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New research could force a fundamental rethink of the nature of space and time.

Nature, Published online: 19 March 2025; doi:10.1038/d41586-025-00837-2

Physicists had long assumed that the elusive force has constant strength. But the latest results from a project to map the Universe’s expansion challenge this idea.

Our current best theories of the universe suggest that dark energy is making it expand faster and faster, but new observations from the Dark Energy Spectroscopic Instrument suggest this mysterious force is actually growing weaker

Title to be confirmed

Abstract not available

• Speaker: Piero Madau (UCSC)
• Friday 30 May 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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arXiv:2503.11752v1 Announce Type: new Abstract: JWST spectroscopy has revealed a population of compact objects at redshifts $z=2$-9 with `v'-shaped spectral energy distributions, broad permitted lines, and, often, hydrogen Balmer absorption. Among these `Little Red Dots' (LRDs), Abell2744-QSO1 at $z=7.04$ has been confirmed to have time-variable equivalent width (EW) in its broad emission lines, confirming its AGN nature. We extend the analysis of NIRSpec/IFS data from the BlackTHUNDER survey to the H$\alpha$ line. The broad-line profile in Abell2744-QSO1 is manifestly non-Gaussian, requiring at least two Gaussian components with full width at half maximum FWHM=$450\pm50$ and $1800\pm100$ km s$^{-1}$. Crucially, we also detect a narrow-line Gaussian component, and strong H$\alpha$ absorption (EW relative to the continuum $\approx 30^{+15}_{-9}$ A), confirming a connection between the strong Balmer break and line absorption. The absorber is at rest with respect to broad H$\alpha$, suggesting that the gas cannot be interpreted as an inflow or outflow, forming instead a long-lived structure. Its velocity dispersion is $\sigma_{abs} = 100\pm10$ km s$^{-1}$, consistent with the value inferred from the analysis of the Balmer break. Based on H$\alpha$, we infer a black hole mass of log(M$_{BH}$/M$_\odot$)=6.3-6.7, 0.9-1.3 dex smaller than previous estimates based on H$\beta$. The Eddington ratio is 0.7-1.6. Combining the high signal-to-noise ratio of the narrow H$\alpha$ line with the spectral resolution R=3,700 of the G395H grating, we infer a narrow-line dispersion $\sigma_n = 22^{+5}_{-6}$ km s$^{-1}$, which places a stringent constraint on the black-hole-to-dynamical-mass ratio of this system to be M$_{BH}$/M$_{dyn}$>0.02-0.4. If M$_{BH}$ is near the low-mass end of our estimates, the SMBH would be accreting at a super-Eddington rate. Alternatively, at the high-M$_{BH}$ end, there would be minimal room for a host galaxy.

arXiv:2503.11740v1 Announce Type: new Abstract: We present and analyse the results of the Science data challenge 3a (SDC3a, https://sdc3.skao.int/challenges/foregrounds), an EoR foreground-removal community-wide exercise organised by the Square Kilometre Array Observatory (SKAO). The challenge ran for 8 months, from March to October 2023. Participants were provided with realistic simulations of SKA-Low data between 106 MHz and 196 MHz, including foreground contamination from extragalactic as well as Galactic emission, instrumental and systematic effects. They were asked to deliver cylindrical power spectra of the EoR signal, cleaned from all corruptions, and the corresponding confidence levels. Here we describe the approaches taken by the 17 teams that completed the challenge, and we assess their performance using different metrics. The challenge results provide a positive outlook on the capabilities of current foreground-mitigation approaches to recover the faint EoR signal from SKA-Low observations. The median error committed in the EoR power spectrum recovery is below the true signal for seven teams, although in some cases there are some significant outliers. The smallest residual overall is $4.2_{-4.2}^{+20} \times 10^{-4}\,\rm{K}^2h^{-3}$cMpc$^{3}$ across all considered scales and frequencies. The estimation of confidence levels provided by the teams is overall less accurate, with the true error being typically under-estimated, sometimes very significantly. The most accurate error bars account for $60 \pm 20$\% of the true errors committed. The challenge results provide a means for all teams to understand and improve their performance. This challenge indicates that the comparison between independent pipelines could be a powerful tool to assess residual biases and improve error estimation.

