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1. The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics

   https://doi.org/10.1016/j.dark.2025.101965  

   Di_Valentino, Eleonora; Said, Jackson Levi; Riess, Adam; Pollo, Agnieszka; Poulin, Vivian; Gómez-Valent, Adrià; Weltman, Amanda; Palmese, Antonella; Huang, Caroline D; Bruck, Carsten_van de; et al (September 2025, Physics of the Dark Universe)

   The standard model of cosmology has provided a good phenomenological description of a wide range of observations both at astrophysical and cosmological scales for several decades. This concordance model is constructed by a universal cosmological constant and supported by a matter sector described by the standard model of particle physics and a cold dark matter contribution, as well as very early-time inflationary physics, and underpinned by gravitation through general relativity. There have always been open questions about the soundness of the foundations of the standard model. However, recent years have shown that there may also be questions from the observational sector with the emergence of differences between certain cosmological probes. In this White Paper, we identify the key objectives that need to be addressed over the coming decade together with the core science projects that aim to meet these challenges. These discordances primarily rest on the divergence in the measurement of core cosmological parameters with varying levels of statistical confidence. These possible statistical tensions may be partially accounted for by systematics in various measurements or cosmological probes but there is also a growing indication of potential new physics beyond the standard model. After reviewing the principal probes used in the measurement of cosmological parameters, as well as potential systematics, we discuss the most promising array of potential new physics that may be observable in upcoming surveys. We also discuss the growing set of novel data analysis approaches that go beyond traditional methods to test physical models. These new methods will become increasingly important in the coming years as the volume of survey data continues to increase, and as the degeneracy between predictions of different physical models grows. There are several perspectives on the divergences between the values of cosmological parameters, such as the model-independent probes in the late Universe and model-dependent measurements in the early Universe, which we cover at length. The White Paper closes with a number of recommendations for the community to focus on for the upcoming decade of observational cosmology, statistical data analysis, and fundamental physics developments 

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2. Probing gravity with the DES-CMASS sample and BOSS spectroscopy

   https://doi.org/10.1093/mnras/stab3129  

   Lee, S.; Huff, E. M.; Choi, A.; Elvin-Poole, J.; Hirata, C.; Honscheid, K.; MacCrann, N.; Ross, A. J.; Troxel, M. A.; Eifler, T. F.; et al (November 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The DES-CMASS sample (DMASS) is designed to optimally combine the weak lensing measurements from the Dark Energy Survey (DES) and redshift-space distortions (RSD) probed by the CMASS galaxy sample from the Baryonic Oscillation Spectroscopic Survey. In this paper, we demonstrate the feasibility of adopting DMASS as the equivalent of CMASS for a joint analysis of DES and BOSS in the framework of modified gravity. We utilize the angular clustering of the DMASS galaxies, cosmic shear of the DES metacalibration sources, and cross-correlation of the two as data vectors. By jointly fitting the combination of the data with the RSD measurements from the CMASS sample and Planck data, we obtain the constraints on modified gravity parameters $$\mu _0=-0.37^{+0.47}_{-0.45}$$ and $$\Sigma _0=0.078^{+0.078}_{-0.082}$$. Our constraints of modified gravity with DMASS are tighter than those with the DES Year 1 redMaGiC sample with the same external data sets by 29 per cent for μ0 and 21 per cent for Σ0, and comparable to the published results of the DES Year 1 modified gravity analysis despite this work using fewer external data sets. This improvement is mainly because the galaxy bias parameter is shared and more tightly constrained by both CMASS and DMASS, effectively breaking the degeneracy between the galaxy bias and other cosmological parameters. Such an approach to optimally combine photometric and spectroscopic surveys using a photometric sample equivalent to a spectroscopic sample can be applied to combining future surveys having a limited overlap such as DESI and LSST. 

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3. What is flat ΛCDM, and may we choose it?

   https://doi.org/10.1088/1475-7516/2023/02/049  

   Anselmi, Stefano; Carney, Matthew F.; Giblin, John T.; Kumar, Saurabh; Mertens, James B.; O'Dwyer, Marcio; Starkman, Glenn D.; Tian, Chi (February 2023, Journal of Cosmology and Astroparticle Physics)

