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1. Stepped Partially Acoustic Dark Matter, Large Scale Structure, and the Hubble Tension

   Buen-Abad, Manuel A.; Chacko, Zackaria; Kilic, Can; Marques-Tavares, Gustavo; Youn, Taewook (June 2023, The journal of high energy physics)

   We propose a new interacting dark sector model, Stepped Partially Acoustic Dark Matter (SPartAcous), that can simultaneously address the two most important tensions in current cosmological data, the H0 and S8 problems. As in the Partially Acoustic Dark Matter (PAcDM) scenario, this model features a subcomponent of dark matter that interacts with dark radiation at high temperatures, suppressing the growth of structure at small scales and thereby addressing the S8 problem. However, in the SPartAcous model, the dark radiation includes a component with a light mass that becomes non-relativistic close to the time of matter-radiation equality. As this light component annihilates away, the remaining dark radiation heats up and its interactions with dark matter decouple. The heating up of the dark sector results in a step-like increase in the relative energy density in dark radiation, significantly reducing the H0 tension, while the decoupling of dark matter and dark radiation ensures that the power spectrum at larger scales is identical to ΛCDM. 

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2. Stepped partially acoustic dark matter: likelihood analysis and cosmological tensions

   https://doi.org/10.1088/1475-7516/2023/11/005  

   Buen-Abad, Manuel A; Chacko, Zackaria; Kilic, Can; Marques-Tavares, Gustavo; Youn, Taewook (November 2023, Journal of Cosmology and Astroparticle Physics)

   Abstract We generalize the recently proposed Stepped Partially Acoustic Dark Matter (SPartAcous) model by including additional massless degrees of freedom in the dark radiation sector. We fit SPartAcous and its generalization against cosmological precision data from the cosmic microwave background, baryon acoustic oscillations, large-scale structure, supernovae type Ia, and Cepheid variables. We find that SPartAcous significantly reduces theH₀tension but does not provide any meaningful improvement of theS₈tension, while the generalized model succeeds in addressing both tensions, and provides a better fit than ΛCDM and other dark sector models proposed to address the same tensions. In the generalized model,H₀can be raised to 71.4 km/s/Mpc (the 95% upper limit), reducing the tension, if the fitted data does not include the direct measurement from the SH0ES collaboration, and to 73.7 km/s/Mpc (95% upper limit) if it does. A version ofCLASSthat has been modified to analyze this model is publicly available athttps://github.com/ManuelBuenAbad/class_spartacous. 

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3. Mirror twin Higgs cosmology: constraints and a possible resolution to the H0 and S8 tensions

   https://doi.org/10.1007/JHEP05(2022)050  

   Bansal, Saurabh; Kim, Jeong Han; Kolda, Christopher; Low, Matthew; Tsai, Yuhsin (May 2022, Journal of High Energy Physics)

   A bstract The mirror twin Higgs model (MTH) is a solution to the Higgs hierarchy problem that provides well-predicted cosmological signatures with only three extra parameters: the temperature of the twin sector, the abundance of twin baryons, and the vacuum expectation value (VEV) of twin electroweak symmetry breaking. These parameters specify the behavior of twin radiation and the acoustic oscillations of twin baryons, which lead to testable effects on the cosmic microwave background (CMB) and large-scale structure (LSS). While collider searches can only probe the twin VEV, through a fit to cosmological data we show that the existing CMB (Planck18 TTTEEE+lowE+lowT+lensing) and LSS (KV450) data already provide useful constraints on the remaining MTH parameters. Additionally, we show that the presence of twin radiation in this model can raise the Hubble constant H 0 while the scattering twin baryons can reduce the matter fluctuations S 8 , which helps to relax the observed H 0 and S 8 tensions simultaneously. This scenario is different from the typical ΛCDM + ∆ N eff model, in which extra radiation helps with the Hubble tension but worsens the S 8 tension. For instance, when including the SH0ES and 2013 Planck SZ data in the fit, we find that a universe with ≳ 20% of the dark matter comprised of twin baryons is preferred over ΛCDM by ∼ 4 σ . If the twin sector is indeed responsible for resolving the H 0 and S 8 tensions, future measurements from the Euclid satellite and CMB Stage 4 experiment will further measure the twin parameters to O (1 − 10%)-level precision. Our study demonstrates how models with hidden naturalness can potentially be probed using precision cosmological data. 

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4. Constraints from high-precision measurements of the cosmic microwave background: the case of disintegrating dark matter with Λ or dynamical dark energy

   https://doi.org/10.1088/1475-7516/2022/02/012  

   Liu, Wenzhong; Anchordoqui, Luis A.; Valentino, Eleonora Di; Pan, Supriya; Wu, Yabo; Yang, Weiqiang (February 2022, Journal of Cosmology and Astroparticle Physics)

   Abstract In recent years discrepancies have emerged in measurements of the present-day rate of expansion of the universe H 0 and in estimates of the clustering of matter S 8 . Using the most recent cosmological observations we reexamine a novel model proposed to address these tensions, in which cold dark matter disintegrates into dark radiation. The disintegration process is controlled by its rate Q  = αℋρ ddm , where α is a (constant) dimensionless parameter quantifying the strength of the disintegration mechanism and ℋ is the conformal Hubble rate in the spatially flat Friedmann-Lemaître-Robertson-Walker universe and ρ ddm is the energy density of the disintegrating cold dark matter. We constrain this model with the latest 2018 Planck temperature and polarization data, showing that there is no evidence for α≠ 0 and that it cannot solve the H 0 tension below 3σ, clashing with the result obtained by analyzing the Planck 2015 temperature data. We also investigate two possible extensions of the model in which the dark energy equation-of-state parameter w ≠ -1. In this case it is possible to combine Planck data with the SH0ES measurement, and we demonstrate that in both these models the H 0 tension is resolved at the 1σ level, but the condition w ≠ -1 exacerbates the S 8 tension. We also demonstrate that the addition of intermediate-redshift data (from the Pantheon supernova type Ia dataset and baryon acoustic oscillations) weakens the effectiveness of all these models to address the H 0 and S 8 tensions. 

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5. Stepped partially acoustic dark matter: likelihood analysis and cosmological tensions

   Buen-Abad, Manuel A; Chacko, Zackaria; Kilic, Can; Marques-Tavares, Gustavo; Youn, Taewook (November 2023, Journal of cosmology and astroparticle physics)

   We generalize the recently proposed Stepped Partially Acoustic Dark Matter (SPartAcous) model by including additional massless degrees of freedom in the dark radiation sector. We fit SPartAcous and its generalization against cosmological precision data from the cosmic microwave background, baryon acoustic oscillations, large-scale structure, supernovae type Ia, and Cepheid variables. We find that SPartAcous significantly reduces the H0 tension but does not provide any meaningful improvement of the S8 tension, while the generalized model succeeds in addressing both tensions, and provides a better fit than ΛCDM and other dark sector models proposed to address the same tensions. In the generalized model, H0 can be raised to 71.4 km/s/Mpc (the 95% upper limit), reducing the tension, if the fitted data does not include the direct measurement from the SH0ES collaboration, and to 73.7 km/s/Mpc (95% upper limit) if it does. A version of CLASS that has been modified to analyze this model is publicly available at https://github.com/ManuelBuenAbad/class_spartacous. 

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