When a star dies, a black hole is born. This has been the textbook origin story for most black holes. At the end of a massive star’s life, its outer layers blast away in a brilliant supernova, and its core collapses into a gravitationally tight and dense region, forming a black hole.

Recent discoveries from gravitational-wave detectors have revealed hundreds of merging black holes across the universe. Many of them have been thought to come directly from exploding stars. But black holes can also come from other, smaller black holes. The products of previous black hole mergers can, in principle, merge again, creating a more massive black hole. This alternative, black-holes-birthing-black-holes pathway is known as “hierarchical merging.”

Now MIT scientists are finding that a good number of merging black holes may have indeed merged before. They carried out a new analysis of recent data from the LIGO, Virgo, and KAGRA observatories, containing 155 pairs of binary black holes, and found about 14 percent of merging black holes in the universe may in fact be second-generation black holes that formed from the previous merging of two smaller black holes. 

The results, which the team reports this week in Physical Review Letters, suggest that repeated hierarchical merging is a significant pathway by which black holes form. 

“We’re finding that, for some of these merging black holes, it’s not their first rodeo,” says the study’s first author, Cailin Plunkett, a graduate student in MIT’s Department of Physics. “Overall in the universe, black holes are merging all the time. The question of how often are they repeatedly merging was pretty uncertain. Now we’re seeing a relatively consistent picture where there’s a decent percentage of black holes that are coming from this repeated pathway.”

The study’s co-authors are Salvatore Vitale, associate professor of physics at MIT; Thomas Callister of Williams College; and Michael Zevin of Adler Planetarium and Northwestern University.

Lopsided pairs

When a massive star collapses and dies, the resulting black hole should have very little spin. In addition to losing a huge amount of mass when it explodes, the star should also lose much of its inherent spin, or angular momentum. The black hole left over should then have little to no spin. 

In contrast, when two black holes merge, the collision should create a new, wildly spinning second-generation black hole. 

“They would be spinning very fast, at about 70 percent their maximum possible spin,” Vitale says. 

Scientists suspect that hierarchical mergers occur in dense stellar environments, where stars are so tightly packed together that multiple neighboring stars could die and collapse to form black holes that are then close enough to merge with each other to form second-generation black holes. 

“You might have a ton of stars whizzing around each other, and if some are massive and explode, they become black holes. The black holes continue to whizz around, and can capture each other and merge,” Plunkett says. “This process can repeat potentially ad infinitum, by virtue of the fact that you have a ton of stars and black holes in this really dense environment.”

One sign of a hierarchical merger is that one black hole in a pair of merging black holes has a much higher spin, and higher mass, than the other. Such a lopsided duo would signal that at least one of the black holes came from the collision of two previous black holes. 

In 2024, scientists detected two such lopsided mergers in signals recorded by the LIGO, Virgo, and KAGRA observatories. The observatories detect incoming gravitational waves — incredibly small wobbles in the fabric of space and time — that are the reverberations from distant cosmic phenomena, such as colliding black holes. 

The observatories detected two gravitational-wave signals, labeled GW241011 and GW241110, each of which likely contain a black hole spinning much faster than its partner. The hierarchical mergers were discovered by analyzing each signal in detail to tease out the specific masses and spins of the black holes involved in each merger.

That work inspired Plunkett and Vitale to do a search of similar hierarchical mergers using all the gravitational-wave signals that the observatories have captured to date. 

A pattern of wobbles

For their new study, the team analyzed the LIGO-Virgo-KAGRA Gravitational Wave Transient Catalog 4.0 (GWTC-4.0), which comprises gravitational-wave detections from the observatories’ fourth observing run. Rather than analyze each gravitational-wave signal one by one, which is what scientists did for GW241011 and GW241110, Plunkett and Vitale searched for a characteristic pattern of hierarchical mergers across the data overall, to see if any matching signals popped out.

The pattern they searched for represents a range of orbital “wobbles.” Just before they merge, two black holes spiral toward each other in a disk-like, orbital plane. When the spins of the pair are perpendicular to the plane, this remains relatively steady. But when one or both spins are not perpendicular to the plane, the disk will wobble. The degree to which the whole plane wobbles, or “precesses,” can tell scientists about the balance of masses and spins between the two spiraling black holes. 

