We present the discovery of SN 2025mkn, a gravitationally lensed Type II supernova. First detected as a blue transient in Zwicky Transient Facility (ZTF), 0."83 from a z = 0.42 elliptical galaxy, the follow-up SNIFS/UH2.2 m and LRIS/Keck spectra revealed absorption lines at z = 1.371. Later JWST NIRCam imaging shows that the bright transient is a close pair of point sources separated by ~ 0."07, and a 30 times fainter counterimage opposite the lens, for which NIRSpec reveals strong Hα emission also at z = 1.371. The lightcurves and spectra are consistent with the Type II supernova source being magnified ≳100 times, with ∼250 required to reconcile its luminosity with that of nearby events such as SN 2023ixf. Lens models are consistent with such high magnifications, and always show that the faint image arrived first (undetected in earlier ZTF imaging), consistent with the later spectral phase of this fainter image. A fourth image is also predicted and possibly detected in the NIRSpec data. Lightcurve-based time-delay measurements are not possible due to the first image being the faintest; however, the resolved NIRSpec spectra offer a future opportunity for time-delay cosmography through supernova phase measurements.

In many data analyses, each measurement may come with a simple yes/no correction – for example, belonging to one of two populations or being contaminated or not. Ignoring such binary effects may bias the results, while accounting for them explicitly quickly becomes infeasible as each of the N data points introduces an additional parameter, resulting in an exponentially growing number of possible configurations ((Formula presented) ). We show that, under generic conditions, an exact treatment of these binary corrections leads to a mathematical form identical to the well-known Ising model from statistical physics. This connection opens up a powerful set of tools developed for the Ising model, enabling fast and accurate likelihood calculations. We present efficient approximation schemes with minimal computational cost and demonstrate their effectiveness in applications, including Type Ia supernova calibration, where we show that the uncertainty in host-galaxy mass classification has negligible impact on the inferred value of the Hubble constant.

Cepheid-based determinations of the Hubble constant stand in significant tension with early-Universe inferences from the cosmic microwave background. While this tension is often discussed in terms of new physics or unmodelled systematics, the role of the assumed priors on the model parameters has received comparatively little attention. Recent work has pointed out that the commonly adopted flat prior on distance moduli upweights smaller distances and systematically favours high inferred values of the Hubble constant. Motivated by this observation, we perform a comprehensive Bayesian recalibration of the distance ladder, applying physically motivated priors uniformly to all distances, including the Milky Way Cepheids, which are incorporated directly into the joint fit. Together with a conservative treatment of the Gaia EDR3 residual parallax offset, the Hubble constant shifts from H₀ = 73.0±1.0 km s⁻¹ Mpc⁻¹ to ⁠H₀ = 70.6±1.0 km s⁻¹ Mpc⁻¹, reducing the Hubble tension from 5σ to 2σ⁠. Our results show that the assumed priors – often treated as innocuous defaults – can play a central role in the Hubble tension. Because all local distance ladders rely on the calibration of distances, similar prior-driven effects are expected to arise across distance-ladder methods.

We investigate whether the latest combination of the dark energy spectroscopic instrument (DESI) Year 3 data release (DR2) baryon acoustic oscillation measurements, cosmic microwave background (CMB) data (Planck 2018+ACT), and Type Ia supernovae (SNe Ia) compilations (Pantheon+, Union3, and DES Y5) favor a dynamical dark energy component, and explore if such a scenario can simultaneously help resolve the Hubble tension. We contrast two frameworks: the widely used phenomenological 𝑤₀⁢𝑤_𝑎 cold dark matter (CDM) model, and bimetric gravity—a fundamental modification of general relativity that naturally gives rise to phantom dark energy. The 𝑤_0⁢𝑤_𝑎⁢CDM model is moderately preferred over Λ⁢CDM, at the 2–4⁢𝜎 level, when fitting DESI DR2 + CMB+SNe Ia, but it exacerbates the Hubble tension. By comparison, bimetric gravity provides a modest improvement in fit quality, at the 1⁢𝜎 level, but, by inferring 𝐻₀ =69.0 ±0.4  km/s/Mpc, it partially eases the Hubble tension, from a 5⁢𝜎 discrepancy to a 3.7⁢𝜎 tension. Including locally calibrated SNe Ia brings the overall preference for the bimetric model over Λ⁢CDM to the 2⁢𝜎 level, comparable to that of the 𝑤₀⁢𝑤_𝑎⁢CDM model when including the local SN Ia calibration.

