the$radiaon,dominated$era - caltech astronomygeorge/ay127/hirata_lec3_radera.pdf · =1.95(1+z)$k.$...
TRANSCRIPT
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The Radia)on-‐Dominated Era
Ay 127 April 9, 2013
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Outline
1. Radia)on-‐Dominated Plasmas, Expansion of the Universe
2. Neutrino Decoupling 3. Synthesis of the Light Elements
4. Observa)ons of D, He, Li
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Radia)on-‐Dominated Plasmas
• Consider blackbody spectrum:
g = number of polariza)ons (2 for photons) − = bosons; + = fermions
• Energy density
g* = g (bosons) or ⅞ g (fermions)
€
Iν =ghν 3
c 21
ehν / kT ±1
€
u =4πc
Iν dν =π 2
303c 3g*(kT)
40
∞
∫
3
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Thermodynamic proper)es
• Effec)ve ma[er density:
• Pressure:
• Entropy density:
€
p = 13 u =
π 2
903c 3g*(kT)
4
€
ρ =uc 2
=π 2
303c 5g*(kT)
4
€
s =duT0
u∫ =
2π 2
453c 3g*k
4T 3
4
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Expansion of Universe
• Friedmann equa)on rela)ng H to T:
• Solu)on: H ~ T2 ~ a-‐2 a ~ t1/2 H=1/(2t).
• More convenient form:
€
H 2 =83πGρ =
4π 2G453c 5
g*(kT)4
€
t =3 53 / 2c 5 / 2
4πG1/ 2g*1/ 2(kT)2
€
t =1g*
1.56 MeVkT
⎛
⎝ ⎜
⎞
⎠ ⎟
2
sec5
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What is g*?
• Consider the Universe at t~1 second. • Photons only: g*=2. • Also neutrinos: 3 flavors, ×2 for an)neutrinos: ⅞ × 3 × 2 = 21/4.
• At 3kT≥mec2, e+e− are “massless.” 2 spins each: ⅞ × 2 × 2 = 7/2.
• At MeV temperatures, g* = 43/4. 6
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Evolu)on (slightly idealized)
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g*
kT 10 MeV 100 MeV 1 GeV 10 GeV 100 GeV
t
γ, ν, e+e−
10¾ 10
100
+μ, π 17¼
−π+udsg 61¾
+c, τ 75¾
+b 86¼
+WZHt 106¾
??? + MSSM 228¾
quark-‐gluon plasma hadrons
1 ms 1 μs 1 ns
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Neutrino Decoupling
• Neutrinos have weak interac)ons with e+e−:
• Typical E ~ 3kT so can find the number of neutrino interac)ons in the life)me of the Universe:
• At kT~1.6 MeV, universe transi)ons from being neutrino-‐opaque to transparent.
8
€
ne+e- ~ 5 ×1031(kT)MeV3 cm−3
€
σweak ~ 10−44 EMeV2 cm2
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nσct ~ 0.1(kT)MeV5
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Epoch of e+e− Annihila)on
• At kT<mec2 almost all the electrons and positrons annihilate. (~1ppb more e− than e+. These few e− survive.)
• Energy of annihila)on heats the photons but not the neutrinos, so Tγ>Tν.
• Can do computa)on assuming annihila)on is adiaba)c (conserves entropy of e+e−γ).
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e+ + e− →≥ 2γ ~ 100%ν e + ν e (almost) neglgible⎧ ⎨ ⎩
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Annihila)on Part 2
• Entropy before annihila)on:
• Entropy ayer annihila)on:
• Equa)ng gives:
(Remains true since both temperatures scale as 1/a.) 10
€
s(e±γ) =2π 2
453c 3g*(e
±γ)k 4Tν3 =
11π 2
453c 3k 4Tν
3
€
s(γ) =2π 2
453c 3g*(γ)k
4Tγ3 =
4π 2
453c 3k 4Tγ
3
€
Tγ = 114
3 Tν
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Post-‐annihila)on
• Can combine neutrino and photon energy densi)es to get radia)on energy density:
• Factor of 1.68 from neutrinos. Affects BBN, CMB, etc.
• Equivalent to “effec)ve” g*=3.36. • From temperature ra)o, Tν=1.95(1+z) K.
• They’re s)ll here today: ~ 300/cm3. 11
€
u =π 2
303c 32(kTγ )
4 + 214 (kTν )
4[ ] =1.68π 2(kTγ )
4
153c 3
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Nucleosynthesis
• Universe has net baryon number:
• We don’t know why. • At high temperature the thermodynamically favored state of baryons is p + n.
