knit pair and the third star are separated by more than 100 astronomical units (an astronomical unit, or au, is the distance between the Sun and Earth).

Looking back at the Gaia data, the researchers found agreement: The triples with CVs did exhibit wider separations on average than typical systems.

"For the past 50 years, people were using the spiral-in common-envelope evolution model to explain CV formation," El-Badry says. "Nobody had noticed before that this was largely happening in triples!"

The study titled "Cataclysmic Variables in Triples: Formation Models and New Discoveries" was funded by the Joshua and Beth Friedman Foundation Fund, NASA, the National Science Foundation, and Howard and Astrid Preston. The project was done in collaboration with Smadar Naoz, a researcher at UCLA who specializes in theoretical studies of triples. Other authors include Antonio Rodriguez, a graduate student at Caltech, and Jan van Roestel of the University of Amsterdam.

Written by Whitney Clavin

Source: Caltech/News

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Contact:

Whitney Clavin
(626) 395‑1944
wclavin@caltech.edu

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Cellular Coordinate System Reveals Secrets of Active Matter

Left: Active matter composed of filaments and motors. Center: Active matter overlaid with a fluorescence-cancelling grid, creating a coordinate system to measure deformation. Right: As the system contracts, the coordinate system deforms as well. Credit: Courtesy of S. Hirokawa

An example of bioengineered microtubules being directed by light
Credit: Caltech

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All humans who have ever lived were once each an individual cell, which then divided countless times to produce a body made up of around 10 trillion cells. These cells have busy lives, executing all kinds of dynamic movement: contracting every time we flex a muscle, migrating toward the site of an injury, and rhythmically beating for decades on end.

Cells are an example of active matter. As inanimate matter must burn fuel to move, like airplanes and cars, active matter is similarly animated by its consumption of energy. The basic molecule of cellular energy is adenosine triphosphate (ATP), which catalyzes chemical reactions that enable cellular machinery to work.

Caltech researchers have now developed a bioengineered coordinate system to observe the movement of cellular machinery. The research enables a better understanding of how cells create order out of chaos, such as during embryonic development or in the organized movements of chromosomes that are a prerequisite to faithful cell division.

The work was conducted in the laboratories of Rob Phillips, the Fred and Nancy Morris Professor of Biophysics, Biology, and Physics, and Matt Thomson, Professor of Computational Biology and Heritage Medical Research Institute Investigator. A paper describing the study appears in the journal Proceedings of the National Academy of Sciences.

The basic units of cellular machinery are motors and filaments made of proteins, which act like the muscles and skeleton of the cell. These structures self-assemble, like little protein robots, to enable cells to move. In 2018, former graduate student Tyler Ross (PhD '21) engineered a system of these components that can be controlled by light in a lab setting, enabling researchers to observe and experiment upon their movements. Each experimental system is only the width of a human hair, containing thousands of individual motors and filaments.

In the new work, led by former graduate student Soichi Hirokawa (PhD '23), the team developed additional light patterns that create a grid, or coordinate system, upon the mixture of motors and filaments. To understand this, imagine a sheet of rubber with a grid patterned on it—as the rubber stretches and deforms, the grid does as well. Once a set of regularly spaced squares, the grid's deformation gives a measure of which regions are being stretched or squeezed and by how much. In this way, the team can track the movements of a collection of filaments and motors—they are too small to be seen themselves, but the light-patterned grid, each square about 12-by-12 micrometers, is visible with a microscope.

"The system allows us to observe how these biomolecules reorganize as they collectively form a structure," says Hirokawa. "With it, we can distinguish the processes that contribute to the deformations that we observe on these squares."

This new system enabled the team to measure the competing dynamics of active shrinking and a process that influences cellular self-assembly, called diffusion. Taking a mixture of motors and filaments, the researchers triggered the components to contract inward, like a shrinking circle. But each component naturally still experiences some random movement, or diffusion, jiggling every which way as the whole contracts. The deforming coordinate system enabled the team to watch this competition between active contraction and random diffusion, and characterize it. Interestingly, they found that the more ATP is in the system, the more the molecules randomly diffuse.

"The formation of patterns and structure in biology has to fight against this randomness," says Phillips. "The system is able to organize despite the forces of chaos."

The dynamic coordinate system introduced here could be used in other contexts as well.

"Order is particularly important in processes like embryonic development," says staff scientist and co-author Heun Jin Lee. "An early embryo gastrulates, folding into a tube that ultimately becomes the digestive tract. You could imagine decorating the surface of an embryo with a coordinate system that stretches as the embryo folds."

The paper is titled "Motor-driven microtubule diffusion in a photobleached dynamical coordinate system." In addition to Hirokawa, Lee, Thomson, and Phillips, Caltech co-authors are former graduate student Rachel Banks (PhD '22), graduate student Ana Duarte, and postdoctoral scholar Bibi Najma. Funding was provided by the Maximizing Investigators Research Awards and the Foundational Questions Institute. Matt Thomson is an affiliated faculty member with the Tianqiao and Chrissy Chen Institute for Neuroscience at Caltech.

Written by Lori Dajose

Source: Caltech/News

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Star Quakes and Monster Shock Waves

Artist's concept of a black hole orbited by a cracked neutron star. Before a black hole consumes a neutron star, tidal forces from its immense gravity shears the star's surface, causing quakes and the opening of rifts. In this artwork, the gravity of both the black hole and neutron star can be seen bending our view of the background. The neutron star, though less massive than the black hole, has strong enough gravity to warp the view of the black hole as well. Credit: Caltech/R. Hurt (IPAC)

This snapshot from a simulation shows a magnetized outflow of plasma launched following the merger of a black hole and a magnetized neutron star. The light blue color maps show the strength of magnetic fields within this wind. The magnetized outflow is powered by the spin of the remnant black hole, like a rotating fan pushing air around. Credit: Yoonsoo Kim/Caltech

A side view from a simulation of a "black hole pulsar," a hypothetical object in which a black hole launches magnetized outflows that sweep around the black hole, like a lighthouse beacon, as it spins. The yellow lines show where magnetic fields that are pointing in different directions meet up. Electric currents flow along this interface and heat up plasma, which takes on a characteristic "ballerina's skirt" geometry. Credit: Yoonsoo Kim/Caltech

A series of three simulated images showing a black hole eat a neutron star. These three panels are taken from a supercomputer simulation of a merger between a black hole (large black circle) and a neutron star (colored blob). The images, which move forward in time from left to right, show how the intense gravity of the black hole stretches the neutron star, before the black hole ultimately consumes it. Credit: Elias Most/Caltech

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Across the cosmos, many stars can be found in pairs, gracefully circling one another. Yet one of the most dramatic pairings occurs between two orbiting black holes, formed after their massive progenitor stars exploded in supernova blasts. If these black holes lie close enough together, they will ultimately collide and form an even more massive black hole. Sometimes a black hole is orbited by a neutron star—the dense corpse of a star also formed from a supernova explosion but which contains less mass than a black hole. When these two bodies finally merge, the black hole will typically swallow the neutron star whole.

