Fast Radio Bursts Poised to Help with Biggest Cosmic Mysteries

This illustration shows a fast radio burst (FRB) arriving at a radio telescope array on Earth. The FRB originates during an energetic event in a distant galaxy, but as it passes through intervening clouds of gas, a process known as optical refraction spreads the colors of the burst out much like a prism turns sunlight into a rainbow. This causes the shorter, bluer wavelengths to arrive before the longer, redder wavelengths. This illustration was made by artists in collaboration with researchers to ensure technical accuracy. Credit: Caltech/Robert Hurt & Keith Miller (IPAC - SELab)

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The future Deep Synoptic Array will catch tens of thousands of FRBs, enhancing the power of this cosmology tool

Intense, brief flashes of radio light called fast radio bursts (FRBs) travel across billions of light-years to reach Earth, passing through a fog of matter along the way. The bursts' origins are unclear but are thought to possibly come from highly magnetized dead stars called magnetars. The denser the fog the FRBs travel through, the more their signals will become dispersed—similar to the way a prism splits white light into a rainbow of colors.

Thanks to this dispersing trait, FRBs make excellent tracers of how ordinary matter is distributed in the universe; ordinary matter is the same stuff that makes up people, planets, stars, and anything made of subatomic particles called baryons. As the FRB radio beams pass through this matter in our universe, they can essentially map out how much is present and how clumpy it is.

Using Fast Radio Burstos to Map Matter in the Universe

Professor of Astronomy Vikram Ravi talks about fast radio bursts, or FRBs, and how these intense flashes of radio waves can be used to map out the distribution of ordinary matter in our universe.

The new study, which analyzed a sample of about 100 FRBs, is the first to directly measure the impact of feedback on clumpy matter in the large-scale regions around and between galaxies. The results show that galactic feedback does indeed smooth surrounding material, making it less clumpy. However, it does so less than what has been measured previously by state-of-the-art surveys, including the eROSITA X-ray telescope, and the former microwave-based Atacama Cosmology Telescope in Chile, which ended in 2022.

"Our analysis of FRBs reveals how gas ejected by astrophysical feedback suppresses cosmic structure, delivering constraints competitive with X-ray and microwave surveys," says coauthor Elisabeth Krause (PhD '12), a professor of astronomy and physics at The University of Arizona. "This is amazing considering we only had about 100 FRBs in our sample. It's only the beginning."

Caltech's Deep Synoptic Array (DSA), a powerful radio telescope scheduled to be built by 2029 in a remote valley in Nevada, is expected to find tens of thousands of FRBs, vastly enhancing the cosmic events' power to improve cosmology measurements. Data from the DSA, which is funded by Schmidt Sciences, will work synergistically with several cosmology experiments, including the European Euclid mission, in which NASA's Jet Propulsion Laboratory and Caltech's IPAC astronomy center play key roles (Caltech manages JPL for NASA); the Dark Energy Spectroscopic Instrument (DESI) in Arizona; the Vera Rubin Observatory in Chile; and NASA's newly launched Nancy Grace Roman Telescope, in which JPL and Caltech's IPAC also play roles.

"The DSA will be a game changer for the field," says Ravi, who is the co-principal investigator on the DSA project.

The Nature Astronomy paper is titled "Signatures of Suppressed Matter Clustering revealed by Fast Radio Bursts." This work builds on a series of related studies from the team, including "Baryons in the Darkest Sites of the Universe," "Backlighting the Cosmic Web with Fast Radio Bursts: An Anthology of Dispersion Measure Cross-Correlations with Large-Scale Structure and Baryon Tracers," "Quantifying the Impact of Selection Effects on FRB DM–z Relation Cosmological Inference," "Probing Baryonic Feedback and Cosmology with the 3×2-point Statistic of FRBs and Galaxies," and "A Hydrodynamical Simulations-based Model that Connects the FRB DM-Redshift Relation to Suppression of the Matter Power Spectrum via Feedback."

Fast Radio Burst (FRB) Dispersion

This animation shows how the signals from fast radio bursts (FRBs) become dispersed as they travel through clouds of gas around and between galaxies. Animation credit: Caltech/Robert Hurt (IPAC - SELab)

In a new Nature Astronomy study, researchers show how these FRB measurements can help to solve some of the biggest questions in cosmology.

"We've established that FRBs are a leading probe of the distribution of matter in the universe," says Kritti Sharma (MS '24), lead author of the new study and a graduate student working with Vikram Ravi, a professor of astronomy at Caltech and a coauthor of the paper. "These FRB data can be used to enhance cosmology experiments that are trying to answer questions about dark matter, dark energy, and the mass of neutrinos."

Many questions persist about the nature of dark energy, a repulsive force or substance that is causing our universe to fly apart at increasing speeds, and about dark matter, a substance that far outweighs matter in our universe but cannot be seen. Mysteries about neutrinos, ghostly particles that pass freely through ordinary matter, also endure—including the particles' mass, a measurement that could help reveal how large-scale galactic structures in the universe formed.

Dark energy, dark matter, and neutrinos are all predicted to influence how matter clumps together, so scientists use sky surveys to map this clumping and gain clues to the nature of these cosmological phenomena. The problem is that feedback processes inside galaxies can also affect how smooth or clumpy matter is, muddying the researchers' ability to precisely measure the cosmological effects.