arXiv:2503.14454v1 Announce Type: new Abstract: We use new cosmic microwave background (CMB) primary temperature and polarization anisotropy measurements from the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) to test foundational assumptions of the standard cosmological model and set constraints on extensions to it. We derive constraints from the ACT DR6 power spectra alone, as well as in combination with legacy data from Planck. To break geometric degeneracies, we include ACT and Planck CMB lensing data and baryon acoustic oscillation data from DESI Year-1, and further add supernovae measurements from Pantheon+ for models that affect the late-time expansion history. We verify the near-scale-invariance (running of the spectral index $d n_s/d\ln k = 0.0062 \pm 0.0052$) and adiabaticity of the primordial perturbations. Neutrino properties are consistent with Standard Model predictions: we find no evidence for new light, relativistic species that are free-streaming ($N_{\rm eff} = 2.86 \pm 0.13$, which combined with external BBN data becomes $N_{\rm eff} = 2.89 \pm 0.11$), for non-zero neutrino masses ($\sum m_\nu < 0.082$ eV at 95% CL), or for neutrino self-interactions. We also find no evidence for self-interacting dark radiation ($N_{\rm idr} < 0.134$), early-universe variation of fundamental constants, early dark energy, primordial magnetic fields, or modified recombination. Our data are consistent with standard BBN, the FIRAS-inferred CMB temperature, a dark matter component that is collisionless and with only a small fraction allowed as axion-like particles, a cosmological constant, and the late-time growth rate predicted by general relativity. We find no statistically significant preference for a departure from the baseline $\Lambda$CDM model. In general, models introduced to increase the Hubble constant or to decrease the amplitude of density fluctuations inferred from the primary CMB are not favored by our data.

String theory may be our best attempt at a theory of everything, except that it can't describe an expanding universe like ours. Now a radical new twist on the idea could finally fix that – but it requires us to completely reimagine reality

6 min read

Preparations for Next Moonwalk Simulations Underway (and Underwater) This image shows about 1.5% of Euclid’s Deep Field South, one of three regions of the sky that the telescope will observe for more than 40 weeks over the course of its prime mission, spotting faint and distant galaxies. One galaxy cluster near the center is located almost 6 billion light-years away from Earth. ESA/Euclid/Euclid Consortium/NASA; image processing by J.-C. Cuillandre, E. Bertin, G. An-selmi

With contributions from NASA, the mission is looking back into the universe’s history to understand how the universe’s expansion has changed. 

The Euclid mission — led by ESA (European Space Agency) with contributions from NASA — aims to find out why our universe is expanding at an accelerating rate. Astronomers use the term “dark energy” to refer to the unknown cause of this phenomenon, and Euclid will take images of billions of galaxies to learn more about it. A portion of the mission’s data was released to the public by ESA released on Wednesday, March 19.

This new data has been analyzed by mission scientists and provides a glimpse of Euclid’s progress. Deemed a “quick” data release, this batch focuses on select areas of the sky to demonstrate what can be expected in the larger data releases to come and to allow scientists to sharpen their data analysis tools in preparation.

The data release contains observations of Euclid’s three “deep fields,” or areas of the sky where the space telescope will eventually make its farthest observations of the universe. Featuring one week’s worth of viewing, the Euclid images contain 26 million galaxies, the most distant being over 10.5 billion light-years away. Launched in July 2023, the space telescope is expected to observe more than 1.5 billion galaxies during its six-year prime mission.