To better understand the extreme physics underlying such a grisly demise, researchers at Caltech are using supercomputers to simulate black hole–neutron star collisions. In one study appearing in The Astrophysical Journal Letters, the team, led by Elias Most, a Caltech assistant professor of theoretical astrophysics, developed the most detailed simulation yet of the violent quakes that rupture a neutron star's surface roughly a second before the black hole consumes it.

"The neutron star's crust will crack open just like the ground in an earthquake," Most says. "The black hole's gravity first shears the surface, causing quakes in the star and the opening of rifts."

While cracks in the crust of a neutron star had been predicted before, the simulation is the first to demonstrate what kinds of light flares astronomers might see in the future when pointing telescopes in space and on the ground at such an event.

"This goes beyond educated models for the phenomenon—it is an actual simulation that includes all the relevant physics taking place when the neutron star breaks like an egg," says co-author Katerina Chatziioannou, assistant professor of physics at Caltech and a William H. Hurt Scholar.

In a second, more recent paper in The Astrophysical Journal Letters, published March 31 of this year, the team used a supercomputer to simulate what happens after the neutron star fractures—a brief milliseconds-long window when monster shock waves, the most powerful predicted shock waves in the universe, shoot outward from the star. These monster shock waves had only recently been predicted by co-author Andrei Beloborodov of Columbia University. Now, the simulation, along with another from a different study published by the team last year, are the first to show how they form.

What is more, the most recent simulation does not stop when the monster shock waves form—it proceeds to show the neutron star being swallowed, which then triggers the creation of an exotic object called a "black hole pulsar."

A classic pulsar is a highly magnetized neutron star that emits beams of radiation, which sweep around like a lighthouse beacon as the star spins on its axis. A black hole pulsar is a hypothetical object in which a black hole launches magnetic winds that would also sweep around it as it spins, mimicking the appearance of a pulsar. While black hole pulsars had been previously conjectured, the simulation is the first to show how such a rare object could actually form in nature from the collision of a neutron star and black hole.

"When the neutron star plunges into the black hole, the monster shock waves are launched," says Yoonsoo Kim (MS '24), a Caltech graduate student working with Most, and lead author of the study on monster shock waves and black hole pulsars. "After the star is sucked in, whipping winds are formed, creating the black hole pulsar. But the black hole cannot sustain its winds and will become quiet again within seconds."

This snippet from a supercomputer simulation shows the aftermath of a collision between a black hole and a neutron star. After the black hole consumes the magnetized neutron star, a hypothetical object called a "black hole pulsar" is formed, in which magnetic outflows sweep around the black hole as it spins. The thin yellow lines represent the interface where magnetic fields pointing in opposite directions meet. Electric currents form at this interface and heat up plasma, which can power bright gamma and X-ray emissions. This movie covers a period of about eight milliseconds.

Like the simulation depicting how a neutron star cracks, this one also predicts the characteristics of the resulting flares astronomers might see through telescopes. In the fleeting moments when monster shock waves rip outward and a black hole pulsar forms, telescopes may be able to catch outbursts of radio waves or a combination of X-rays and gamma rays. In short, the simulations performed by Most and colleagues provide a deeper understanding of the physics driving some of the most energetic events in the universe

Undulating Space and Time

When two black holes collide, they generate not only shock waves and flares of light but also another type of radiation known as gravitational waves. These ripples in the fabric of space and time itself were first predicted more than 100 years ago by Albert Einstein. The Caltech- and MIT-led LIGO (Laser Interferometer Gravitational-wave Observatory), which is funded by the National Science Foundation (NSF), famously made the first direct detection of gravitational waves, generated from the coalescence of two black holes, in 2015. The achievement would later earn three of the collaboration's leading teammates the 2017 Nobel Prize in Physics.

In 2017, LIGO and Virgo, its European sister observatory, observed a different kind of collision: that between two neutron stars. The fiery explosion, called a kilonova, unleashed a spray of metals, including the element gold. That event emitted both gravitational waves and light. LIGO–Virgo first caught the blast in gravitational waves and then notified astronomers around the world who followed up with telescopes in space and on the ground to detect a broad range of electromagnetic, or light, wavelengths, ranging from high-energy gamma rays to low-energy radio waves.

Whether a neutron star–black hole collision would also produce a similar light show is not clear, but so far none have been seen. Still, it is possible that the neutron star–black hole mergers, even if they fail to produce a cloud of glowing material, might flash with brief radio and/or other electromagnetic signals right before and during the collisions. Simulations like those from Most and his colleagues help astronomers know which electromagnetic signals to look for.

To aid in the hunt for these precursor signals, the LIGO team is working to detect mergers up to a minute before they occur, which would give astronomers more time to point their telescopes at the blasts and search for tell-tale signs of an impending crash.

LIGO can detect mergers before they happen because the pair of colliding objects emit gravitational waves in the frequency band that LIGO detects as they spiral closer and closer together," says Chatziioannou, who is part of the LIGO team. "Currently, we can detect the collisions just seconds before they occur, and we are working up to a full minute. The gravitational waves are one piece of the puzzle while the electromagnetic radiation is another. We want to put the puzzle pieces together."

The Most Advanced Computers

A major factor in the success of the team's recent neutron star–black hole simulations is the use of supercomputers containing GPUs (graphics processing units). For these recent studies, the team used the Perlmutter supercomputer located at the Lawrence Berkeley National Laboratory in Berkeley (named after astronomer Saul Perlmutter, who won the 2011 Nobel Prize in Physics with two other scientists for discovering that the universe is accelerating). GPUs provide processing power for video games and AI programs like ChatGPT; in this case, the massive parallel computing power of GPUs allowed the Perlmutter supercomputer to handle the codes needed to simulate the intricate interactions between a converging neutron star and black hole.

"When you simulate two black holes merging," Most says, "you need the equations of general relativity to describe the gr

avitational waves. But when you have a neutron star, there's a lot more physics taking place including the complex nuclear physics of the star and plasma dynamics around it."

The actual simulations take about four to five hours to run. Most and his team had been working on similar simulations for about two years using supercomputers without GPUs before they ran them on Perlmutter. "That's what unlocked the problem," Most says. "With GPUs, suddenly, everything worked and matched our expectations. We just did not have enough computing power before to numerically model these highly complex physical systems in a sufficient detail.