All galaxies harbor supermassive black holes at the centers, which voraciously feed on nearby matter while also ejecting winds of hot, ionized—or charged—gas into their surroundings. Exploding stars can also expel energy into the galactic neighborhoods. This feedback has a role in smoothing out the material outside the galaxies, making it less clumpy.

"The feedback process thins the gas around the galaxies, redistributing matter across vast distances. It smooths out clumps of matter in a way that looks astonishingly similar to what massive neutrinos do, or what dark energy or dark matter theories predict," Ravi says. "Unless scientists can independently measure this contribution from feedback, they can't tell these effects apart."

Coauthors on the study include Pranjal R. S. (University of Arizona); Dhayaa Anbajagane (University of Chicago); and Liam Connor (Harvard and Smithsonian Center for Astrophysics). Coauthors on related studies include Assistant Professor Kimmy Wu and staff scientist Casey Law (Caltech); Simone Ferraro (UC Berkeley); Sebastian Grandis (University of Innsbruck); David Alonso and William Coulton (University of Oxford); Yi-Kuan Chiang (Academia Sinica Institute of Astronomy and Astrophysics); Samuel McCarty (Harvard and Smithsonian Center for Astrophysics); Nico Schuster (Aix-Marseille University); Alice Pisani (Princeton University); Shivam Pandey (University of Arizona); Nico Hamaus (University Observatory Munich); and Robert Reischke (University of Bonn).

This research was supported by Schmidt Sciences; the National Science Foundation; the Kavli Institute for Theoretical Physics; the David and Lucile Packard Foundation; the European Research Council; Aix-Marseille University's French Initiative of Excellence; the Austrian Research Promotion Agency; Austria's Federal Ministry of Innovation, Mobility, and Infrastructure; and the Austrian Space Applications Program.; and the Austrian Space Applications Program.

Written by Whitney Clavin

Source: Caltech/News

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Contact:

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

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Authors:

Kritti Sharma (Lead author and a graduate student)

Vikram Ravi (Professor of Astronomy at Caltech and a Coauthor of the paper)

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Thermal Anomaly Discovered Below Mars's South Pole

Artist concept of Mars's warm southern interior
Credit: Artist concept: NASA / Theophilus Britt Griswold

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Researchers have discovered a thermal asymmetry deep beneath the surface throughout Mars's southern hemisphere.

Based on gravitational measurements that give clues to the Red Planet's interior structure, Mars's interior southern hemisphere is around 200 to 400 degrees Celsius warmer than the northern half of the planet and partially molten. The surprising finding adds additional context to Martian history and the periods of time in which it may have hosted conditions favorable for life.

The research was led by Caltech alumnus Alexander Berne (PhD '26), who is now a postdoctoral associate at the University of Arizona. The findings are reported in a paper appearing in the journal Nature on August 27.

During his graduate studies at Caltech, Berne developed a model that uses variations in gravitational data to infer the structure of a planetary body's interior. Berne and his collaborators then aimed to apply his model to understanding Mars's interior. Using data collected over decades from three different Mars missions—Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter—the team measured tiny variations in these spacecrafts' velocities and used them to reconstruct the gravitational field around Mars. The gravitational forces exerted by the Sun on Mars vary over seasonal timescales as a result of Mars's slightly elliptical orbit and its tilted rotation axis. A technique called tidal tomography measures how those gravitational signatures vary over time and results in a model of the planet's interior.

"Scientists usually assume that the interiors of planetary bodies are generally spherically symmetric, but this is not necessarily true," Berne says. "As we get more gravity data, we can determine the three-dimensional intricacies of a planet's interior structure. These inferences in turn give us a blueprint for designing future missions and scientific exploration of these worlds. Understanding the interior structure of planetary bodies helps us unravel the processes that shaped their formation and evolution."

On the surface, Mars is a geologically asymmetrical planet: Its southern hemisphere contains towering mountains and deep craters, whereas the northern hemisphere is composed of low flat lands. In the new study, the team was surprised to discover that Mars's interior is also thermally asymmetric—the southern hemisphere is hundreds of degrees hotter than the north.

The new observation also happens to suggest explanations for other phenomena observed on Mars, such as magnetic anomalies found in iron minerals in the south. A thermal anomaly in the southern mantle could mean that a magnetic field existed strong enough to cause magnetic differences between the north and south. Additionally, NASA's InSight mission had previously discovered that seismic waves dissipate more quickly in the south, which could be explained if the region were hotter.

"The dichotomy that we see between north and south is important to understand because it gives information about processes that may have influenced the hydrology of Mars, including the formation of basins that may have held water," says Amirhossein Bagheri, a postdoctoral scholar at Caltech and co-author on the paper. Bagheri is also a former member of the InSight team.

It is still unclear what created the thermal anomaly, and there are several hypotheses for its origins, including a giant impact releasing heat from the north, past spontaneous convection in the Martian southern mantle, and thick geological features trapping excess heat from escaping.

The paper is titled "Tidal Tomography Reveals a Thermal Anomaly Beneath Mars's Crustal Dichotomy." In addition to Berne and Bagheri, co-authors are Nicholas Wagner and Harriet Lau of Brown University, Isamu Matsuyama and Angela Marusiak of the University of Arizona, Sander Goossens of NASA Goddard Space Flight Center, Karwai Cheng of the Institute of Astronomy and Astrophysics at Academia Sinica in Taiwan, Antonio Genova of the University of Rome in Italy, Marc Rovira-Navarro of the Delft University of Technology in the Netherlands, Chuan Qin of UCLA, Douglas Hemingway of the University of Texas at Austin, Shijie Zhong of the University of Colorado Boulder, and Francis Nimmo of UC Santa Cruz. Funding was provided by NASA.