Context. Gravitationally lensed type Ia supernovae (glSNe Ia) are unique astronomical tools that can be used to study cosmological parameters, distributions of dark matter, the astrophysics of the supernovae, and the intervening lensing galaxies themselves. A small number of highly magnified glSNe Ia have been discovered by ground-based telescopes such as the Zwicky Transient Facility (ZTF), but simulations predict that a fainter, undetected population may also exist. Aims. We present a systematic search for glSNe Ia in the ZTF archive of alerts distributed from June 1 2019 to September 1 2022. Methods. Using the AMPEL platform, we developed a pipeline that distinguishes candidate glSNe Ia from other variable sources. Initial cuts were applied to the ZTF alert photometry (with constraints on the peak absolute magnitude and the distance to a catalogue-matched galaxy, as examples) before forced photometry was obtained for the remaining candidates. Additional cuts were applied to refine the candidates based on their light curve colours, lens galaxy colours, and the resulting parameters from fits to the SALT2 SN Ia template. The candidates were also cross-matched with the DESI spectroscopic catalogue. Results. Seven transients were identified that passed all the cuts and had an associated galaxy DESI redshift, which we present as glSN Ia candidates. Although superluminous supernovae (SLSNe) cannot be fully rejected as contaminants, two events, ZTF19abpjicm and ZTF22aahmovu, are significantly different from typical SLSNe and their light curves can be modelled as two-image glSN Ia systems. From this two-image modelling, we estimate time delays of 22 ± 3 and 34 ± 1 days for the two events, respectively, which suggests that we have uncovered a population of glSNe Ia with longer time delays. Conclusions. The pipeline is efficient and sensitive enough to parse full alert streams. It is currently being applied to the live ZTF alert stream to identify and follow-up future candidates while active. This pipeline could be the foundation for glSNe Ia searches in future surveys, such as the Rubin Observatory Legacy Survey of Space and Time.

We present the discovery of SN 2025wny (ZTF25abnjznp/GOTO25gqt) and spectroscopic classification of this event as the first gravitationally lensed Type I superluminous supernova (SLSN-I). Deep ground-based follow-up observations resolve four images of the supernova with ∼ 1 . ″ 7 angular separation from the main lens galaxy, each coincident with the lensed images of a background galaxy seen in archival imaging of the field. Spectroscopy of the brightest image shows narrow features matching absorption lines at a redshift of z = 2.010 and broad features matching those seen in superluminous SNe with far-UV coverage. We infer a magnification factor of μ ∼ 20-50 for the brightest image in the system, based on photometric and spectroscopic comparisons to other SLSNe-I. SN 2025wny demonstrates that gravitationally lensed SNe are in reach of ground-based facilities out to redshifts far higher than previously assumed, and provide a unique window into studying distant supernovae and the internal properties of dwarf galaxies, as well as for time-delay cosmography.

The Hubble constant (H0) is a key parameter in cosmology, yet its precise value remains contentious due to discrepancies between early- and late-universe measurement methods, a problem known as the ‘Hubble tension’. In this study, we revisit the Cepheid-based distance ladder calibration, focusing on two potential sources of bias in the period–luminosity relation (PLR): (1) the assumed prior for the residual parallax offset of the Milky Way Cepheids and (2) systematic differences between Cepheid periods in anchor galaxies versus supernova host galaxies. To address the latter, we adopt two different strategies alongside a renewed MW Cepheid calibration. The first strategy involves resampling anchor and host Cepheids from a common distribution of periods. This approach provides a conservative estimate of H0 = (72.18 ± 1.76) km s−1 Mpc−1, including the renewed MW analysis. The increased uncertainty reflects the reduced sample size – about 700 Cepheids per resampling compared to 3200 in the original data set. This method reduces the Hubble tension from 5.4 σ (as reported by the SH0ES collaboration with H0 = (73.17 ± 0.86) km s−1 Mpc−1) to 2.4 σ. The second strategy allows the PLR slope to vary with the period, yielding H0 = (72.35 ± 0.91) km s−1 Mpc−1, including the renewed MW analysis, and the tension reduced to 4.4 σ. A statistical comparison of the model with the single-linear PLR shows a significant preference for the broken PLR (p-value < 0.001). Both strategies consistently indicate a downward shift of approximately −1 km s−1 Mpc−1 in H0. Our findings underscore the importance of careful consideration of Cepheid population characteristics for precise H0 calibrations.