Simulation Secrets

The first cracking simulation reveals the drama of what unfolds as the neutron star gets close to its partner black hole. First, gravitational forces from the massive black hole shear the dead star's surface, causing it to shatter. Neutron stars are surrounded by an intense magnetic field, and when their surface shatters due to these so-called tidal forces, the magnetic field wiggles around. This leads to magnetic ripples called Alfvén waves, named after the Swedish physicist Hannes Alfvén who won the 1970 Nobel Prize in Physics for his work on magnetohydrodynamics, a theory that describes how electromagnetic fields behave in a plasma.

"The magnetic field can be thought of as strings attached to the neutron star," Most says. "The neutron star's quake violently shakes these strings like a whip, and then it makes a cracking sound."

The Alfvén waves eventually transform into a blast wave that produces a burst of radio waves about a second before the neutron star is swallowed. In the future, Caltech's planned Deep Synoptic Array-2000, or DSA-2000—an array of 2,000 radio dishes to be built in the Nevada desert—may be able to pick up these radio wave bursts, (called fast radio bursts or FRBs), indicating the death of the neutron star.

"Before this simulation, people thought you could crack a neutron star like an egg, but they never asked if you could hear the cracking," Most says. "Our work predicts that, yes, you could hear or detect it as a radio signal."

The team's second simulation reveals what happens further along in the neutron's star demise. When the dead star is slurped up by the black hole, some of the strongest shock waves in the universe are produced.

"It's like an ocean wave," Kim says. "The ocean is initially quiet, but as the waves come ashore, they steepen until they finally break. In our simulation, we can see the magnetic field waves break into a monster shock wave."

Those monster shock waves would convert into blast waves that are stronger than the ones generated by the neutron star's cracking, and they too would produce radio signals. That means astronomers observing a neutron star and black hole in the second before they collide might detect two radio signals, one after the other

"What this means is that a neutron star-black hole collision, while it might not erupt with material like a neutron star–neutron star collision, could power strong signals that telescopes can detect," Most says.

Brief Beacons

Finally, after the neutron star is gulped down by the black hole, the second simulation shows how a black hole pulsar is born.

"If the black hole eats up the neutron star, it's also eating up its magnetic field," Most explains. "And it needs to get rid of that. The black hole doesn't want the magnetic field; it repels it. What the simulation shows is that it actually does that in a way that forms a state that looks like a pulsar."

The black hole essentially drags the unwanted magnetic field around with it, and this creates magnetic winds that whip around the black hole, making it resemble a pulsar for a brief period lasting just under a second. The data show that such an event would emit a short burst of high-energy X-rays and/or higher-energy gamma rays.

In the future, the researchers hope to explore whether this same phenomenology extends to other types of binary systems. With the help of supercomputers, they aim to unravel the wondrous physics driving the universe's most cataclysmic events.

The neutron-star cracking study, titled "Nonlinear Alfvén-wave Dynamics and Premerger Emission from Crustal Oscillations in Neutron Star Mergers," was funded by NSF and the Simons Foundation. Other authors include Caltech graduate student Isaac Legred (MS '24).

The monster shock waves and black hole pulsar study, titled "Black Hole Pulsars and Monster Shocks as Outcomes of Black Hole–Neutron Star Mergers," was funded by the Sherman Fairchild Foundation, NSF, NASA, Natural Sciences & Engineering Research Council of Canada, the Canadian Space Agency, and the Simons Foundation. Other authors include Bart Ripperda from the Canadian Institute for Theoretical Astrophysics.

Written by Whitney Clavin

Source: Caltech/News

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Astronomers Find Giant Dinosaur of a Galaxy

This JWST image shows the Big Wheel galaxy (in the center) and its cosmic environment. The galaxy is a gigantic rotating disk lying 11.7 billion light-years away. Its spiral disk stretches across 100,000 light-years, making it larger than any other galaxy disk confirmed at this epoch of the universe. The blue blob and some of the other larger objects in the image are galaxies in the nearby universe. The smaller objects tend to be distant galaxies; however, the larger galaxy to the lower left of Big Wheel is part of the same remote galactic structure as Big Wheel. Credit: NASA/ESA

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Newfound galaxy is one of the biggest ever found in distant universe

A team of astronomers has stumbled upon a humungous spiral galaxy, about five times more massive than our Milky Way galaxy and covering an area two times as big, making it among the largest known galaxies. The most surprising trait of the galaxy, however, is not its colossal size but the fact it existed in the early cosmos when the universe was only 2 billion years old.

"This galaxy is spectacular for being among the largest spiral galaxies ever found, which is unprecedented for this early era of the universe," says Charles (Chuck) Steidel (PhD '90), the Lee A. DuBridge Professor of Astronomy at Caltech. "Ultimately, this galaxy would have been stripped of gas and would not have survived to the modern day. It is like finding a live dinosaur, before it became extinct."

Steidel was part of an international team of astronomers, led by the University of Milano-Bicocca, that made the discovery and published its findings on March 17 in Nature Astronomy. The team's observations were made using James Webb Space Telescope (JWST), a partnership between NASA, the European Space Agency, and the Canadian Space Agency.

The researchers serendipitously noticed the large anomalous galaxy in JWST images taken of a nearby quasar—a powerful, active supermassive black hole. The team then followed up with JWST to learn more about the object's size, precise distance, rotation speed, and mass. Because the speed of light is finite, observations made of objects in the distant universe capture light from a bygone era. The JWST data revealed that the colossal specimen is not only surprisingly large, but also spins at great speeds. This led the team to nickname the galaxy "Big Wheel."

Prior to the discovery, it was thought that disk-shaped galaxies in the early universe were considerably smaller. (Disk galaxies include spiral galaxies as well as other flat, circular galaxies without spiral arms). Big Wheel is about three times larger than any previously discovered galaxies with similar masses at similar cosmic times, and it is also at least three times larger than what is predicted by current cosmological simulations. The galaxy's radius stretches across 100,00 light-years.

The finding begs the question: How did the galaxy get this big so fast? The team is not sure but suspects the answer has to do with the fact that it lives in a very dense area of space packed with a lot of young galaxies that will eventually coalesce into a giant cluster of gravitationally bound galaxies.

"Exceptionally dense environments such as the one hosting the Big Wheel are still a relatively unexplored territory," concludes co-author Sebastiano Cantalupo of the University of Milano-Bicocca. "Further targeted observations are needed to build a statistical sample of giant disks in the early universe and thus open a new window on the early stages of galaxy formation."

The Nature Astronomy study titled "A giant disk galaxy two billion years after the Big Bang," was funded by the European Research Council, the Fondazione Cariplo foundation, NASA, and the Australian Research Council.

