Richard Muller, a physicist at the University of California, Berkeley, was in his office when someone called Liz showed up who’d once taken one of his classes. She said her family had invited a physicist over for dinner, who touted controlled nuclear fusion as a future energy source. When Liz suggested solar power was a better option, the guest grew patronizing. “If you wanted to power California,” he told her, “you’d have to plaster the entire state with solar cells.”
Fortunately, Liz remembered what she’d learned on Muller’s course, entitled “Physics for Future Presidents”, and explained why the dinner guest was wrong. “There’s a kilowatt in a square metre of sunlight,” she told him, “which means a gigawatt in a square kilometre – only about the space of a nuclear power plant.” Stunned, the physicist grew silent. “Your numbers don’t sound wrong,” he finally said. “Of course, today’s solar cells are only 15% efficient. But I’ll take a look again.”
It’s a wonderful story that Muller told me when I visited him a few months ago to ask about his 2008 book Physics for Future Presidents: the Science Behind the Headlines. Based on the course that Liz took, the book tries to explain physics concepts underpinning key issues including energy and climate change. “She hadn’t just memorized facts,” Muller said. “She knew enough to shut up an expert who hadn’t done his homework. That’s what presidents should be able to do.” A president, Muller believes, should know enough science to have a sense for the value of expert advice.
Dissenting minds
Muller’s book was published shortly before Barack Obama’s two terms as US president. Obama was highly pro-science, appointing the Nobel-prize-winning physicist Steven Chu as his science adviser. With Donald Trump in the White House, I had come to ask Muller what advice – if any – he would change in the book. But it wasn’t easy for me to keep Muller on topic, as he derails easily with anecdotes of fascinating situations and extraordinary people that he’s encountered in his remarkable life.
Talking physics Richard Muller explaining antimatter to students at the University of California, Berkeley, in 2005. (Courtesy: WikiCommons)
Born in New York City, Muller, 81, attended Bronx High School of Science and Columbia University, joining the University of California, Berkeley as a graduate student in the autumn of 1964. A few weeks after entering, he joined the Free Speech Movement to protest against the university’s ban on campus political activities. During a sit-in, Muller was arrested and dragged down the steps of Sproul Hall, Berkeley’s administration building.
As a graduate student, Muller worked with Berkeley physicist Luis Alvarez – who would later win the 1968 Nobel Prize for Physics – to send a balloon with a payload of cosmic-ray detectors over the Pacific. Known as the High Altitude Particle Physics Experiment (HAPPE), the apparatus crashed in the ocean. Or so Muller thought.
As Muller explained in a 2023 article in the Wall Street Journal, US intelligence recovered a Chinese surveillance device, shot down over Georgia by the US military, with a name that translated as “HAPI”. Muller found enough other similarities to conclude that the Chinese had recovered the device and copied it as a model for their balloons. But by then Muller had switched to studying negative kaon particles using bubble chambers. After his PhD, he stayed at Berkeley as a postdoc, eventually becoming a professor in 1980.
Muller is a prominent contrarian, publishing an article advancing the controversial – though some now argue that it’s plausible – view that the COVID-19 virus originated in a Chinese lab. For a long time he was a global-warming sceptic, but in 2012, after three years of careful analysis, he publicly changed his mind via an article in the New York Times. Former US President Bill Clinton cited Muller as “one of my heroes because he changed his mind on global warming”. Muller loved that remark, but told me: “I’m not a hero. I’m just a scientist.”
Muller was once shadowed by a sociology student for a week for a course project. “She was like [the anthropologist] Diane Fosse and I was a gorilla,” Muller recalls. She was astonished. “I thought physicists spent all their time thinking and experimenting,” the student told him. “You spend most of your time talking.” Muller wasn’t surprised. “You don’t want to spend your time rediscovering something somebody already knows,” he said. “So physicists talk a lot.”
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I tried again to steer Muller back to the book. He said it was based on a physics course at Berkeley known originally as “Qualitative physics” and informally as physics for poets or dummies. One of the first people to teach it had been the theorist and “father of the fusion bomb” Edward Teller. “Teller was exceedingly popular,” Muller told me, “possibly because he gave everyone in class an A and no exams.”
After Teller, fewer and fewer students attended the course until enrolment dropped to 20. So when Muller took over in 1999 he retitled it “Physics for future presidents”, he refocused it on contemporary issues, and rebuilt the enrolment until it typically filled a large auditorium with about 500 students. He retired in 2010 after a decade of teaching the course.
Making a final effort, I handed Muller a copy of his book, turned to the last page where he listed a dozen or so specific recommendations for future presidents, and asked him to say whether he had changed his mind in the intervening 17 years.
Fund strong programmes in energy efficiency and conservation? “Yup!”
Raise the miles-per-gallon of autos substantially? “Yup.”
Support efforts at sequestering carbon dioxide? “I’m not much in favour anymore because the developing world can’t afford it.”
Encourage the development of nuclear power? “Yeah. Particularly fission; fusion’s too far in the future. Also, I’d tell the president to make clear that nuclear waste storage is a solved problem, and make sure that Yucca mountain is quickly approved.”
See that China and India are given substantial carbon credits for building coal-fired power stations and nuclear plants? “Nuclear power plants yes, carbon credits no. Over a million and a half people in China die from coal pollution each year.”
Encourage solar and wind technologies? “Yes.” Cancel subsidies on corn ethanol? “Yes”. Encourage developments in efficient lighting? “Yes.” Insulation is better than heating? “Yes.” Cool roofs save more energy than air conditioners and often better than solar cells? “Yes.”
The critical point
Muller’s final piece of advice to the future president was that the “emphasis must be on technologies that the developing world can afford”. He was adamant. “If what you are doing is buying expensive electric automobiles that will never sell in the developing world, it’s just virtue signalling in luxury.”
I kept trying to find some new physics Muller would tell the president, but it wasn’t much. “Physics mostly stays the same,” Muller concluded, “so the advice mainly does, too.” But not everything remains unvarying. “What changes the most”, he conceded, “is how the president listens”. Or even whether the president is listening at all.
NASA has successfully launched a mission to explore the interactions between the Sun’s and Earth’s magnetic fields. The Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) craft was sent into low-Earth orbit on 23 July from Vandenberg Space Force Base in California by a SpaceX Falcon 9 rocket. Following a month of calibration, the twin-satellite mission is expected to operate for a year.
