A Step in Understanding the Hubble Tension

Abstract

As cosmological data have improved, tensions have arisen. One such tension is the difference between the locally measured Hubble constant H0 and the value inferred from the cosmic microwave background (CMB). Interacting radiation has been suggested as a solution, but studies show that conventional models are precluded by high-CMB polarization data. It seems at least plausible that a solution may be provided by related models that distinguish between high-and low-multipoles. When interactions of strongly-coupled radiation are mediated by a force-carrier that becomes nonrelativistic, the dark radiation undergoes a "step" in which its relative energy density increases as the mediator deposits its entropy into the lighter species. If this transition occurs while CMB-observable modes are inside the horizon, high-and low-peaks are impacted differently, corresponding to modes that enter the horizon before or after the step. These dynamics are naturally packaged into the simplest supersymmetric theory, the Wess-Zumino model, with the mass of the scalar mediator near the eV-scale. We investigate the cosmological signatures of such "Wess-Zumino Dark Radiation" (WZDR) and find that it provides an improved fit to the CMB alone, favoring larger values of H0. If supernovae measurements from the SH0ES collaboration are also included in the analysis, the inferred value of H0 is yet larger, but the preference for dark radiation and the location of the transition is left nearly unchanged. Utilizing a standardized set of measures, we compare to other models and find that WZDR is among the most successful at addressing the H0 tension and the best of those with a Lagrangian formulation.

New early dark energy

“…On the other hand, it has been proposed that an extra component of dark energy, which decays just before recombination, shows more promise towards resolving the tension. The first attempt in this direction, called early dark energy (EDE) [22][23][24][25][26][27], proposed that the decay of the extra EDE component happened through a second order rollover phase transition (see [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] for different studies building up on this idea and [45][46][47][48][49][50][51][52][53] for other early-time approaches). This model, which, for the reminder of this paper, we will refer to as old EDE 1 , is however not completely without problems.…”

“…In fact, other proposals relying on the dark sector radiation plasma to provide the early energy injection, like the step model in[53], typically require T d Tν . Whether this can be achieved in a minimal dark sector model without fine-tuning remains to be seen.…”

X-ray morphology of cluster-mass haloes in self-interacting dark matter

“…In addition, the ΛCDM paradigm has seen challenges on cosmological scales, most notably a tension between direct and indirect measurements of the Hubble parameter today, 𝐻 0 (for a summary see, e.g., Verde et al 2019), but also a discrepancy between late time cosmic shear measurements and Cosmic Microwave Background measurements in the Ω 𝑚 − 𝜎 8 parameter plane (see, e.g., Asgari et al 2021;Loureiro et al 2021;Secco et al 2021;Amon et al 2021, and references therein), involving the total matter content of the universe and a measure for the amplitude of matter clustering. A plethora of attempts have been made to address the 𝐻 0 and Ω 𝑚 − 𝜎 8 discrepancies (for summaries see, e.g., Di Valentino et al 2021;Schöneberg et al 2021), many involving more complex dark particle sectors with additional interactions between dark relics (e.g., Cyr-Racine & Sigurdson 2014; Archidiacono et al 2015Archidiacono et al , 2020Baumann et al 2016;Forastieri et al 2015Forastieri et al , 2017Forastieri et al , 2019Lancaster et al 2017;Choi et al 2018;Kreisch et al 2020;Escudero & Witte 2020;Blinov & Marques-Tavares 2020;Das & Ghosh 2021;Roy Choudhury et al 2021;Brinckmann et al 2021;Esteban & Salvado 2021;Aloni et al 2021), including models of dark matter interacting with light dark relics or dark radiation (e.g., van den Aarssen et al 2012;Buckley et al 2014;Buen-Abad et al 2015Cyr-Racine et al 2016;Lesgourgues et al 2016;Archidiacono et al 2017Archidiacono et al , 2019Di Valentino et al 2018;Bose et al 2019;…”

Two Sides of the Same Coin: Sterile Neutrinos and Dark Radiation, Status and Perspectives

“…In Section 3, we focus on neutrino nonstandard interactions as a mean to accommodate sterile neutrinos in cosmology, also in light of the infamous Hubble constant problem [37,38]. In Section 4, then, we extend the discussion to the case of dark radiation as part of some new physics extension of the standard model (see, e.g., [39][40][41][42][43][44][45][46][47]), which may also be connected to dark matter within a complex dark sector. Finally, Section 5 summarizes our conclusions.…”