Source: Caltech/News

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Contact:

Caltech Media Relations
mr@caltech.edu

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Gravitational-wave observatories release new catalog of detections

Binary Black Hole Merger

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When the densest objects in the universe collide and merge, the violence sets off gravitational waves that reverberate across space and time over hundreds of millions and even billions of years. By the time they pass through Earth, such cosmic ripples are barely discernible.

Thanks to a global network of gravitational-wave observatories—the US-based National Science Foundation-funded Laser Interferometer Gravitational-wave Observatory (LIGO), the Virgo interferometer in Italy, and the Kamioka Gravitational Wave Detector (KAGRA) in Japan—scientists can "listen" for faint wobbles in the gravitational field that could have come from far-off astrophysical smashups.

Now, the LIGO–Virgo–KAGRA (LVK) Collaboration is publishing its fourth major update to a catalog of detections since gravitational waves were first observed by LIGO in 2015. The latest findings, published in Astrophysical Journal Letters, indicate that the universe is echoing all over with a kaleidoscope of cosmic collisions

"Each new gravitational-wave detection allows us to unlock another piece of the universe's puzzle in ways we couldn't just a decade ago," says Lucy Thomas, who led part of the analysis of the catalog and is a postdoc in the Caltech LIGO lab.

The LVK's Gravitational-wave Transient Catalog-4.0 (GWTC-4) comprises detections of gravitational waves from a portion of the observatories' fourth and most recent observing run. The observatories detected 128 new gravitational-wave events, meaning signals that are likely from exotic, far-off astrophysical sources. This newest crop more than doubles the size of the gravitational-wave catalog, which previously contained 90 events compiled from all three previous observing runs.

"The GWTC-4 catalog is a real benchmark for gravitational-wave astronomy," says David Reitze, the executive director of LIGO and a research professor at Caltech. "The abundance of black holes that LIGO and its partners have detected is beginning to have a real impact on our understanding of stellar evolution and black hole formation."

The merger of a pair of black holes was the source of the very first gravitational-wave detection and colliding black holes are the source of most of the gravitational waves detected since then. In addition to the black hole binaries, the updated catalog includes the heaviest black hole binary, a binary with black holes having asymmetric masses, and a binary where both black holes have exceptionally high spins. The catalog also holds two examples of black hole–neutron star mergers.

"The message from this catalog is: We are expanding into new parts of what we call 'parameter space' and a whole new variety of black holes," says paper co-author Daniel Williams, a research fellow at the University of Glasgow and a member of the LVK Collaboration. "We are really pushing the edges and are seeing things that are more massive, spinning faster, and are more astrophysically interesting and unusual."

Among the more unusual signals that LIGO detected in the first phase of the fourth observing run was an event called GW231123_135430. As previously reported, it is the heaviest black hole binary detected to date. Scientists estimate that the signal arose from the collision of two heavier-than-normal black holes, each roughly 130 times as massive as the Sun.

Another standout is GW231028_153006, which is a black hole binary with the highest recorded inspiral spin: Both black holes appear to be spinning at about 40 percent the speed of light. "This dataset has increased our belief that black holes that collided earlier in the history of the universe could more easily have had larger spins than the ones that collided later," says LVK member Salvatore Vitale, associate professor of physics at MIT and member of the MIT LIGO Lab.

The new detections have also allowed scientists to test Albert Einstein's general theory of relativity, which describes gravity as a geometric property of space and time, using an event called GW230814_230901, which is one of the "loudest" gravitational-wave signals observed to date. The surprisingly clear signal pushed the limits of scientists' tests of general relativity, passing most with flying colors.

In addition, the updated catalog is helping scientists to nail down a key mystery in cosmology: How fast is the universe expanding today? "It's incredibly exciting to think about what astrophysical mysteries and surprises we can uncover with future observing runs," Thomas says.

"Black holes are one of the most iconic and mind-bending predictions of general relativity," says co-author and LVK member Aaron Zimmerman (PhD '13), associate professor of physics at the University of Texas at Austin, adding that when black holes collide, they "shake up space and time more intensely than almost any other process we can imagine observing. When testing our physical theories, it's good to look at the most extreme situations we can, since this is where our theories are most likely to break down, and where we have the best chance of discovery."

Read the full version of this story from the LVK Collaboration.

The paper is titled "GWTC-4.0: An Introduction to Version 4.0 of the Gravitational-Wave Transient Catalog." LIGO is funded by the NSF and is operated by Caltech and MIT, which conceived and built the project. Financial support for the Advanced LIGO project was led by NSF with Germany (Max Planck Society), the UK (Science and Technology Facilities Council), and Australia (Australian Research Council) making significant commitments and contributions to the project. More than 1,600 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration.

Additional studies describing the GWTC-4.0 methods and results are online.

