The key to the achievement is one of the strangest energy levels known in nuclear physics. The thorium-229 isotope possesses an exceptionally low-lying isomeric state, first proposed in the 1970s and only recently pinned down at an energy of about 8.3 electronvolts, corresponding to a wavelength of roughly 148 nanometres in the vacuum ultraviolet. Because this energy is so low, the transition can be driven with laser light, something impossible for essentially any other nuclear transition, which typically requires energies thousands of times higher. The idea of exploiting this quirk for timekeeping was proposed in 2003 by Ekkehard Peik and Christian Tamm, who envisioned a nuclear clock with a potential resonance quality factor on the order of 10 to the 19th power, far beyond what ordinary atomic clocks can offer.

The path to that vision spanned half a century of incremental discoveries. Gamma spectroscopy experiments gradually narrowed the transition energy from a conjectured value below 100 electronvolts to around 8 electronvolts. In 2016, the existence of the isomer was proven through its internal conversion decay channel, and in 2022 researchers achieved the first optical detection of the radiative nuclear decay, refining the energy to 8.338 electronvolts. Resonant laser excitation of the nucleus followed in 2024 and 2025 in thorium-doped calcium fluoride and other hosts, and the nuclear quadrupole structure of the transition in the crystal was subsequently resolved. The final missing ingredient was a continuous-wave laser at 148 nanometres, developed in 2025, which enabled absorption spectroscopy of the nuclear transition and, with it, the possibility of true clock operation.

Absorption spectroscopy is the technical heart of the new clock. Rather than waiting for excited nuclei to fluoresce, a slow process limited by the roughly ten-minute lifetime of the isomeric state, the team shines the vacuum ultraviolet laser through the crystal and measures how much light the nuclei absorb. A photomultiplier tube with a cesium iodide photocathode, operated in photon-counting mode behind the crystal, records the transmitted photons. This approach delivers three orders of magnitude more signal photons per second than fluorescence detection and allows the transition to be probed continuously. In the experiment, 65 picowatts of vacuum ultraviolet power reached the detector, and the thorium nuclei absorbed about 0.75 percent of the light on the investigated transition, yielding a signal-to-noise ratio of approximately 17 after one second of measurement.

The clock architecture combines two control loops. A high-finesse cavity-stabilized external-cavity diode laser operating at 1,187 nanometres provides short-term stability, while the nuclear resonance provides the long-term reference. The 1,187-nanometre seed laser is frequency-quadrupled, with the final doubling stage performed in a randomly quasi-phase-matched strontium tetraborate crystal to generate the 148-nanometre radiation. By modulating the offset lock frequency between two points on either side of the absorption line, the researchers generate an error signal with a zero crossing at the line centre. Each clock cycle, the measured deviation is inverted and applied in a single adjustment step to an electro-optical modulator, steering the laser back onto the nuclear resonance and closing the feedback loop. Depending on the chosen integration time, feedback steps occur every 20 seconds or every 30 minutes.

The performance, quantified by the Allan deviation, shows a shot-noise-limited fractional frequency instability of 3 times 10 to the minus 12 divided by the square root of the averaging time in seconds, approaching instabilities of 10 to the minus 15 over one day of continuous operation. The clock ran unattended for a full day, unperturbed by crystal temperature fluctuations of about 0.1 kelvin, consistent with the known temperature sensitivity of the transition of 1.8 kilohertz per kelvin near room temperature. The nuclear transition frequency itself is 2.0204 times 10 to the 15th hertz, with a measured linewidth of approximately 100 kilohertz. To benchmark the device, the team compared it with a ytterbium-ion single-ion clock at BEV via a Doppler-compensated fibre link and a pair of frequency combs, allowing the ratio of the nuclear and atomic transition frequencies to be monitored continuously.

One limitation emerged from the solid-state nature of the clock. Reproducibility between runs on different days was limited to about 5 times 10 to the minus 13, and measurements at four different positions on the crystal revealed line-centre deviations of up to 1.7 kilohertz, well above the 0.13-kilohertz statistical spread. Because the laser beam must be realigned before each run, it probes slightly different local regions of the crystal, and the team conjectures that local strains from structural inhomogeneity cause these shifts. Remeasuring the same position gave reproducible frequencies. Future improvements could constrain the optical path to a fixed volume or improve doping homogeneity through optimized crystal growth, potentially restoring full reproducibility.