The entirety of the Euclid mission’s Deep Field South region is shown here. It is about 28.1 square degrees on the sky. Euclid will observe this and two other deep field regions for a total of about 40 weeks during its 6-year primary mission. ESA/Euclid/Euclid Consortium/NASA; image processing by J.-C. Cuillandre, E. Bertin, G. An-selmi

By the end of that prime mission, Euclid will have observed the deep fields for a total of about 40 weeks in order to gradually collect more light, revealing fainter and more distant galaxies. This approach is akin to keeping a camera shutter open to photograph a subject in low light.

The first deep field observations, taken by NASA’s Hubble Space Telescope in 1995, famously revealed the existence of many more galaxies in the universe than expected. Euclid’s ultimate goal is not to discover new galaxies but to use observations of them to investigate how dark energy’s influence has changed over the course of the universe’s history.

In particular, scientists want to know how much the rate of expansion has increased or slowed down over time. Whatever the answer, that information would provide new clues about the fundamental nature of this phenomenon. NASA’s Nancy Grace Roman Space Telescope, set to launch by 2027, will also observe large sections of the sky in order to study dark energy, complementing Euclid’s observations.

The location of the Euclid deep fields are shown marked in yellow on this all-sky view from ESA’s Gaia and Planck missions. The bright horizontal band is the plane of our Milky Way galaxy. Euclid’s Deep Field South is at bottom left.ESA/Euclid/Euclid Consortium/NASA; ESA/Gaia/DPAC; ESA/Planck Collaboration Looking Back in Time

To study dark energy’s effect throughout cosmic history, astronomers will use Euclid to create detailed, 3D maps of all the stuff in the universe. With those maps, they want to measure how quickly dark energy is causing galaxies and big clumps of matter to move away from one another. They also want to measure that rate of expansion at different points in the past. This is possible because light from distant objects takes time to travel across space. When astronomers look at distant galaxies, they see what those objects looked like in the past.

For example, an object 100 light-years away looks the way it did 100 years ago. It’s like receiving a letter that took 100 years to be delivered and thus contains information from when it was written. By creating a map of objects at a range of distances, scientists can see how the universe has changed over time, including how dark energy’s influence may have varied.

But stars, galaxies, and all the “normal” matter that emits and reflects light is only about one-fifth of all the matter in the universe. The rest is called “dark matter” — a material that neither emits nor reflects light. To measure dark energy’s influence on the universe, astronomers need to include dark matter in their maps.  

Bending and Warping

Although dark matter is invisible, its influence can be measured through something called gravitational lensing. The mass of both normal and dark matter creates curves in space, and light traveling toward Earth bends or warps as it encounters those curves. In fact, the light from a distant galaxy can bend so much that it forms an arc, a full circle (called an Einstein ring), or even multiple images of the same galaxy, almost as though the light has passed through a glass lens.

In most cases, gravitational lensing warps the apparent shape of a galaxy so subtly that researchers need special tools and computer software to see it. Spotting those subtle changes across billions of galaxies enables scientists to do two things: create a detailed map of the presence of dark matter and observe how dark energy influenced it over cosmic history.

It is only with a very large sample of galaxies that researchers can be confident they are seeing the effects of dark matter. The newly released Euclid data covers 63 square degrees of the sky, an area equivalent to an array of 300 full Moons. To date, Euclid has observed about 2,000 square degrees, which is approximately 14% of its total survey area of 14,000 square degrees. By the end of its mission, Euclid will have observed a third of the entire sky.

The dataset released this month is described in several preprint papers available today. The mission’s first cosmology data will be released in October 2026. Data accumulated over additional, multiple passes of the deep field locations will also be included in the 2026 release.

More About Euclid

Euclid is a European mission, built and operated by ESA, with contributions from NASA. The Euclid Consortium — consisting of more than 2,000 scientists from 300 institutes in 15 European countries, the United States, Canada, and Japan — is responsible for providing the scientific instruments and scientific data analysis. ESA selected Thales Alenia Space as prime contractor for the construction of the satellite and its service module, with Airbus Defence and Space chosen to develop the payload module, including the telescope. Euclid is a medium-class mission in ESA’s Cosmic Vision Programme.

