Dark energy at early times, the Hubble parameter, and the string axiverse

By Tanvi Karwal, M. Kamionkowski. 2016

242 citations recorded by the source corpus.

Precise measurements of the cosmic microwave background (CMB) power spectrum are in excellent agreement with the predictions of the standard $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ cosmological model. However, there is some tension between the value of the Hubble parameter ${H}_{0}$ inferred from the CMB and that inferred from observations of the Universe at lower redshifts, and the unusually small value of the dark-energy density is a puzzling ingredient of the model. In this paper, we explore a scenario with a new exotic energy density that behaves like a cosmological constant at early times and then decays quickly at some critical redshift ${z}_{c}$. An exotic energy density like this is motivated by some string-axiverse-inspired scenarios for dark energy. By increasing the expansion rate at early times, the very precisely determined angular scale of the sound horizon at decoupling can be preserved with a larger Hubble constant. We find, however, that the Planck temperature power spectrum tightly constrains the magnitude of the early dark-energy density and thus any shift in the Hubble constant obtained from the CMB. If the reionization optical depth is required to be smaller than the Planck 2016 $2\ensuremath{\sigma}$ upper bound $\ensuremath{\tau}\ensuremath{\lesssim}0.0774$, then early dark energy allows a Hubble-parameter shift of at most $1.6\text{ }\mathrm{km}\text{ }{\mathrm{s}}^{\ensuremath{-}1}\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}$ (at ${z}_{c}\ensuremath{\simeq}1585$), too small to fully alleviate the Hubble-parameter tension. Only if $\ensuremath{\tau}$ is increased by more than $5\ensuremath{\sigma}$ can the CMB Hubble parameter be brought into agreement with that from local measurements. In the process, we derive strong constraints to the contribution of early dark energy at the time of recombination---it can never exceed $\ensuremath{\sim}2%$ of the radiation/matter density for $10\ensuremath{\lesssim}{z}_{c}\ensuremath{\lesssim}1{0}^{5}$.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/42963b3f4b4fe7fb98c093654b0e51514429dd0a

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Citations in the corpus

The Ups and Downs of Early Dark Energy solutions to the Hubble tension: A review of models, hints and constraints circa 2023 (2023, background): “[74] Tanvi Karwal and Marc Kamionkowski, “Dark energy at early times, the Hubble parameter, and the string axiverse,” Phys.”
KiDS-450: testing extensions to the standard cosmological model (2016, background): “…the Hubble constant can be resolved by invoking non-standard physics in the dark energy and dark radiation sectors (most recently, e.g. Archidiacono et al. 2016; Bernal, Verde & Riess 2016; Di Valentino, Melchiorri & Silk 2016a,b; Grandis et al. 2016; Karwal & Kamionkowski 2016; Riess et al. 2016).”
Alleviating the H0 and σ8 anomalies with a decaying dark matter model (2019, background): “sa. Solutions of the Hubble tension must resolve the tension by reducing the size r sof the sound horizon to remain consistent with data [29–31]. This requires a modiﬁcation of early-universe physics [19,20]. On the other hand, a solution to the S 8 tension would require late-universe physics that l”