9. Baryogenesis & Nucleosynthesis

Question 1

What dominated the Universe in the first three minutes?

Answer 1

Radiation

a is prop. t^1/2

Question 2

Why was there no last scattering of the baryons which existed in the first three minutes?

Answer 2

The mean energy was > baryon rest mass

• Behaved like a particle accelerator well before last scattering took place
• Cannot be observed

Question 3

What was the relationship between quarks and photons in the first three minutes?

Answer 3

There was equilibrium between photons and quarks through pair production and annihilation

Question 4

Why is the universe matter dominated?

Answer 4

Due to very small asymmetries between matter and anti matter which annihilate leaving a net result of matter

Question 5

When did Nucleosynthesis occur?

Answer 5

After the energies lowered and the universe started to expand

Question 6

How do free and bound neutrons decay?

Answer 6

Free neutrons decay with a half life of 880s

- Bound neutrons don’t decay

Question 7

How are the masses of protons and neutrons related?

Answer 7

Neutron is slightly heavier than a proton

- Different mass of 1.29MeV (Q_n)

Question 8

How did particles behave at around 0.1s when T ~ 3x10^10 K and the photon energy was 10MeV?

Answer 8

All particles were in kinetic equilibrium with energy kT ~ 3MeV &laquo_space;m_p c^2

Question 9

Give some examples of decays at 0.1s when T ~ 3x10^10 K and the photon energy was 10MeV

Answer 9

2 photon < - > electron + positron
n + elec. neutrino < - > proton + electron
neutron + positron < - > proton + anti. elec. neutrino

Question 10

How can the number densities of neutrons and protons be derived in the early times?

Answer 10

Maxwell Boltzmann distribution
At early times, kT&raquo_space; Q_n (mass diff. between neutron and proton)
n_n / n_p ~ exp(-Q_n / kT)

Question 11

How does the ratio of neutrons to protons change over time?

Answer 11

Early universe: number of protons ~ number neutrons

- As universe cools and expands, fewer n’s wrt p’s until freeze out

Question 12

What is freeze out?

Answer 12

When the reaction rate drops below the Hubble parameter

- n_n / n_p is “frozen” at t_freeze = 1s, kT_freeze = 0.8MeV

Question 13

What is the freeze out ratio equal to?

Answer 13

n_n / n_p ~ 0.2 but neutrons can decay after this

Question 14

When does fusion start to occur?

Answer 14

After the freezeout time with all neutrons in atomic nuclei

Question 15

Give examples of fusion interactions where the neutrons are in atomic nuclei

Answer 15

p + n -> D + photon
D + p -> 3He + photon
3He + D -> 4He + p

Question 16

What is the theoretical value for the ratio of Helium 4 to baryons, and why is this an overestimate?

Answer 16

Y_p = 1/3

Actual is 0.24 as some neutrons decayed and go into other elements