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A bumper crop of measurements of the expansion rate of the universe have stretched the Hubble tension as taut as it has ever been, with scientists grappling with trying to find a solution.

Over 500 researchers have come together in the “CosmoVerse” consortium to produce a new white paper that delves into the various cosmological tensions between theory and observation. These include the Hubble tension, which is the bewildering discrepancy in the expansion rate of the universe, referred to as the Hubble constant (H₀).

Predictive measurements made by applying the standard model of cosmology to the cosmic microwave background (CMB) give H₀ as 67.4 km/s/Mpc. In other words, every volume of space a million parsecs across (one parsec is 3.26 light years) should be expanding by 67.4 kilometres every second.

Yet that’s not what Hubble’s law – which tells us the expansion rate based on a given object’s velocity away from us and its distance – says, as demonstrated by the CosmoVerse White Paper.

“The paper’s been getting a lot of attention in our field,” Joe Jensen of Utah Valley University tells Physics World. “You can easily see that the vast majority of measurements fall around 73 km/s/Mpc, with varying uncertainties.”

There’s no known reason why local measurements of H₀ (based on supernovae observations) should differ from the CMB measurement. This discrepancy leads to two possibilities. Either there are unknown systematic uncertainties in measurements that skew the results, or cosmology’s standard model is wrong and new physics is needed.

A lot at stake

The highest rung on the cosmic distance ladder is a type Ia supernova – a white dwarf explosion. They have a standardizable brightness that makes them perfect for judging how far away they are, based on their luminosity curve. These measurements are calibrated by lower rungs on the ladder, such as Cepheid variable stars or the peak brightness of red giant stars (referred to as the “tip of the red giant branch”, or TRGB).

If the tension is real, then different calibrators should still give the same result. One of the few outliers is found in a new paper published in The Astrophysical Journal by the Chicago–Carnegie Hubble Program (CCHP) led by the University of Chicago’s Wendy Freedman.

CCHP’s latest paper uses the TRGB to arrive at a best value of 70.39 km/s/Mpc when combining measurements from the James Webb Space Telescope (JWST) – which is able to better resolve red giant stars in other galaxies – with Hubble Space Telescope data.

The CCHP team argue that this result is in line with the CMB measurements and removes the tension. However, their conclusion has met opposition.

“Their result is sort of in the middle of the Hubble tension, so I’m surprised that they would say they rule it out,” Dan Scolnic, an astrophysicist at Duke University in the United States, tells Physics World.

At a meeting of the American Astronomical Society in January 2025, Scolnic declared that the Hubble tension was now a crisis. CCHP’s results do not dissuade him from this conclusion.

“For some reason they don’t include a number of supernovae in their sample that they could have,” says Scolnic. “Siyang Li [of Johns Hopkins University] led a paper [on which Scolnic is a co-author] that showed that if one uses their TRGB measurements, and the complete sample of supernovae, one goes back to higher H₀.”

Freedman did not respond to Physics World‘s request for an interview.

Different approaches

Jensen has also led a team that recently conducted measurements of H₀ using TRGB stars, but in a different way by looking for surface brightness fluctuations (SBF).

“SBF is a statistical method that measures the brightnesses of red giant stars even when they cannot be measured individually,” says Jensen.

Individual stars in galaxies cannot be resolved at great distance – their light blends together, and the more distant the galaxy, the smoother this blend is. We describe this blended light as the galaxy’s surface brightness, and fluctuations are statistical in nature and result from the discrete nature of stars.

In old elliptical galaxies, the surface brightness is dominated by red giant stars, which are evolved Sun-like stars. Measuring the SBF therefore provides a value for the TRGB, from which a distance can be determined.

Using JWST images to measure the SBF of 14 elliptical galaxies, then using those to calibrate the distances to 60 more distant ellipticals, and then using that calibration to determine H₀, Jensen’s team arrived at a value of 73.8 km/s/Mpc.

“The reason that we don’t get the same answer [as CCHP] is that we are not using the same JWST calibrators, and we don’t use type Ia to measure H₀,” says Jensen.

This contradicts CCHP’s main assertion, which is that there must be unknown systematic uncertainties in either the type Ia supernovae or the Cepheids. Jensen’s team use neither, yet still find a tension.