Plunkett and Vitale developed a model for the range of wobbling that should be a sign of a hierarchical merger, specifically between a first-generation and a second-generation black hole. 

The team applied the model to the entire GWTC-4.0 catalog, which comprises gravitational-wave signals from 153 black hole mergers, in addition to the signals from GW241011 and GW241110. Their analysis revealed that a number of mergers fit the pattern for orbital wobbling that was likely caused by the colliding of first- and second-generation black holes. 

Specifically, they found that roughly 14 percent of merging black holes in the universe may have merged before, and that these second-generation black holes had very particular masses: Black holes of around 10 solar masses (10 times the mass of the sun) and 30 solar masses were run-of-the-mill star-born black holes, while second-generation black holes had masses of around 20 solar masses or 40 solar masses and above. 

“One of the reasons why the 40-and-above regime is interesting is, stellar evolution theory predicts you shouldn’t be able to form black holes in that mass range at all from just a supernova,” Plunkett says. “We think supernovae from really massive stars end up being so violent that they leave no black holes at all above roughly 45 solar masses. Yet we have seen black holes that are that massive. And the question is: Where did they come from?”

The team’s new analysis provides support for the idea that black holes can form from the repeated merging of other black holes, and that this alternate origin story could explain some of the curious black holes that we can detect today. 

This work was supported, in part, by the National Science Foundation, and the Brinson Foundation.

Many experts refer to hydrogen as “the fuel of the future.” It is expected to help decarbonize the global economy in two main ways: burning it or feeding it into a fuel cell produces storable energy with no carbon emissions, just water. And it can be used in place of fossil fuels or as a chemical feedstock in hard-to-decarbonize industrial processes such as steel and cement production.

But for hydrogen to realize its potential, two challenges must be overcome. Researchers worldwide are now working to address the first: finding a method of producing pure hydrogen that’s both cheap and low in carbon emissions.

Just as critical is finding a good means of transporting and storing hydrogen. A team led by researchers at the MIT Energy Initiative (MITEI) has been tackling that less-discussed but important challenge. The location where the pure hydrogen is produced is likely to be far away from where it will be used, so moving it will be critical — and difficult.

The problem stems from two characteristics of hydrogen: It’s the lightest gas there is, and it has low energy density per volume. Therefore, delivering a given amount of energy requires a large volume of hydrogen and a container that’s sealed so tightly that the hydrogen molecules can’t escape. Suffice it to say, moving a liquid fuel such as gasoline is easier. And without a good means of storing and transporting hydrogen, it can’t fulfill its promise as the world’s clean fuel of the future.

In 2024, with funding provided by ExxonMobil Technology and Engineering Co. through MITEI, a team of MITEI researchers and their Exxon colleagues began examining various approaches to transporting hydrogen. The researchers have now concluded that there’s no single answer; the cost and carbon emissions from a given transportation method will vary from one location to another. Therefore, instead of presenting a table showing the “best” outcome, the team created a tool that enables users to understand the various options and choose the best option for their particular use case. 

The researchers present their study and the tool they developed in a new paper published in the journal Fuel.    

The study was led by former MITEI postdocs Gasim Ibrahim, now an R&D engineer/scientist at Honeywell, and Guiyan Zang, former MITEI group lead who is now an associate professor at Washington State University. Additional MIT co-authors include former postdocs Bosong Lin, Jacqueline Garrido, Woojae Shin, and Haoxiang Lai.

The hydrogen challenge and hydrogen “carriers” that can help

The team’s starting assumption was that for hydrogen to become a viable fuel for the world, it would need to be transported over long distances — specifically, overseas, across continents, or across large water bodies. Given the properties of hydrogen gas, it would be best to convert it to some liquid form before shipping.

There are known ways to do that, but what would be best for shipping? How much would various methods cost, and how much would they add to the carbon intensity of the delivered hydrogen?

“There hasn’t been a lot of attention paid to addressing those questions,” Ibrahim says. While some studies have been done, their conclusions are inconsistent and many uncertainties remain, both because the cost and carbon emissions will differ from place to place and because there’s not a lot of data to inform how the large-scale transportation of hydrogen will work.