Even at this early stage, the nuclear clock delivers competitive results in the search for dark matter. Many theories predict that dark matter may consist of ultralight scalar bosons whose feeble couplings to photons, gluons or quarks would cause oscillations in fundamental constants such as the fine structure constant, the QCD scale parameter and quark masses. The thorium-229 transition is extraordinarily sensitive to such variations because its anomalously low energy arises from a near-cancellation of mega-electronvolt-scale electromagnetic and strong-force contributions, giving it sensitivity factors predicted to exceed those of the best atomic clocks by orders of magnitude. Analysing roughly 23 hours of clock data with the Lomb-Scargle method, the team found no oscillation amplitudes above a 5 percent detection threshold established through Monte Carlo simulations, and used the resulting upper limits to constrain dark matter couplings.

The constraints are striking. For couplings to photons, the nuclear clock already rivals the best atomic clock comparisons, and for couplings to the strong force, probed through variations of the QCD scale and quark masses, it reaches a factor of 10 to 1,000 deeper into parameter space than previous experiments. The fast feedback of the absorption scheme also allowed direct probing of boson masses that previously could only be restricted through line-shape analyses. A linear fit of the frequency ratio data over the run gave a drift of (2 plus or minus 4) times 10 to the minus 14 per day, consistent with zero, corresponding to drift limits on the fundamental constants scaled by the thorium sensitivity factors.

The outlook for the technology is ambitious. Increasing the vacuum ultraviolet laser power through enhancement cavities, four-wave mixing in cadmium vapour or patterned nonlinear crystals could push the instability toward 10 to the minus 15 per square root second, and longer crystals or vacuum ultraviolet cavities could raise the signal-to-noise ratio by an order of magnitude. Alternative host crystals such as thorium fluoride, where thorium is part of the stoichiometric structure rather than a dopant, promise narrower linewidths and better reproducibility, while spinless solids could suppress magnetic broadening from neighbouring fluorine nuclei. With a kilohertz linewidth and modest laser power, the researchers estimate that a solid-state nuclear clock could reach instabilities near 10 to the minus 16 per square root second, matching state-of-the-art optical atomic clocks while retaining the compact form factor of a crystal at room temperature, and delivering a four-order-of-magnitude boost in sensitivity to variations of the fundamental constants that may reveal new physics beyond the Standard Model.

Subject of Research: A solid-state optical nuclear clock based on the thorium-229 nuclear transition with laser feedback stabilization

Article Title: A thorium-229 optical nuclear clock with feedback loop

Article References: Toscani De Col, L., Riebner, T., Morawetz, I., Schneider, F., Sempelmann, N., Schlachet-Lépinay, J., Schaden, F., Bartokos, M., Kazakov, G. A., Beeks, K., Gerstenecker, B., Pimon, M., Lahs, S., Hellerschmied, A., Lercher, T., Denker, H., Premper, J., Niessner, A., Matus, M., … Schumm, T. (2026). A thorium-229 optical nuclear clock with feedback loop. Nature. https://doi.org/10.1038/s41586-026-11084-4

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11084-4

Keywords: thorium-229, nuclear clock, optical clock, vacuum ultraviolet laser, absorption spectroscopy, calcium fluoride crystal, dark matter, fundamental constants, frequency metrology, isomeric transition, feedback loop, quantum metrology

The baryon cycle is the engine of galactic evolution. Stars forge heavy elements such as carbon, oxygen and silicon, and stellar feedback in the form of supernova explosions and intense radiation drives those metals out of galaxies into the circumgalactic medium, from which the gas can eventually cool and fall back to fuel new generations of stars. Models have long predicted that this redistribution should begin almost as soon as star formation does, but direct observational constraints at the earliest cosmic epochs have been extraordinarily hard to obtain. Previous measurements of metal enrichment during reionization relied mostly on absorption lines imprinted on the light of distant quasars, which probe random sightlines through the intergalactic medium and are only indirectly connected to individual galaxies.

