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Neutrinos in Cosmology
Jim Rich
SPP-IRFUCEA-Saclay
91191 Gif-sur-Yvette
November, 2015
Jim Rich (IRFU) Neutrinos in Cosmology November, 2015 1 / 17
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History of the universe:
Density and temperature decrease with time
Jim Rich (IRFU) Neutrinos in Cosmology November, 2015 2 / 17
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History of the universe:
Interesting things happen!
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The role of neutrinos in cosmology: summary
Early Universe: effects of number of neutrino species (νe , νµ, ντ ....)
Each species: one black-body spectra in the primordial soupEffect on Nucleosynthesis ⇒ Nν ∼ 3Effect on Recombination ⇒ Nν ∼ 3
Late Universe: effects of neutrino mass
mν “complicates” late-time expansion and structure formationNo complications seen ⇒ mν < 0.2− 0.1eV
A dark matter candidate that is easy to kill:mν ∼ 15eV (hot dark matter)
An intriguing unorthodox dark matter candidate:mν ∼ 5keV (non-thermal relic, warm dark matter)
A super-challenge: detect cosmological neutrinos!
Jim Rich (IRFU) Neutrinos in Cosmology November, 2015 4 / 17
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The role of neutrinos in cosmology: summary
Early Universe: effects of number of neutrino species (νe , νµ, ντ ....)
Each species: one black-body spectra in the primordial soupEffect on Nucleosynthesis ⇒ Nν ∼ 3Effect on Recombination ⇒ Nν ∼ 3
Late Universe: effects of neutrino mass
mν “complicates” late-time expansion and structure formationNo complications seen ⇒ mν < 0.2− 0.1eV
A dark matter candidate that is easy to kill:mν ∼ 15eV (hot dark matter)
An intriguing unorthodox dark matter candidate:mν ∼ 5keV (non-thermal relic, warm dark matter)
A super-challenge: detect cosmological neutrinos!
Jim Rich (IRFU) Neutrinos in Cosmology November, 2015 4 / 17
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The role of neutrinos in cosmology: summary
Early Universe: effects of number of neutrino species (νe , νµ, ντ ....)
Each species: one black-body spectra in the primordial soupEffect on Nucleosynthesis ⇒ Nν ∼ 3Effect on Recombination ⇒ Nν ∼ 3
Late Universe: effects of neutrino mass
mν “complicates” late-time expansion and structure formationNo complications seen ⇒ mν < 0.2− 0.1eV
A dark matter candidate that is easy to kill:mν ∼ 15eV (hot dark matter)
An intriguing unorthodox dark matter candidate:mν ∼ 5keV (non-thermal relic, warm dark matter)
A super-challenge: detect cosmological neutrinos!
Jim Rich (IRFU) Neutrinos in Cosmology November, 2015 4 / 17
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The role of neutrinos in cosmology: summary
Early Universe: effects of number of neutrino species (νe , νµ, ντ ....)
Each species: one black-body spectra in the primordial soupEffect on Nucleosynthesis ⇒ Nν ∼ 3Effect on Recombination ⇒ Nν ∼ 3
Late Universe: effects of neutrino mass
mν “complicates” late-time expansion and structure formationNo complications seen ⇒ mν < 0.2− 0.1eV
A dark matter candidate that is easy to kill:mν ∼ 15eV (hot dark matter)
An intriguing unorthodox dark matter candidate:mν ∼ 5keV (non-thermal relic, warm dark matter)
A super-challenge: detect cosmological neutrinos!
Jim Rich (IRFU) Neutrinos in Cosmology November, 2015 4 / 17
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Early universe: Neutrinos = 3 more black bodies
Same as photons except
Decouple much earlier (T ∼MeV instead of 0.25eV)[Only weak interactions]
fermions instead of bosons[Slightly different thermal distribution]
only left-handed ν and right-handed ν reach thermal equilibrium[Wrong-helicity states not expected to be present.]
Gold mine of questions for exams:1. Show that Tν = (4/11)1/3Tγ (as long as T � mν)2. Show that nν = (3/11)nγ ∼ 110cm−3 (each species)
Jim Rich (IRFU) Neutrinos in Cosmology November, 2015 5 / 17
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Early universe: Neutrinos = 3 more black bodies
Same as photons except
Decouple much earlier (T ∼MeV instead of 0.25eV)[Only weak interactions]
fermions instead of bosons[Slightly different thermal distribution]
only left-handed ν and right-handed ν reach thermal equilibrium[Wrong-helicity states not expected to be present.]
Gold mine of questions for exams:1. Show that Tν = (4/11)1/3Tγ (as long as T � mν)2. Show that nν = (3/11)nγ ∼ 110cm−3 (each species)
Jim Rich (IRFU) Neutrinos in Cosmology November, 2015 5 / 17
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What can a neutrino do in the early universe
They have only
weak interactions
gravitational interactions.
Both interactions play essential roles.
