Impact of theoretical assumptions in the determination of the neutrino effective number from future CMB measurements

Ludovico Capparelli, Eleonora Di Valentino, Alessandro Melchiorri, Jens Chluba

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    Abstract

    One of the major goals of future cosmic microwave background (CMB) measurements is the accurate determination of the effective number of neutrinos Neff. Reaching an experimental sensitivity of ΔNeff=0.013 could indeed falsify the presence of any nonstandard relativistic particles at 95% C.L. In this paper, we test how this future constraint can be affected by the removal of two common assumptions: a negligible running of the inflationary spectral index nrun and a precise determination of the neutron lifetime τn. We first show that the constraints on Neff could be significantly biased by the unaccounted presence of a running of the spectral index. Considering the Stage-IV experiment, a negative running of dn/dlnk=-0.002 could mimic a positive variation of ΔNeff=0.03. Moreover, given the current discrepancies between experimental measurements of the neutron lifetime τn, we show that the assumption of a conservative error of Δτn∼10 s could cause a systematic error of ΔNeff=0.02. Complementary cosmological constraints on the running of the spectral index and a solution to the neutron lifetime discrepancy are therefore needed for an accurate and reliable future CMB bound of Neff at the percent level.

    Original languageEnglish
    Article number063519
    JournalPhysical Review D
    Volume97
    Issue number6
    Early online date19 Mar 2018
    DOIs
    Publication statusPublished - 2018

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