Perhaps the most convincing evidence for the tension comes from the TDCOSMO (time-delay cosmography) team, who utilize gravitationally lensed quasars to measure H₀.

Quasars fluctuate in brightness over a matter of days. When light from a quasar takes paths of varying lengths around a lensing object, it produces multiple images that have time lags relative to one another. The expansion of space can extend this time delay, providing a completely independent measure of H₀.

In 2019 the H0LiCOW project used six gravitational lenses to arrive at a value of 73.3 km/s/Mpc. This result came with some scepticism. So they formed the new TDCOSMO consortium and “went on a six-year journey to see if their original measurement was okay,” says Scolnic.

TDCOSMO’s final conclusion is 72.1 km/Mpc/s, strongly supporting the tension. However, in all these measurements there’s wriggle room from various known measuring uncertainties.

“It’s important to remember that the uncertainties put us in only mild disagreement,” says Jensen. “I expect that we will soon know if the disagreement can be explained by the mundane choices of calibration galaxies and processing techniques.”

If it cannot, then the inescapable conclusion is that there’s something wrong with our understanding of the universe. Figuring that out could be the next great quest in cosmology.

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While Arianespace is committed to moving to its peak launch rate of the Ariane 6 “as soon as possible,” it may take several years to do so.

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Pictures are a prelude to a transformative, all-sky survey set to begin in months

The Vera C. Rubin Observatory is poised to discover billions of new astronomical objects, revolutionizing understanding of everything from the history of the solar system to the workings of dark energy.

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The first spectacular images from the Vera C Rubin Observatory have been released today showing millions of galaxies and Milky Way stars and thousands of asteroids in exquisite detail.

Based in Cerro Pachón in the Andes, the Vera C Rubin Observatory contains the Legacy Survey of Space and Time (LSST) – the largest camera ever built. Taking almost two decades to build, the 3200 megapixel instrument forms the heart of the observatory’s 8.4 m Simonyi Survey Telescope.

The imagery released today, which took just 10 hours of observations, is a small preview of the Observatory’s upcoming 10-year scientific mission.

The image above is of the Trifid and Lagoon nebulas. This picture combines 678 separate images taken by the Vera C. Rubin Observatory in just over seven hours of observing time. It reveals otherwise faint or invisible details, such as the clouds of gas and dust that comprise the Trifid nebula (top right) and the Lagoon nebula, which are several thousand light-years away from Earth.

The image below is of the Virgo cluster. It shows a small section of the Virgo cluster, featuring two spiral galaxies (lower right), three merging galaxies (upper right) and several groups of distant galaxies.

Star mapper

Later this year, the Vera C Rubin Observatory, which is funded by the National Science Foundation and the Department of Energy’s Office of Science, will begin a decade-long survey of the southern hemisphere sky.

The LSST will take a complete picture of the southern night sky every 3-4 nights. It will then replicate this process over a decade to produce almost 1000 full images of sky.

This will be used to plot the positions and measure the brightness of objects in the sky to help improve our understanding of dark matter and dark energy. It will examine 20 billion galaxies as well as produce the most detailed star map of the Milky Way, imaging 17 billion stars and cataloguing some six million small objects within our solar system including asteroids.

Cosmic pioneer

The observatory is named in honour of the US astronomer Vera C. Rubin. In 1970, working with Kent Ford Jr, they observed that outer stars orbiting in the Andromeda galaxy were all doing so at the same speed.

Examining more galaxies still, they found that their rotation curves – the orbital speed of visible stars within the galaxy compared with their radial distance to the galaxy centre – contradicted Kepler’s law.

They also found that stars near the outer edges of the galaxies were orbiting so fast that they should be falling apart.

Rubin and Ford Jr’s observation led them to predict that there was some mass, dubbed “dark matter”, inside the galaxies responsible for the anomalous motions, something their telescopes couldn’t see but was there in quantities about six times the amount of the luminous matter present.

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'RocketGPT' among first deployments of government-compliant ChatGPT designed for sensitive aerospace data

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Independent measurements of the charge radius of the helium-3 nucleus using two different  methods have yielded significantly different results – prompting a re-evaluation of underlying theory to reconcile them. The international CREMA Collaboration used muonic helium-3 ions to determine the radius, whereas a team in the Netherlands used a quantum-degenerate gas of helium-3 atoms.