Source: Caltech/News

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Contact:

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

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Related Links

LIGO Lab website

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Black Hole Flare is Biggest and Most Distant Seen

This artist's concept depicts a supermassive black hole in the process of shredding a massive star—at least 30 times the mass of our Sun—to pieces. Scientists propose this is what happened around the distant black hole referred to as J2245+3743, which in 2018, brightened dramatically to create the brightest black hole flare ever recorded, shining with the light of 10 trillion suns. This is likely the most massive star-shredding event (formally called a tidal disruption event or TDE) ever observed and one of only a few to have occurred in an active galactic nucleus (AGN), which is an active supermassive black hole "feeding" off material in a surrounding disk. The stunning flare was discovered by the Zwicky Transient Facility (ZTF), based at Caltech's Palomar Observatory, and the Caltech-led Catalina Real-Time Transient Survey. Credit: Caltech/R. Hurt (IPAC)

The 48-inch Samuel Oschin Telescope at Palomar Observatory, where ZTF resides.
Credit: Palomar/Caltech

Matthew Graham
Credit: Nick Macdonald

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The flare, co-discovered by the Zwicky Transient Facility, may be the result of a mega black-hole meal

The most massive stars in the universe are destined to explode as brilliant supernova before collapsing into black holes. Yet one huge star appears to have never fulfilled its destiny; in a twist of irony, the star wandered too close to a gargantuan black hole, which gobbled it up, shredding the star to bits and pieces.

That is the most likely explanation to come from authors of a new Nature Astronomy report describing the most powerful and most distant flare of energy ever recorded from a supermassive black hole. The cosmic object was first observed in 2018 by the US National Science Foundation (NSF)-funded Zwicky Transient Facility (ZTF), based at Caltech's Palomar Observatory, and the Caltech-led Catalina Real-Time Transient Survey, which is also funded by NSF. The flare rapidly brightened by a factor of 40 over a period of months, and, at its peak, was 30 times more luminous than any previous black hole flare seen to date. At its brightest, the flare shined with the light of 10 trillion suns.

The supermassive black hole behind the flare is a type of accreting, or feeding, black hole called an active galactic nucleus (AGN). Referred to as J2245+3743, this AGN is estimated to be 500 million times more massive than our Sun. It resides 10 billion light-years away in the remote universe. Because light has a finite speed and takes time to reach us, astronomers observe distant events like this one in the past, when the universe was young.

"The energetics show this object is very far away and very bright," says study lead author Matthew Graham, research professor of astronomy at Caltech, as well as the project scientist for ZTF, and a co-principal investigator of the project. "This is unlike any AGN we've ever seen."

Astronomers are continuing to monitor the black hole flare though it is fading over time. In fact, in addition to witnessing the object in the past, time itself runs slower at the remote site of the black hole compared to our own experience of time. "It's a phenomenon called cosmological time dilation due to stretching of space and time. As the light travels across expanding space to reach us, its wavelength stretches as does time itself," Graham explains, noting that long-lived surveys like ZTF and Catalina are important to fully witness events in the past because, in this case, "seven years here is two years there. We are watching the event play back at quarter speed."

To determine what could cause such a dramatic burst of light in the cosmos, the researchers thoroughly examined a list of possibilities, concluding that the most likely culprit is a tidal disruption event (TDE). This phenomenon occurs when a supermassive black hole's gravity shears a star that comes too close, slowly consuming the star over time as it spirals into the black hole. The fact that the black hole flare J2245+3743 is still going indicates that we are witnessing a star not yet fully devoured but rather like "a fish only halfway down the whale's gullet," Graham says.

If the flare is from a TDE, the scientists estimate that the supermassive black hole gobbled a star with a mass at least 30 times greater than that of our Sun. The previous record holder for the largest candidate TDE, an event nicknamed Scary Barbie after its initial ZTF classification as ZTF20abrbeie, was not nearly as intense. That TDE, which is also thought to have originated from an AGN, was 30 times weaker than that of J2245+3743, and its doomed star is estimated to have been between three and 10 solar masses.

Stellar Snack Within a Black Hole's Disk

Most of the roughly 100 TDEs seen to date do not take place around AGN—massive structures that consist of supermassive black holes surrounded by large, swirling disks of material that feed the central black hole. The AGN burble along, flaring up with their own feeding activity, which can mask TDE bursts and makes them harder to find. The recent jumbo flare J2245+3743, on the other hand, was so large that it was easier to see.

However, at first, J2245+3743 did not seem to be anything special. In 2018, after the object was first spotted, the researchers used the 200-inch Hale Telescope at Caltech's Palomar Observatory to obtain a spectrum of the object's light, but it did not reveal anything unusual. In 2023, the team noticed the flare was decaying slower than expected, so they obtained another spectrum from the W. M. Keck Observatory in Hawai‘i, which indicated the extreme brightness of this particular AGN.

"At first, it was important to establish that this extreme object was truly this bright," explains co-author K. E. Saavik Ford, a professor at the City University of New York (CUNY) Graduate Center and Borough of Manhattan Community College and American Museum of Natural History (AMNH). It was possible, she says, that the object could have been beaming the light toward us rather than glowing in all directions, but data from NASA's former Wide-field Infrared Survey Explorer (WISE) mission helped rule that out. In the end, after other scenarios were also ruled out, the researchers concluded that J2245+3743 was indeed the brightest black hole flare ever recorded.

"If you convert our entire Sun to energy, using Albert Einstein's famous formula E = mc2, that's how much energy has been pouring out from this flare since we began observing it," Ford says.

Once the team established the unprecedented brightness of the event, they looked at what could possibly have caused it. "Supernovae are not bright enough to account for this," Ford says, referring to one possibility. Instead, the team's favored explanation is a supermassive black hole slowly ripping a huge star to death.

