Presentation - Cosmology - The Early Universe (37p)

Astronomy 1: Cosmology

2002/2003

The Early Universe

Thermal history 3K

1010 yr

temperature is insignificant

3000 K

106 yr

hydrogen is ionized

104 K

105 yr

radiation dominates dynamics

109 K

10 s

nuclei are dissociated

1013 K

10−8 s

baryons are dissociated

1015 K

10−12 s

theoretical physicists’ playground

1031 K

10−43 s

“Planck scale”, end of physics

Figure 4.1 Thermal history of the universe.

Page 32

Astronomy 1: Cosmology

2002/2003

Planck Scale QM: de Broglie wavelength LP = h/mP c GR: event horizon REH = 2GmP /c2 Limit of current physics when LP = REH Planck mass mP ∼ 10−8 kg ∼ 1019 GeV/c2 Planck length LP ∼ 10−35 m Planck time tP = LP /c ∼ 10−43 s

High-energy Era t < 10−8 s, T > 1013 K Universe consists of photons, quarks, leptons, . . . in thermal equilibrium. Particle–antiparticle pair production, e.g. γ+γ * ) e+ + e− * ) ν + ν¯ γ+γ * ) q + ¯q

Inflation At t ∼ 10−35 –10−32 s, universe expanded very rapidly by a factor ∼ 1025 . Possibly caused by the separation of the strong & electroweak forces. Explains: • horizon problem (why homogeneous?) • flatness problem (why Ωtotal = 1?)

Particle Physics At t ∼ 10−8 s, T ∼ 1013 K, baryons–antibaryons annihilate b + ¯b → γ + γ Baryon asymmetry problem – for every 109 antibaryons, must be 109 + 1 baryons

Page 33

Astronomy 1: Cosmology ⇒

2002/2003

Baryon number not conserved; “CP violation” in QM

At t ∼ 0.1 s, T ∼ 1010 K, universe is transparent to neutrinos ⇒ they “decouple” from the radiation ⇒ number of ν, ν¯ stays constant (ρν0 ∼ 108 m−3 ) At t ∼ 1 s, T ∼ 1010 K, electrons–positrons annihilate e− + e+ → γ + γ

Nucleosynthesis t ∼ 1–120 s, T ∼ 109 K Thermal energy, kT ∼ 0.1 MeV Nuclear binding energy, EB ∼ 1 MeV kT  EB

⇒

nuclei form

Figure 4.2 Nucleosynthesis of the light elements.

Page 34

Astronomy 1: Cosmology

2002/2003

Main products: 4 He, 2 H, 3 He, 3 H, 7 Be, 7 Li Reaction rates depend on T and ρb – nucleosynthesis stops after t ∼ 3 minutes: • T too low to overcome Coulomb barrier • no stable nuclei with A = 5 or 8 • ρ too low for 12 C production ⇒

no heavy elements

Helium: • observed abundance ≥23%, including oldest stars • agrees with prediction of 23–25% Deuterium: • cannot be produced in stars • absorption line systems in quasars: abundance is 0.002–0.02% Baryon density parameter: Ωb =

ρb ρc

Deuterium abundance ⇒ Ωb = 0.045 CMB “power spectrum” results are consistent with this. Ωb /Ω0 = 0.15 ⇒ 85% of the mass density is “non-baryonic”. Figure 4.3 Light element abundances (Schramm & Turner, 1998, Rev. Mod. Phys., 70, 303–318)

Page 35

Astronomy 1: Cosmology

2002/2003

Epoch of Recombination t ∼ 106 yr, T ∼ 3000 K, z ∼ 1100 Previously, photons scattered by electrons ⇒ universe was opaque. Photon energy, kT ∼ 0.3 eV Hydrogen ionization energy, EI = 13.6 eV kT  EI

⇒

electrons “recombine” with protons/nuclei

⇒ universe becomes transparent ⇒ radiation no longer in thermal equilibrium with matter ⇒ origin of the CMB

Page 36

Astronomy 1: Cosmology

2002/2003

Summary Planck Scale: t ∼ 10−43 s, QM/GR limit High-energy Era: t < 10−8 s, particle “soup” Inflation: t ∼ 10−35 –10−32 s, rapid expansion of the universe Particle Physics: t ∼ 10−8 –1 s; baryon asymmetry problem Nucleosynthesis: ⇒ t ∼ 1–120 s, T ∼ 109 K ⇒ right T & ρ for nuclear fusion to occur ⇒ main products: 4 He (23%), 2 H (0.002%), 3 He (0.001%), 3 H, 7 Be, 7 Li (all <∼ 10−6 ) ⇒ density parameter in baryons Ωb = ρb /ρ0 = 0.045 Recombination: t ∼ 106 yr, T ∼ 3000 K, z ∼ 1100; electrons “recombine” with nuclei

Page 37