States who received funding under an EPA project aimed to expand solar energy for low-income communities

Nearly two dozen states are suing the Trump administration over its cancellation of a $7bn grant program aimed at expanding solar energy in low-income communities, according to court papers.

In a statement on Thursday, California’s attorney general, Rob Bonta, announced two lawsuits by a group of states that received grants under the Environmental Protection Agency’s Solar for All program. The EPA’s administrator, Lee Zeldin, announced the termination of the program in August. The agency said in an email that it would not comment on pending litigation.

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© Photograph: Bing Guan/Reuters

© Photograph: Bing Guan/Reuters

© Photograph: Bing Guan/Reuters

Lobbyists attended 48% more meetings than Tories, as Labour accused of giving them ‘backstage pass’

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Government ministers met representatives from the fossil fuel industry more than 500 times during their first year in power – equivalent to twice every working day, according to research.

The analysis found that fossil fuel lobbyists were present at 48% more ministerial meetings during Labour’s first year in power than under the Conservatives in 2023.

Ministers at the Department for Energy Security and Net Zero (DESNZ) met fossil fuel lobbyists 274 times, with industry figures present at almost a quarter of meetings.

During the same period DESNZ ministers met trade union representatives 61 times.

Ed Miliband, the secretary for energy and climate change, met fossil fuel lobbyists 91 times – with a third of all his meetings attended by industry figures.

Three fossil fuel companies: BP, Shell and Equinor, met ministers 100 times between them.

Fossil fuel lobbyists attended almost every government meeting about the energy profits levy, a temporary windfall tax on the “extraordinary profits” of North Sea oil and gas companies.

This article was amended on 16 October 2025. Owing to an error in supplied information, an earlier version said Ed Miliband met fossil fuel lobbyists 250 times during Labour’s first year in power; this should have said 91 meetings.

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© Photograph: Nature Picture Library/Alamy

© Photograph: Nature Picture Library/Alamy

© Photograph: Nature Picture Library/Alamy

Climate advisers warn that current plans to protect against extreme weather are inadequate

Britain must prepare for global heating far in excess of the level scientists have pegged as the limit of safety, the government’s climate advisers have warned, as current plans to protect against extreme weather are inadequate.

Heatwaves will occur in at least four of every five years in England by 2050, and time spent in drought will double. The number of days of peak wildfire conditions in July will nearly treble for the UK, while floods will increase in frequency throughout the year, with some peak river flows increasing by 40%.

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© Photograph: Yui Mok/PA

© Photograph: Yui Mok/PA

© Photograph: Yui Mok/PA

The world is changing rapidly – economically, geopolitically, technologically, militarily and environmentally. But when it comes to the environment, many people feel the world is on the cusp of catastrophe. That’s especially true for anyone directly affected by endemic environmental disasters, such as drought or flooding, where mass outmigration is the only option possible.

The challenges are considerable and the crisis is urgent. But we know that physics has already contributed enormously to society – and I believe that environmental physics can make a huge difference by identifying, addressing and alleviating the problems at stake. However, physicists will only be able to make a difference if we put environmental physics at the centre of our university teaching.

Grounded in physics

Environmental physics is defined as the response of living organisms to their environment within the framework of the physics principles and processes. It examines the interactions within and between the biosphere, the hydrosphere, the cryosphere, the lithosphere, the geosphere and the atmosphere. Stretching from geophysics, meteorology and climate change to renewable energy and remote sensing, it also covers soils and vegetation, the urban and built environment, and the survival of humans and animals in extreme environments.

Environmental physics was pioneered in the UK in the 1950s by the physicists Howard Penman and John Monteith, who were based at the Rothamsted Experimental Station, which is one of the oldest agricultural research institutions in the world. In recent decades, environmental physics has become more prevalent in universities across the world.

Some UK universities either teach environmental physics in their undergraduate physics degrees or have elements of it within environmental science degrees. That’s the approach taken, for example, by University College London as well as well as the universities of Cambridge, Leicester, Manchester, Oxford, Reading, Strathclyde and Warwick.

When it comes to master’s degrees in environmental physics, there are 17 related courses in the UK, including nuclear and environmental physics at Glasgow and radiation and environmental protection at Surrey. Even the London School of Economics has elements of environmental physics in some of its business, geography and economics degrees via a “physics of climate” course.

But we need to do more. The interdisciplinary nature of environmental physics means it overlaps with not just physics and maths but agriculture, biology, chemistry, computing, engineering, geology and health science too.