"Stars this massive are rare," Ford says, "but we think stars within the disk of an AGN can grow larger. The matter from the disk is dumped onto stars, causing them to grow in mass."

Finding a black hole meal with such mega proportions indicates that other events like this are likely taking place across the cosmos. The researchers hope to mine through more ZTF data to find others, and the NSF and Department of Energy's Vera C. Rubin Observatory may likewise find unusually large TDEs.

"We never would have found this rare event in the first place if it weren't for ZTF," Graham says. "We've been observing the sky with ZTF for seven years now, so when we see anything flare or change, we can see what it has done in the past and how it will evolve."

The Nature Astronomy study titled "An Extremely Luminous Flare Recorded from a Supermassive Black Hole" was funded by the NSF, the Simons Foundation, NASA, and the German Research Foundation. Other Caltech authors include Andrew Drake, Yuanze Ding (MS '25), Mansi Kasliwal (PhD '11), Sam Rose, Jean Somalwar (now a postdoc at UC Berkeley), George Djorgovski, Shri Kulkarni, and Ashish Mahabal; Tracy Chen and Steven Groom of Caltech's IPAC astronomy center; and Daniel Stern of NASA's Jet Propulsion Laboratory (which is managed by Caltech). Additional authors are Barry McKernan of CUNY Graduate Center and Borough of Manhattan Community Collegeand AMNH; Matteo Cantiello of the Simons Foundation's Flatiron Institute and Princeton University; Mike Koss of Eureka Scientific; Raffaella Margutti of UC Berkeley; Phil Wiseman of University of Southampton, UK; Patrik Veres of Ruhr University in Bochum, Germany; and Eric Bellm of the University of Washington.

Caltech's ZTF is funded by the NSF and an international collaboration of partners. Additional support comes from the Heising-Simons Foundation and from Caltech. ZTF data are processed and archived by Caltech's IPAC. NASA supports ZTF's search for near-Earth objects through the Near-Earth Object Observations program.

Written by Whitney Clavin

Source: Caltech/News

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Contact:

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

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A New Model of Water in Jupiter's Atmosphere

An illustration of Jupiter's atmosphere at varying depths. Deeper below the cloud layer, water concentrations increase.
Credit: H. Ge

Caltech researchers have developed a new simulation of the hydrological cycle on Jupiter, modeling how water vapor condenses into clouds and falls as rain throughout the giant planet's swirled, turbulent atmosphere. The research shows that Jupiter's water is not uniformly distributed, giving missions like NASA's Juno orbiter important guidance about where to look for water on the planet.

An illustration of Jupiter's atmosphere at varying depths. Deeper below the cloud layer, water concentrations increase. Credit: H. Ge

Jupiter was considered the first planet in our solar system to form, and its massive gravitational influence shaped the orbital architecture of Earth and the other planets in the solar system. Understanding how much water Jupiter has, and where to look for it, gives clues to how water arrived on Earth, which is still an open question in planetary science.

The research is described in a paper appearing in the journal Proceedings of the National Academy of Sciences on September 29. The study's first author is Huazhi Ge, a postdoctoral scholar in the group of Andrew P. Ingersoll, professor of planetary science, emeritus.

"While we are focusing on Jupiter, ultimately we are trying to create a theory about water and atmospheric dynamics that can broadly be applied to other planets, including exoplanets," Ge says.

Jupiter's swirled appearance results from its atmospheric dynamics, which, while visually striking, make it difficult to determine the abundances of chemical species such as water and metals. The Galileo mission first detected water on Jupiter near its equator in the 1990s, but it remained uncertain if that water was distributed evenly across the giant planet. The new model accounts for Jupiter's rapid rotation—one full rotation, or one day, on Jupiter takes only about 10 Earth hours. This fast rotation causes the turbulent stripes visible on Jupiter's atmosphere. The new model suggests that this turbulence in the subtropic and mid-latitudes leads to rain that draws water deeper beneath the cloud layer, making the planet's lower atmosphere more humid tens of kilometers beneath the clouds.

Jupiter is different from Earth in many ways, so modeling its atmospheric dynamics—and then comparing those models with observations—leads to a better understanding of a diverse range of planets more broadly. Next, the team plans to create a more global model, expanding past the mid-latitudes. Ideally, the theory can be applied to other gas giants like Uranus and Neptune that also have nonuniform distributions of chemical species like methane rather than water.

The paper is titled "Non-Uniform Water Distribution in Jupiter's Mid-Latitudes: Influence of Precipitation and Planetary Rotation." In addition to Ge and Ingersoll, coauthors are Cheng Li of the University of Michigan, Xi Zhang of UC Santa Cruz, and Caltech graduate student Sihe Chen. Funding was provided by NASA, UC Santa Cruz, the Heising-Simons Foundation, and the National Science Foundation.