The spacecraft will observe particles and electromagnetic fields in the Earth’s northern magnetic “cusp region”, which encircles the North Pole where the Earth’s magnetic field lines curve down toward Earth.
This unique vantage point allows researchers to study how magnetic reconnection — when field lines connect and explosively reconfigure — affects the space environment. Such observations will help researchers understand how processes change over both space and time.
The two satellites will collect data from over 3000 cusp crossings during the one-year mission with the information being used to understand space-weather phenomena that can disrupt satellite operations, communications and power grids on Earth.
Each nearly identical octagonal satellite – weighing less than 200 kg each – features six instruments including magnetomers, electric-field instruments and devices to measure the energy of ions and electrons in plasma around the spacecraft.
It will operate in a Sun-synchronous orbit about 590 km above ground with the satellites following one behind the other in close separation, passing through regions of space at least 10 seconds apart.
“TRACERS is an exciting mission,” says Stephen Fuselier from the Southwest Research Institute in Texas, who is the mission’s deputy principal investigator. “The data from that single pass through the cusp were amazing. We can’t wait to get the data from thousands of cusp passes.”
Driven by global warming The researchers identified which factors influence the jet stream in the southern hemisphere. (Courtesy: Leipzig University/Office for University Communications)
An international team of meteorologists has found that half of the recently observed shifts in the southern hemisphere’s jet stream are directly attributable to global warming – and pioneered a novel statistical method to pave the way for better climate predictions in the future.
Prompted by recent changes in the behaviour of the southern hemisphere’s summertime eddy-driven jet (EDJ) – a band of strong westerly winds located at a latitude of between 30°S and 60°S – the Leipzig University-led team sifted through historical measurement data to show that wind speeds in the EDJ have increased, while the wind belt has moved consistently toward the South Pole. They then used a range of innovative methods to demonstrate that 50% of these shifts are directly attributable to global warming, with the remainder triggered by other climate-related changes, including warming of the tropical Pacific and the upper tropical atmosphere, and the strengthening of winds in the stratosphere.
“We found that human fingerprints on the EDJ are already showing,” says lead author Julia Mindlin, research fellow at Leipzig University’s Institute for Meteorology. “Global warming, springtime changes in stratospheric winds linked to ozone depletion, and tropical ocean warming are all influencing the jet’s strength and position.”
“Interestingly, the response isn’t uniform, it varies depending on where you look, and climate models are underestimating how strong the jet is becoming. That opens up new questions about what’s missing in our models and where we need to dig deeper,” she adds.
Storyline approach
Rather than collecting new data, the researchers used existing, high-quality observational and reanalysis datasets – including the long-running HadCRUT5 surface temperature data, produced by the UK Met Office and the University of East Anglia, and a variety of sea surface temperature (SST) products including HadISST, ERSSTv5 and COBE.
“We also relied on something called reanalysis data, which is a very robust ‘best guess’ of what the atmosphere was doing at any given time. It is produced by blending real observations with physics-based models to reconstruct a detailed picture of the atmosphere, going back decades,” says Mindlin.
To interpret the data, the team – which also included researchers at the University of Reading, the University of Buenos Aires and the Jülich Supercomputing Centre – used a statistical approach called causal inference to help isolate the effects of specific climate drivers. They also employed “storyline” techniques to explore multiple plausible futures rather than simply averaging qualitatively different climate responses.
“These tools offer a way to incorporate physical understanding while accounting for uncertainty, making the analysis both rigorous and policy-relevant,” says Mindlin.
Future blueprint
For Mindlin, these findings are important for several reasons. First, they demonstrate “that the changes predicted by theory and climate models in response to human activity are already observable”. Second, she notes that they “help us better understand the physical mechanisms that drive climate change, especially the role of atmospheric circulation”.
“Third, our methodology provides a blueprint for future studies, both in the southern hemisphere and in other regions where eddy-driven jets play a role in shaping climate and weather patterns,” she says. “By identifying where and why models diverge from observations, our work also contributes to improving future projections and enhances our ability to design more targeted model experiments or theoretical frameworks.”
The team is now focused on improving understanding of how extreme weather events, like droughts, heatwaves and floods, are likely to change in a warming world. Since these events are closely linked to atmospheric circulation, Mindlin stresses that it is critical to understand how circulation itself is evolving under different climate drivers.
One of the team’s current areas of focus is drought in South America. Mindlin notes that this is especially challenging due to the short and sparse observational record in the region, and the fact that drought is a complex phenomenon that operates across multiple timescales.
“Studying climate change is inherently difficult – we have only one Earth, and future outcomes depend heavily on human choices,” she says. “That’s why we employ ‘storylines’ as a methodology, allowing us to explore multiple physically plausible futures in a way that respects uncertainty while supporting actionable insight.”
Collaborative insights: The UK Quantum Hackathon, organized by the NQCC for the fourth consecutive year and a cornerstone of the Quantum Fringe festival, allowed industry experts to work alongside early-career researchers to explore practical use cases for quantum computing. (Courtesy: NQCC)
The International Year of Quantum Science and Technology (IYQ) has already triggered an explosion of activities around the world to mark 100 years since the emergence of quantum mechanics. In the UK, the UNESCO-backed celebrations have provided the perfect impetus for the University of Edinburgh’s Quantum Software Lab (QSL) to work with the National Quantum Computing Centre (NQCC) to organize and host a festival of events that have enabled diverse communities to explore the transformative power of quantum computing.
Known collectively as the Quantum Fringe, in a clear nod to Edinburgh’s famous cultural festival, some 16 separate events have been held across Scotland throughout June and July. Designed to make quantum technologies more accessible and more relevant to the outside world, the programme combined education and outreach with scientific meetings and knowledge exchange.
The Quantum Fringe programme evolved from several regular fixtures in the quantum calendar. One of these cornerstones was the NQCC’s flagship event, the UK Quantum Hackathon, which is now in its fourth consecutive year. In common with previous editions, the 2025 event challenged teams of hackers to devise quantum solutions to real-world use cases set by mentors from different industry sectors. The teams were supported throughout the three-day event by the industry mentors, as well as by technical experts from providers of various quantum resources.