“So we decided the best thing to do was to develop an adaptive tool that would enable users to perform their own assessments — a tool that could be updated very easily,” Ibrahim explains. “And we would make it open source, so anyone can see and update the numbers that we used in formulating and testing it. As the industry develops, and as scale becomes more a factor, the assumptions made in [our initial] assessments of the economics and the carbon intensity [of different shipping methods] will need to be updated.”

To focus on the transportation and storage issues, their model — called the Hydrogen Carrier Analysis Tool, or HyCAT — doesn’t consider how the starting hydrogen is produced, or how the hydrogen is used after it’s delivered. HyCAT focuses on determining the costs and carbon emissions incurred as the hydrogen is transported and delivered. In addition, while a full life-cycle assessment would include all environmental impacts, HyCAT focuses on emissions of greenhouse gases (GHGs).

The tool is easy to use, says Ibrahim. Built into it is a user interface with drop-down menus for inputting assumptions, and results from an analysis are presented in simple bar charts that include links to tables presenting the details.

Ibrahim clarifies that, while HyCAT has a well-defined boundary — “incoming hydrogen to outgoing hydrogen” — in an analysis of a specific situation, the user will input various factors about the local situation, including the carbon intensity and cost associated with production of the incoming hydrogen. “So that will inform the final values that come out of a HyCAT analysis,” says Ibrahim, and in part explains why the results vary from place to place.

Based on the user’s assumptions, HyCAT calculates the cost and GHG emissions at five steps in the “supply chain”:

• converting the hydrogen into liquid form at the “export” terminal;
• storing the hydrogen-rich liquid;
• shipping it when an empty tanker becomes available;
• storing it at the “import” terminal; and
• releasing the hydrogen as a gas suitable for burning or being fed into a pipeline for distribution.  

Options for liquifying hydrogen gas

The main decision in analyzing the cost and emissions of a proposed hydrogen transport plan is how to convert the gaseous hydrogen to a liquid, and then how to recover the hydrogen gas at the end.

One approach is to simply change the gaseous hydrogen into an easily transportable liquid. But turning hydrogen gas into a liquid requires making it very, very cold. Indeed, notes Ibrahim, “you would need to consume about a third of the energy content of the hydrogen to make the gaseous hydrogen cold enough to liquify.” A further problem arises as the liquified hydrogen is being stored and moved. Unless the vessel containing the liquid hydrogen is properly insulated, the liquid hydrogen can re-gasify and escape. The upside of hydrogen liquefaction is that no chemical reactions are required.

Other options involve using a hydrogen “carrier.” Some liquid chemical compounds will absorb hydrogen atoms under certain conditions, and under other conditions will release them. Therefore, one approach to solving the hydrogen transportation problem is to make a carrier compound absorb the hydrogen where it’s made and then release it when it reaches its destination. This approach therefore involves two chemical reactions — one to bind the hydrogen to the carrier and the other to release it.  

In their demonstration runs, the researchers looked at the hydrogen carriers involving three potential compounds, each of which has known advantages and disadvantages.

One of those carriers is produced by adding hydrogen to toluene. That chemical reaction hasn’t been studied a lot, but there’s one known drawback: the source of toluene is typically the oil and gas industry, so the toluene itself has a relatively high carbon intensity when it picks up the hydrogen. Moreover, over time some of the toluene is lost, so more toluene must be added.
    
The researchers also looked at “synthetic methane,” which is made by reacting hydrogen with carbon dioxide. That reaction has been known for some time. Ibrahim notes that making synthetic methane actually consumes carbon dioxide, often captured from the atmosphere. On the negative side, however, one of the products of the reaction is water, so some of the hydrogen is lost each time the reaction occurs.

The final option they analyzed is ammonia, which forms when hydrogen reacts with nitrogen from the air. That reaction is very well-studied and is used commercially. “We’ve been producing ammonia for a long time,” says Ibrahim. And the infrastructure for transporting and storing it is well established. While Ibrahim refers to ammonia as the “most promising option,” the reaction needed to release the hydrogen has not received much attention.

Varying conclusions and future plans

Based on their sample runs, the researchers observed that the best path to follow will vary from place to place and from situation to situation. “As we developed the tool, we saw that the ‘best’ carrier was very specific to the supply chain at hand,” says Ibrahim. “It’s a function of how far you’re trying to ship your hydrogen, energy and shipping costs at your exporting and importing countries, the capital cost of building the needed facilities at both ends, and more.”