Indeed, recent developments in machine learning, digital technology and artificial intelligence (AI) have had an impact on environmental physics – for example, through the use of drones in environmental monitoring and simulations – while AI algorithms can catalyse modelling and weather forecasting. AI could also in future be used to predict natural disasters, such as earthquakes, tsunamis, hurricanes and volcanic eruptions, and to assess the health implications of environmental pollution.

Environmental physics is exciting and challenging, has solid foundations in mathematics and the sciences via experiments both in the lab and field. Environmental measurements are a great way to learn about the use of uncertainties, monitoring and modelling, while providing scope for project and teamwork. A grounding in environmental physics can also open the door to lots of exciting career opportunities, with ongoing environmental change meaning lots of ongoing environmental research will be vital.

Solving major regional and global environmental problems is a key part of sociopolitics and so environmental physics has a special role to play in the public arena. It gives students the chance to develop presentational and interpersonal skills that can be used to influence decision makers at local and national government level.

Taken together, I believe a module on environmental physics should be a component of every undergraduate degree as a minimum, ideally having the same weight as quantum or statistical physics or optics. Students of environmental physics have the potential to be enabled, engaged and, ultimately, to be empowered to meet the demands that the future holds.

The post Environmental physics should be on a par with quantum physics or optics appeared first on Physics World.

An unconventional approach to solving the dark energy problem called the cosmologically coupled black hole (CCBH) hypothesis appears to be compatible with the observed masses of neutrinos. This new finding from researchers working at the DESI collaboration suggests that black holes may represent little Big Bangs played in reverse and could be used as a laboratory to study the birth and infancy of our universe. The study also confirms that the strength of dark energy has increased along with the formation rate of stars.

The Dark Energy Spectroscopic Instrument (DESI) is located on the Nicholas U Mayall four-metre Telescope at Kitt Peak National Observatory in Arizona. Its raison d’être is to shed more light on the “dark universe” – the 95% of the mass and energy in the universe that we know very little about. Dark energy is a hypothetical entity invoked to explain why the rate of expansion of the universe is (mysteriously) increasing – something that was discovered at the end of the last century.

According to standard theories of cosmology, matter is thought to comprise cold dark matter (CDM) and normal matter (mostly baryons and neutrinos). DESI can observe fluctuations in the matter density of the universe known as baryonic acoustic oscillations (BAOs), which are density fluctuations that were created after the Big Bang in the hot plasma of baryons and electrons that prevailed then. BAOs expand with the growth of the universe and represent a sort of “standard ruler” that allows cosmologists to map the universe’s expansion by statistically analysing the distance that separates pairs of galaxies and quasars.

Largest 3D map

DESI has produced the largest such 3D map of the universe ever and it recently published the first set of BAO measurements determined from observations of over 14 million extragalactic targets going back 11 billion years in time.

In the new study, the DESI researchers combined measurements from these new data with cosmic microwave background (CMB) datasets (which measure the density of dark matter and baryons from a time when the universe was less than 400,000 years old) to search for evidence of matter converting into dark energy. They did this by focusing on a new hypothesis known as the cosmologically coupled black hole (CCBH), which was put forward five years ago by DESI team member Kevin Croker, who works at Arizona State University (ASU), and his colleague Duncan Farrah at the University of Hawaii. This physical model builds on a mathematical description of black holes as bubbles of dark energy in space that was introduced over 50 years ago. CCBH describes a scenario in which massive stars exhaust their nuclear fuel and collapse to produce black holes filled with dark energy that then grows as the universe expands. The rate of dark energy production is therefore determined by the rate at which stars form.

Neutrino contribution

Previous analyses by DESI scientists suggested that there is less matter in the universe today compared to when it was much younger. When they then added the additional, known, matter source from neutrinos, there appeared to be no “room” and the masses of these particles therefore appeared negative in their calculations. Not only is this unphysical, explains team member Rogier Windhorst of the ASU’s School of Earth and Space Exploration, it also goes against experimental measurements made so far on neutrinos that give them a greater-than-zero mass.

When the researchers re-interpreted the new set of data with the CCBH model, they were able to resolve this issue. Since stars are made of baryons and black holes convert exhausted matter from stars into dark energy, the number of baryons today has decreased in comparison to the CMB measurements. This means that neutrinos can indeed contribute to the universe’s mass, slowing down the expansion of the universe as the dark energy produced sped it up.

“The new data are the most precise measurements of the rate of expansion of the universe going back more than 10 billion years,” says team member Gregory Tarlé at the University of Michigan, “and it results from the hard work of the entire DESI collaboration over more than a decade. We undertook this new study to confront the CCBH hypothesis with these data.”