Source: Caltech/News

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The Evolution of the Trappist-1 Planetary System

Orbital frequencies of the exoplanets in the Trappist-1 solar system

This video shows the distances between the planets in the Trappist-1 system (labeled b-h) and their orbital frequencies, showing where and when various planets' orbits come into brief alignment with one another. Credit: Gabriele Pichierri

All seven planets discovered in orbit around the red dwarf star TRAPPIST-1 could easily fit inside the orbit of Mercury, the innermost planet of our solar system. In fact, they would have room to spare. TRAPPIST-1 also is only a fraction of the size of our sun; it isn't much larger than Jupiter. So, the TRAPPIST-1 system's proportions look more like Jupiter and its moons than those of our solar system. Credit: NASA/JPL-Caltech/R. Hurt, T. Pyle (IPAC)

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Planets are bodies that orbit a star and have sufficient gravitational mass that they form themselves into roughly spherical shapes that, in turn, exert gravitational force on smaller objects around them, such as asteroids and moons. For most of human history, the only planets our ancestors knew of were those they could see in the night sky. But in the last 30 years, telescopes sensitive enough to infer the presence of exoplanets—planets outside our own solar system—have been developed.

Exoplanets are, of course, much more difficult to directly observe than stars and galaxies. Almost all exoplanet discoveries, particularly starting around 2010, have been based on photometric measurements (the amount of light received) of the exoplanets' host stars, rather than of the planets themselves. This is called the transit method. Now, with the help of the Spitzer Space Telescope, which made its own first exoplanet detection in 2005; the Kepler/KW Space Telescope, specifically designed to search for exoplanets; and the James Webb Space Telescope, launched in 2021, the transit method and other techniques have confirmed the existence of more than 5,000 exoplanets inhabiting thousands of star systems

"When we had only our own solar system to analyze, one could just assume that the planets formed in the places where we find them today," says Gabriele Pichierri, postdoctoral scholar research associate in planetary science at Caltech, working in the group of Professor of Planetary Science Konstantin Batygin. "However, when we discovered even the first exoplanet in 1995, we had to reconsider this assumption. We are developing better models for how planets are formed and how they come to be in the orientations we find them in."

Most exoplanets form out of the disc of gas and dust around newly formed stars and are then expected to migrate inward approaching the inner boundary of this disc. This assembles planetary systems that are much closer to the host star than is the case in our own solar system.

In the absence of other factors, planets will tend to space themselves apart from one another at characteristic distances based on their masses and gravitational forces between the planets and their host star. "This is the standard migration process," Pichierri explains. "The positions of the planets form resonances between their orbital periods. If you take the orbital period of one planet and then you divide it by the orbital period of its neighboring planet, you get a ratio of simple integers, such as 3:2." So, for example, if one planet takes two days to orbit around its star, the next planet, farther out, will take three days. If that second planet and a third one farther out are also in a 3:2 resonance, then the third planet's orbital period will be 4.5 days.

Trappist-1 system, which hosts seven planets and is located about 40 light-years from Earth, is a special one for multiple reasons. "The outer planets behave properly, so to speak, with the simpler expected resonances," Pichierri says. "But the inner ones have resonances that are a bit spicier." The ratio between planet b and c's orbits is 8:5, for example, and that between c and d is 5:3. "This narrow discrepancy in the outcome of Trappist-1's assembly is puzzling and represents a wonderful opportunity to figure out in detail what other processes were at play in its assembly," he says.

"In addition, most planetary systems are thought to have started in these resonant states but have encountered significant instabilities in their lifespan before we observe them today," Pichierri explains. "Most planets go unstable or collide with one another, and everything gets shuffled. Our own solar system, for example, was affected by such an instability. But we know of a few systems that have remained stable, that are more or less pristine specimens. They, in effect, exhibit a record of their entire dynamical history that we can then attempt to reconstruct. Trappist-1 is one of these."

The challenge then was to develop a model that could explain the orbits of the Trappist-1 planets and how they reached their current configuration.

The resulting model suggests that the inner four planets initially evolved alone in the expected 3:2 resonance chain. It was only as the disc's inner boundary expanded outward that their orbits relaxed out of the tighter 3:2 chain into the configuration we observe today. The fourth planet, which originally sat on the inner boundary of the disc, moving farther out along with it, was later pushed back inward when three additional outer planets joined the planetary system at a later stage.

"By looking at Trappist-1, we have been able to test exciting new hypotheses for the evolution of planetary systems," Pichierri says. "Trappist-1 is very interesting because it is so intricate; it's a long planetary chain. And it's a great exemplar for testing alternative theories about planetary system formation."

The paper containing this research, titled "Forming the Trappist-1 system in two steps during the recession of the disc inner edge," is published in Nature Astronomy. Authors are Pichierri; Alessandro Morbidelli of the Observatoire de la Cote d'Azur, formerly a Moore Distinguished Scholar at Caltech; Konstantin Batygin (PhD '12) of Caltech; and Ramon Brasser of the University of Oslo. This work was supported by the European Research Council, the Barr Foundation, the David and Lucile Packard Foundation, the National Science Foundation, and the Research Council of Norway.

Written by Cynthia Eller

Source: Caltech/News

Contact:

Cynthia Eller
celler@caltech.edu

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Sun-Like Stars Found Orbiting Hidden Companions

Astronomers have discovered 21 stars like our Sun in orbit around neutron stars—heavy, compact remains of massive stars that previously exploded. The hidden neutron stars were discovered through their gravitational effects alone. Though the neutron stars are heavier than Sun-like stars, the two objects mutually orbit one another around a common center of mass. As the neutron stars orbit around, they tug on the Sun-like stars, causing them to wobble. The European Space Agency's Gaia mission detected this wobble by observing the orbits of the Sun-like stars (yellow dots) over a period of three years. The Sun-like stars are green in this animation, and the neutron stars (and their orbits) are purple. Credit: Caltech/Kareem El-Badry

This illustration depicts a binary star system consisting of a dense neutron star and a normal Sun-like star (upper left). Using data from the European Space Agency's Gaia mission, astronomers found several systems like this one, in which the two bodies are widely separated. Because the bodies in these systems are far apart, with separations on average 300 times the size of a Sun-like star, the neutron star is dormant—it is not actively stealing mass from its companion and is thus very faint. To find these hidden neutron stars, the scientists used Gaia observations to look for a wobble in the Sun-like stars caused by a tugging action of the orbiting neutron stars. These are the first neutron stars discovered purely due to their gravitational effects. As depicted in this illustration, the intense gravity of the compact neutron star—which is about 100,000 times smaller than the Sun-like star yet heavier—warps our view of the sky around it, producing a distorted mirrored view of the nearby star. Credit: Caltech/R. Hurt (IPAC)

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Most stars in our universe come in pairs. While our own Sun is a loner, many stars like our Sun orbit similar stars, while a host of other exotic pairings between stars and cosmic orbs pepper the universe. Black holes, for example, are often found orbiting each other. One pairing that has proved to be quite rare is that between a Sun-like star and a type of dead star called a neutron star.