Time constrained: the teams of hackers were given two days to formulate their solution and test it on simulators, annealers and physical processors. (Courtesy: NQCC)
This year, perhaps buoyed by the success of previous editions, there was a significant uptick in the number of use cases submitted by end-user organizations. “We had twice as many applications as we could accommodate, and over half of the use cases we selected came from newcomers to the event,” said Abby Casey, Quantum Readiness Delivery Lead at the NQCC. “That level of interest suggests that there is a real appetite among the end-user community for understanding how quantum computing could be used in their organizations.”
Reflecting the broader agenda of the IYQ, this year the NQCC particularly encouraged use cases that offered some form of societal benefit, and many of the 15 that were selected aimed to align with the UN’s Sustainable Development Goals. One team investigated the accuracy of quantum-powered neural networks for predicting the progression of a tumour, while another sought to optimize the performance of graphene-based catalysts for fuel cells. Moonbility, a start-up firm developing digital twins to optimize the usage of transport and infrastructure, challenged its team to develop a navigation system capable of mapping out routes for people with specific mobility requirements, such as step-free access or calmer environments for those with anxiety disorders.
During the event the hackers were given just two days to explore the use case, formulate a solution, and generate results using quantum simulators, annealers and physical processors. The last day provided an opportunity for the teams to share their findings with their peers and a five-strong judging panel that was chaired by Sir Peter Knight, one of the architects of the UK’s National Quantum Technologies Programme and co-chair of the IYQ’s Steering Committee a prime mover in the IYQ celebrations. “Your effort, energy and passion have been quite extraordinary,” commented Sir Peter at the end of the event. “It’s truly impressive to see what you have achieved in just two days.”
From the presentations it was clear that some of the teams had adapted their solution to reflect the physical constraints of the hardware platform they had been allocated. Those explorations were facilitated by the increased participation of mentors from hardware developers, including QuEra and Pasqal for cold-atom architectures, and Rigetti and IBM for gate-based superconducting processors. “Cold atoms offer greater connectivity than superconducting platforms, which may make them more suited to solving particular types of problems,” said Gerard Milburn of the University of Sussex, who has recently become a Quantum Fellow at the NQCC.
Results day: The final day of the hackathon allowed the teams to share their results with the other participants and a five-strong judging panel. (Courtesy: NQCC)
The winning team, which had been challenged by Aioi R&D Lab to develop a quantum-powered solution for scheduling road maintenance, won particular praise for framing the problem in a way that recognized the needs of all road users, not just motorists. “It was really interesting that they thought about the societal value right at the start, and then used those ethical considerations to inform the way they approached the problem,” said Knight.
The wider impact of the hackathon is clear to see, with the event providing a short, intense and collaborative learning experience for early-career researchers, technology providers, and both small start-up companies and large multinationals. This year, however, the hackathon also provided the finale to the Quantum Fringe, which was the brainchild of Elham Kashefi and her team at the QSL. Taking inspiration from the better-known Edinburgh Fringe, the idea was to create a diverse programme of events to engage and inspire different audiences with the latest ideas in quantum computing.
“We wanted to celebrate the International Year of Quantum in a unique way,” said Mina Doosti, one of the QSL’s lead researchers. “We had lots of very different events, many of which we hadn’t foreseen at the start. It was very refreshing, and we had a lot of fun.”
One of Doosti’s favourite events was a two-day summer school designed for senior high-school students. As well as introducing the students to the concepts of quantum computing, the QSL researchers challenged them to write some code that could be run on IBM’s free-to-access quantum computer. “The organizers and lecturers from the QSL worked hard to develop material that would make sense to the students, and the attendees really grabbed the opportunity to come and learn,” Doosti explained. “From the questions they were asking and the way they tackled the games and challenges, we could see that they were interested and that they had learnt something.”
From the outset the QSL team were also keen for the Quantum Fringe to become a focal point for quantum-inspired activities that were being planned by other organizations. Starting from a baseline of four pillar events that had been organized by the NQCC and the QSL in previous years, the programme eventually swelled to 16 separate gatherings with different aims and outcomes. That included a public lecture organized by the new QCi3 Hub – a research consortium focused on interconnected quantum technologies – which attracted around 200 people who wanted to know more about the evolution of quantum science and its likely impact across technology, industry, and society. An open discussion forum hosted by Quantinuum, one of the main sponsors of the festival, also brought together academic researchers, industry experts and members of the public to identify strategies for ensuring that quantum computing benefits everyone in society, not just a privileged few.
Quantum researchers also had plenty of technical events to choose from. The regular AIMday Quantum Computing, now in its third year, enabled academics to work alongside industry experts to explore a number of business-led challenges. More focused scientific meetings allowed researchers to share their latest results in quantum cryptography and cybersecurity, algorithms and complexity, and error correction in neutral atoms. For her part, Doosti co-led the third edition of Foundations in Quantum Computing, a workshop that combines invited talks with dedicated time for focused discussion. “The speakers are briefed to cover the evolution of a particular field and to highlight open challenges, and then we use the discussion sessions to brainstorm ideas around a specific question,” she explained.
Those scientific meetings were complemented by a workshop on responsible quantum innovation, again hosted by the QCi3 Hub, and a week-long summer school on the Isle of Skye that was run by Heriot-Watt University and the London School of Mathematics. “All of our partners ran their events in the way they wanted, but we helped them with local support and some marketing and promotion,” said Ramin Jafarzadegan, the QSL’s operations manager and the chair of the Quantum Fringe festival. “Bringing all of these activities together delivered real value because visitors to Edinburgh could take part in multiple events.”
Indeed, one clear benefit of this approach was that some of the visiting scientists stayed for longer, which also enabled them to work alongside the QSL team. That has inspired a new scheme, called QSL Visiting Scholars, that aims to encourage scientists from other institutions to spend a month or so in Edinburgh to pursue collaborative projects.
As a whole, the Quantum Fringe has helped both the NQCC and the QSL in their ambitions to bring diverse stakeholders together to create new connections and to grow the ecosystem for quantum computing in the UK. “The NQCC should have patented the ‘quantum hackathon’ name,” joked Sir Peter. “Similar events are popping up everywhere these days, but the NQCC’s was among the first.”
Topological insulators have generated a lot of interest in recent years because of their potential applications in quantum computing, spintronics and information processing.
