Nucleosynthesis

Question 1

What is nucleosynthesis?

Answer 1

The formation of lighter elements e.g.lithium, helium and deterium

Question 2

Why are high temperatures of 10^9 K needed to form elements in early universe?

Answer 2

Stellar cores are 10^10 times denser than early universe

Question 3

What is the temperature of the universe?

Answer 3

T_universe &laquo_space;1GeV

Question 4

What happens at high enough temperatures?

Answer 4

k_B T is greater than Δmc^2
the neutron-to-proton ratio becomes 1 so there is equilibrium

Question 5

When are neutrons and protons in balance when T > 1MeV?

Answer 5

Through weak interactions:

n + v_e <=> p + e-
n + e+ <=> p + -v_e

Question 6

What happens below T ~ 1 MeV/ end of equilibrium?

Answer 6

The weak interactions freeze out as the reaction rate of these reactions is dependent on temperature

Neutrons locked into atoms are stable
Free neutrons decay (they have a very short timescale)

Question 7

What does a neutron decay into?

Answer 7

proton, electron and electron anti neutrino

Question 8

What happens at the end of equilibrium (0.8MeV)?

Answer 8

There is a finite timescale in which all interactions can happen before free neutrons decay as they have short half lives and elements won’t be able to form

n_n/n_p = 0.2

Question 9

Which is the first element to be made and how is it made?

Answer 9

deuterium
proton + neutron
it is highly unstable and has low binding energy

Question 10

Which elements are created using deuterium?

Answer 10

Lighter elements: 3He and 4He

Question 11

What is created using 3He and 4He?

Answer 11

Lithium and Beryllium

Question 12

Why is nothing heavier than 7Be?

Answer 12

There are no stable elements with atomic weight 5 or 8

Density (temperature) drops too quickly by the time 4He is formed

Question 13

When does nucleosynthesis occur?

Answer 13

at t_universe = 360s

There are too many energetic photons due to annihilation of particle and antiparticle so the formation of deuterium is delayed until k_B T ~ 0.06MeV

Question 14

What is the final neutron-to-proton ratio from decay?

Answer 14

0.13

Question 15

What do the number of protons and neutrons deduce?

Answer 15

How much helium is in the universe as it is formed from deuterium

Every neutron takes up a proton with it into helium

Question 16

What is the ratio of helium to hydrogen?

Answer 16

4/14
(for 1 neutron there are 8 protons)

Question 17

What causes the ratio of helium and hydrogen to change?

Answer 17

radiation density (time and temperature)

Question 18

What happens at 10^3 seconds?

Answer 18

The universe becomes too cool for elements to form

Question 19

What happens to the universe as more radiation is added?

Answer 19

Temperature goes up and universe is younger as there are more neutrons and more 4He

Question 20

What influence do baryons (baryon density) have?

Answer 20

The more baryons you have the more reactions you have and more 4He is formed

As the density rises 7Be is converted back into 7Li (as they have the same weight and just swap neutrons and protons (it gets hot enough to allow fusion of light elements)

Question 21

Is dark matter baryonic?

Answer 21

No

Question 22

What fraction of total energy density is baryonic?

Answer 22

1/8

Question 23

What is the critical baryonic density parameter (Ω_b h^2)?

Answer 23

0.04

Question 24

What is the optimal baryonic density parameter (Ω_b h^2)?

Answer 24

0.02 (observations give tight limits for deuterium)

Question 25

Where is helium created and how is it observed?

Answer 25

In stars (dwarf galaxies)

Observed in galaxies with little previous star formation as stars with previous star formation create oxygen so abundances of the two need to be compared and corrected for oxygen

Question 26

Is Helium-3 easy to detect?

Answer 26

No

Question 27

Why is deuterium (heavy hydrogen) not formed in stars?

Answer 27

It is too easily destroyed and all deuterium is primordial

Question 28

Is deuterium easy to detect?

Answer 28

No as primordial deuterium is also destroyed in stars

Question 29

Why does deuterium appear less abundant than it is?

Answer 29

It gets locked into dust grains, making it appear less abundant than actually is

Question 30

How is deuterium seen?

Answer 30

As an absorption from gas along line of sight to a background star or quasar

Question 31

Which D/H ratio (ppm) should be used to find deuterium?

Answer 31

25 ppm at 275 light years

Question 32

Where is lithium destroyed and where is it not?

Answer 32

Destroyed in stellar cores but not destroyed in outer atmosphere

Question 33

Where are metal poor stars located?

Answer 33

In our galaxy’s halo - absorption lines due to lithium

(they are closest to primordial abundance as abundance is smaller than expected from other elements)

Question 34

What cannot be dark matter?

Answer 34

cannot be “ordinary neutrinos”

If it was considered to be made of very heavy neutrinos, the early universe would be matter dominated. Light neutrinos remain relativistic until late

cannot be baryonic as different density parameter values (Ω_matter = 0.3)

Question 35

What do dark matter particles have to be?

Answer 35

Non-relativistic cold particle which is either low mass and rare
(this is to avoid making the universe matter dominated)