The new work takes a fundamentally different approach: measuring absorption directly against the ultraviolet glow of the galaxies themselves. The team exploited ultra-deep, medium-resolution near-infrared spectroscopy from JWST’s NIRSpec instrument, obtained as part of the Cycle 4 programme SPURS, the SPectroscopic Ultra-deep Reionization-era Survey, in the Abell 2744 field. Only three galaxies in the survey met the demanding selection criterion: their rest-frame ultraviolet continuum near 1,450 angstroms had to be detected at a signal-to-noise ratio above 10, allowing faint absorption lines to be measured against the stellar light. Crucially, the galaxies were chosen purely on continuum brightness, with no prior knowledge of whether metal absorption would be present.

The three targets span a remarkable range of cosmic time. Galaxy A sits at a systemic redshift of 9.31, corresponding to roughly 530 million years after the Big Bang, while Galaxies B and C lie at redshifts 7.88 and 7.29, near the midpoint of reionization at around 650 to 750 million years. In all three systems, the team identified a suite of absorption transitions spanning more than two orders of magnitude in ionization potential: neutral oxygen (O I), low-ionization silicon and carbon (Si II and C II), and highly ionized species (Si IV and C IV). The absorption features appear strikingly similar to those seen in the well-studied spectra of star-forming galaxies at redshift around 3, when the universe was more than three times older.

The kinematics tell the most compelling story. By anchoring each galaxy’s rest frame to the centroid of the strong nebular emission line [O III] 5008, the researchers found that the metal absorption is consistently blueshifted by roughly 50 to 250 kilometres per second relative to the systemic velocity. Because Hubble expansion cannot produce a velocity offset relative to a galaxy’s own rest frame, these blueshifts indicate gas that is physically moving, most plausibly in outflows driven by stellar feedback, although turbulent interstellar motions or more complex internal kinematics may also contribute. The neutral, low-ionization and high-ionization lines all share broadly overlapping velocity structure, typically extending to about minus 200 kilometres per second, pointing to a common reservoir of disturbed, metal-enriched gas.

The coexistence of ions spanning such a wide range of ionization energies within the same blueshifted complex is difficult to reconcile with a single quiescent component of cold interstellar gas. Instead, it points to a multiphase structure, the signature expected when feedback-driven winds shred and accelerate material across cold, warm and hot phases. The data even hint at ionization-dependent stratification: the high-ionization absorption appears more blueshifted than the neutral and low-ionization lines by roughly 60 to 210 kilometres per second depending on the system, a pattern consistent with simulations such as FIRE-2, in which more highly ionized gas traces a faster or more extended component of a multiphase outflow. Higher-resolution spectra would be needed to confirm this ordering in detail.

Independent lines of evidence reinforce the picture. Interpreting the measured line widths as purely thermal broadening yields upper limits on the gas temperature of order ten million kelvin, far above the temperatures at which the detected ions can survive, meaning non-thermal motions such as turbulence and unresolved velocity substructure must dominate the profiles. Equivalent-width ratios between high- and low-ionization species, and between neutral and low-ionization species, place the three galaxies in the same parameter space occupied by lower-redshift star-forming galaxies, with Galaxy A, the most distant, showing somewhat stronger high-ionization absorption relative to its low-ionization lines. The spectra show no broad emission lines or other definitive signatures of an active galactic nucleus, so the feedback is most naturally attributed to intense star formation.

The host galaxies themselves are extreme objects. Spectral energy distribution modelling of Hubble and JWST photometry yields stellar masses between about one and six billion solar masses, strikingly blue ultraviolet slopes between minus 2.12 and minus 2.26, and gas-phase metallicities of roughly one-third to one-tenth of the solar value, subsolar but far from pristine. The youngest stellar populations are forming stars at rates of 19 to 39 solar masses per year. That chemically enriched, outflowing gas could coexist with such blue, dust-poor ultraviolet continua connects these systems to the so-called blue monster galaxies recently identified at even higher redshifts, in which efficient star formation and feedback-driven redistribution of gas and dust shape the observed light.