Now, astronomers led by Caltech's Kareem El-Badry have uncovered what appear to be 21 neutron stars orbiting in binary systems with stars like our Sun. Neutron stars are dense burned-out cores of massive stars that exploded. On their own, they are extremely faint and usually cannot be detected directly. They are heavier than Sun-like stars, but the two objects mutually orbit each other around a common center of mass. As the neutron stars orbit, they tug on the Sun-like stars, causing their companions to shift back and forth in the sky. Using the European Space Agency's Gaia mission, the astronomers were able to catch these telltale wobbles to reveal a new population of dark neutron stars.

"Gaia is continuously scanning the sky and measuring the wobbles of more than a billion stars, so the odds are good for finding even very rare objects," says El-Badry, an assistant professor of astronomy at Caltech and an adjunct scientist at the Max Planck Institute for Astronomy in Germany.

Neutron Star and Sun-like Star in Orbit Around Each Other

This animation depicts a binary star system in which a massive compact neutron star is orbiting a larger Sun-like star. The intense gravity of this high-density neutron star produces significant warping effects that distort the view of the sky around it, not unlike what occurs around more compact black holes. Animation credit: Caltech/R. Hurt (IPAC)

The new study, which includes a team of co-authors from around the world, was published in The Open Journal for Astrophysics. Data from several ground-based telescopes, including the W. M. Keck Observatory on Maunakea, Hawai‘i; La Silla Observatory in Chile; and the Whipple Observatory in Arizona, were used to follow up the Gaia observations and learn more about the masses and orbits of the hidden neutron stars.

While neutron stars have previously been detected in orbit around stars like our Sun, those systems have all been more compact. With little distance separating the two bodies, a neutron star (which is heavier than a Sun-like star) can steal mass away from its partner. This mass transfer process makes the neutron star shine brightly at X-ray or radio wavelengths. In contrast, the neutron stars in the new study are much farther from their partners—on the order of one to three times the distance between Earth and the Sun.

That means the newfound stellar corpses are too far from their partners to be stealing material from them. They are instead quiescent and dark. "These are the first neutron stars discovered purely due to their gravitational effects," El-Badry says.

The discovery comes as somewhat of a surprise because it is not clear how an exploded star winds up next to a star like our Sun.

"We still do not have a complete model for how these binaries form," explains El-Badry. "In principle, the progenitor to the neutron star should have become huge and interacted with the solar-type star during its late-stage evolution." The huge star would have knocked the little star around, likely temporarily engulfing it. Later, the neutron star progenitor would have exploded in a supernova, which, according to models, should have unbound the binary systems, sending the neutron stars and Sun-like stars careening in opposite directions.

"The discovery of these new systems shows that at least some binaries survive these cataclysmic processes even though models cannot yet fully explain how," he says.

Gaia was able to find the unlikely companions due to their wide orbits and long periods (the Sun-like stars orbit around the neutron stars with periods of six months to three years). "If the bodies are too close, the wobble will be too small to detect," El-Badry says. "With Gaia, we are more sensitive to the wider orbits." Gaia is also most sensitive to binaries that are relatively nearby. Most of the newly discovered systems are located within 3,000 light-years of Earth—a relatively small distance compared, for example, to the 100,000 light-year-diameter of the Milky Way Galaxy.

The new observations also suggest just how rare the pairings are. "We estimate that about one in a million solar-type stars is orbiting a neutron star in a wide orbit," he said

El-Badry also has an interest in finding unseen dormant black holes in orbit with Sun-like stars. Using Gaia data, he has found two of these quiet black holes hidden in our galaxy. One, called Gaia BH1, is the closest known black hole to Earth at 1,600 light-years away.

"We don't know for sure how these black hole binaries formed either," El-Badry says. "There are clearly gaps in our models for the evolution of binary stars. Finding more of these dark companions and comparing their population statistics to predictions of different models will help us piece together how they form."

The paper titled "A population of neutron star candidates in wide orbits from Gaia astrometry" was funded by the National Science Foundation, the European Research Council, and the Gordon and Betty Moore Foundation. Other Caltech authors include graduate student Natsuko Yamaguchi and Professor of Astronomy Andrew Howard. Additional authors include Hans-Walter Rix and René Andrae of the Max-Planck Institute for Astronomy, David Latham and Allyson Bieryla of the Center for Astrophysics/Harvard & Smithsonian, Sahar Shahaf of the Weizmann Institute for Science, Tsevi Mazeh of Tel Aviv University; Lars Buchhave of the Technical University of Denmark, Howard Isaacson of UC Berkeley and University of Southern Queensland; Alessandro Savino of UC Berkeley, and Ilya Ilyin of Leibniz Institute for Astrophysics Potsdam.

Written by Whitney Clavin

Contact:

Whitney Clavin
(626) 395‑1944
wclavin@caltech.edu

Source: Caltech/News

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Cosmic Simulation Reveals How Black Holes Grow and Evolve

This still from the simulation shows a supermassive black hole, or quasar, surrounded by a swirling disk of material called an accretion disk. Credit: Caltech/Phil Hopkins group

An earlier still from the simulation shows a tangle of merging galaxies.
Credit: Caltech/Phil Hopkins group

Hopkins Group: Simulation Zooms Into Black Hole

The new simulation flies into a tangle of merging galaxies, ultimately zooming into an active supermassive black hole, or quasar, surrounded by a swirling disk of material called an accretion disk. A filamentary stream of gas has been wound up into the disk, funneling gas in at a rate sufficient to fuel the brightest known quasars in the universe. Near the end of the simulation, magnetic fields rip away the angular momentum from the rotating disk, which allows material to spiral in further and further until it reaches the event horizon of the black hole, where it can't escape. During this simulation, which represents one moment in time, the scale zooms in by a factor of a billion. The colors show the density of the gas, with brighter colors representing higher densities. Credit: Caltech/Phil Hopkins group

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A team of astrophysicists led by Caltech has managed for the first time to simulate the journey of primordial gas dating from the early universe to the stage at which it becomes swept up in a disk of material fueling a single supermassive black hole. The new computer simulation upends ideas about such disks that astronomers have held since the 1970s and paves the way for new discoveries about how black holes and galaxies grow and evolve.

"Our new simulation marks the culmination of several years of work from two large collaborations started here at Caltech," says Phil Hopkins, the Ira S. Bowen Professor of Theoretical Astrophysics.