The charge radius is a statistical measure of how far the electric charge of a particle extends into space. Both groups were mystified by the discrepancy in the values – which hints at physics beyond the Standard Model of particle physics. However, new theoretical calculations inspired by the results may have already resolved the discrepancy.

Both groups studied the difference between the charge radii of the helium-3 and helium-4 nuclei. CREMA used muonic helium ions, in which the remaining electrons replaced by muons. Muons are much more massive than electrons, so they spend more time near the nucleus – and are therefore more sensitive to the charge radius.

Shorter wavelengths

Muonic atoms have spectra at much shorter wavelengths than normal atoms. This affects values such as the Lamb shift. This is the energy difference in the 2S_1/2 and 2P_1/2 atomic states, which are split by interactions with virtual photons and vacuum polarization. This is most intense near the nucleus. More importantly, a muon in an S orbital becomes more sensitive to the finite size of the nucleus.

In 2010, CREMA used the charge radius of muonic hydrogen to conclude that the charge radius of the proton is significantly smaller than the current accepted value. The same procedure was then used with muonic helium-4 ions. Now, CREMA has used pulsed laser spectroscopy of muonic helium-3 ions to extract several key parameters including the Lamb shift and used them to calculate the charge radius of muonic helium-3 nuclei. They then calculated the difference with the charge radius in helium-4. The value they obtained was 15 times more accurate than any previously reported.

Meanwhile, at the Free University of Amsterdam in the Netherlands, researchers were taking a different approach, using conventional helium-3 atoms. This has significant challenges, because the effect of the nucleus on electrons is much smaller. However, it also means that an electron affects the nucleus it measures less than does a muon, which mitigates a source of theoretical uncertainty.

The Amsterdam team utilized the fact that the 2S triplet state in helium is extremely long-lived. ”If you manage to get the atom up there, it’s like a new ground state, and that means you can do laser cooling on it and it allows very efficient detection of the atoms,” explains Kjeld Eikema, one of the team’s leaders after its initial leader Wim Vassen died in 2019. In 2018, the Amsterdam group created an ultracold Bose–Einstein condensate (BEC) of helium-4 atoms in the 2S triplet state in an optical dipole trap before using laser spectroscopy to measure the ultra-narrow transition between the 2S triplet state and the higher 2S singlet state.

Degenerate Fermi gas

In the new work, the researchers turned to helium-3, which does not form a BEC but instead forms a degenerate Fermi gas. Interpreting the spectra of this required new discoveries itself. “Current theoretical models are insufficiently accurate to determine the charge radii from measurements on two-electron atoms,” Eikema explains. However, “the nice thing is that if you measure the transition directly in one isotope and then look at the difference with the other isotope, then most complications from the two electrons are common mode and drop out,” he says. This can be used to the determine the difference in the charge radii.

The researchers obtained a value that was even more precise than CREMA’s and larger by 3.6σ. The groups could find no obvious explanation for the discrepancy. “The scope of the physics involved in doing and interpreting these experiments is quite massive,” says Eikema; “a comparison is so interesting, because you can say ‘Well, is all this physics correct then? Are electrons and muons the same aside from their mass? Did we do the quantum electrodynamics correct for both normal atoms and muonic atoms? Did we do the nuclear polarization correctly?’” The results of both teams are described in Science (CREMA, Amsterdam).

While these papers were undergoing peer review, the work attracted the attention of two groups of theoretical physicists – one led by Xiao-Qiu Qi f the Wuhan Institute of Physics and Mathematics in China, and the other by Krzysztof Pachucki of the University of Warsaw in Poland. Both revised the calculation of the hyperfine structure of helium-3, finding that incorporating previously neglected higher orders into the calculation produced an unexpectedly large shift.

“Suddenly, by plugging this new value into our experiment – ping! – our determination comes within 1.2σ of theirs,” says Eikema; “which is a triumph for all the physics involved, and it shows how, by showing there’s a difference, other people think, ‘Maybe we should go and check our calculations,’ and it has improved the calculation of the hyperfine effect.” In this manner the ever improving experiments and theory calculations continue to seek the limits of the Standard Model.

Xiao-Qiu Qi and colleagues describe their calculations in Physical Review Research, while Pachucki’s team have published in Physical Review A.

