Journal
PHYSICAL REVIEW D
Volume 97, Issue 6, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevD.97.063519
Keywords
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Funding
- European Research Council [681431]
- Royal Society
- European Research Council through Consolidator Grant, UK [725456]
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One of the major goals of future cosmic microwave background (CMB) measurements is the accurate determination of the effective number of neutrinos N-eff. Reaching an experimental sensitivity of Delta N-eff = 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 n(run) and a precise determination of the neutron lifetime tau(n). We first show that the constraints on N-eff 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/d ln k = -0.002 could mimic a positive variation of Delta N-eff = 0.03. Moreover, given the current discrepancies between experimental measurements of the neutron lifetime tau(n), we show that the assumption of a conservative error of Delta tau(n) similar to 10 s could cause a systematic error of Delta N-eff = 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 N-eff at the percent level.
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