The first collaboration, nicknamed FIRE (Feedback in Realistic Environments), has focused on the larger scales in the universe, studying questions such as how galaxies form and what happens when galaxies collide. The other, dubbed STARFORGE, was designed to examine much smaller scales, including how stars form in individual clouds of gas. "But there was this big gap between the two," Hopkins explains. "Now, for the first time, we have bridged that gap." To do that, the researchers had to build a simulation with a resolution that is more than 1,000 times greater than the previous best in the field.

To the team's surprise, as reported in The Open Journal of Astrophysics, the simulation revealed that magnetic fields play a much larger role than previously believed in forming and shaping the huge disks of material that swirl around and feed the supermassive black holes. "Our theories told us the disks should be flat like crepes," Hopkins says. "But we knew this wasn't right because astronomical observations reveal that the disks are actually fluffy—more like an angel cake. Our simulation helped us understand that magnetic fields are propping up the disk material, making it fluffier."

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Cosmic Simulation Reveals How Black Holes Grow and Evolve

Visualizing the Activity Around Supermassive Black Holes Using "Super Zoom-Ins"

In the new simulation, the researchers performed what they call a "super zoom-in" on a single supermassive black hole, a monstrous object that lies at the heart of many galaxies, including our own Milky Way. These ravenous, mysterious bodies contain anywhere from thousands to billions of times the mass of the Sun, and thus exert a huge effect on anything that comes near.

Astronomers have known for decades that as gas and dust are pulled in by the tremendous gravity of these black holes, they are not immediately sucked in. Instead, the material first forms a rapidly swirling disk called an accretion disk. And as the material is just about to fall in, it radiates a huge amount of energy, shining with a brilliance unmatched by just about anything in the universe. But much is still not known about these active supermassive black holes, called quasars, and how the disks that feed them form and behave.

While disks around supermassive black holes have been imaged previously—the Event Horizon Telescope imaged disks circling black holes at the heart of our own galaxy in 2022 and Messier 87 in 2019—these disks are much closer and more tame than the ones that churn around quasars. To visualize what happens around these more active and distant black holes, astrophysicists turn to supercomputer simulations. They feed information about the physics at work in these galactic settings—everything from the basic equations that govern gravity to how to treat dark matter and stars—into thousands of computing processors that work in parallel. This input includes many algorithms, or series of instructions, for the computers to follow to recreate complicated phenomena. So, for example, the computers know that once gas becomes dense enough, a star forms. But the process is not that straightforward.

"If you just say gravity pulls everything down and then eventually the gas forms a star and stars just build up, you'll get everything wildly wrong," Hopkins explains. After all, stars do many things that affect their surroundings. They shine radiation that can heat up or push surrounding gas. They blow winds like the solar wind created by our own Sun, which can sweep up material. They explode as supernovae, sometimes launching material clear out of galaxies or changing the chemistry of their surroundings. So, the computers must know all the ins and outs of this "stellar feedback" as well, as it regulates how many stars a galaxy can actually form.

Building a Simulation that Spans Multiple Scales

But at these larger scales, the set of physics that are most important to include and what approximations can be made differ from those at smaller scales. For example, on the galactic scale, the complicated details of how atoms and molecules behave are extremely important and must be built into any simulation. However, scientists agree that when simulations focus on the more immediate area around a black hole, molecular chemistry can be mostly ignored because the gas there is too hot for atoms and molecules to exist. Instead, what is exists there is hot ionized plasma.

Creating a simulation that could cover all the relevant scales down to the level of a single accretion disk around a supermassive black hole was a huge computational challenge—one that also required a code that could handle all the physics. "There were some codes that had the physics that you needed to do the small-scale part of the problem and some codes that had the physics that you needed to do the larger, cosmological part of the problem, but nothing that had both," Hopkins says.

The Caltech-led team used a code they call GIZMO for both the large- and small-scale simulation projects. Importantly, they built the FIRE project so that all the physics they added to it could work with the STARFORGE project, and vice versa. "We built it in a very modular way, so that you could flip on and off any of the pieces of physics that you wanted for a given problem, but they were all cross compatible," Hopkins says.

This allowed the scientists in the latest work to simulate a black hole that is about 10 million times the mass of our Sun, beginning in the early universe. The simulation then zooms in on that black hole at a moment when a giant stream of material is torn off a cloud of star-forming gas and begins to swirl around the supermassive black hole. The simulation can continue zooming in, resolving a finer area at each step as it follows the gas on its way toward the hole.

Surprisingly Fluffy, Magnetic Disks

"In our simulation, we see this accretion disk form around the black hole," Hopkins says. "We would have been very excited if we had just seen that accretion disk, but what was very surprising was that the simulated disk doesn't look like what we've thought for decades it should look like." In two seminal papers from the 1970s that described the accretion disks fueling supermassive black holes, scientists assumed that thermal pressure—the change in pressure caused by the changing temperature of the gas in the disks—played the dominant role in preventing such disks from collapsing under the tremendous gravity they experience close to the black hole. They acknowledged that magnetic fields might play a minor role in helping to shore up the disks. In contrast, the new simulation found that the pressure from the magnetic fields of such disks was actually 10,000 times greater than the pressure from the heat of the gas.

"So, the disks are almost completely controlled by the magnetic fields," Hopkins says. "The magnetic fields serve many functions, one of which is to prop up the disks and make the material puffy."

This realization changes a host of predictions scientists can make about such accretion disks, such as their mass, how dense and thick they should be, how fast material should be able to move from them into a black hole, and even their geometry (such as whether the disks can be lopsided).

Looking forward, Hopkins hopes this new ability to bridge the gap in scales for cosmological simulations will open many new avenues of research. For example, what happens in detail when two galaxies merge? What types of stars form in the dense regions of galaxies where conditions are unlike those in our Sun's neighborhood? What might the first generation of stars in the universe have looked like? "There's just so much to do," he says.

The new simulation is detailed in a paper entitled "FORGE'd in FIRE: Resolving the End of Star Formation and Structure of AGN Accretion Disks from Cosmological Initial Conditions," which appears in The Open Journal of Astrophysics. Additional authors on the paper include Michael Grudic (PhD '19) of Carnegie Observatories, Kung-Yi Su (PhD '19) of Harvard University, Sarah Wellons of Wesleyan University, Daniel Angles-Alcazar of the University of Connecticut and the Flatiron Institute, Ulrich Steinwandel of the Flatiron Institute, David Guszeinov (PhD '18) of the University of Texas at Austin, Norman Murray (BS '79) of the University of Toronto, Claude-Andre Faucher-Giguere of Northwestern University, Eliot Quatert of Princeton University, and Dusan Keres of UC San Diego. Hopkins's work was supported by funding from the National Science Foundation and NASA.

