2016/08/31 by Tanvi Karwal, Marc Kamionkowski · 2 citations
Physics and Astronomy · #Age of the universe #Astrophysics #Cosmic microwave background #Cosmological constant #Cosmology #Cosmology and Gravitation Theories #Dark Matter and Cosmic Phenomena #Dark energy #Decoupling (probability) #Galaxy #Hubble volume #Hubble's law #Mathematical physics #Physics #Planck #Quantum mechanics #Radio Astronomy Observations and Technology #Redshift #Reionization #Spectral density #astro-ph.CO
paper · pdf · doi:10.1103/physrevd.94.103523
published as Phys. Rev. D 94, 103523 (2016) · 10 pages, 13 figures, 1 table
openalex created_date 2016/08/23 · arxiv created 2016/11/06 · openalex publication_date 2016/11/28 · arxiv updated 2016/12/07 · openalex updated_date 2026/08/06
Precise measurements of the cosmic microwave background (CMB) power spectrum are in excellent agreement with the predictions of the standard \mathrm\ensuremathΛCDM cosmological model. However, there is some tension between the value of the Hubble parameter H0 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 zc. 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\ensuremathσ upper bound \ensuremathτ\ensuremath\lesssim0.0774, then early dark energy allows a Hubble-parameter shift of at most 1.6 km s^\ensuremath-1 Mpc^\ensuremath-1 (at zc\ensuremath≃1585), too small to fully alleviate the Hubble-parameter tension. Only if \ensuremathτ is increased by more than 5\ensuremathσ 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∼2% of the radiation/matter density for 10\ensuremath\lesssimzc\ensuremath\lesssim105.