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1. The Aemulus Project. VI. Emulation of Beyond-standard Galaxy Clustering Statistics to Improve Cosmological Constraints

   https://doi.org/10.3847/1538-4357/ad0ce8  

   Storey-Fisher, Kate; Tinker, Jeremy L; Zhai, Zhongxu; DeRose, Joseph; Wechsler, Risa H; Banerjee, Arka (January 2024, The Astrophysical Journal)

   Abstract There is untapped cosmological information in galaxy redshift surveys in the nonlinear regime. In this work, we use theAemulussuite of cosmologicalN-body simulations to construct Gaussian process emulators of galaxy clustering statistics at small scales (0.1–50h⁻¹Mpc) in order to constrain cosmological and galaxy bias parameters. In addition to standard statistics—the projected correlation functionw_p(r_p), the redshift-space monopole of the correlation functionξ₀(s), and the quadrupoleξ₂(s)—we emulate statistics that include information about the local environment, namely the underdensity probability functionP_U(s) and the density-marked correlation functionM(s). This extends the model ofAemulusIII for redshift-space distortions by including new statistics sensitive to galaxy assembly bias. In recovery tests, we find that the beyond-standard statistics significantly increase the constraining power on cosmological parameters of interest: includingP_U(s) andM(s) improves the precision of our constraints on Ω_mby 27%,σ₈by 19%, and the growth of structure parameter,fσ₈, by 12% compared to standard statistics. We additionally find that scales below ∼6h⁻¹Mpc contain as much information as larger scales. The density-sensitive statistics also contribute to constraining halo occupation distribution parameters and a flexible environment-dependent assembly bias model, which is important for extracting the small-scale cosmological information as well as understanding the galaxy–halo connection. This analysis demonstrates the potential of emulating beyond-standard clustering statistics at small scales to constrain the growth of structure as a test of cosmic acceleration. 

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2. The quality/cosmology tension for a post-inflation QCD axion

   https://doi.org/10.1007/JHEP07(2024)227  

   Lu, Qianshu; Reece, Matthew; Sun, Zhiquan (July 2024, Journal of High Energy Physics)

   A<sc>bstract</sc> It is difficult to construct a post-inflation QCD axion model that solves the axion quality problem (and hence the Strong CP problem) without introducing a cosmological disaster. In a post-inflation axion model, the axion field value is randomized during the Peccei-Quinn phase transition, and axion domain walls form at the QCD phase transition. We emphasize that the gauge equivalence of all minima of the axion potential (i.e., domain wall number equals one) is insufficient to solve the cosmological domain wall problem. The axion string on which a domain wall ends must exist as an individual object (as opposed to a multi-string state), and it must be produced in the early universe. These conditions are often not satisfied in concrete models. Post-inflation axion models also face a potential problem from fractionally charged relics; solving this problem often leads to low-energy Landau poles for Standard Model gauge couplings, reintroducing the quality problem. We study several examples, finding that models that solve the quality problem face cosmological problems, and vice versa. This is not a no-go theorem; nonetheless, we argue that it is much more difficult than generally appreciated to find a viable post-inflation QCD axion model. Successful examples may have a nonstandard cosmological history (e.g., multiple types of cosmic axion strings of different tensions), undermining the widespread expectation that the post-inflation QCD axion scenario predicts a unique mass for axion dark matter. 

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3. Catching the Nebular Needle in a Polluted Haystack: Line-emission Signatures from Population III-forming Pockets around Massive Galaxies at the End of Reionization

   https://doi.org/10.3847/1538-4357/ae7b2c  

   Venditti, Alessandra; Graziani, Luca; Schneider, Raffaella; Bromm, Volker; Muñoz, Julian B; Di_Cesare, Claudia; Valiante, Rosa; Calabrò, Antonello; Maiolino, Roberto; Finkelstein, Steven L; et al (July 2026, The Astrophysical Journal)