Written by Lori Dajose

Source: Caltech/News

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Contact:

Lori Dajose
(626) 395‑1217
ldajose@caltech.edu

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Ten Years Later, LIGO is a Black-Hole Hunting Machine

This plot shows gravitational-wave signals recorded by the LIGO Hanford detector almost ten years apart. The top shows data from LIGO's first-ever detection of gravitational waves, an event called GW150914, captured in 2015. The bottom shows the signal known as GW250114, captured in 2025. Both events involve colliding black holes about 1.3 billion light-years away with masses between 30 to 40 times that of our Sun. The purple line shows the data, which are a combination of the signal plus background detector noise. The noise comes from a variety of sources, including seismic motions that jiggle giant mirrors inside LIGO. The green line shows the best-fit prediction from general relativity for each signal. The much lower noise seen today is thanks to cutting-edge improvements made to the LIGO detectors that hush unwanted noise. Credit: LIGO/J. Tissino (GSSI)/R. Hurt (Caltech-IPAC)

GW250114 – The Clearest of Chirps

This video compares a newly detected gravitational-wave signal called GW250114 with the first gravitational-wave signal ever detected, GW150914, in 2015. Both signals came from colliding black holes, each between 30 to 40 times the mass of the Sun. The video converts the signals to sounds (called "chirps") and plays each detection twice. The first round is played at the original frequencies, in which the gravitational-wave frequencies have been converted directly into sound waves. In the second round, the pitch has been increased by 30 percent to make the chirps easier to hear.

Celebrating 10 years of gravitational waves with GW250114

A numerical relativity simulation of the recently observed GW250114 event. The blue and white surface shows a two-dimensional slice of the gravitational waves spiraling outward as the black holes orbit one another. Throughout this inspiral, the gravitational waves grow in magnitude, peaking as the black holes merge, and then decreasing rapidly as the newly formed remnant black hole settles. Credit: Deborah Ferguson, Derek Davis, Rob Coyne (URI) / LIGO / MAYA Collaboration. Simulation performed with NSF's TACC Frontera supercomputer.

This artwork imagines the ultimate front-row seat for GW250114, a powerful collision between two black holes observed in gravitational waves by the US National Science Foundation LIGO. It depicts the view from one of the black holes as it spirals toward its cosmic partner. Ten years after LIGO's landmark detection of gravitational waves, the observatory's improved detectors allowed it to "hear" this celestial collision with unprecedented clarity. The gravitational-wave data enabled scientists to distinguish multiple subtle tones ringing out like a cosmic bell across the universe (imagined here as intertwining musical threads spiraling toward the center). Credit: Aurore Simonnet (SSU/EdEon)/LVK/URI

Infographic explaining the significance of "overtones" detected by LIGO during a black hole merger. Credit: Lucy Reading-Ikkanda/Simons Foundation

Caltech professors Barry Barish, Kip Thorne, and Fiona Harrison with Caltech President Tom Rosenbaum at a press conference for the 2017 Nobel Prize in Physics.

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LIGO, Virgo, and KAGRA celebrate anniversary, announce verification of Stephen Hawking's Black Hole Area Theorem

On September 14, 2015, a signal arrived on Earth, carrying information about a pair of remote black holes that had spiraled together and merged. The signal had traveled about 1.3 billion years to reach us at the speed of light—but it was not made of light. It was a different kind of signal: a quivering of space-time called gravitational waves first predicted by Albert Einstein 100 years prior. On that day 10 years ago, the twin detectors of the US National Science Foundation Laser Interferometer Gravitational-wave Observatory (NSF LIGO) made the first-ever direct detection of gravitational waves, whispers in the cosmos that had gone unheard until that moment.

The historic discovery meant that researchers could now sense the universe through three different means. Light waves, such as X-rays, optical, radio, and other wavelengths of light, as well as high-energy particles called cosmic rays and neutrinos, had been captured before, but this was the first time anyone had witnessed a cosmic event through the gravitational warping of space-time. For this achievement, first dreamed up more than 40 years prior, three of the team's founders won the 2017 Nobel Prize in Physics: MIT's Rainer Weiss, professor of physics, emeritus (who recently passed away at age 92); Caltech's Barry Barish, the Ronald and Maxine Linde Professor of Physics, Emeritus; and Caltech's Kip Thorne, the Richard P. Feynman Professor of Theoretical Physics, Emeritus.

Today, LIGO, which consists of detectors in both Hanford, Washington and Livingston, Louisiana, routinely observes roughly one black hole merger every three days. LIGO now operates in coordination with two international partners, the Virgo gravitational-wave detector in Italy and KAGRA in Japan. Together, the gravitational-wave-hunting network, known as the LVK (LIGO, Virgo, KAGRA), has captured a total of about 300 black hole mergers, some of which are confirmed while others await further analysis. During the network's current science run, the fourth since the first run in 2015, the LVK has discovered more than 200 candidate black hole mergers, more than double the number caught in the first three runs.

LIGO Hanford in Washington and LIGO Livingston in Louisiana.

The dramatic rise in the number of LVK discoveries over the past decade is owed to several improvements to their detectors—some of which involve cutting-edge quantum precision engineering. The LVK detectors remain by far the most precise rulers for making measurements ever created by humans. The space-time distortions induced by gravitational waves are incredibly miniscule. For instance, LIGO detects changes in space-time smaller than 1/10,000 the width of a proton. That's 700 trillion times smaller than the width of a human hair. "Rai Weiss proposed the concept of LIGO in 1972, and I thought, 'This doesn't have much chance at all of working,'" recalls Thorne, an expert on the theory of black holes. "It took me three years of thinking about it on and off and discussing ideas with Rai and Vladimir Braginsky [a Russian physicist], to be convinced this had a significant possibility of success. The technical difficulty of reducing the unwanted noise that interferes with the desired signal was enormous. We had to invent a whole new technology. NSF was just superb at shepherding this project through technical reviews and hurdles."

