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1. The Dark Energy Survey: Cosmology Results with ∼1500 New High-redshift Type Ia Supernovae Using the Full 5 yr Data Set

   https://doi.org/10.3847/2041-8213/ad6f9f  

   Abbott, DES_Collaboration_T_M_C; Acevedo, M.; Aguena, M.; Alarcon, A.; Allam, S.; Alves, O.; Amon, A.; Andrade-Oliveira, F.; Annis, J.; Armstrong, P.; et al (October 2024, The Astrophysical Journal Letters)

   Abstract We present cosmological constraints from the sample of Type Ia supernovae (SNe Ia) discovered and measured during the full 5 yr of the Dark Energy Survey (DES) SN program. In contrast to most previous cosmological samples, in which SNe are classified based on their spectra, we classify the DES SNe using a machine learning algorithm applied to their light curves in four photometric bands. Spectroscopic redshifts are acquired from a dedicated follow-up survey of the host galaxies. After accounting for the likelihood of each SN being an SN Ia, we find 1635 DES SNe in the redshift range 0.10 <z< 1.13 that pass quality selection criteria sufficient to constrain cosmological parameters. This quintuples the number of high-qualityz> 0.5 SNe compared to the previous leading compilation of Pantheon+ and results in the tightest cosmological constraints achieved by any SN data set to date. To derive cosmological constraints, we combine the DES SN data with a high-quality external low-redshift sample consisting of 194 SNe Ia spanning 0.025 <z< 0.10. Using SN data alone and including systematic uncertainties, we find Ω_M= 0.352 ± 0.017 in flat ΛCDM. SN data alone now require acceleration (q₀< 0 in ΛCDM) with over 5σconfidence. We find $({\mathrm{\Omega }}_{\mathrm{M}},w)=({0.264}_{-0.096}^{+0.074},-{0.80}_{-0.16}^{+0.14})$in flatwCDM. For flatw₀w_aCDM, we find $({\mathrm{\Omega }}_{\mathrm{M}},w_0,w_a)=({0.495}_{-0.043}^{+0.033},-{0.36}_{-0.30}^{+0.36},-{8.8}_{-4.5}^{+3.7})$, consistent with a constant equation of state to within ∼2σ. Including Planck cosmic microwave background, Sloan Digital Sky Survey baryon acoustic oscillation, and DES 3 × 2pt data gives (Ω_M,w) = (0.321 ± 0.007, −0.941 ± 0.026). In all cases, dark energy is consistent with a cosmological constant to within ∼2σ. Systematic errors on cosmological parameters are subdominant compared to statistical errors; these results thus pave the way for future photometrically classified SN analyses. 

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2. The Dark Energy Survey Supernova Program: Cosmological Analysis and Systematic Uncertainties

   https://doi.org/10.3847/1538-4357/ad5e6c  

   Vincenzi, M; Brout, D; Armstrong, P; Popovic, B; Taylor, G; Acevedo, M; Camilleri, R; Chen, R; Davis, T M; Lee, J; et al (October 2024, The Astrophysical Journal)

   Abstract We present the full Hubble diagram of photometrically classified Type Ia supernovae (SNe Ia) from the Dark Energy Survey supernova program (DES-SN). DES-SN discovered more than 20,000 SN candidates and obtained spectroscopic redshifts of 7000 host galaxies. Based on the light-curve quality, we select 1635 photometrically identified SNe Ia with spectroscopic redshift 0.10 <z< 1.13, which is the largest sample of supernovae from any single survey and increases the number of knownz> 0.5 supernovae by a factor of 5. In a companion paper, we present cosmological results of the DES-SN sample combined with 194 spectroscopically classified SNe Ia at low redshift as an anchor for cosmological fits. Here we present extensive modeling of this combined sample and validate the entire analysis pipeline used to derive distances. We show that the statistical and systematic uncertainties on cosmological parameters are ${\sigma }_{{\mathrm{\Omega }}_M,\mathrm{stat}+\mathrm{sys}}^{\mathrm{\Lambda }\mathrm{CDM}}=$0.017 in a flat ΛCDM model, and $({\sigma }_{{\mathrm{\Omega }}_M},{\sigma }_w{)}_{\mathrm{stat}+\mathrm{sys}}^{w\mathrm{CDM}}$= (0.082, 0.152) in a flatwCDM model. Combining the DES SN data with the highly complementary cosmic microwave background measurements by Planck Collaboration reduces by a factor of 4 uncertainties on cosmological parameters. In all cases, statistical uncertainties dominate over systematics. We show that uncertainties due to photometric classification make up less than 10% of the total systematic uncertainty budget. This result sets the stage for the next generation of SN cosmology surveys such as the Vera C. Rubin Observatory's Legacy Survey of Space and Time. 

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3. Constraining Cosmological Parameters Using the Cluster Mass–Richness Relation

   https://doi.org/10.3847/1538-4357/ace773  

   Abdullah, Mohamed H.; Wilson, Gillian; Klypin, Anatoly; Ishiyama, Tomoaki (September 2023, The Astrophysical Journal)

   Abstract The cluster mass–richness relation (MRR) is an observationally efficient and potentially powerful cosmological tool for constraining the matter density Ω_mand the amplitude of fluctuationsσ₈using the cluster abundance technique. We derive the MRR relation usingGalWCat19, a publicly available galaxy cluster catalog we created from the Sloan Digital Sky Survey-DR13 spectroscopic data set. In the MRR, cluster mass scales with richness as $\log M_{200}=\alpha +\beta \log N_{200}$. We find that the MRR we derive is consistent with both the IllustrisTNG and mini-Uchuu cosmological numerical simulations, with a slope ofβ≈ 1. We use the MRR we derived to estimate cluster masses from theGalWCat19catalog, which we then use to set constraints on Ω_mandσ₈. Utilizing the all-member MRR, we obtain constraints of Ω_m= ${0.31}_{-0.03}^{+0.04}$andσ₈= ${0.82}_{-0.04}^{+0.05}$, and utilizing the red member MRR only, we obtain Ω_m= ${0.31}_{-0.03}^{+0.04}$andσ₈= ${0.81}_{-0.04}^{+0.05}$. Our constraints on Ω_mandσ₈are consistent and very competitive with the Planck 2018 results. 