The defining property of these materials is that their interior behaves as an electrical insulator while their surface behaves as an electrical conductor. In other words, electrons can only move along the material’s surface.
In some cases however, known as strongly correlated systems, the strong interactions between electrons cause this relatively simple picture to break down.
Understanding and modelling strongly correlated topological insulators, it turns out, is extremely challenging.
A team of researchers from the Kavli Institute for Theoretical Sciences, China, have recently tackled this challenge by using a new approach employing fermionic tensor states.
Their framework notably reduces the number of parameters needed in numerical simulations. This should lead to a greatly improved computational efficiency when modelling these systems.
By combining their methods with advanced numerical techniques, the researchers expect to be able to overcome the challenges posed by strong interaction effects.
This will lead to a deeper understanding of the properties of strongly correlated systems and could also enable the discovery of new materials with exciting new properties.
Dark exciton control: Researchers assemble a large cryostat in an experimental physics laboratory, preparing for ultra-low temperature experiments with quantum dots on a semiconductor chip. (Courtesy: Universität Innsbruck)
Physicists in Austria and Germany have developed a means of controlling quasiparticles known as dark excitons in semiconductor quantum dots for the first time. The new technique could be used to generate single pairs of entangled photons on demand, with potential applications in quantum information storage and communication.
Excitons are bound pairs of negatively charged electrons and positively charged “holes”. When these electrons and holes have opposite spins, they recombine easily, emitting a photon in the process. Excitons of this type are known as “bright” excitons. When the electrons and holes have parallel spins, however, direct recombination by emitting a photon is not possible because it would violate the conservation of spin angular momentum. This type of exciton is therefore known as a “dark” exciton.
Because dark excitons are not optically active, they have much longer lifetimes than their bright cousins. For quantum information specialists, this is an attractive quality, because it means that dark excitons can store quantum states – and thus the information contained within these states – for much longer. “This information can then be released at a later time and used in quantum communication applications, such as optical quantum computing, secure communication via quantum key distribution (QKD) and quantum information distribution in general,” says Gregor Weihs, a quantum photonics expert at the Universität Innsbruck, Austria who led the new study.
The problem is that dark excitons are difficult to create and control. In semiconductor quantum dots, for example, Weihs explains that dark excitons tend to be generated randomly, for example when a quantum dot in a higher-energy state decays into a lower-energy state.
Chirped laser pulses lead to reversible exciton production
In the new work, which is detailed in Science Advances, the researchers showed that they could control the production of dark excitons in quantum dots by using laser pulses that are chirped, meaning that the frequency (or colour) of the laser light varies within the pulse. Such chirped pulses, Weihs explains, can turn one quantum dot state into another.
“We first bring the quantum dot to the (bright) biexciton state using a conventional technique and then apply a (storage) chirped laser pulse that turns this biexciton occupation (adiabatically) into a dark state,” he says. “The storage pulse is negatively chirped – its frequency decreases with time, or in terms of colour, it turns redder.” Importantly, the process is reversible: “To convert the dark exciton back into a bright state, we apply a (positively chirped) retrieval pulse to it,” Weihs says.
One possible application for the new technique would be to generate single pairs of entangled photons on demand – the starting point for many quantum communication protocols. Importantly, Weihs adds that this should be possible with almost any type of quantum dot, whereas an alternative method known as polarization entanglement works for only a few quantum dot types with very special properties. “For example, it could be used to create ‘time-bin’ entangled photon pairs,” he tells Physics World. “Time-bin entanglement is particularly suited to transmitting quantum information through optical fibres because the quantum state stays preserved over very long distances.”
The study’s lead author, Florian Kappe, and his colleague Vikas Remesh describe the project as “a challenging but exciting and rewarding experience” that combined theoretical and experimental tools. “The nice thing, we feel, is that on this journey, we developed a number of optical excitation methods for quantum dots for various applications,” they say via e-mail.
The physicists are now studying the coherence time of the dark exciton states, which is an important property in determining how long they can store quantum information. According to Weihs, the results from this work could make it possible to generate higher-dimensional time-bin entangled photon pairs – for example, pairs of quantum states called qutrits that have three possible values.
“Thinking beyond this, we imagine that the technique could even be applied to multi-excitonic complexes in quantum dot molecules,” he adds. “This could possibly result in multi-photon entanglement, such as so-called GHZ (Greenberger-Horne-Zeilinger) states, which are an important resource in multiparty quantum communication scenarios.”
The space scientist Michele Dougherty from Imperial College London has been appointed the next Astronomer Royal – the first woman to hold the position. She will succeed the University of Cambridge cosmologist Martin Rees, who has held the role for the past three decades.
The title of Astronomer Royal dates back to the creation of the Royal Observatory in Greenwich in 1675, when it mostly involved advising Charles II on using the stars to improve navigation at sea. John Flamsteed from Derby was the first Astronomer Royal and since then 15 people have held the role.
Dougherty will now act as the official adviser to King Charles III on astronomical matters. She will hold the role alongside her Imperial job as well as being executive chair of the Science and Technology Facilities Council and the next president of the Institute of Physics (IOP), a two-year position she will take up in October.
After gaining a PhD in 1988 from the University of Natal in South Africa, Dougherty moved to Imperial in 1991, where she was head of physics from 2018 until 2024. She has been principal investigator of the magnetometer on the Cassini-Huygens mission to Saturn and its moons and also for the magnetometer for the JUICE craft, which is currently travelling to Jupiter to study its three icy moons.
She was made Commander of the Order of the British Empire in the 2018 New Year Honours for “services to UK Physical Science Research”. Dougherty is also a fellow of the Royal Society, who won its Hughes medal in 2008 for studying Saturn’s moons and had a Royal Society Research Professorship from 2014 to 2019.
“I am absolutely delighted to be taking on the important role of Astronomer Royal,” says Dougherty. “As a young child I never thought I’d end up working on planetary spacecraft missions and science, so I can’t quite believe I’m actually taking on this position. I look forward to engaging the general public in how exciting astronomy is, and how important it and its outcomes are to our everyday life.”
Tom Grinyer, IOP group chief executive officer, offered his “warmest congratulations” to Dougherty. “As incoming president of the IOP and the first woman to hold this historic role [of Astronomer Royal], Dougherty is an inspirational ambassador for science and a role model for every young person who has gazed up at the stars and imagined a future in physics or astronomy.”
