1. Jan Hamann Rencontres de Moriond (Cosmology) 21st March 2016
Current and future constraints on neutrino physics from cosmology
Jan Hamann Rencontres de Moriond (Cosmology) 21st March 2016

2. What do we not know about neutrinos?
We only know their mass-squared differences…
… but what is their absolute mass scale?
solar neutrinos
atmospheric neutrinos
β-decay
experiments
oscillations

3. What do we not know about neutrinos?
What is their mass hierarchy?
normal m3
mass m2 m1
inverted m1
mass m2 m3

4. What do we not know about neutrinos?
Are there additional (light) sterile neutrinos?
Anomalies observed by
Accelerator experiments
Reactor experiments
Gallium experiments
Hints for additional state with Δm2 ≈ eV2?
[Aguilar-Arevalo+ 2013]
[Mention+ 2010] [An+ 2016]
[Giunti&Laveder 2011]
[Giunti+ 2014]

5. neutrinos decouple
e+e--annihilation

6. The Cosmic Neutrino Background (CνB)
Neutrino decoupling around T = 1 MeV, shortly before goes out of equilibrium
Annihilation heats CMB relative to CνB
Neutrino mixing equilibrates momentum distributions
If Treheating > 10 MeV, all three flavours populated

7. Impact of cosmological neutrinos
Structure formation
Gravity
Neutrinos
Weak
interaction
Gravity
Big Bang Nucleosynthesis

8. How much energy density do neutrinos contribute…
Neutrino parameters
How much energy density do neutrinos contribute…
… at early times?
photon
energy density
Fermi-Dirac vs.
Bose-Einstein
lower neutrino
temperature
radiation
energy density
Effective number of
neutrino species
ΛCDM: Neff = 3.046
(small deviation from
Fermi-Dirac)

9. Neutrino parameters How much energy density do neutrinos contribute…
… at early times?
… at late times?
photon
energy density
Fermi-Dirac vs.
Bose-Einstein
lower neutrino
temperature
neutrino
energy density
radiation
energy density
Effective number of
neutrino species
Sum of
neutrino masses
ΛCDM: Neff = 3.046
ΛCDM: Σmν= 0.06 eV
(small deviation from
Fermi-Dirac)
(assumes lightest mass
state is massless)

10. Impact of cosmological neutrinos
Structure formation
Background
evolution
Evolution of
perturbations
Gravity
Neutrinos
Background
evolution
Nuclear
reactions
Weak
interaction
Gravity
Big Bang Nucleosynthesis

11. Free streaming Velocity dispersion large wrt size of potential well
gravitational
potential
initial time x
Cold dark matter
Neutrinos escape from potential well,
density perturbations get washed out
neutrino
gravitational
potential
later time x

12. Structure formation with massive neutrinos
Σmν = 0 eV
Σmν = 7 eV
[simulation and movie by T. Haugbølle]

13. Matter power spectrum with massive neutrinos
Suppression of the matter power spectrum
wrt massless neutrino case
wavenumber
[Figure from Abazajian+ 2013]

14. Neutrino masses and the CMB angular power spectrum
[Planck collaboration 2015]
Changing neutrino mass affects zeq and dA(zrec)
Can shift CMB peaks back in place by tweaking ωc and H0 (geometric degeneracy of the CMB)
Remaining effects (early ISW, late ISW, lensing) rather subtle for sub-eV masses
For better sensitivity, combine with external data (or CMB lensing)

15. Planck constraints on the sum of neutrino masses
No sign of non-zero neutrino masses…
[Planck collaboration 2015]

16. Effective number of neutrinos and the CMB angular power spectrum
[Planck collaboration 2015]
Also subject to geometric degeneracy
In addition, changes damping scale, anisotropic stress (partially degenerate with spectral index/amplitude of primordial spectrum)
Planck measurement of damping tail greatly improved sensitivity

17. Planck constraints on the effective number of relativistic species
Data confirm standard model expectation
(CνB only, no more hints of additional light particles)
[Planck collaboration 2015]

18. Planck results vs. BBN
Deuterium abundance from
damped Ly-α system
4He abundance from
H II-regions
Excellent match with BBN expectation + astrophysical element abundance measurements
[Planck collaboration 2015]

19. Planck constraints on eV-mass sterile neutrinos
Planck data not compatible with a fully thermalised eV-mass neutrino
Want to save the scenario?
Need to suppress production of steriles (e.g., lepton asymmetry,
new interactions, etc.)
[Planck collaboration 2015]

20. Matter power spectrum with massive neutrinos
Suppression of the matter power spectrum
wrt massless neutrino case
wavenumber
[Figure from Abazajian+ 2013]

21. Matter power spectrum with massive neutrinos
Suppression of the matter power spectrum
wrt massless neutrino case
Non-linear regime
Linear regime
wavenumber
[Figure from Abazajian+ 2013]

22. Nonlinear structure formation with massive neutrinos
0.15 eV
0.3 eV
linear
theory
0.45 eV
0.6 eV
Theoretical prediction of matter power spectrum with massive neutrinos in the non-linear regime is a big challenge
Analytical methods [see previous talks]
N-body simulations with neutrino particles, grid-based, hybrid approach…
Simulations with
CDM and neutrino
particles
[Brandbyge+ 2008,2009,2010;
Viel+ 2010, Ali-Haimoud+ 2012]

23. Probes of the matter power spectrum
CMB lensing
21 cm
Lyman-α forest
Galaxy clustering
Cluster counts
Cosmic shear

24. Future sensitivity: CMB Stage-IV + CMB lensing (+BAO)
[Abazajian+ 2013]

25. Future Large Scale Structure surveys
BAO scale
Cosmic shear
Type Ia supernovae
Cluster counts
Galaxy clustering
Geometric observables
Perturbation-based observables

26. Future sensitivity: Planck + EUCLID shear PS/galaxy PS/clusters
Sensitivity up to 10 meV for sum of neutrino masses, and up to 0.02 for effective number of neutrino species when observables are combined
Can cleanly distinguish between effects of dark energy and neutrinos
[Basse+ 2013]

27. Future sensitivity: Combining LSS observables
Shear
Galaxies
(pessimistic)
Galaxies
(optimistic)
CMB
Beautiful complementarity between different observables:
combination breaks parameter degeneracies of individual probes
[Hamann+ 2012]

28. Direct sensitivity to mass hierarchy?
Assume minimal mass in inverted hierarchy
Probably not…
[Hamann+ 2012]

29. Conclusions
The Universe continues to be boring: no evidence for anything unexpected in the cosmological neutrino sector
With the next generation of large-volume galaxy surveys and CMB lensing measurements, a detection of the sum of neutrino masses is extremely likely, provided non-linear growth can be understood sufficiently well