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4. Cosmography with the Double-source-plane Strong Gravitational Lens AGEL 150745+052256

   https://doi.org/10.3847/1538-4357/adf442  

   Sahu, Nandini; Shajib, Anowar J; Tran, Kim-Vy; Skobe, Hannah; Rhoades, Sunny; Jones, Tucker; Glazebrook, Karl; Collett, Thomas E; Suyu, Sherry H; Vasan_G_C, Keerthi; et al (September 2025, The Astrophysical Journal)

   Strong gravitational lenses with two background sources at widely separated redshifts are a promising independent probe of cosmological parameters. We can use these systems, known as double-source-plane lenses (DSPLs), to measure the ratio (β) of angular-diameter distances of the sources, which is sensitive to the matter density (Ω_m) and the equation-of-state parameter for dark-energy (w). However, DSPLs are rare and require high-resolution imaging and spectroscopy for detection, lens modeling, and measuringβ. Here, we report only the second DSPL ever used to measure cosmological parameters. We model the DSPLAGEL150745+052256 from the ASTRO 3D Galaxy Evolution with Lenses (AGEL) survey using Hubble Space Telescope/Wide-Field Camera 3 imaging and Keck Cosmic Web Imager spectroscopy. The spectroscopic redshifts for the deflector and two sources inAGEL1507 arez_defl= 0.594,z_S1 =  2.163, andz_S2= 2.591. We measure a stellar velocity dispersion ofσ_obs = 109 ± 27 km s⁻¹for the nearer source (S1). Usingσ_obsfor the main deflector (from literature) and S1, we test the robustness of our DSPL model. We measure $\beta =0.953_{-0.010}^{+0.008}$forAGEL1507 and infer Ω_m $=0.33_{-0.23}^{+0.38}$for ΛCDM cosmology. CombiningAGEL1507 with the published model of the Jackpot lens improves the precision on Ω_m(ΛCDM) andw(wCDM) by ∼10%. The inclusion of DSPLs significantly improves the constraints when combined with Planck’s cosmic microwave background observations, enhancing the precision onwby 30%. This paper demonstrates the potential constraining power of DSPLs and their complementarity to other standard cosmological probes. Tighter future constraints from larger DSPL samples discovered from ongoing and forthcoming large-area sky surveys would provide insights into the nature of dark energy. 

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5. Stellar Metallicities and Gradients in the Faint M31 Satellites Andromeda XVI and Andromeda XXVIII

   https://doi.org/10.3847/1538-4357/ad76a2  

   Fu, Sal_Wanying; Weisz, Daniel_R; Starkenburg, Else; Martin, Nicolas; Collins, Michelle_L_M; Savino, Alessandro; Boylan-Kolchin, Michael; Côté, Patrick; Dolphin, Andrew_E; Longeard, Nicolas; et al (October 2024, The Astrophysical Journal)

   Abstract We present ∼300 stellar metallicity measurements in two faint M31 dwarf galaxies, Andromeda XVI (M_V= −7.5) and Andromeda XXVIII (M_V= –8.8), derived using metallicity-sensitive calcium H and K narrowband Hubble Space Telescope imaging. These are the first individual stellar metallicities in And XVI (95 stars). Our And XXVIII sample (191 stars) is a factor of ∼15 increase over literature metallicities. For And XVI, we measure $〈\mathrm{[Fe/H]}〉=-{2.17}_{-0.05}^{+0.05}$, ${\sigma }_{\mathrm{[Fe/H]}}={0.33}_{-0.07}^{+0.07}$, and ∇_[Fe/H]= −0.23 ± 0.15 dex $R_e^{-1}$. We find that And XVI is more metal-rich than Milky Way ultrafaint dwarf galaxies of similar luminosity, which may be a result of its unusually extended star formation history. For And XXVIII, we measure $〈\mathrm{[Fe/H]}〉=-{1.95}_{-0.04}^{+0.04}$, ${\sigma }_{\mathrm{[Fe/H]}}={0.34}_{-0.05}^{+0.05}$, and ∇_[Fe/H]= −0.46 ± 0.10 dex $R_e^{-1}$, placing it on the dwarf galaxy mass–metallicity relation. Neither galaxy has a metallicity distribution function (MDF) with an abrupt metal-rich truncation, suggesting that star formation fell off gradually. The stellar metallicity gradient measurements are among the first for faint (L≲ 10⁶L_⊙) galaxies outside the Milky Way halo. Both galaxies’ gradients are consistent with predictions from the FIRE simulations, where an age–gradient strength relationship is the observational consequence of stellar feedback that produces dark matter cores. We include a catalog for community spectroscopic follow-up, including 19 extremely metal-poor ([Fe/H] < –3.0) star candidates, which make up 7% of And XVI’s MDF and 6% of And XXVIII’s. 

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