MIT's Nergis Mavalvala, the Curtis and Kathleen Marble Professor of Astrophysics and dean of the School of Science, says that the challenges the team overcame to make the first discovery are still very much at play. "From the exquisite precision of the LIGO detectors to the astrophysical theories of gravitational-wave sources, to the complex data analyses, all these hurdles had to be overcome, and we continue to improve in all of these areas," Mavalvala says. As the detectors get better, we hunger for farther, fainter sources. LIGO continues to be a technological marvel."

This chart plots discoveries made by the LIGO-Virgo-KAGRA (LVK) network since LIGO's first detection, in 2015, of gravitational waves emanating from a pair of colliding black holes. The detections consist mainly of black hole mergers, but a handful involve neutron stars (either black hole-neutron star collisions or neutron star-neutron star collisions). Credit: LIGO/Caltech/MIT/R. Hurt (IPAC)

The Clearest Signal Yet

LIGO's improved sensitivity is exemplified in a recent discovery of a black hole merger referred to as GW250114 (the numbers denote the date the gravitational-wave signal arrived at Earth: January 14, 2025). The event was not that different from LIGO's first-ever detection (called GW150914)—both involve colliding black holes about 1.3 billion light-years away with masses between 30 to 40 times that of our Sun. But thanks to 10 years of technological advances reducing instrumental noise, the GW250114 signal is dramatically clearer.

"We can hear it loud and clear, and that lets us test the fundamental laws of physics," says LIGO team member Katerina Chatziioannou, Caltech assistant professor of physics and William H. Hurt Scholar, and one of the authors of a new study on GW250114 published in the Physical Review Letters.

By analyzing the frequencies of gravitational waves emitted by the merger, the LVK team provided the best observational evidence captured to date for what is known as the black hole area theorem, an idea put forth by Stephen Hawking in 1971 that says the total surface areas of black holes cannot decrease. When black holes merge, their masses combine, increasing the surface area. But they also lose energy in the form of gravitational waves. Additionally, the merger can cause the combined black hole to increase its spin, which leads to it having a smaller area. The black hole area theorem states that despite these competing factors, the total surface area must grow in size.

Later, Hawking and physicist Jacob Bekenstein concluded that a black hole's area is proportional to its entropy, or degree of disorder. The findings paved the way for later groundbreaking work in the field of quantum gravity, which attempts to unite two pillars of modern physics: general relativity and quantum physics.

In essence, the LIGO detection allowed the team to "hear" two black holes growing as they merged into one, verifying Hawking's theorem. (Virgo and KAGRA were offline during this particular observation.) The initial black holes had a total surface area of 240,000 square kilometers (roughly the size of Oregon), while the final area was about 400,000 square kilometers (roughly the size of California)—a clear increase. This is the second test of the black hole area theorem; an initial test was performed in 2021 using data from the first GW150914 signal, but because that data was not as clean, the results had a confidence level of 95 percent compared to 99.999 percent for the new data.

Thorne recalls Hawking phoning him to ask whether LIGO might be able to test his theorem immediately after he learned of the 2015 gravitational-wave detection. Hawking died in 2018 and sadly did not live to see his theory observationally verified. "If Hawking were alive, he would have reveled in seeing the area of the merged black holes increase," Thorne says.

The trickiest part of this type of analysis had to do with determining the final surface area of the merged black hole. The surface areas of pre-merger black holes can be more readily gleaned as the pair spiral together, roiling space-time and producing gravitational waves. But after the black holes coalesce, the signal is not as clear-cut. During this so-called ringdown phase, the final black hole vibrates like a struck bell.

In the new study, the researchers precisely measured the details of the ringdown phase, which allowed them to calculate the mass and spin of the black hole and, subsequently, determine its surface area. More specifically, they were able, for the first time, to confidently pick out two distinct gravitational-wave modes in the ringdown phase. The modes are like characteristic sounds a bell would make when struck; they have somewhat similar frequencies but die out at different rates, which makes them hard to identify. The improved data for GW250114 meant that the team could extract the modes, demonstrating that the black hole's ringdown occurred exactly as predicted by math models based on the Teukolsky formalism—devised in 1972 by Saul Teukolsky, now a professor at Caltech and Cornell.

Another study from the LVK, submitted to Physical Review Letters today, places limits on a predicted third, higher-pitched tone in the GW250114 signal, and performs some of the most stringent tests yet of general relativity's accuracy in describing merging black holes.

Ringing Black Hole Animation (GW250114)

Visualization of the binary black hole merger called GW250114. The animation shows the inspiral and merger of the two black holes, then continues a few milliseconds into the ringdown phase. At that point the gravitational waves are separated into the two modes of the ringing remnant black hole that were identified in the observation. A predicted third tone (that the data place limits on) is also shown. Credit: H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), K. Mitman (Cornell University)

"A decade of improvements allowed us to make this exquisite measurement," Chatziioannou says. "It took both of our detectors, in Washington and Louisiana, to do this. I don't know what will happen in 10 more years, but in the first 10 years, we have made tremendous improvements to LIGO's sensitivity. This not only means we are accelerating the rate at which we discover new black holes, but we are also capturing detailed data that expand the scope of what we know about the fundamental properties of black holes."

Jenne Driggers, detection lead senior scientist at LIGO Hanford, adds, "It takes a global village to achieve our scientific goals. From our exquisite instruments, to calibrating the data very precisely, vetting and providing assurances about the fidelity of the data quality, searching the data for astrophysical signals, and packaging all that into something that telescopes can read and act upon quickly, there are a lot of specialized tasks that come together to make LIGO the great success that it is."