   Abstract The Universe is neither homogeneous nor isotropic, but it is close enough that we can reasonably approximate it as such on suitably large scales.The inflationary-Λ-Cold Dark Matter (ΛCDM) concordance cosmology builds on these assumptions to describe the origin and evolution of fluctuations. With standard assumptions about stress-energy sources, this system is specified by just seven phenomenological parameters,whose precise relations to underlying fundamental theories are complicated and may depend on details of those fields.Nevertheless, it is common practice to set the parameter that characterizes the spatial curvature, Ω_K, exactly to zero.This parameter-fixed ΛCDM is awarded distinguished status as separate model, “flat ΛCDM.”Ipso factothis places the onus on proponents of “curved ΛCDM” to present sufficient evidence that Ω_K≠ 0, and is needed as a parameter.While certain inflationary model Lagrangians, with certain values of their parameters, and certain initial conditions, will lead to a present-day universe well-described as containing zero curvature, this does not justify distinguishing that subset of Lagrangians, parameters and initial conditions into a separate model.Absent any theoretical arguments, we cannot use observations that suggest small Ω_Kto enforce Ω_K= 0.Our track record in picking inflationary models and their parametersa priorimakes such a choice dubious, andconcerns about tensions in cosmological parameters and large-angle cosmic-microwave-background anomalies strengthens arguments against this choice.We argue that Ω_Kmust not be set to zero, and that ΛCDM remains a phenomenological model with at least 7 parameters. 

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4. High-accuracy emulators for observables in ΛCDM, N eff, Σ m ν, and w cosmologies

   https://doi.org/10.1093/mnras/stae1201  

   Bolliet, Boris; Spurio Mancini, Alessio; Hill, J_Colin; Madhavacheril, Mathew; Jense, Hidde_T; Calabrese, Erminia; Dunkley, Jo (May 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We use the emulation framework CosmoPower to construct and publicly release neural network emulators of cosmological observables, including the cosmic microwave background (CMB) temperature and polarization power spectra, matter power spectrum, distance-redshift relation, baryon acoustic oscillation (BAO) and redshift-space distortion (RSD) observables, and derived parameters. We train our emulators on Einstein–Boltzmann calculations obtained with high-precision numerical convergence settings, for a wide range of cosmological models including ΛCDM, wCDM, ΛCDM + Neff, and ΛCDM + Σmν. Our CMB emulators are accurate to better than 0.5 per cent out to ℓ = 104, which is sufficient for Stage-IV data analysis, and our P(k) emulators reach the same accuracy level out to $$k=50 \, \, \mathrm{Mpc}^{-1}$$, which is sufficient for Stage-III data analysis. We release the emulators via an online repository (CosmoPower Organisation), which will be continually updated with additional extended cosmological models. Our emulators accelerate cosmological data analysis by orders of magnitude, enabling cosmological parameter extraction analyses, using current survey data, to be performed on a laptop. We validate our emulators by comparing them to class and camb and by reproducing cosmological parameter constraints derived from Planck TT, TE, EE, and CMB lensing data, as well as from the Atacama Cosmology Telescope Data Release 4 CMB data, Dark Energy Survey Year-1 galaxy lensing and clustering data, and Baryon Oscillation Spectroscopic Survey Data Release 12 BAO and RSD data. 

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5. Galaxy bias in the era of LSST: perturbative bias expansions

   https://doi.org/10.1088/1475-7516/2024/02/015  

   Nicola, Andrina; Hadzhiyska, Boryana; Findlay, Nathan; García-García, Carlos; Alonso, David; Slosar, Anže; Guo, Zhiyuan; Kokron, Nickolas; Angulo, Raúl; Aviles, Alejandro; et al (February 2024, Journal of Cosmology and Astroparticle Physics)

   Upcoming imaging surveys will allow for high signal-to-noise measurements of galaxy clustering at small scales. In this work, we present the results of the Rubin Observatory Legacy Survey of Space and Time (LSST) bias challenge, the goal of which is to compare the performance of different nonlinear galaxy bias models in the context of LSST Year 10 (Y10) data. Specifically, we compare two perturbative approaches, Lagrangian perturbation theory (LPT) and Eulerian perturbation theory (EPT) to two variants of Hybrid Effective Field Theory (HEFT), with our fiducial implementation of these models including terms up to second order in the bias expansion as well as nonlocal bias and deviations from Poissonian stochasticity. We consider a variety of different simulated galaxy samples and test the performance of the bias models in a tomographic joint analysis of LSST-Y10-like galaxy clustering, galaxy-galaxy-lensing and cosmic shear. We find both HEFT methods as well as LPT and EPT combined with non-perturbative predictions for the matter power spectrum to yield unbiased constraints on cosmological parameters up to at least a maximal scale ofk_max = 0.4 Mpc^-1for all samples considered, even in the presence of assembly bias. While we find that we can reduce the complexity of the bias model for HEFT without compromising fit accuracy, this is not generally the case for the perturbative models. We find significant detections of non-Poissonian stochasticity in all cases considered, and our analysis shows evidence that small-scale galaxy clustering predominantly improves constraints on galaxy bias rather than cosmological parameters. These results therefore suggest that the systematic uncertainties associated with current nonlinear bias models are likely to be subdominant compared to other sources of error for tomographic analyses of upcoming photometric surveys, which bodes well for future galaxy clustering analyses using these high signal-to-noise data. 

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