Understanding Type Ia supernovae (SNe Ia) and the empirical standardisation relations that make them excellent distance indicators is vital to improving cosmological constraints. SN Ia ‘siblings, i.e. two or more SNe Ia in the same host or parent galaxy, offer a unique way to infer the standardisation relations and their scatter across the population. We analysed a sample of 25 SN Ia pairs observed homogeneously by the Zwicky Transient Facility (ZTF) to infer the SNe Ia light curve width-luminosity and colour-luminosity parameters, α and β. Using the pairwise constraints from siblings, which allow for a scatter in the standardisation relations, we found α = 0.218 ± 0.055 and β = 3.084 ± 0.312, respectively, with a dispersion in α and β of ≤0.195 and ≤0.923, respectively, at a 95% confidence level. While the median dispersion is large, the values within ∼1σ are consistent with no dispersion. Hence, fitting for a single global standardisation relation, we found α = 0.228 ± 0.029 and β = 3.160 ± 0.191. We also found a very small intrinsic scatter of the siblings sample σ_int ≤ 0.10 mag at a 95% confidence level compared to σ_int = 0.22 ± 0.04 mag when computing the scatter using the Hubble residuals without comparing them as siblings. When comparing to large samples used in cosmological measurements, we found an α that is ∼2-3 σ higher, while the β values are consistent. The high α is driven by low x₁ pairs, potentially suggesting that the slow and fast declining SN Ia have different slopes for the width-luminosity relation. We found no difference in α and β when dividing the sample by host galaxy mass. The finding of a higher α with increased statistics can be confirmed or refuted through upcoming time-domain surveys. If confirmed, this finding can improve the cosmological inference from SNe Ia and be used to infer properties of the progenitors for subpopulations of SNe Ia.

Observations of galaxy-scale strong gravitational lensing systems enable unique tests of departures from general relativity at the kilo- to megaparsec scale. In this work, the gravitational slip parameter γ_PN, measuring the amplitude of a hypothetical fifth force, is constrained using 130 elliptical galaxy lens systems. We implement a lens model with a power-law total mass density and a deprojected De Vaucouleurs luminosity density, favored over a power-law luminosity density. To break the degeneracy between the lens velocity anisotropy β and the gravitational slip, we introduce a new prior on the velocity anisotropy based on recent dynamical data. For a constant gravitational slip, we find γ_PN= in agreement with general relativity at the 68% confidence level. Introducing a Compton wavelength λ_g, effectively screening the fifth force at small and large scales, the best fit is obtained for λ_g∼0.2  Mpc and γ_PN=. A local minimum is found at λ_g∼100  Mpc and γ_PN=. We conclude that there is no evidence in the data for a significant departure from general relativity and that using accurate assumptions and having good constraints on the lens galaxy model is key to ensure reliable constraints on the gravitational slip.

Recent surveys have discovered a population of faint supernovae, known as Ca-rich gap transients, inferred to originate from explosive ignitions of white dwarfs. In addition to their unique spectra and luminosities, these supernovae have an unusual spatial distribution and are predominantly found at large distances from their presumed host galaxies. We show that the locations of Ca-rich gap transients are well matched to the distribution of dwarf spheroidal galaxies surrounding large galaxies, in a scenario where dark matter interactions induce thermonuclear explosions among low-mass white dwarfs that may be otherwise difficult to ignite with standard stellar or binary evolution mechanisms. A plausible candidate to explain the observed event rate are primordial black holes with masses above 1021 grams.