TY - JOUR
T1 - Constraints on scalar and tensor spectra from Neff
AU - Ben-Dayan, Ido
AU - Keating, Brian
AU - Leon, David
AU - Wolfson, Ira
N1 - Publisher Copyright:
© 2019 IOP Publishing Ltd and Sissa Medialab.
PY - 2019/6/4
Y1 - 2019/6/4
N2 - At the linear level, the gravitational wave (GW) spectrum predicted by inflation, and many of its alternatives, can have arbitrarily small amplitude and consequently an unconstrained tilt. However, at second order, tensor fluctuations are sourced by scalar fluctuations that have been measured in the cosmic microwave background (CMB). These second order fluctuations generically produce a minimum amount of tensor perturbations corresponding to a tensor-to-scalar ratio of r∼ 10-6. Inverting this relationship yields a bound on the tensor tilt sourced by scalar fluctuations. Since this induced GW spectrum depends on the scalar spectrum, we derive a new indirect bound that involves all scales of the scalar spectrum based on CMB observations. This bound comes from the constraint on the number of effective relativistic degrees of freedom, Neff. We estimate the bound using current data, and the improvements expected by future CMB experiment. The bound forces the running and running of running to conform to standard slow-roll predictions of α,β (ns-1)2 where α≡ d ns/d k and β ≡ d ns 2/d k2, improving on current CMB measurements by an order of magnitude. This bound has further implications for the possibility of primordial black holes as dark matter candidates. Performing a likelihood analysis including this new constraint, we find that positive α and/or β are disfavored at least at 1σ. Even using conservative analysis, β + 0.074 α>8.6× 10-4 are ruled out at 3σ. Finally, using bounds on the fractional energy density of gravitational waves today obtained by LIGO and the Pulsar Timing Array, we obtain a bound on the primordial scalar spectrum on these scales and give forecast for future measurements.
AB - At the linear level, the gravitational wave (GW) spectrum predicted by inflation, and many of its alternatives, can have arbitrarily small amplitude and consequently an unconstrained tilt. However, at second order, tensor fluctuations are sourced by scalar fluctuations that have been measured in the cosmic microwave background (CMB). These second order fluctuations generically produce a minimum amount of tensor perturbations corresponding to a tensor-to-scalar ratio of r∼ 10-6. Inverting this relationship yields a bound on the tensor tilt sourced by scalar fluctuations. Since this induced GW spectrum depends on the scalar spectrum, we derive a new indirect bound that involves all scales of the scalar spectrum based on CMB observations. This bound comes from the constraint on the number of effective relativistic degrees of freedom, Neff. We estimate the bound using current data, and the improvements expected by future CMB experiment. The bound forces the running and running of running to conform to standard slow-roll predictions of α,β (ns-1)2 where α≡ d ns/d k and β ≡ d ns 2/d k2, improving on current CMB measurements by an order of magnitude. This bound has further implications for the possibility of primordial black holes as dark matter candidates. Performing a likelihood analysis including this new constraint, we find that positive α and/or β are disfavored at least at 1σ. Even using conservative analysis, β + 0.074 α>8.6× 10-4 are ruled out at 3σ. Finally, using bounds on the fractional energy density of gravitational waves today obtained by LIGO and the Pulsar Timing Array, we obtain a bound on the primordial scalar spectrum on these scales and give forecast for future measurements.
KW - Cosmological parameters from CMBR
KW - Gravitational waves / sources
KW - Gravitational waves / theory
UR - http://www.scopus.com/inward/record.url?scp=85069483307&partnerID=8YFLogxK
U2 - 10.1088/1475-7516/2019/06/007
DO - 10.1088/1475-7516/2019/06/007
M3 - Article
AN - SCOPUS:85069483307
SN - 1475-7516
VL - 2019
JO - Journal of Cosmology and Astroparticle Physics
JF - Journal of Cosmology and Astroparticle Physics
IS - 6
M1 - 7
ER -