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.

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.

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.

Fifth forces are ubiquitous in modified theories of gravity. In this paper, we analyze their effect on the Cepheid-calibrated cosmic distance ladder, specifically with respect to the inferred value of the Hubble constant (H₀). We consider a variety of effective models where the strength, or amount of screening, of the fifth force is estimated using proxy fields related to the large-scale structure of the Universe. To quantify the level of tension between the local distance ladder and the Planck value for H₀, we calculate the probability of obtaining a test result at least as extreme as the observed one, assuming that the model is correct (the p-value). For all models considered, the level of agreement is ≳20%, relieving the tension compared to the concordance model, exhibiting an agreement of only 1%. The alleviated discrepancy comes partially at the cost of an increased tension between distance estimates from Cepheids and the tip of the red-giant branch (TRGB). Demanding also that the consistency between Cepheid and TRGB distance estimates is not impaired, some fifth force models can still accommodate the data with a probability ≳20%. This provides incentive for more detailed investigations of fundamental theories on which the effective models are based and their effect on the Hubble tension.

Fifth forces are ubiquitous in modified theories of gravity. To be compatible with observations, such a force must be screened on Solar System scales but may still give a significant contribution on galactic scales. If this is the case, the fifth force can influence the calibration of the cosmic distance ladder, hence changing the inferred value of the Hubble constant H₀. In this paper, we analyze symmetron screening and show that it generally increases the Hubble tension. On the other hand, by doing a full statistical analysis, we show that cosmic distance ladder data are able to constrain the theory to a level competitive with Solar System tests—currently the most constraining tests of the theory. For the standard coupling case, the constraint on the symmetron Compton wavelength is λ_C≲2.5  Mpc. Thus, distance ladder data constitutes a novel and powerful way of testing this, and similar, types of theories.

We set out to rederive the 8 Hubble parameter values obtained from estimated relative galaxy ages by Simon et al. We find that to obtain the level of precision claimed in H(⁠z⁠), unrealistically small galaxy age uncertainties have to be assumed. Also, some parameter values will be correlated. In our analysis we find that the uncertainties in the Hubble parameter values are significantly larger when 8 independent H(⁠z⁠) are obtained using Monte Carlo sampling. Smaller uncertainties can be obtained using Gaussian processes, but at the cost of strongly correlated results. We do not obtain any useful constraints on the Hubble parameter from the galaxy data employed.

The nature of dark matter (DM) is a central question in cosmology today. While elementary particles could explain DM, compact astrophysical objects such as black holes formed in the early Universe offer a theoretically appealing alternate route. Here, we constrain the fraction of DM that can be made up of primordial black holes (PBHs) with masses M≳0.01M_⊙⁠, with Type Ia supernovae. Utilizing the Dyer–Roeder distance relation, we find a maximum fractional amount of DM in compact objects (f_p) of 0.50 at 95 per cent confidence level (C.L.), in the flat Lambda cold dark matter model and 0.49 when marginalizing over a constant dark energy equation of state or spatial curvature, demonstrating robustness to the cosmological model. With a prior on the homogeneity parameter, η, including values >1, we derive η = 1.08 ± 0.17, hence, f_p < 0.32 at 95 per cent C.L., showing that the prior assumption of η ≤ 1 gives a conservative upper limit on f_p. The Hubble constant we infer is consistent with the homogeneous case, showing that inhomogeneities like compact DM cannot account for the observed Hubble tension. In conclusion, we can exclude stellar masses PBHs as comprising all of the observed DM.

Detecting gravitationally lensed supernovae is among the biggest challenges in astronomy. It involves a combination of two very rare phenomena: catching the transient signal of a stellar explosion in a distant galaxy and observing it through a nearly perfectly aligned foreground galaxy that deflects light towards the observer. Here we describe how high-cadence optical observations with the Zwicky Transient Facility, with its unparalleled large field of view, led to the detection of a multiply imaged type Ia supernova, SN Zwicky, also known as SN 2022qmx. Magnified nearly 25-fold, the system was found thanks to the standard candle nature of type Ia supernovae. High-spatial-resolution imaging with the Keck telescope resolved four images of the supernova with very small angular separation, corresponding to an Einstein radius of only θ_E = 0.167″ and almost identical arrival times. The small θ_E and faintness of the lensing galaxy are very unusual, highlighting the importance of supernovae to fully characterize the properties of galaxy-scale gravitational lenses, including the impact of galaxy substructures.

A potential solution to the Hubble tension is the hypothesis that the Milky Way is located near the center of a matter underdensity. We model this scenario through the Lemaître-Tolman-Bondi formalism with the inclusion of a cosmological constant (ΛLTB) and consider a generalized Gaussian parametrization for the matter density profile. We constrain the underdensity and the background cosmology with a combination of data sets: the Pantheon Sample of type Ia supernovae (both the full catalogue and a redshift-binned version of it), a collection of baryon acoustic oscillations data points and the distance priors extracted from the latest Planck data release. The analysis with the binned supernovae suggests a preference for a -13 % density drop with a size of approximately 300 Mpc, interestingly matching the prediction for the so-called KBC void already identified on the basis of independent analyses using galaxy distributions. The constraints obtained with the full Pantheon Sample are instead compatible with a homogeneous cosmology and we interpret this radically different result as a cautionary tale about the potential bias introduced by employing a binned supernova data set. We quantify the level of improvement on the Hubble tension by analyzing the constraints on the B-band absolute magnitude of the supernovae, which provides the calibration for the local measurements of H₀. Since no significant difference is observed with respect to an analogous fit performed with a standard ΛCDM cosmology, we conclude that the potential presence of a local underdensity does not resolve the tension and does not significantly degrade current supernova constraints on H₀.