The clash between dark matter and modified Newtonian dynamics (MOND) can get a little heated at times. On one side is the vast majority of astronomers who vigorously support the concept of dark matter and its foundational place in cosmology’s standard model. On the other side is the minority – a group of rebels convinced that tweaking the laws of gravity rather than introducing a new particle is the answer to explaining the composition of our universe.
Both sides argue passionately and persuasively, pointing out evidence that supports their view while discrediting the other side. Often it seems to come down to a matter of perspective – both sides use the same results as evidence for their cause. For the rest of us, how can we tell who is correct?
As long as we still haven’t identified what dark matter is made of, there will remain some ambiguity, leaving a door ajar for MOND. However, it’s a door that dark-matter researchers hope will be slammed shut in the not-too-distant future.
Crunch time for WIMPs
In part two of this series, where I looked at the latest proposals from dark-matter scientists, we met University College London’s Chamkaur Ghag, who is the spokesperson for Lux-ZEPLIN. This experiment is searching for “weakly interacting massive particles” or WIMPs – the leading dark-matter candidate – down a former gold mine in South Dakota, US. A huge seven-tonne tank of liquid xenon, surrounded by an array of photomultiplier tubes, watches patiently for the flashes of light that may occur when a passing WIMP interacts with a xenon atom.
Running since 2021, the experiment just released the results of its most recent search through 280 days of data, which uncovered no evidence of WIMPs above a mass of 9 GeV/c2 (Phys. Rev. Lett.135 011802). These results help to narrow the range of possible dark-matter theories, as the new limits impose constraints on WIMP parameters that are almost five times more rigorous than the previous best. Another experiment at the INFN Laboratori Nazionali del Gran Sasso in Italy, called XENONnT, is also hoping to spot the elusive WIMPs – in its case by looking for rare nuclear recoil interactions in a liquid xenon target chamber.
Deep underground The XENON Dark Matter Project is hosted by the INFN Gran Sasso National Laboratory in Italy. The latest detector in this programme is the XENONnT (pictured) which uses liquid xenon to search for dark-matter particles. (Courtesy: XENON Collaboration)
Lux-ZEPLIN and XENONnT will cover half the parameter space of masses and energies that WIMPs could in theory have, but Ghag is more excited about a forthcoming, next-generation xenon-based WIMP detector dubbed XLZD that might settle the matter. XLZD brings together both the Lux-ZEPLIN and XENONnT collaborations, to design and build a single, common multi-tonne experiment that will hopefully leave WIMPs with no place to hide. “XLZD will probably be the final experiment of this type,” says Ghag. “It’s designed to be much larger and more sensitive, and is effectively the definitive experiment.”
I think none of us are ever going to fully believe it completely until we’ve found a WIMP and can reproduce it in a lab
Richard Massey
If WIMPs do exist, then this detector will find them, and it could happen on UK shores. Several locations around the world are in the running to host the experiment, including Boulby Mine Underground Laboratory near Whitby Bay on the north-east coast of England. If everything goes to plan, XLZD – which will contain between 40 and 100 tonnes of xenon – will be up and running and providing answers by the 2030s. It will be a huge moment for dark matter, and a nervous one for its researchers.
“I think none of us are ever going to fully believe it completely until we’ve found [a WIMP] and can reproduce it in a lab and show that it’s not just some abstract stuff that we call dark matter, but that it is a particular particle that we can identify,” says astronomer Richard Massey of the University of Durham, UK.
But if WIMPs are in fact a dead-end, then it’s not a complete death-blow for dark matter – there are other dark-matter candidates and other dark-matter experiments. For example, the Forward Search Experiment (FASER) at CERN’s Large Hadron Collider is looking for less massive dark-matter particles such as axions (read more about them in part 2). However, WIMPs have been a mainstay of dark-matter models since the 1980s. If the xenon-based experiments turn up empty-handed it will be a huge blow, and the door will creak open just a little bit more for MOND.
Galactic frontier
MOND’s battleground isn’t in particle detectors – it’s in the outskirts of galaxies and galaxy clusters, and its proof lies in the history of how our universe formed. This is dark matter’s playground too, with the popular models for how galaxies grow being based on a universe in which dark matter forms 85% of all matter. So it’s out in the depths of space where the two models clash.
The current standard model of cosmology describes how the growth of the large-scale structure of the universe, over the past 13.8 billion years of cosmic history since the Big Bang, is influenced by a combination of dark matter and dark energy (responsible for the accelerated expansion of the universe). Essentially, density fluctuations in the cosmic microwave background (CMB) radiation reflect the clumping of dark matter in the very early universe. As the cosmos aged, these clumps thinned out into the cosmic web of matter. This web is a universe-spanning network of dark-matter filaments, where all the matter lies, between which are voids that are comparatively less densely packed with matter than the filaments. Galaxies can form inside “dark matter haloes”, and at the densest points in the dark-matter filaments, galaxy clusters coalesce.
Simulations in this paradigm – known as lambda cold dark matter (ΛCDM) – suggest that galaxy and galaxy-cluster formation should be a slow process, with small galaxies forming first and gradually merging over billions of years to build up into the more massive galaxies that we see in the universe today. And it works – kind of. Recently, the James Webb Space Telescope (JWST) peered back in time to between just 300 and 400 million years after the Big Bang and found the universe to be populated by tiny galaxies perhaps just a thousand or so light-years across (ApJ970 31). This is as expected, and over time they would grow and merge into larger galaxies.