   Abstract Finding the first generation of (Population III or Pop III) stars is one of the most ambitious and exciting challenges of astrophysics. JWST opened concrete prospects for their detection during the Epoch of Reionization, where increasing evidence suggests that residual Pop III formation may persist, even within pristine pockets of high-mass halos, due to inhomogeneous enrichment. However, the identification of Pop III stars within globally enriched environments will be challenging. We investigate the detectability of a subdominant Pop III component in/around massive (M_⋆ ≳ 10⁹M_⊙) galaxies atz ≈ 6.5–9 from thedustyGadgetcosmological simulation suite, and the confusion arising from second-generation (Pop II) stars in their surroundings. We find that young (≲1 Myr), massive (M_III ∼ 6 × 10⁵M_⊙) Pop III clusters forming within these galaxy environments are responsible for strong HeII1640 line emission (L_HeII1640 ≳ 10⁴¹erg s⁻¹), which would be detectable with ≈10(50) hr of medium-resolution observations with NIRSpec/IFU atz ≈ 6(10). These bright luminosities cannot be produced by standard Pop II populations alone. On the other hand, the dominant Pop II component within massive “hybrid” Pop III hosts powers strong metal line emission (L_[OIII]5007 ≳ 10⁴²erg s⁻¹), indicating that the detection of metal lines alone cannot exclude the presence of Pop IIIs in high-zgalaxy environments. We further discuss candidate selection strategies based on Lyα, Hα,and Hβemission, and how spatially resolved observations may enable the detection of isolated, pristine pockets in the outskirts of massive halos. 

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4. The ASTRID Simulation at z = 0: From Massive Black Holes to Large-scale Structure

   https://doi.org/10.3847/1538-4357/ae3c08  

   Zhou_周, Yihao 亦豪; Di_Matteo, Tiziana; Bird, Simeon; Croft, Rupert; Ni, Yueying; Yang, Yanhui; Chen, Nianyi; Lachance, Patrick; Zhang, Xiaowen; Hafezianzadeh, Fatemeh (February 2026, The Astrophysical Journal)

   Abstract We present thez= 0 results for the cosmological simulationASTRID. Hosting 2 × 5500³≈ 0.33 trillion particles in a box of 370 Mpc per side,ASTRIDis one of the largest cosmological hydrodynamic simulations evolved toz= 0.ASTRIDfeatures a large population of massive black holes (MBHs), covering a wide mass range 4 × 10⁴∼ 2 × 10¹¹M_⊙. The adopted dynamical friction model provides a relatively accurate description of MBH dynamics, makingASTRIDa powerful tool to study MBH growth and mergers in a cosmological context.ASTRIDsuccessfully captures the coevolution of MBHs and their host galaxies, producingM_BH–M_⋆andM_BH–σrelations in good agreement with observations. Notably,ASTRIDgenerates scatter in these relations that is more consistent with observations than previous simulations, indicating a more realistic MBH diversity. The galaxy stellar mass function atz= 0 is generally consistent with observational constraints. When dust attenuation is applied, the galaxy luminosity function also agrees well with observations, and the bimodality in galaxy colors is reproduced as well.ASTRIDhosts a large population of massive galaxy groups and clusters: seven halos haveM_200c> 10¹⁵M_⊙, and 9709 halos haveM_200c> 10¹³M_⊙. We quantify the stellar mass content in these halos, and find that the correlations between the stellar and halo mass match well with observational constraints. Finally, we present thez= 0 power spectra of MBH and galaxies, as well as their bias with respect to the matter power spectrum. We find that MBHs withM_BH≥ 10⁸M_⊙and galaxies withM_⋆≥ 10^10.5M_⊙serve as good tracers of large-scale structure. 

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5. A closer look in the mirror: reflections on the matter/dark matter coincidence

   https://doi.org/10.1007/JHEP06(2024)052  

   Bodas, Arushi; Buen-Abad, Manuel A; Hook, Anson; Sundrum, Raman (June 2024, Journal of High Energy Physics)

   A<sc>bstract</sc> We argue that the striking similarity between the cosmic abundances of baryons and dark matter, despite their very different astrophysical behavior, strongly motivates the scenario in which dark matter resides within a rich dark sector parallel in structure to that of the standard model. The near cosmic coincidence is then explained by an approximateℤ₂exchange symmetry between the two sectors, where dark matter consists of stable dark neutrons, with matter and dark matter asymmetries arising via parallel WIMP baryogenesis mechanisms. Taking a top-down perspective, we point out that an adequateℤ₂symmetry necessitates solving the electroweak hierarchy problem in each sector, without our committing to a specific implementation. A higher-dimensional realization in the far UV is presented, in which the hierarchical couplings of the two sectors and the requisiteℤ₂-breaking structure arise naturally from extra-dimensional localization and gauge symmetries. We trace the cosmic history, paying attention to potential pitfalls not fully considered in previous literature. Residualℤ₂-breaking can very plausibly give rise to the asymmetric reheating of the two sectors, needed to keep the cosmological abundance of relativistic dark particles below tight bounds. We show that, despite the need to keep inter-sector couplings highly suppressed after asymmetric reheating, there can naturally be order-one couplings mediated by TeV scale particles which can allow experimental probes of the dark sector at high energy colliders. Massive mediators can also induce dark matter direct detection signals, but likely at or below the neutrino floor. 

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