Black holes as a laboratory

“We found that the standard assumptions currently employed for cosmological analyses simply did not work and we had to carefully revisit and rewrite massive amounts of a lot of cosmological computer code,” adds Croker.

“If dark energy is being sourced by black holes, these structures may be used as a laboratory to study the birth and infancy of our own universe,” he tells Physics World. “The formation of black holes may represent little Big Bangs played in reverse, and to make a biological analogy, they may be the ‘offspring’ of our universe.”

The researchers say they studied the CCBH scenario in its simplest form in this work, and found that it performs very well. “The next big observational test will involve a new layer of complexity, where consistency with the large-scale features of the Big Bang relic radiation, or CMB, and the statistical properties of the distribution of galaxies in space will make or break the model,” says Tarlé.

The research is described in Physical Review Letters.

The post Unconventional approach to dark energy problem gives observed neutrino masses appeared first on Physics World.

A new transition-metal oxide crystal that reversibly and repeatedly absorbs and releases oxygen could be ideal for use in fuel cells and as the active medium in clean energy technologies such as thermal transistors, smart windows and new types of batteries. The “breathing” crystal, discovered by scientists at Pusan National University in Korea and Hokkaido University in Japan, is made from strontium, cobalt and iron and contains oxygen vacancies.

Transition-metal oxides boast a huge range of electrical properties that can be tuned all the way from insulating to superconducting. This means they can find applications in areas as diverse as energy storage, catalysis and electronic devices.

Among the different material parameters that can be tuned are the oxygen vacancies. Indeed, ordering these vacancies can produce new structural phases that show much promise for oxygen-driven programmable devices.

Element-specific behaviours

In the new work, a team of researchers led by physicist Hyoungjeen Jeen of Pusan and materials scientist Hiromichi Ohta in Hokkaido studied SrFe_0.5Co_0.5O_x. The researchers focused on this material, they say, since it belongs to the family of topotactic oxides, which are the main oxides being studied today in solid-state ionics. “However, previous work had not discussed which ion in this compound was catalytically active,” explains Jeen. “What is more, the cobalt-containing topotactic oxides studied so far were fragile and easily fractured during chemical reactions.”

The team succeeded in creating a unique platform from a solid solution of epitaxial SrFe_0.5Co_0.5O_2.5 in which both the cobalt and iron ions bathed in the same chemical environment. “In this way, we were able to test which ion was better for reduction reactions and whether or not it sustained its structural integrity,” Jeen tells Physics World. “We found that our material showed element-specific reduction behaviours and reversible redox reactions.”

The researchers made their material using a pulsed laser deposition technique, ideal for the epitaxial synthesis of multi-element oxides that allowed them to grow SrFe_0.5Co_0.5O_2.5 crystals in which the iron and cobalt ions were randomly located in the crystal. This random arrangement was key to the material’s ability to repeatedly release and absorb oxygen, they say.

“It’s like giving the crystal ‘lungs’ so that it can inhale and exhale oxygen on command,” says Jeen.

Stable and repeatable

This simple breathing picture comes from the difference in the catalytic activity of cobalt and iron in the compound, he explains. Cobalt ions prefer to lose and gain oxygen and these ions are the main sites for the redox activity. However, since iron ions prefer not to lose oxygen during the reduction reaction, they serve as pillars in this architecture. This allows for stable and repeatable oxygen release and uptake.

Until now, most materials that absorb and release oxygen in such a controlled fashion were either too fragile or only functioned at extremely high temperatures. The new material works under more ambient conditions and is stable. “This finding is striking in two ways: only cobalt ions are reduced, and the process leads to the formation of an entirely new and stable crystal structure,” explains Jeen.

The researchers also showed that the material could return to its original form when oxygen was reintroduced, so proving that the process is fully reversible. “This is a major step towards the realization of smart materials that can adjust themselves in real time,” says Ohta. “The potential applications include developing a cathode for intermediate solid oxide fuel cells, an active medium for thermal transistors (devices that can direct heat like electrical switches), smart windows that adjust their heat flow depending on the weather and even new types of batteries.”

Looking ahead, Jeen, Ohta and colleagues aim to investigate the material’s potential for practical applications.

They report their present work in Nature Communications.

The post ‘Breathing’ crystal reversibly releases oxygen appeared first on Physics World.

Dark matter could be accumulating inside planets close to the galactic centre, potentially even forming black holes that might consume the afflicted planets from the inside-out, new research has predicted.

According to the standard model of cosmology, all galaxies including the Milky Way sit inside huge haloes of dark matter, with the greatest density at the centre. This dark matter primarily interacts only through gravity, although some popular models such as weakly interacting massive particles (WIMPS) do imply that dark-matter particles may occasionally scatter off normal matter.