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νe weak interactions
⇒ initial conditions for nucleosynthesis
neutron-proton ratio vs. temperature:
..
thermal equilibrium viaνen↔ e−p..
n→ pe−νe
nucleosynthesis (T = 60keV)(number of neutrons reducedby decay)
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νe weak interactions
⇒ initial conditions for nucleosynthesis
neutron-proton ratio vs. temperature:
..
thermal equilibrium viaνen↔ e−p..
n→ pe−νe
nucleosynthesis (T = 60keV)(number of neutrons reducedby decay)
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νe, νµ, ντ gravitational interactions
.
expansion rate (squared) proportional to density
a2
a2=
8πG
3(ρmatter + ργ + ρν + ....)
(Friedman)
⇒In radiation epoch expansion rate (squared)proportional to number of black-body spectra.
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νe, νµ, ντ gravitational interactions
. Expansion rate in Early universe ∼ (Nγ +Nν) (Nγ = 1).
Nν > 3⇒ temperaturedrops faster
Less time for neutrondecay ⇒ more heliumthan the observed 25%
Recombination faster⇒ sharper CMB image
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1977: Observed helium abundance ⇒ Nν < 5
2015: 1.8 < Nν < 4.5 (95%CL)PDG, R.H. Cyburt et al., Astropart. Phys. 23, 313 (2005)
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2015: CMB ⇒ Nν = 3
Planck image of our “last-scattering surface”:
The image is blurred because photons random-walk in the 100,000yrbefore recombination. Increasing the expansion rate, reduces the timefor random walking, and makes the image sharper.
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Photon random walk on last-scattering surface
Diffusion distance proportional to geometric mean of photonmean-free-path and ctwalk ∼ 1/expansion rate.
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Planck damping for ` > 1000 ⇒ Nν < 4
0
1000
2000
3000
4000
5000
6000
DTT
`[µ
K2]
30 500 1000 1500 2000 2500`
-60-3003060
∆DTT
`
2 10-600-300
0300600
Nν = 2.99± 0.20(CMB only)
Nν < 4 at ∼ 3σ
⇒ Any sterile neutrino must not have thermalized.
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Late universe: massive neutrinos become
non-relativistic kT ∼ mνc2
Neutrino oscillation experiments ⇒ at least one neutrino withmν > 0.05eV!
mν modifies density⇒ modified expansion rate(Friedman eqn.)⇒ modifieddistance-redshift relation
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Limits on neutrino mass
When T ∼ mν :
Energy per neutrino becomes constant (= mν)⇒modified the expansion rate⇒modified distance-redshift relationship⇒ mν < 0.23eV (Planck plus BAO)
Start to contribute to structure formationModify predicted inhomogeneities⇒ mν < 0.12eV (Planck plus Lyα forest)Palanque-Delabrouille et al [2015]
Question: If it turns out that mν > 0.2eV, how must we modify thecosmological model to recover agreement? (e.g. time-varying darkenergy).
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Excluded: mν ∼ 15eV: hot dark matter
mν ∼ 15eV gives the right matter density (for 110cm−3) but causesproblems for CMB spectrum and structure formation.
mν modifies expansion ratebefore recombination ⇒CMB spectrum modified
Neutrinos “free stream” atv = c until T = 15eV ⇒inhomogeneities on galacticscales removed.
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Neutrino free-streaming destroys short wavelength
perturbations
Neutrino perturbation destroyed if ct > λ where t is the timebetween neutrino decoupling and T = mν .
mν = 15eV ⇒ galaxy-size perturbations destroyed
mν = 4keV ⇒ small-galaxy-size perturbations destroyed
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mν ∼keV: warm dark matter
Matter density too high if nν = 100cm−3 ⇒ must not have been inthermal equilibrium in early universe.Possibility: sterile neutrino produced by oscillations
Neutrinos “free stream” atv = c until T = 4keV ⇒inhomogeneities on scalesof small galaxies removed.This is good because fewsmall galaxies are seen!
⇒ fine-tuning once again:neutrino mass just bigenough allow for theexistence of galaxies!
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mν ∼keV: warm dark matter
Matter density too high if nν = 100cm−3 ⇒ must not have been inthermal equilibrium in early universe.Possibility: sterile neutrino produced by oscillations
Neutrinos “free stream” atv = c until T = 4keV ⇒inhomogeneities on scalesof small galaxies removed.This is good because fewsmall galaxies are seen!
⇒ fine-tuning once again:neutrino mass just bigenough allow for theexistence of galaxies!
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The ultimate challenge: detection of νcosmo
Charged current interactions only on radioactive targets.Tritium β-decay (T1/2 = 12yr):
3H → 3He e− νe Ee(max) = MHe −MH −mν ∼ 17keV
Capture of cosmological neutrinos on tritium:
ν 3H → 3He e− Ee = MHe −MH + mν
Capture electrons separated from β electrons by 2mν (S. Weinberg)
100g tritium ⇒∼ 10 captures per year ( and ∼ 3× 1024 decays!)
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Tritium β-decay plus capture spectrum
Long et al. arXiv:1405:7654:
Ptolemy project: arXiv:1307.4738
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Neutrinos in cosmology: conclusion
Neutrinos have an essential role in cosmologyNucleosynthesisMaybe dark matter....
Cosmological observations consistent with three lightneutrinos.
Direct observation somewhat difficult
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