1 Step back in time
a (Courtesy: NASA/ESA/CSA/STScI/ Brant Robertson, UC Santa Cruz/ Ben Johnson, CfA/ Sandro Tacchella, University of Cambridge/ Phill Cargile, CfA)
b (Courtesy: NASA/ESA/CSA/ Joseph Olmsted, STScI/ S Carniani, Scuola Normale Superiore/ JADES Collaboration)
Data from the James Webb Space Telescope (JWST) form the basis of the JWST Advanced Deep Extragalactic Survey (JADES). (a) This infrared image from the JWST’s NIRCam highlights galaxy JADES-GS-z14-0. (b) The JWST’s NIRSpec (Near-Infrared Spectrograph) obtained this spectrum of JADES-GS-z14-0. A galaxy’s redshift can be determined from the location of a critical wavelength known as the Lyman-alpha break. For JADES-GS-z14-0 the redshift value is 14.32 (+0.08/–0.20), making it the second most distant galaxy known at less than 300 million years after the Big Bang. The current record holder, as of August 2025, is MoM-z14, which has a redshift of 14.4 (+0.02/–0.02), placing it less than 280 million years after the Big Bang (arXiv:2505.11263). Both galaxies belong to an era referred to as the “cosmic dawn”, following the epoch of reionization, when the universe became transparent to light. JADES-GS-z14-0 is particularly interesting to researchers not just because of its distance, but also because it is very bright. Indeed, it is much more intrinsically luminous and massive than expected for a galaxy that formed so soon after the Big Bang, raising more questions on the evolution of stars and galaxies in the early universe.
Yet the deeper we push into the universe, the more we observe challenges to the ΛCDM model, which ultimately threatens the very existence of dark matter. For example, those early galaxies that the JWST has observed, while being quite small, are also surprisingly bright – more so than ΛCDM predicts. This has been attributed to an initial mass function (IMF – the property that determines the average mass of stars that form) that skews more towards higher-mass stars and therefore more luminous stars than today. It does sound reasonable, except that astronomers still don’t understand why the IMF is what it is today (favouring the smallest stars; massive stars are rare) never mind what it might have been over 13 billion years ago.
Not everyone is convinced, and this is compounded by slightly later galaxies, seen around a billion years after the Big Bang, which continue the trend of being more luminous and more massive than expected. Indeed, some of these galaxies sport truly enormous black holes hundreds of times more massive than the black hole at the heart of our Milky Way. Just a couple of billion years later and significantly large galaxy clusters are already present, earlier than one would have surmised with ΛCDM.
The fall of ΛCDM?
Astrophysicist and MOND advocate Pavel Kroupa, from the University of Bonn in Germany, highlights giant elliptical galaxies in the early universe as an example of what he sees as a divergence from ΛCDM.
“We know from observations that the massive elliptical galaxies formed on shorter timescales than the less massive ellipticals,” he explains. This phenomenon has been referred to as “downsizing”, and Kroupa declares it is “a big problem for ΛCDM” because the model says that “the big galaxies take longer to form, but what we see is exactly the opposite”.
To quantify this problem, a 2020 study (MNRAS498 5581) by Australian astronomer Sabine Bellstedt and colleagues showed that half the mass in present-day elliptical galaxies was in place 11 billion years ago, compared with other galaxy types that only accrued half their mass on average about 6 billion years ago. The smallest galaxies only accrued that mass as recently as 4 billion years ago, in apparent contravention of ΛCDM.
Observations (ApJ905 40) of a giant elliptical galaxy catalogued as C1-23152, which we see as it existed 12 billion years ago, show that it formed 200 billion solar masses worth of stars in just 450 million years – a huge firestorm of star formation that ΛCDM simulations just can’t explain. Perhaps it is an outlier – we’ve only sampled a few parts of the sky, not conducted a comprehensive census yet. But as astronomers probe these cosmic depths more extensively, such explanations begin to wear thin.
Kroupa argues that by replacing dark matter with MOND, such giant early elliptical galaxies suddenly make sense. Working with Robin Eappen, who is a PhD student at Charles University in Prague, they modelled a giant gas cloud in the very early universe collapsing under gravity according to MOND, rather than if there were dark matter present.
“It is just stunning that the time [of formation of such a large elliptical] comes out exactly right,” says Kroupa. “The more massive cloud collapses faster on exactly the correct timescale, compared to the less massive cloud that collapses slower. So when we look at an elliptical galaxy, we know that thing formed from MOND and nothing else.”
Elliptical galaxies are not the only thing with a size problem. In 2021 Alexia Lopez, a PhD student at the University of Central Lancashire, UK, discovered a “Giant Arc” of galaxies spanning 3.3 billion light-years, some 9.2 billion light-years away. And in 2023 Lopez spotted another gigantic structure, a “Big Ring” (shaped more like a coil) of galaxies 1.3 billion light-years in diameter, but with a circumference of about 4 billion light-years. The opposite of these giant structures are the massive under-dense voids that take up space between the filaments of the cosmic web. The KBC Void (sometimes called the “Local Hole”), for example, is about two billion light-years across and the Milky Way among a host of other galaxies sits inside it. The trouble is, simulations in ΛCDM, with dark matter at the heart of it, cannot replicate structures and voids this big.
“We live in this huge under-density; we’re not at the centre of it but we are within it and such an under-density is completely impossible in ΛCDM,” says Kroupa, before declaring, “Honestly, it’s not worthwhile to talk about the ΛCDM model anymore.”
A bohemian model
Such fighting talk is dismissed by dark-matter astronomers because although there are obviously deficiencies in the ΛCDM model, it does such a good job of explaining so many other things. If we’re to kill ΛCDM because it cannot explain a few large ellipticals or some overly large galaxy groups or voids, then there needs to be a new model that can explain not only these anomalies, but also everything else that ΛCDM does explain.
“Ultimately we need to explain all the observations, and some of those MOND does better and some of those ΛCDM does better, so it’s how you weigh those different baskets,” says Stacy McGaugh, a MOND researcher from Case Western Reserve University in the US.
As it happens, Kroupa and his Bonn colleague Jan Pflamm-Altenburg are working on a new model that they think has what it takes to overthrow dark matter and the broader ΛCDM paradigm. Calling it the Bohemian model (the name has a double meaning – Kroupa is originally from Czechia), it incorporates MOND as its main pillar and Kroupa describes the results they are getting from their simulations in this paradigm as “stunning” (A&A698 A167).
A lot of experts at Ivy League universities will say it’s all completely impossible. But I know that part of the community is just itching to have a completely different model
Pavel Kroupa
But Kroupa admits that not everybody will be happy to see it published. “If it’s published, a lot of experts at Ivy League universities will say it’s all completely impossible,” he says. “But I know for a fact that there is part of the community, the ‘bright part’ as I call them, which is just itching to have a completely different model.”