Written by Kimm Fesenmaier

Source: Caltech/News

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Contact:

Whitney Clavin
(626) 395‑1944
wclavin@caltech.edu

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High School Student Creates Soundscape of Exploding Stars

A sound explosion of cosmic explosions: ZTF supernovae sonified

Vanya Agrawal creates her sonification with a computer and MIDI board.

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Vanya Agrawal discusses her experience of creating a sonification of supernova data

Agrawal's full 28-minute sonification of the ZTF supernova dataset.

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Using data from the Zwicky Transient Facility, Southern California high school student Vanya Agrawal creates new "space music."

In September 2023, Vanya Agrawal, a senior at Palos Verdes High School, was searching for a science research project. "I've been interested in music since I was very young, and, over the past few years, I've also become interested in physics and astronomy," Agrawal says. "I was planning on pursuing both as separate disciplines, but then I began to wonder if there might be a way to combine the two."

Enter data sonification. Just as researchers design graphs or diagrams or scatterplots to create a visual mapping of their data, they may also develop an audiomapping of their data by rendering it as sound. Instead of drawing a dot (or any other visual symbol) to correspond to a point of data, they record a tone.

Granted, this is highly unusual in scientific research, but it has been done. Her curiosity piqued, Agrawal soon found examples of these sonifications. For example, in 1994, an auditory researcher, Gregory Kramer, sonified a geoseismic dataset, resulting in detections of instrument error, while in 2014 the CEO and co-founder of Auralab Technologies, Robert Alexander, rendered a spectral dataset into sound and found that participants could consistently identify wave patterns simply by listening.

Do these scientific sonifications make you want to sit yourself down in a concert hall to be swept away by the music they create? Well, when you see a scatterplot of supernovae in an astrophysics journal, do you think, "What is that doing in an academic journal? It belongs on the wall of a museum!" Probably not often.

Here is where the artistic effort comes in: representing scientific information in ways that delight the eye or the ear. This was Agrawal's goal, using an astrophysical dataset to make music that could draw in nonscientific audiences and help them to engage with new discoveries about the universe.

Agrawal first approached Professor of Astronomy Mansi Kasliwal (PhD '11), a family friend, to see about finding an appropriate dataset to sonify. She was quickly put in touch with Christoffer Fremling, a staff scientist working with the Zwicky Transient Facility (ZTF) team. Using a wide-field-of-view camera on the Samuel Oschin Telescope at Caltech's Palomar Observatory, ZTF scans the entire sky visible from the Northern Hemisphere every two days, weather permitting, observing dynamic events in space.

Many of the dynamic events observed by ZTF are supernovae, the explosions of dying stars. In the dataset Agrawal received from Fremling of supernova observations from March 2018 to September 2023, there were more than 8,000 of these. She decided that each supernova detection would be one note in the music she was composing.

"I knew the things that I could modify about the music were when the note occurred, its duration, its pitch, its volume, and the instrument that played the note," Agrawal says. "Then it was a matter of looking at the parameters measured in the dataset of supernova observations and deciding which were most significant and how they should be matched up to musical features."

With Fremling's input, Agrawal decided that the five measurements associated with supernova observations that she would sonify would be discovery date, luminosity, redshift (a quantifiable change in the wavelength of light indicating the light source's distance from us), duration of explosion, and supernova type.

"Discovery date of a supernova has an obvious correlation with the time in which its associated note appears in the music," Agrawal says, "and matching the duration of a supernova with the duration of the note and the type of supernova with the type of instrument playing the note also made the most sense." As for the remaining parameters, Agrawal "flip-flopped back and forth with redshift and luminosity, which would go with pitch or volume. But I ultimately decided on having the luminosity correlate to volume because you can think of volume as the auditory equivalent to brightness. If something emits a dim light, that's like a quiet sound, but if it emits a bright light, that correlates to a loud sound. That left redshift to be translated into pitch."

Once parameters had been translated, the pitch values were modified to enhance the sound. Redshift had to be condensed into a tight range of pitches such that the result would be in the most audible range for human ears.

The initial result, according to Agrawal, was less than euphonius. Fremling, who had tried his own hand at setting down sounds in relationship to each supernova, had the same result: The music, he said, "did not sound good at all."

"I don't think I realized how many notes 8,000 actually is," Agrawal says. "I was definitely picturing it to be a lot slower and more spread out, but after converting the data to sound I heard how densely packed the notes were."

To achieve a sparser texture, Agrawal slowed the tempo of the sound file, extending its length to about 30 minutes, and then set about manipulating and enhancing the musicality of the piece. To ensure that the music would evoke outer space, Agrawal rounded pitches to fit into what is known as the Lydian augmented mode, a scale that begins with whole tones which, Agrawal says, "feel less settled and rooted than ordinary major or minor scales. This resembles the scales in sci-fi music, so I thought it would be beneficial for representing the vastness of space." Agrawal then added a percussion track, a chord track that harmonized dominant pitches in the dataset, and effects such as the sound of wind and distorted chattering.

"There is an element of subjectivity in this," Agrawal says, "because, of course, the music isn't what space actually sounds like, even before I began adding musical tracks. It's my interaction with the universe, my interpretation of it through sound. I would find it interesting to hear how other people sonify the same data, how they interact with the same universe."

Agrawal's composition has already been published on the ZTF website, along with a short video of supernova discoveries that uses portions of Agrawal's composition for background music. But Agrawal's imagination reaches well beyond her first composition: "Obviously the parameters will be different for every dataset, but this type of sonification can be done with any dataset. And with the right algorithms, sonifications can be created automatically and in real time. These compositions could be published on streaming services or played within planetariums, helping astrophysics discoveries to reach wider audiences."

Until those algorithms come along, Fremling, Agrawal, and the outreach coordinator of ZTF have created the resources and tutorials needed to enable anyone to sonify ZTF datasets. The aim is to build a library of sonifications that can be offered to educators, artists, science engagement centers, astronomy visualization professionals, and more to improve and enrich accessibility to science. All resources are available.

Of course, new data will come along to shift our perspective on supernovae, and as a consequence, musical compositions featuring them will change too. "Just within the last year or two we have found a new type of supernova, even though people have been studying supernovae since the 1940s and 1950s," Fremling says. Agrawal will need to introduce another instrument into her orchestra. Also, supernova data can be interpreted in different ways. For example, Fremling notes, "some types of supernovae are inherently always very similar in absolute luminosity. The only reason their luminosity varies in the dataset—which Agrawal has translated into volume in her composition—is because these supernovae are occurring at different distances from our observatory at Palomar."

Agrawal is bound for Washington University in St. Louis in fall 2024, planning to double major in music and astrophysics.

Try your own hand at sonifying supernovae!

Written by Cynthia Eller

Source: Caltech/News

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Contact:

Cynthia Eller
celler@caltech.edu

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Using Polarization to Improve Quantum Imaging

A stained slice of a mouse brain, as imaged with classical imaging (left) and using the Wang group's ICE method (right). The difference in resolution between the two techniques is clear in the side-by-side comparison focused on an area of detail highlighted in boxes E and F.