Eikema adds “Personally I would have adjusted the value in our paper according to these new calculations, but Science preferred to keep the paper as it was at the time of submission and peer review, with an added final paragraph to explain the latest developments.”

Theoretical physicist Marko Horbatsch at Canada’s York University is impressed by the experimental results and bemused by the presentation. “I would say that their final answer is a great success,” he concludes. “There is validity in having the CREMA and Eikema work published side-by-side in a high-impact journal. It’s just that the fact that they agree should not be confined to a final sentence at the end of the paper.”

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Computational physicist Jose Milovich of the Lawrence Livermore National Laboratory (LLNL) and colleagues have been awarded the 2025 Plasma Physics and Controlled Fusion (PPCF) Outstanding Paper Prize for their computational research on capsule implosions during laser fusion.

The work – Understanding asymmetries using integrated simulations of capsule implosions in low gas-fill hohlraums at the National Ignition Facility – is an important part of understanding the physics at the heart of inertial confinement fusion (ICF).

Fusion is usually performed via two types of plasma confinement. Magnetic involves using magnetic fields to hold stable a plasma of deuterium-tritium (D-T), while inertial confinement uses rapid compression, usually by lasers, to create a confined plasma for a short period of time.

The award-winning work was based on experiments carried out at the National Ignition Facility (NIF) based in California, which is one of the leading fusion centres in the world.

During NIF’s ICF experiments, a slight imbalance of the laser can induce motion of the hot central core of an ignition capsule, which contains the D-T fuel. This effect results in a reduced performance.

Experiments at NIF in 2018 found that laser imbalances alone, however, could not account for the motion of the capsule. The simulations carried out by Milovich and colleagues demonstrated that other factors were at play such as non-concentricity of the layers of the material surrounding the D-T fuel as well as “drive perturbations” induced by diagnostic windows on the implosion.

Changes made following the team’s findings then helped towards the recent demonstration of “energy breakeven” at NIF in December 2022.

Awarded each year, the PPCF prize aims to highlight work of the highest quality and impact published in the journal.  The award was judged on originality, scientific quality and impact as well as being based on community nominations and publication metrics. The prize will be presented at the 51st European Physical Society Conference on Plasma Physics in Vilnius, Lithuania, on 7–11 July.

The journal is now seeking nominations for next year’s prize, which will focus on papers in magnetic confinement fusion.

Below, Milovich talks to Physics World about prize, the future of fusion and what advice he has for early-career researchers.

What does winning the 2025 PPCF Outstanding Paper Prize mean to you and for your work?

The award is an incredible honour to me and my collaborators as a recognition of the detailed work required to make inertial fusion in the laboratory a reality and the dream of commercial fusion energy a possibility. The paper presented numerical confirmation of how seemingly small effects can significantly impact the performance of fusion targets.  This study led to target modifications and revised manufacturing specifications for improved performance.  My collaborators and I would like to deeply thank PPCF for granting us this award.

What excites you about fusion?

Nuclear fusion is the process that powers the stars, and achieving those conditions in the laboratory is exciting in many ways.  It is an interesting scientific problem in its own right and it is an incredibly challenging engineering problem to handle the extreme conditions required for successful energy production. This is an exciting time since the possibility of realizing this energy source became tangibly closer two years ago when NIF successfully demonstrated that more energy can be released from D-T fusion than the laser energy delivered to the target.

What are your thoughts on the future direction of ICF and NIF?

While the challenges ahead to make ICF commercially feasible are daunting, we are well positioned to address them by developing new technologies and innovative target configurations. Applications of artificial intelligence to reactor plant designs, optimized operations, and improvements on plasma confinement could potentially lead to improved designs at a fraction of the cost. The challenges are many but the potential for providing a clean and inexhaustible source of energy for the benefit of mankind is invigorating.

What advice would you give to people thinking about embarking on a career in fusion?

This is an exciting time to get involved in fusion. The latest achievements at NIF have shown that fusion is possible. There are countless difficulties to overcome, making it an ideal time to devote one’s career in this area. My advice is to get involved now since, at this early stage, any contribution will have a major and lasting impact on mankind’s future energy needs.

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Developing lenacapavir, the drug newly approved to protect against HIV for six months in one shot, took basic science, sophisticated chemistry, and perseverance

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The European Space Agency has signed an agreement regarding potential use of Orbital Reef as it refines its strategy for use of commercial space stations.

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