Kroupa is staying tight-lipped on the precise details of his new model, but says that according to simulations the puzzle of large-scale structure forming earlier than expected, and growing larger faster than expected, is answered by the Bohemian model. “These structures [such as the Giant Arc and the KBC Void] are so radical that they are not possible in the ΛCDM model,” he says. “However, they pop right out of this Bohemian model.”
Binary battle
Whether you believe Kroupa’s promises of a better model or whether you see it all as bluster, the fact remains that a dark-matter-dominated universe still has some problems. Maybe they’re not serious, and all it will take is a few tweaks to make those problems go away. But maybe they’ll persist, and require new physics of some kind, and it’s this possibility that continues to leave the door open for MOND. For the rest of us, we’re still grasping for a definitive statement one way or another.
For MOND, perhaps that definitive statement could still turn out to be binary stars, as discussed in the first article in this series. Researchers have been particularly interested in so-called “wide binaries” – pairs of stars that are more than 500 AU apart. Thanks to the vast distance between them, the gravitational impact of each star on the other is weak, making it a perfect test for MOND. Idranil Banik, of the University of St Andrews, UK, controversially concluded that there was no evidence for MOND operating on the smaller scales of binary-star systems. However, other researchers such as Kyu-Hyun Chae of Sejong University in South Korea argue that they have found evidence for MOND in binary systems, and have hit out at Banik’s findings.
Indeed, after the first part of this series was published, Chae reached out to me, arguing that Banik had analysed the data incorrectly. Chae specifically points out the fraction of wide binaries (pairs that are more than 500 AU apart, meaning that the gravitational impact of each star on the other is weak, making it a perfect test for MOND) with an extra unseen close stellar companion (a factor designated fmulti) to one or both of the binary stars must be calibrated for when performing the MOND calculations. Often when two stars are extremely close together, their angular separation is so small that we can’t resolve them and don’t realize that they are binary, he explains. So we might mistake a triple system, with two stars so close together that we can’t distinguish them and a third star on a wider circumbinary orbit, for just a wide binary.
“I initially believed Banik’s claim, but because what’s at stake is too big and I started feeling suspicious, I chose to do my own investigation,” says Chae (ApJ952 128). “I came to realize the necessity of calibrating fmulti due to the intrinsic degeneracy between mass and gravity (one cannot simultaneously determine the gravity boost factor and the amount of hidden mass).”
The probability of a wide binary having an unseen extra stellar companion is the same as for shorter binaries (those that we can resolve). But for shorter binaries the gravitational acceleration is high enough that they obey regular Newtonian gravity – MOND only comes into the picture at wider separations. Therefore, the mass uncertainty in the study of wide binaries in a MOND regime can be calibrated for using those shorter-period binaries. Chae argues that Banik did not do this. “I’m absolutely confident that if the Banik et al. analysis is properly carried out, it will reveal MOND’s low-acceleration gravitational anomaly to some degree.”
So perhaps there is hope for MOND in binary systems. Given that dark matter shouldn’t be present on the scale of binary systems, any anomalous gravitational effect could only be explained by MOND. A detection would be pretty definitive, if only everyone could agree upon it.
Bullet time and mass This spectacular new image of the Bullet Cluster was created using NASA’s James Webb Space Telescope and Chandra X-ray Observatory. The new data allow for an improved measurement of the thousands of galaxies in the Bullet Cluster. This means astronomers can more accurately “weigh” both the visible and invisible mass in these galaxy clusters. Astronomers also now have an improved idea of how that mass is distributed. (X-ray: NASA/CXC/SAO; near-infrared: NASA/ESA/CSA/STScI; processing: NASA/STScI/ J DePasquale)
But let’s not kid ourselves – MOND still has a lot of catching up to do on dark matter, which has become a multi-billion-dollar industry with thousands of researchers working on it and space missions such as the European Space Agency’s Euclid space telescope. Dark matter is still in pole position, and its own definitive answers might not be too far away.
“Finding dark matter is definitely not too much to hope for, and that’s why I’m doing it,” says Richard Massey. He highlights not only Euclid, but also the work of the James Webb Space Telescope in imaging gravitational lensing on smaller scales and the Nancy G Roman Space Telescope, which will launch later this decade on a mission to study weak gravitational lensing – the way in which small clumps of matter, such as individual dark matter haloes around galaxies, subtly warp space.
“These three particular telescopes give us the opportunity over the next 10 years to catch dark matter doing something, and to be able to observe it when it does,” says Massey. That “something” could be dark-matter particles interacting, perhaps in a cluster merger in deep space, or in a xenon tank here on Earth.
“That’s why I work on dark matter rather than anything else,” concludes Massey. “Because I am optimistic.”
In the first instalment of this three-part series, Keith Cooper explored the struggles and successes of modified gravity in explaining phenomena at varying galactic scales
In the second part of the series, Keith Cooper explored competing theories of dark matter
A new method for generating high-energy proton beams could one day improve the precision of proton therapy for treating cancer. Developed by an international research collaboration headed up at the National University of Singapore, the technique involves accelerating H2+ ions and then using a novel two-dimensional carbon membrane to split the high-energy ion beam into beams of protons.
One obstacle when accelerating large numbers of protons together is that they all carry the same positive charge and thus naturally repel each other. This so-called space–charge effect makes it difficult to keep the beam tight and focused.
“By accelerating H₂⁺ ions instead of single protons, the particles don’t repel each other as strongly,” says project leader Jiong Lu. “This enables delivery of proton beam currents up to an order of magnitude higher than those from existing cyclotrons.”
Lu explains that a high-current proton beam can deliver more protons in a shorter time, making proton treatments quicker, more precise and targeting tumours more effectively. Such a proton beam could also be employed in FLASH therapy, an emerging treatment that delivers therapeutic radiation at ultrahigh dose rates to reduce normal tissue toxicity while preserving anti-tumour activity.
Industry-compatible fabrication
The key to this technique lies in the choice of an optimal membrane with which to split the H₂⁺ ions. For this task, Lu and colleagues developed a new material – ultraclean monolayer amorphous carbon (UC-MAC). MAC is similar in structure to graphene, but instead of an ordered honeycomb structure of hexagonal rings, it contains a disordered mix of five-, six-, seven and eight-membered carbon rings. This disorder creates angstrom-scale pores in the films, which can be used to split the H₂⁺ ions into protons as they pass through.