Perhaps the most profound implication concerns the speed of chemical enrichment. Detecting carbon and oxygen absorption in a galaxy at redshift 9.3, barely half a billion years after the Big Bang, requires extremely rapid metal production, consistent with highly efficient early star formation. Recent theoretical work shows that such rapid enrichment does not demand a dominant contribution from the hypothetical first generation of Population III stars: ordinary Population II star formation with a high upper-mass cutoff of roughly 200 to 300 solar masses can produce sufficient metal yields on short timescales, in line with expectations that the maximum stellar mass rises at low metallicity. The relative abundance patterns of the three galaxies overlap those of previously studied high-redshift absorbers, without extreme offsets that would signal exotic nucleosynthetic channels.

The observations do not directly constrain which sources drove reionization, and the broader population of high-redshift absorbers remains diverse, ranging from enriched systems to extremely metal-poor gas, indicating that early enrichment proceeded in a patchy, inhomogeneous fashion. But they decisively rule out a picture in which galaxy environments stayed chemically pristine until late times. Within the first several hundred million years of cosmic history, at least a subset of luminous galaxies had already built the full toolkit of the baryon cycle: metals in multiple ionization phases, feedback-driven kinematics, and gas circulating between the interstellar and circumgalactic media. The same processes that today regulate star formation across the universe were evidently at work almost from the beginning.

Subject of Research: Multiphase metal-enriched gas outflows around galaxies during the epoch of reionization

Article Title: Early metal-enriched baryon cycling before the midpoint of cosmic reionization

Article References: Zhu, Y., Ji, Z., Becker, G. D., Ding, J., Egami, E., Fan, X., Jin, X., Liu, W., Lyu, J., Ma, Z., Narisetty, S., Rieke, G. H., Wu, Y., Yue, M., Zhang, J., & Rieke, M. J. (2026). Early metal-enriched baryon cycling before the midpoint of cosmic reionization. Nature Astronomy. https://doi.org/10.1038/s41550-026-02988-2

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02988-2

Keywords: JWST, early universe, cosmic reionization, baryon cycle, galactic outflows, metal enrichment, circumgalactic medium, NIRSpec, high-redshift galaxies, stellar feedback, absorption spectroscopy, Population II stars

The implications reach to the very foundations of galaxy evolution theory. Heavy elements, which astronomers collectively call metals, are produced exclusively inside stars through nuclear fusion and related processes. In the newborn cosmos, gravity slowly gathered clouds of primordial hydrogen and helium into the first stellar furnaces, where extreme temperatures and pressures fused light nuclei into heavier ones. When massive stars exhausted their fuel and died as supernovae, they scattered these newly forged elements into space. Those ashes became the raw material for subsequent generations of stars, for planets, and ultimately for life itself. The carbon in human cells and the oxygen in every breath were manufactured in stars that lived and died long before the Sun existed. What remained uncertain until now was precisely how quickly and by what mechanism these elements escaped from the earliest galaxies into the wider universe.

Yongda Zhu, a postdoctoral researcher at the University of Arizona’s Department of Astronomy and Steward Observatory and first author of the paper, led the investigation into three extraordinarily distant galaxies whose light has been traveling for more than 13 billion years. These objects are observed as they appeared during the Epoch of Reionization, a pivotal cosmic era when the first generations of stars and galaxies flooded the universe with ultraviolet radiation, stripping electrons from the hydrogen gas that pervaded intergalactic space. This ionization process gradually dissolved the fog that had shrouded the cosmos since its so-called dark ages, allowing light to travel freely for the first time. The Epoch of Reionization represents one of the most transformative chapters in cosmic history, and the new findings suggest it was already a chemically dynamic period.

The observations were only possible because of the James Webb Space Telescope’s unparalleled infrared sensitivity. Webb collects light that has been stretched to longer, redder wavelengths by the expansion of the universe, allowing it to detect objects whose emission has been shifted far beyond the reach of earlier instruments. Zhu and colleagues accumulated nearly 30 hours of exposure time, enough to capture the faint absorption patterns imprinted on the spectra of these distant galaxies. In a remarkable feat of manual analysis conducted over the course of a single long night, Zhu combed through publicly available JWST spectra from hundreds of galaxies and identified three whose absorption signatures betrayed the presence of heavy elements, including carbon, oxygen and silicon.