Three NASA-supported science teams contribute to the Euclid mission. In addition to designing and fabricating the sensor-chip electronics for Euclid’s Near Infrared Spectrometer and Photometer (NISP) instrument, JPL led the procurement and delivery of the NISP detectors as well. Those detectors, along with the sensor chip electronics, were tested at NASA’s Detector Characterization Lab at Goddard Space Flight Center in Greenbelt, Maryland. The Euclid NASA Science Center at IPAC (ENSCI), at Caltech in Pasadena, California, supports U.S.-based science investigations, and science data is archived at the NASA / IPAC Infrared Science Archive (IRSA). JPL is a division of Caltech.

For more information about Euclid go to:

science.nasa.gov/mission/euclid/

News Media Contact

ESA Media Relations
media@esa.int

Calla Cofield
Jet Propulsion Laboratory, Pasadena, Calif.
626-808-2469
calla.e.cofield@jpl.nasa.gov

2025-039

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Details Last Updated Mar 19, 2025 Related Terms

• Euclid
• Galaxies, Stars, & Black Holes
• Jet Propulsion Laboratory
• Stars

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arXiv:2412.15100v2 Announce Type: replace Abstract: Model misspecification analysis strategies, such as anomaly detection, model validation, and model comparison are a key component of scientific model development. Over the last few years, there has been a rapid rise in the use of simulation-based inference (SBI) techniques for Bayesian parameter estimation, applied to increasingly complex forward models. To move towards fully simulation-based analysis pipelines, however, there is an urgent need for a comprehensive simulation-based framework for model misspecification analysis. In this work, we provide a solid and flexible foundation for a wide range of model discrepancy analysis tasks, using distortion-driven model misspecification tests. From a theoretical perspective, we introduce the statistical framework built around performing many hypothesis tests for distortions of the simulation model. We also make explicit analytic connections to classical techniques: anomaly detection, model validation, and goodness-of-fit residual analysis. Furthermore, we introduce an efficient self-calibrating training algorithm that is useful for practitioners. We demonstrate the performance of the framework in multiple scenarios, making the connection to classical results where they are valid. Finally, we show how to conduct such a distortion-driven model misspecification test for real gravitational wave data, specifically on the event GW150914.

arXiv:2503.10751v1 Announce Type: new Abstract: We re-analysed ALMA observations of the [OIII]$\lambda$88$\mu$m emission line in JADES-GS-z14.0, so far the most distant spectroscopically confirmed galaxy at z=14.18. Our analysis shows a tentative detection of a velocity gradient of [OIII]$\lambda$88$\mu$m using three independent tests: 1) construction of moment maps; 2) extraction of integrated spectra from a grid of apertures; and 3) spectro-astrometry in both the image and uv planes. We performed kinematical fitting using the KinMS code and estimated a dynamical mass of log$_{10}$(M$_{\rm dyn}$/$\rm M_\odot$)= 9.4$^{+0.8}_{-0.4}$, with the bulk of the uncertainties due to the degeneracy between dynamical mass and inclination. We measure an upper limit on the velocity dispersion ($\sigma_{v}$) of $<40~$ km/s~which results in an estimate of V$_{\rm rot}/\sigma>$ 2.5. This result, if confirmed with higher-resolution observations, would imply that kinematically cold discs are already in place at $z\sim14$. Comparison with mock observations from the SERRA cosmological simulations confirms that even low-resolution observations are capable of detecting a velocity gradient in $z>10$ galaxies as compact as JADES-GS-z14.0. This work shows that deeper ALMA or JWST/NIRSpec IFS observations with high spatial resolution will be able to estimate an accurate dynamical mass for JADES-GS-z14.0, providing an upper limit to the stellar mass of this over-luminous galaxy.