• At low temperature the favored state is 56Fe. • Nucleosynthesis is process of forming the heavier nuclei from p+n. Started in Big Bang, con)nues today in stars.
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€
η =baryons− antibaryons
photons~ 6 ×10−10
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Primordial nucleosynthesis
• The process: 1. n/p ra)o determined at neutrino decoupling 2. Universe expands/cools, some neutrons decay 3. Assembly of D, 3He, 4He, 7Li
• Nuclear processing is not finished ayer BBN, e.g. heavy elements are produced in stars. This more recent processing must be corrected/avoided to infer primordial abundances.
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The ini)al n/p ra)o
• Before neutrino decoupling, neutrons are kept in equilibrium:
• Equilibrium ra)o
where Q = (mn-‐mp)c2 = 1.293 MeV. • n:p=1:1 at high T, then starts decreasing.
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n + e+ ↔ p+ + ν en + ν e ↔ p+ + e−
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n : p = e−Q / kT
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The ini)al n/p ra)o (Part II)
• Neutrino decoupling at kT ~ 1.6 MeV would imply n:p=0.4.
• More detailed calcula)on gives n:p≈0.15.
• Neutron has a half-‐life of ~11 minutes.
• Therefore the neutron abundance declines:
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€
n : (n + p) ≈ 0.152t /11min€
n→ p+ + e− + ν e
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Deuterium (Part I)
• Deuterium is the simplest nucleus. Binding energy is B = 2.22 MeV.
• Saha-‐like equa)on for its abundance:
• Compare with total baryon abundance:
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€
n + p+ ↔D+ + γ
€
n(D+)npnn
=34
2π2
mredkT⎛
⎝ ⎜
⎞
⎠ ⎟
3 / 2
eB / kT
€
nb = 5.6 ×1020Ωbh2T9
3 cm−3
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Deuterium (Part II)
• Using nota)on Xi=ni/nb, and assuming the universe is mostly p (85%) and n (15%) we get
• Deuterium abundance grows exponen)ally. Would become of order unity at T9~0.7 (t ~ 3 min) except that deuterium can burn to heavier nuclei.
17
€
X(D+) ≈ 5 ×10−14Ωbh2T9
3 / 2e25.8 /T9
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Helium
• Deuterium consump)on:
• When deuterium reaches ~1%, these reac)ons package most of the available neutrons into the very )ghtly bound 4He.
18
€
D + p↔ 3He + γ
D + D↔ 3H + pD + D↔ 3He + n
A=23
€
3He + p↔ 3H + n3H + D↔ 4He + n3He + D↔ 4He + p
A=34
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What happens next
• At t ~ 3 minutes, 13% of baryons are neutrons. Packaged into 4He, we get Y~0.26 (4He frac)on by mass). Most of the rest is 1H.
• No new D is produced once free neutrons are gone. Deuterium is burned via D+D and D+p. A small amount, XD~2×10−5, survives.
• Some 3H and 3He are ley over (total X3~10−5). 19
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End Game
• Some A=7 nuclei (X7~4×10−10) are produced by 3He+4He and 3H+4He.
• 6Li produced in much smaller amounts since it is fragile (X6 ~ 10−14).
• Heavy element produc)on e.g. 3α12C negligible at BBN densi)es (10−5 g/cm3).
• Radioac)ve nuclei (n, 3H, 7Be) decay.
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1H
n
2H 3H
3He 4He
7Be
7Li 6Li
(n,γ)
(p,γ)
(D,p)
(p,γ) (D,n) (n,p)
(D,p)
(D,n)
(3H,γ)
(3He,γ) (n,p)
Stable isotope
Radioac)ve isotope
Reac)on
Radioac)ve decay
(3H,n) (3He,p) (D, γ)
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BBN Yield Predic)ons
Cyburt, Fields & Olive 2003 Phys Le> B 567,227
4He frac)on by mass
Abundance rela)ve to H by number
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Dependence on Cosmology
• 4He: Determined by n:p ra)o, so very sensi)ve to physics at neutrino decoupling, e.g. new massless par)cles. Slightly sensi)ve to Ωbh2.
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u(T) increased
H(T) increased
Reduced nσct
ν decoupling at higher T
Larger n:p raMo
More 4He
New massless parMcle
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Dependence on Cosmology
• 2H and 3He: Leyovers of incomplete H burning. Decrease for larger baryon density Ωbh2.
• 7Li: Non-‐monotonic behavior in Ωbh2: – Low Ωbh2: direct produc)on of 7Li, destroyed by
7Li+p at high densi)es. – High Ωbh2: 7Be can be produced, decays to 7Li.