The technique at the heart of the study is elegantly simple in concept. Rather than relying on the galaxies’ own emitted light alone, the researchers used the galaxies themselves as background light sources. As light from each galaxy traveled toward Earth, it passed through the gas surrounding it, and atoms in that gas absorbed light at very specific wavelengths corresponding to particular elements. By examining which wavelengths were missing from the observed spectra, the team could determine which elements were present in the circumgalactic medium, the reservoir of gas enveloping each galaxy. This absorption spectroscopy method has long been a workhorse of observational astronomy, but applying it to galaxies at such extreme distances became feasible only with Webb’s capabilities.

The crucial clue came from the direction of the gas motion. The absorption lines associated with the heavy elements were blueshifted relative to the galaxies’ overall redshift, meaning the light from those absorbing atoms was shifted toward shorter wavelengths compared with the galaxies themselves. In an expanding universe, such a velocity offset indicates that the metal-enriched gas was flowing outward, away from the galaxies and into intergalactic space. In other words, these infant galaxies were not merely factories producing heavy elements; they were distribution centers, dispersing their stellar products across their surroundings and potentially seeding other, younger galaxies with the chemical building blocks of future star formation.

Perhaps most striking is how familiar the chemical fingerprints of these ancient galaxies appeared. The researchers found that the elemental signatures closely resembled those of evolved galaxies billions of years later in cosmic time. Zhu offered an analogy to illustrate the process: think of these elements, which originated from the galaxies’ stars, as food dye dropped into a cup of water. The color begins to spread through the water, and in a similar fashion, heavy elements from early galaxies began to escape into space and enrich their surroundings. That enrichment, according to the study, was underway far earlier than astronomers had expected, meaning the chemical maturation of the universe proceeded at a remarkably brisk pace from the very start.

The discovery carries significant weight for the concept of baryon cycling, the process by which galaxies exchange material with their environments and with one another. Baryon cycling is one of the key reasons astronomers no longer regard galaxies as isolated island universes but as interconnected nodes within a larger galactic ecosystem. Material processed through one generation of stars can be expelled, mixed into the surrounding gas, and later recycled into new galaxies and new stars. Demonstrating that this cycling was already operating before the midpoint of cosmic reionization establishes that the galactic ecosystem was chemically connected almost from its inception, reshaping how models of early galaxy formation and evolution must be constructed.

The findings may also help resolve one of the most stubborn mysteries in modern astronomy: the elusiveness of Population III stars. These hypothetical objects are theorized to be the very first stars ever formed, born from pristine gas containing only hydrogen and helium before any heavier elements had been synthesized. Despite extensive searches, no Population III star has ever been conclusively observed. The new results offer a plausible explanation. If galaxies were already enriching their surroundings just 500 million years after the Big Bang, then truly pristine gas, and the first-generation stars that would have formed from it, may simply have existed for too brief a window to be detectable. As Zhu put it, if you start out with pure vanilla ice cream but begin mixing in sprinkles soon after, it will not be long until you can no longer find any pristine, plain vanilla ice cream. In the same way, the chemical pollution from early galaxies may have erased the observational signature of the universe’s very first stellar generation, closing a chapter of cosmic history almost before it opened.

Published on September 24, 2026, in Nature Astronomy under the title describing early metal-enriched baryon cycling before the midpoint of cosmic reionization, the study marks a milestone in humanity’s ability to probe the chemical state of the infant universe. It demonstrates that the era when astronomers imagined galaxies floating in virgin hydrogen and helium was, in reality, fleeting at best. Within the first half-billion years of cosmic time, stellar nucleosynthesis had already run its course in at least some galaxies, and powerful outflows had carried the products of those stellar furnaces beyond galactic boundaries. As Webb continues to survey the earliest epochs of the universe, astronomers anticipate that the picture of cosmic dawn will grow ever more detailed, and ever more surprising, revealing a young cosmos that was busier, more chemically mature, and more interconnected than anyone had dared to predict.

Subject of Research: Early metal-enriched baryon cycling during the Epoch of Reionization observed with the James Webb Space Telescope

Article Title: JWST finds early galaxies were already seeding the universe with heavy elements

Article References: JWST finds early galaxies were already seeding the universe with heavy elements. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: James Webb Space Telescope, early galaxies, Epoch of Reionization, heavy elements, baryon cycling, metal enrichment, Population III stars, supernovae, absorption spectroscopy, cosmic dawn, University of Arizona, Nature Astronomy