arXiv:2503.09759v1 Announce Type: new Abstract: The next generation of galaxy surveys will provide highly precise measurements of galaxy clustering, therefore requiring a corresponding accuracy. Current approaches, which rely on approximations and idealized assumptions, may fall short in capturing the level of detail required for high-precision observations. In order to increase the modeling accuracy, recently, unequal-time contributions to the galaxy power spectrum have been introduced in order to include the effects of radial correlations. We present a generalization of the formalism for the observed unequal-time power spectrum, that includes Doppler and local general relativistic corrections, plus local primordial non-Gaussianity. We find that unequal time corrections can potentially mimic an effective $f_{\mathrm{NL}}$ of order unity. We provide a first assessment of the significance of unequal-time corrections for future galaxy clustering experiments, estimating a Signal-to-Noise-Ratio of $\sim3$ for Stage IV-like surveys.

arXiv:2412.07765v2 Announce Type: replace Abstract: Cosmic shear, galaxy clustering, and the abundance of massive halos each probe the large-scale structure of the Universe in complementary ways. We present cosmological constraints from the joint analysis of the three probes, building on the latest analyses of the lensing-informed abundance of clusters identified by the South Pole Telescope (SPT) and of the auto- and cross-correlation of galaxy position and weak lensing measurements (3$\times$2pt) in the Dark Energy Survey (DES). We consider the cosmological correlation between the different tracers and we account for the systematic uncertainties that are shared between the large-scale lensing correlation functions and the small-scale lensing-based cluster mass calibration. Marginalized over the remaining $\Lambda$ cold dark matter ($\Lambda$CDM) parameters (including the sum of neutrino masses) and 52 astrophysical modeling parameters, we measure $\Omega_\mathrm{m}=0.300\pm0.017$ and $\sigma_8=0.797\pm0.026$. Compared to constraints from Planck primary cosmic microwave background (CMB) anisotropies, our constraints are only 15% wider with a probability to exceed of 0.22 ($1.2\sigma$) for the two-parameter difference. We further obtain $S_8\equiv\sigma_8(\Omega_\mathrm{m}/0.3)^{0.5}=0.796\pm0.013$ which is lower than the Planck measurement at the $1.6\sigma$ level. The combined SPT cluster, DES 3$\times$2pt, and Planck datasets mildly prefer a nonzero positive neutrino mass, with a 95% upper limit $\sum m_\nu<0.25~\mathrm{eV}$ on the sum of neutrino masses. Assuming a $w$CDM model, we constrain the dark energy equation of state parameter $w=-1.15^{+0.23}_{-0.17}$ and when combining with Planck primary CMB anisotropies, we recover $w=-1.20^{+0.15}_{-0.09}$, a $1.7\sigma$ difference with a cosmological constant. The precision of our results highlights the benefits of multiwavelength multiprobe cosmology.

arXiv:2411.12021v3 Announce Type: replace Abstract: We present the measurements and cosmological implications of the galaxy two-point clustering using over 4.7 million unique galaxy and quasar redshifts in the range $0.1

Title TBC

Abstract TBC

• Speaker: Alicia Anderson (Cavendish Astrophysics)
• Tuesday 13 May 2025, 11:15-12:00
• Venue: Martin Ryle Seminar Room, Kavli Institute.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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arXiv:2503.08658v1 Announce Type: new Abstract: Recent cosmological surveys have provided unprecedented datasets that can be used to reconstruct the history of the dark energy equation of state. In this work, a free-form "flexknot'' parameterisation is employed to represent $w(a)$ as a linear spline between free-moving nodes, the number of which may vary. By combining DESI Baryon Acoustic Oscillation measurements with Pantheon+ or DES5Y supernovae, the functional posteriors of $w(a)$ reveal an unexpected W-shaped structure. While the Bayesian evidence may still favour $\Lambda$CDM, the robustness of these results suggests the structure is indeed present in the data. The tension $R$-statistic and suspiciousness have been marginalised over models, and demonstrate that while the reconstructions from DESI and Pantheon+ agree, DESI and DES5Y do not. We conclude that, while there is no smoking gun for dynamical dark energy, the structure unearthed in this work is generally too complex to be captured by the restrictive $w$CDM or CPL parameterisations.