• 6Li: Primordial contribu)on should be undetectable.
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4He
• Predic)on: Y = 0.2482±0.0003±0.0006 • Usually es)mate He abundance from H II region spectra (H I and He I recombina)on lines). – Determine temperature (self-‐consistent or using [O III]). – Model collisional excita)on, fluorescence. – Op)cal depth effects. He I 23S1 is metastable. – Model reddening. – Model stellar He I absorp)on. – Ioniza)on correc)on factors (H II and He I can coexist). – Extrapolate to zero metallicity to get primordial value.
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Helium
• Predic)on: Y = 0.2482±0.0003±0.0006 • Usually es)mate He abundance from H II region spectra (He I recombina)on lines). Extrapolate to zero metallicity to get primordial value.
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0.232 < Yp < 0.258 Olive & Skillman 2004 ApJ 617,29
✔
predicted
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Examples of Deuterium Measurements
LocaMon D/H (ppm) Method Earth 150
Venus 16000±2000 Mass spec. Donahue et al 1982
Mars 900±400 IR lines (HDO/H2O) Owen et al 1988
Jupiter 22—50 50±20
IR lines (CH3D/CH4) Kunde et al 1982 Mass spec. Niemann et al 1996
Saturn 4—29 IR lines (CH3D/CH4) Cour)n et al 1984
Local ISM ~ 7 – 20 Absorp)on lines in stellar spectra. Depends on line of sight; see tabula)on by Linsky et al 2006
Lyman-‐α absorbers (IGM)
28±4 H vs D Lyman absorp)on lines in QSO spectra Kirkman et al 2003
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Warnings on D/H
• Not all D/H is the primordial value, or even that at the forma)on of the solar system.
• Problems: – Astra)on: burning of D to 3He etc. in stars. – Chemical frac)ona)on: At low temperatures D binds more )ghtly to molecules than H due to vibra)onal zero point energy.
– Frac)ona)on due to depth of planetary gravity well. (This is why Jupiter was once used for BBN D/H.)
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XH+ + D XD+ + H
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Intergalac)c D/H
• Probably most reliable method is low-‐metallicity quasar absorp)on line systems. – Li[le processing by stars – Less opportunity to hide deuterium in molecules or dust grains
• Reduced mass of e−D+ is greater than e−p+ by 1 part in 3600, rescaling all hydrogenic energy levels. Equivalent to velocity shiy of c/3600 = 80 km/s.
• IGM value 28±4 ppm agrees with predicted 25.7(+1.7)(−1.3) ppm.
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✔
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Kirkman et al 2003 ApJS 149, 1 Q1243+3047 zabs = 2.53
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3He • Difficul)es:
– Can be both produced and destroyed in stars. – No IGM measurement.
• Most(?) accepted measurement is 3He+ hyperfine line (λ=3.46cm) low-‐metallicity H II regions. Bania et al (2002) find 3He/H < 15 ppm. – Directly observed 11±2 ppm, argue that stars would lead to net increase in 3He.
• Predicted from WMAP baryon abundance: 3He/H = 10.5±0.3±0.3 ppm. – Aside: Jupiter atmosphere ra)o 3He:4He = (1.1±0.2)×10−4 [Niemann et al 1996]. 31
✔?
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Lithium
• Can be measured in low-‐metallicity stars. – Two mul)plets available for Li I: 6708Å (2s—2p) and 6104Å (2p—3d).
– 7Li destroyed at high T, 7Li + p 24He. Avoid low-‐mass stars due to deep convec)ve zone.
– Slight isotope shiy of 6Li vs. 7Li. • Li can also be produced via cosmic rays: – Spalla)on of CNO elements (also gives Be, B) – 4He + 4He collisions at low energies.
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Li abundance evolu)on
33 Asplund et al (2006) ApJ 644, 229
12 + log10 XLi
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The Lithium Problem(s)
• 7Li: Predicted (5.2±0.7)×10−10. – See update from Cyburt, Fields, Olive 2008.
• Observa)ons give lower values – e.g. (1.1—1.5) ×10−10 from Asplund et al. 2006. – Other determina)ons are typically ~(1—2)×10−10.
• 6Li: there have been reports of a plateau at ~6×10−12, although the accuracy of 6Li/7Li extracted from line profiles has been debated.
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✗
✗?
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Solu)ons
• The 7Li problem: – Errors in nuclear reac)on rates? – Deple)on in convec)on zone? – Stellar atmosphere modeling? – Destroyed in early genera)on of stars? – New physics?
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