Pentagons, hexagons, heptagons, octagons Illustration of disorder-to-disorder synthesis (left); scanning transmission electron microscopy image of UC-MAC (right). (Courtesy: National University of Singapore)
Scaling the manufacture of ultrathin MAC films, however, has previously proved challenging, with no industrial synthesis method available. To address this problem, the researchers proposed a new fabrication approach in which the emergence of long-range order in the material is suppressed, not by the conventional approach of low-temperature growth, but by a novel disorder-to-disorder (DTD) strategy.
DTD synthesis uses plasma-enhanced chemical vapor deposition (CVD) to create a MAC film on a copper substrate containing numerous nanoscale crystalline grains. This disordered substrate induces high levels of randomized nucleation in the carbon layer and disrupts long-range order. The approach enabled wafer-scale (8-inch) production of UC-MAC films within just 3 s – an order of magnitude faster than conventional CVD methods.
Disorder creates precision
To assess the ability of UC-MAC to split H₂⁺ ions into protons, the researchers generated a high-energy H2+ nanobeam and focused it onto a freestanding two-dimensional UC-MAC crystal. This resulted in the ion beam splitting to create high-precision proton beams. For comparison they repeated the experiment (with beam current stabilities controlled within 10%) using single-crystal graphene, non-clean MAC with metal impurities and commercial carbon thin films (8 nm).
Measuring double-proton events – in which two proton signals are detected from a single H2+ ion splitting – as an indicator for proton scattering revealed that the UC-MAC membrane produced far fewer unwanted scattered protons than the other films. Ion splitting using UC-MAC resulted in about 47 double-proton events over a 20 s collection time, while the graphene film exhibited roughly twice this number and the non-clean MAC slightly more. The carbon thin film generated around 46 times more scattering events.
The researchers point out that the reduced double-proton events in UC-MAC “demonstrate its superior ability to minimize proton scattering compared with commercial materials”. They note that as well as UC-MAC creating a superior quality proton beam, the technique provides control over the splitting rate, with yields ranging from 88.8 to 296.0 proton events per second per detector.
“Using UC-MAC to split H₂⁺ produces a highly sharpened, high-energy proton beam with minimal scattering and high spatial precision,” says Lu. “This allows more precise targeting in proton therapy – particularly for tumours in delicate or critical organs.”
“Building on our achievement of producing proton beams with greatly reduced scattering, our team is now developing single molecule ion reaction platforms based on two-dimensional amorphous materials using high-energy ion nanobeam systems,” he tells Physics World. “Our goal is to make proton beams for cancer therapy even more precise, more affordable and easier to use in clinical settings.”
Evidence of the coherent elastic scattering of reactor antineutrinos from atomic nuclei has been reported by the German-Swiss Coherent Neutrino Nucleus Scattering (CONUS) collaboration. This interaction has a higher cross section (probability) than the processes currently used to detect neutrinos, and could therefore lead to smaller detectors. It also involves lower-energy neutrinos, which could offer new ways to look for new physics beyond the Standard Model.
Antineutrinos only occasionally interact with matter, which makes them very difficult to detect. They can be observed using inverse beta decay, which involves the capture of electron antineutrinos by protons, producing neutrons and positrons. An alternative method involves observing the scattering of antineutrinos from electrons. Both these reactions have small cross sections, so huge detectors are required to capture just a few events. Moreover, inverse beta decay can only detect antineutrinos if they have energies above about 1.8 MeV, which precludes searches for low-energy physics beyond the Standard Model.
It is also possible to detect neutrinos by the tiny kick a nucleus receives when a neutrino scatters off it. “It’s very hard to detect experimentally because the recoil energy of the nucleus is so low, but on the other hand the interaction probability is a factor of 100–1000 higher than these typical reactions that are otherwise used,” says Christian Buck of the Max Planck Institute for Nuclear Physics in Heidelberg. This enables measurements with kilogram-scale detectors.
This was first observed in 2017 by the COHERENT collaboration using a 14.6 kg caesium iodide crystal to detect neutrinos from the Spallation Neutron Source at the Oak Ridge National Laboratory in the US. These neutrinos have a maximum energy of 55 MeV, making them ideal for the interaction. Moreover, the neutrinos come in pulses, allowing the signal to be distinguished from background radiation.
Reactor search
Multiple groups have subsequently looked for signals from nuclear reactors, which produce lower-energy neutrinos. These include the CONUS collaboration, which operated at the Brokdorf nuclear reactor in Germany until 2022. However, the only group to report a strong hint of a signal included Juan Collar of the University of Chicago. In 2022 it published results suggesting a stronger than expected signal at the Dresden-2 power reactor in the US.
Now, Buck and his CONUS colleagues present data from the CONUS+ experiment conducted at the Leibstadt reactor in Switzerland. They used three 1 kg germanium diodes sensitive to energies as low as 160 eV. They extracted the neutrino spectrum from background radiation by taking data when the reactor was running and when it was not. Writing in Nature, the team conclude that 395±106 neutrinos had been detected during 119 days of operation, which is consistent with the Standard Model 3.7σ away from zero. The experiment is currently in its second run, with the detector masses increased to 2.4 kg to provide better statistics and potentially a lower threshold energy.
Collar, however, is sceptical of the result. “[The researchers] seem to have an interest in dismissing the limitations of these detectors – limitations that affect us too,” he says. “The main difference between our approach and theirs is that we have made a best effort to demonstrate that our data are not contaminated by residual sources of low-energy noise dominant in this type of device prior to a careful analysis.” His group will soon release data taken at the Vandellòs reactor in Spain. “When we release these, we will take the time to point out the issues visible in their present paper,” he says. “It is a long list.”
Buck accepts that, if the previous measurements by Collar’s group are correct, the CONUS+ researchers should have detected least 10 times more neutrinos than they actually did. “I would say the control of backgrounds at our site in Leibstadt is better because we do not have such a strong neutron background. We have clearly demonstrated that the noise Collar has in mind is not dominant in the energy region of interest in our case.”
Patrick Huber at Virginia Tech in the US says, “Let’s see what Collar’s new result is going to be. I think this is a good example of the scientific method at work. Science doesn’t care who’s first – scientists care, but for us, what matters is that we get it right. But with the data that we have in hand, most experts, myself included, think that the current result is essentially the result we have been looking for.”
