UMICH-2015:Neutrino and Light Relativisic Species Summary

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Forecasts for CMB Stage IV

Planck 2015 versus CMB S4


Very sensitive probe of the thermal history. Forecasts at a very exciting level

E.g. a field in thermal equilibrium that decouples above 100 GeV :
Real scalar - ΔNeff = 0.027
Weyl fermion - ΔNeff = 0.047 (Dirac - 0.094)
Vector field - ΔNeff = 0.054


Sensitive to lots of exciting ideas in BSM physics 
axions, gravitinos, dark radiation, etc.
Can test our thermal history by comparing with BBN 
e.g. time evolution of Neff
BBN and the CMB test different things!
E.g. we can test interacting (Nfluid versus non-interacting dark radiation


How do we make the most of the measurement?

Most optimistic measurements use lmax > 3000
Need to understand foregrounds for E-modes at small scales
Delensing E-modes improves the constraints
What are our requirements from optimizing Neff measurement

But a lot of information is coming from the phase shift, which is less sensitive to lmax

What do we gain from relic abundances and BBN
D/H measurements will also be very good. How does this impact CMB S4?
Do we need lab measurements to help with BBN predictions?
Standard model prediction of Neff is still uncertain at 0.01 level. Do we need to improve this?

Things to do


CMB S4 is very sensitive -- do we need a broader parameterization of BSM physics?

Sterile neutrinos, non-thermal relics in general 
are there good targets

Can we really achieve this sensitivity ?

How realistic are these forecasts? What is the impact of foregrounds
Will we be able to use the high l modes?
What is the tradeoff between lmax and sensitivity?

Neutrino Masses


We don't see the shape in lensing

Neutrino masses signature can be spoiled by degeneracy with τ


Need a good measurement of τ to achieve forecasts


Unrealistic to get this from the ground
How do we get around this issue?

Relation to Lab Experiments

Lab experiments are complimentary
measure different parameters
BSM physics can hide in any disagreement between lab based and cosmology based measurements
Non-standard energy density / thermal history / laws of physics will alter cosmology and lab measurements differently

What needs to be done?

How do non-linearities, baryonic effects enter lensing power spectrum
How is lensing reconstruction affected by foregrounds
Are estimators bias by foregrounds?
We are assuming that only lensing + BAO are the observables. Would be believe constraints from RSD, clustering, etc.?
Opportunities with other observables (e.g. clusters) and cross-correlation (e.g. LSST, DESI, Euclid)
How would we confirm the signature (checks for robustness) ?
Is there a good parameterization of BSM physics that is captured by cosmological neutrino mass constraints (versus lab constraints)?

Action Items

More realistic forecasts
how do things depend on lmin?
How sensitive are we to foregrounds?
Can we reach ΔNeff=0.01-0.02?
How do we deal with the tau degeneracy for mν?
What is the right space of models? Are we missing opportunities?
e.g. non-thermal relics
BSM physics associated with neutrinos
What more can we learn from CMB lensing?
Are models computed to the necessary accuracy
neutrinos in the CMB
Neff from the Standard model
Can we leverage improvements in relic abundance measurements