This has led PhD student Mehrdad Phoroutan Mehr and Tara Fetherolf of the University of California, Riverside, to make an extraordinary proposal: that dark matter could elastically scatter off molecules inside planets, lose energy and become trapped inside those planets, and then grow so dense that they collapse to form a black hole. In some cases, a black hole could be produced in just ten months, according to Mehr and Fetherolf’s calculations, reported in Physical Review D.

Even more remarkable is that while many planets would be consumed by their parasitic black hole, it is feasible that some planets could actually survive with a black hole inside them, while in others the black hole might evaporate, Mehr tells Physics World.

“Whether a black hole inside a planet survives or not depends on how massive it is when it first forms,” he says.

This leads to a trade-off between how quickly the black hole can grow and how soon the black hole can evaporate via Hawking radiation – the quantum effect that sees a black hole’s mass radiated away as energy.

The mass of a dark-matter particle remains unknown, but the less massive it is, and the more massive a planet is, then the greater the chance a planet has of capturing dark matter, and the more massive a black hole it can form. If the black hole starts out relatively massive, then the planet is in big trouble, but if it starts out very small then it can evaporate before it becomes dangerous. Of course, if it evaporates, another black hole could replace it in the future.

“Interestingly,” adds Mehr, “There is also a special in-between mass where these two effects balance each other out. In that case, the black hole neither grows nor evaporates – it could remain stable inside the planet for a long time.”

Keeping planets warm

It’s not the first time that dark matter has been postulated to accumulate inside planets. In 2011 Dan Hooper and Jason Steffen of Fermilab proposed that dark matter could become trapped inside planets and that the energy released through dark-matter particles annihilating could keep a planet outside the habitable zone warm enough for liquid water to exist on its surface.

Mehr and Fetherolf’s new hypothesis “is worth looking into more carefully”, says Hooper.

That said, Hooper cautions that the ability of dark matter to accumulate inside a planet and form a black hole should not be a general expectation for all models of dark matter. Rather, “it seems to me that there could be a small window of dark-matter models where such particles could be captured in stars at a rate that is high enough to lead to black hole formation,” he says.

Currently there remains a large parameter space for the possible properties for dark matter. Experiments and observations continue to chip away at this parameter space, but there remain a very wide range of possibilities. The ability of dark matter to self-annihilate is just one of those properties – not all models of dark matter allow for this.

If dark-matter particles do annihilate at a sufficiently high rate when they come into contact, then it is unlikely that the mass of dark matter inside a planet would ever grow large enough to form a black hole. But if they don’t self-annihilate, or at least not at an appreciable rate, then a black hole formed of dark matter could still keep a planet warm with its Hawking radiation.

Searching for planets with black holes inside

The temperature anomaly that this would create could provide a means of detecting planets with black holes inside them. It would be challenging – the planets that we expect to contain the most dark matter would be near the centre of the galaxy 26,000 light years away, where the dark-matter concentration in the halo is densest.

Even if the James Webb Space Telescope (JWST) could detect anomalous thermal radiation from such a distant planet, Mehr says that it would not necessarily be a smoking gun.

“If JWST were to observe that a planet is hotter than expected, there could be many possible explanations, we would not immediately attribute this to dark matter or a black hole,” says Mehr. “Rather, our point is that if detailed studies reveal temperatures that cannot be explained by ordinary processes, then dark matter could be considered as one possible – though still controversial – explanation.”

Another problem is that black holes cannot be distinguished from planets purely through their gravity. A Jupiter-mass planet has the same gravitational pull as a Jupiter-mass black hole that has just eaten a Jupiter-mass planet. This means that planetary detection methods that rely on gravity, from radial velocity Doppler shift measurements to astrometry and gravitational microlensing events, could not tell a planet and a black hole apart.

The planets in our own Solar System are also unlikely to contain much dark matter, says Mehr. “We assume that the dark matter density primarily depends on the distance from the centre of the galaxy,” he explains.

Where we are, the density of dark matter is too low for the planets to capture much of it, since the dark-matter halo is concentrated in the galactic centre. Therefore, we needn’t worry about Jupiter or Saturn, or even Earth, turning into a black hole.

The post Exoplanets suffering from a plague of dark matter could turn into black holes appeared first on Physics World.

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.

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 (ApJ 970 31). This is as expected, and over time they would grow and merge into larger galaxies.

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 (MNRAS 498 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 (ApJ 905 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&A 698 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 f_multi) 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 (ApJ 952 128). “I came to realize the necessity of calibrating f_multi 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.

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

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