A zebrafish is shown classically imaged (left) and using the ICE technique (right), in the presence of unwanted, or stray light, that could interfere with the quality of an image. The black dots in the classical image are imperfections caused by stray light.

Since quantum entanglement allows paired photons to be linked no matter how far apart they might be, Wang is already imagining how his new system could be used to make birefringence measurements in space.

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Quantum imaging is a growing field that takes advantage of the counterintuitive and "spooky" ability of light particles, or photons, to become linked, or entangled, under specialized circumstances. If the state of one photon in the entangled duo gets tweaked, so does the other, regardless of how far apart the two photons might be.

Caltech researchers demonstrated last May how such entanglement could double the resolution of classical light microscopes while also preventing an imaging system's light from damaging fragile biological samples. Now the same team has improved upon the technique, making it possible to quantum image whole organ slices and even small organisms.

Led by Lihong Wang, the Bren Professor of Medical Engineering and Electrical Engineering, the new work uses entanglement—what Albert Einstein once famously described as "spooky action at a distance"— to control not only the color and brightness of the light hitting a sample, but also the polarization of that light.

"Our new technique has the potential to pave the way for quantum imaging in many different fields, including biomedical imaging and potentially even remote space sensing," says Wang, who is also the Andrew and Peggy Cherng Medical Engineering Leadership Chair and executive officer for medical engineering.

Like wavelength and intensity, polarization is a fundamental property of light and represents which direction the electric component of a light wave is oriented with respect to the wave's general direction of travel. Most light, including sunlight, is unpolarized, meaning that its electromagnetic waves move and travel in all directions. However, filters called polarizers can be used to create light beams with one specific polarization. A vertical polarizer, for example, only allows photons with vertical polarization to pass through. Those with horizontal polarization (meaning that the electric component of the light wave is oriented horizontally with respect to the direction of travel) will be blocked. Any light with other polarization angles (between vertical and horizontal), will partially pass through. The outcome is a stream of vertically polarized light.

This is how polarized sunglasses reduce glare. They use a vertically polarizing chemical coating to block sunlight that has become horizontally polarized by reflecting off a horizontal surface, such as a lake or snowy field. This means that the wearer only observes vertically polarized light.

When changes in light intensity or color are not enough to give scientists quality images of certain objects, controlling the polarization of the light in an imaging system can sometimes provide more information about the sample and offer a different way to identify contrast between a sample and its background. Detecting the changes in polarization caused by certain samples can also give researchers information about the internal structure and behavior of those materials.

Wang's newest microscopy technique, dubbed quantum imaging by coincidence from entanglement (ICE), takes advantage of entangled photon pairs to obtain higher-resolution images of biological materials, including thicker samples, and to make measurements of materials that have what scientists call birefringent properties.

Rather than consistently bending incoming light waves in the same way, as most materials do, birefringent materials bend those waves to different degrees depending on the light's polarization and the direction in which it is traveling. The most common birefringent materials studied by scientists are calcite crystals. But biological materials, such as cellulose, starch, and many types of animal tissue, including collagen and cartilage, are also birefringent.

If a sample with birefringent properties is placed between two polarizers oriented at 90-degree angles to each other, some of the light going through the sample will be altered in its polarization and will therefore make it through to the detector, even though all the other incoming light should be blocked by the two polarizers. The detected light can then provide information about the structure of the sample. In materials science, for example, scientists use birefringence measurements to get a better understanding of the areas where mechanical stress builds up in plastics.

In Wang's ICE setup, light is passed first through a polarizer and then through a pair of special barium borate crystals, which will occasionally create an entangled photon pair; about one pair is produced for every million photons that pass through the crystals. From there, the two entangled photons will branch off and follow one of the system's two arms: one will travel straight ahead, following what is called the idler arm, while the other traces a more circuitous path called the signal arm that causes the photon to pass through the object of interest. Finally, both photons go through an additional polarizer before reaching two detectors, which record the time of arrival of the detected photons. Here, though, occurs a "spooky" quantum effect because of the entangled nature of the photons: the detector in the idler arm can act as a virtual "pinhole" and "polarization selector" on the signal arm, instantly affecting the location and polarization of the photon incident on the object in the signal arm.

"In the ICE setup, the detectors in the signal and idler arms function as 'real' and 'virtual' pinholes, respectively," says Yide Zhang, lead author of the new paper and a postdoctoral scholar fellowship trainee in medical engineering at Caltech. "This dual pinhole configuration enhances the spatial resolution of the object imaged in the signal arm. Consequently, ICE achieves higher spatial resolution than conventional imaging that utilizes a single pinhole in the signal arm."

"Since each entangled photon pair always arrives at the detectors at the same time, we can suppress noises in the image caused by random photons," adds Xin Tong, co-author of the study and a graduate student in medical and electrical engineering at Caltech.

To determine the birefringent properties of a material with a classical microscopy setup, scientists typically switch through different input states, illuminating an object separately with horizontally, vertically, and diagonally polarized light, and then measuring the corresponding output states with a detector. The goal is to measure how the birefringence of the sample alters the image that the detector receives in each of those states. This information informs scientists about the structure of the sample and can provide images that would not otherwise be possible.

Since quantum entanglement allows paired photons to be linked no matter how far apart they might be, Wang is already imagining how his new system could be used to make birefringence measurements in space. Consider a situation where something of interest, perhaps an interstellar medium, is located light years away from Earth. A satellite in space might be positioned such that it could emit entangled photon pairs using the ICE technique, with two ground stations acting as detectors. The large distance to the satellite would make it impractical to send any kind of signal to adjust the device's source polarization. However, due to entanglement, changing the polarization state in the idler arm would be equivalent to changing the polarization of the source light before the beam hits the object. "Using quantum technology, nearly instantaneously, we can make changes to the polarization state of the photons no matter where they are," Wang says. "Quantum technologies are the future. Out of scientific curiosity, we need to explore this direction."

A paper describing the work, "Quantum imaging of biological organisms through spatial and polarization entanglement," appears in the March 8 issue of the journal Science Advances. In addition to Wang, Zhang, and Tong, the paper's co-authors are medical engineering graduate student David Garrett, postdoctoral scholar research associate Rui Cao, and former postdoctoral scholar research associate Zhe He, who is now at the Shandong Institute of Advanced Technology. The work was supported by funding from Caltech's Center for Sensing to Intelligence and the National Institutes of Health.

Written by Kimm Fesenmaier

Source: Caltech

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Contact:

Kimm Fesenmaier

(626) 395‑1217

kfesenma@caltech.edu

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