Pushing the Limits

LIGO and Virgo have also unveiled neutron stars over the past decade. Like black holes, neutron stars form from the explosive deaths of massive stars, but they weigh less and glow with light. Of note, in August 2017, LIGO and Virgo witnessed an epic collision between a pair of neutron stars—a kilonova—that sent gold and other heavy elements flying into space and drew the gaze of dozens of telescopes around the world, which captured light ranging from high-energy gamma rays to low-energy radio waves. The "multi-messenger" astronomy event marked the first time that both light and gravitational waves had been captured in a single cosmic event. Today, the LVK continues to alert the astronomical community to potential neutron star collisions, who then use telescopes to search the skies for signs of kilonovae.

"The LVK has made big strides in recent years to make sure we're getting high quality data and alerts out to the public in under a minute, so that astronomers can look for multi-messenger signatures from our gravitational-wave candidates," Driggers says.

"The global LVK network is essential to gravitational-wave astronomy," says Gianluca Gemme, Virgo spokesperson and director of research at the National Institute of Nuclear Physics in Italy. "With three or more detectors operating in unison, we can pinpoint cosmic events with greater accuracy, extract richer astrophysical information, and enable rapid alerts for multi-messenger follow-up. Virgo is proud to contribute to this worldwide scientific endeavor."

Other LVK scientific discoveries include the first detection of collisions between one neutron star and one black hole; asymmetrical mergers, in which one black hole is significantly more massive than its partner black hole; the discovery of the lightest black holes known, challenging the idea that there is a "mass gap" between neutron stars and black holes; and the most massive black hole merger seen yet with a merged mass of 225 solar masses. For reference, the previous record holder for the most massive merger had a combined mass of 140 solar masses.

Even in the decades before LIGO began taking data, scientists were building foundations that made the field of gravitational-wave science possible. Breakthroughs in computer simulations of black hole mergers, for example, allow the team to extract and analyze the feeble gravitational-wave signals generated across the universe.

LIGO's technological achievements, beginning as far back as the 1980s, include several far-reaching innovations, such as a new way to stabilize lasers using the so-called Pound–Drever–Hall technique. Invented in 1983 and named for contributing physicists Robert Vivian Pound, the late Ronald Drever of Caltech (a founder of LIGO), and John Lewis Hall, this technique is widely used today in other fields, such as the development of atomic clocks and quantum computers. Other innovations include cutting-edge mirror coatings that almost perfectly reflect laser light; "quantum squeezing" tools that enable LIGO to surpass sensitivity limits imposed by quantum physics; and new AI methods that could further hush certain types of unwanted noise.

"What we are ultimately doing inside LIGO is protecting quantum information and making sure it doesn't get destroyed by external factors," Mavalvala says. "The techniques we are developing are pillars of quantum engineering and have applications across a broad range of devices, such as quantum computers and quantum sensors."

In the coming years, the scientists and engineers of LVK hope to further fine tune their machines, expanding their reach deeper and deeper into space. They also plan to use the knowledge they have gained to build another gravitational-wave detector, LIGO India. Having a third LIGO observatory would greatly improve the precision with which the LVK network can localize gravitational-wave sources.

Looking farther into the future, the team is working on a concept for an even larger detector, called Cosmic Explorer, which would have arms 40 kilometers long (the twin LIGO observatories have 4-kilometer arms). A European project, called Einstein Telescope, also has plans to build one or two huge underground interferometers with arms more than 10 kilometers long. Observatories on this scale would allow scientists to hear the earliest black hole mergers in the universe.

"Just 10 short years ago, LIGO opened our eyes for the first time to gravitational waves and changed the way humanity sees the cosmos," says Aamir Ali, a program director in the NSF Division of Physics, which has supported LIGO since its inception. "There's a whole universe to explore through this completely new lens and these latest discoveries show LIGO is just getting started."

The LIGO-Virgo-KAGRA Collaboration

LIGO is funded by the US National Science Foundation and operated by Caltech and MIT, which together conceived and built the project. Financial support for the Advanced LIGO project was led by NSF with Germany (Max Planck Society), the United Kingdom (Science and Technology Facilities Council), and Australia (Australian Research Council) making significant commitments and contributions to the project. More than 1,600 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. Additional partners are listed at my.ligo.org/census.php.

The Virgo Collaboration is currently composed of approximately 1,000 members from 175 institutions in 20 different (mainly European) countries. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa, Italy, and is funded by the French National Centre for Scientific Research, the National Institute of Nuclear Physics in Italy, the National Institute of Subatomic Physics in the Netherlands, The Research Foundation – Flanders, and the Belgian Fund for Scientific Research. A list of the Virgo Collaboration groups can be found at: https://www.virgo-gw.eu/about/scientific-collaboration/. More information is available on the Virgo website at https://www.virgo-gw.eu.

KAGRA is the laser interferometer with 3-kilometer arm length in Kamioka, Gifu, Japan. The host institute is the Institute for Cosmic Ray Research of the University of Tokyo, and the project is co-hosted by the National Astronomical Observatory of Japan and the High Energy Accelerator Research Organization. The KAGRA collaboration is composed of more than 400 members from 128 institutes in 17 countries/regions. KAGRA's information for general audiences is at the website gwcenter.icrr.u-tokyo.ac.jp/en/. Resources for researchers are accessible from gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA.

Written by Whitney Clavin

Source: Caltech/News

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Contact:

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

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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