Ibrahim and his team are now planning a follow-up study in which they use HyCAT to analyze specific supply chains under certain conditions. They’ll then select assumptions that are highly uncertain and look at the range of possible values for those assumptions. “Then we’ll be able to say, ‘under these conditions, this carrier is better than that one,’ or ‘this carrier is better at cost, but worse at carbon intensity,’” says Ibrahim.

For now, the main conclusion of the study, says Ibrahim, is that “there’s no conclusion.” He warns decision-makers not to assume that anything they see in the literature can easily be generalized or extrapolated to their specific conditions. Instead, decision-makers should use HyCAT to explore the options available to them. Guided by their results and the objectives and values of their company, they will be able to optimize their supply chains and make clean-burning hydrogen a reality.

Professor Jesse Thaler has been named director of the MIT Laboratory for Nuclear Science (LNS), effective Aug. 1. He succeeds Professor Bolek Wyslouch, who directed LNS for the past decade. Thaler is a theoretical particle physicist who combines techniques from quantum field theory and machine learning to address outstanding questions in fundamental physics. 

“In his research, Jesse has done pioneering work on particle jets at the Large Hadron Collider and is a leader in combining AI and machine learning with fundamental particle physics,” says Nergis Mavalvala, dean of the MIT School of Science and the Curtis and Kathleen Marble Professor of Astrophysics. “The collaborative nature of his research programs will serve the Laboratory for Nuclear Science as science enters a new era of AI-driven discovery.”

Thaler is the William and Emma Rogers Professor of Physics in the MIT Center for Theoretical Physics — a Leinweber Institute (CTP-LI). Since 2020, he has served as inaugural director of the National Science Foundation (NSF) AI Institute for Artificial Intelligence and Fundamental Interactions, or IAIFI, which was recently renewed for another five years. Mike Williams, professor of physics, will succeed Thaler as IAIFI director. LNS is also poised to pursue new research projects through the Department of Energy’s Genesis Mission, which has a focus on AI-enabled scientific discovery.

“In my own field of particle physics, researchers are developing cutting-edge AI algorithms to handle the data deluge from collider experiments and to perform heroic theoretical calculations. This work has direct implications for discovering new physics, but the algorithms themselves turn out to be valuable well beyond our field,” says Thaler. “I’m excited to bring LNS into the next wave of discoveries supported by AI-driven capabilities.”

At IAIFI, Thaler has championed education and research activities at the intersection of physics and AI. With the MIT Institute for Data, Systems, and Society, IAIFI leadership created a doctoral program in physics, statistics, and data science. IAIFI also created dedicated postdoctoral fellowships to give early-career researchers the freedom to pursue interdisciplinary work. 

“Giving young scientists space to build connections across domains, universities, and career stages has been transformative within IAIFI,” says Thaler, who hopes to bring this type of framework to LNS. Established in 1946 to support nuclear and particle physics, LNS now encompasses research spanning cosmology, gravity, field theory, and quantum information science.

As head of LNS, Thaler will also oversee his home center of CTP-LI, which last year received a donation from the Leinweber Foundation to establish a network of theoretical physics research institutes. According to the Science Philanthropy Alliance, a nonprofit organization that promotes philanthropy for science, this constitutes the largest philanthropic commitment ever for this field.

Thaler received his PhD in physics from Harvard University in 2006, and his BS in math/physics from Brown University in 2002. From 2006 to 2009, he was a fellow at the Miller Institute for Basic Research in Science at the University of California at Berkeley. He joined the MIT faculty in 2010.

Not sure this will have any effect, but I support the effort:

According to Google’s legal filing, Outsider Enterprise operates through Telegram. The group offers phishing-as-a-service to individuals who may not be technically savvy enough to set up fraudulent websites and text campaigns on their own. In its Telegram channels, Outsider Enterprise reportedly provided instructions on how to use Google’s Gemini AI to create websites that imitate those of Google, YouTube, and government agencies such as New York’s E-ZPass. The group offered nearly 300 scam templates...

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Nature Climate Change, Published online: 07 July 2026; doi:10.1038/s41558-026-02702-0

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Here the authors show that a common interpretation of the Paris Agreement’s ‘well below 2° C’ target is changing with time, which could lead to higher overall warming. They propose that using median temperatures instead of probability ranges is more robust.