Nucleosynthesis Flashcards
What is nucleosynthesis?
The formation of lighter elements e.g.lithium, helium and deterium
Why are high temperatures of 10^9 K needed to form elements in early universe?
Stellar cores are 10^10 times denser than early universe
What is the temperature of the universe?
T_universe «_space;1GeV
What happens at high enough temperatures?
k_B T is greater than Δmc^2
the neutron-to-proton ratio becomes 1 so there is equilibrium
When are neutrons and protons in balance when T > 1MeV?
Through weak interactions:
n + v_e <=> p + e-
n + e+ <=> p + -v_e
What happens below T ~ 1 MeV/ end of equilibrium?
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)
What does a neutron decay into?
proton, electron and electron anti neutrino
What happens at the end of equilibrium (0.8MeV)?
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
Which is the first element to be made and how is it made?
deuterium
proton + neutron
it is highly unstable and has low binding energy
Which elements are created using deuterium?
Lighter elements: 3He and 4He
What is created using 3He and 4He?
Lithium and Beryllium
Why is nothing heavier than 7Be?
There are no stable elements with atomic weight 5 or 8
Density (temperature) drops too quickly by the time 4He is formed
When does nucleosynthesis occur?
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
What is the final neutron-to-proton ratio from decay?
0.13
What do the number of protons and neutrons deduce?
How much helium is in the universe as it is formed from deuterium
Every neutron takes up a proton with it into helium
What is the ratio of helium to hydrogen?
4/14
(for 1 neutron there are 8 protons)
What causes the ratio of helium and hydrogen to change?
radiation density (time and temperature)
What happens at 10^3 seconds?
The universe becomes too cool for elements to form
What happens to the universe as more radiation is added?
Temperature goes up and universe is younger as there are more neutrons and more 4He
What influence do baryons (baryon density) have?
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)
Is dark matter baryonic?
No
What fraction of total energy density is baryonic?
1/8
What is the critical baryonic density parameter (Ω_b h^2)?
0.04
What is the optimal baryonic density parameter (Ω_b h^2)?
0.02 (observations give tight limits for deuterium)
Where is helium created and how is it observed?
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
Is Helium-3 easy to detect?
No
Why is deuterium (heavy hydrogen) not formed in stars?
It is too easily destroyed and all deuterium is primordial
Is deuterium easy to detect?
No as primordial deuterium is also destroyed in stars
Why does deuterium appear less abundant than it is?
It gets locked into dust grains, making it appear less abundant than actually is
How is deuterium seen?
As an absorption from gas along line of sight to a background star or quasar
Which D/H ratio (ppm) should be used to find deuterium?
25 ppm at 275 light years
Where is lithium destroyed and where is it not?
Destroyed in stellar cores but not destroyed in outer atmosphere
Where are metal poor stars located?
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)
What cannot be dark matter?
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)
What do dark matter particles have to be?
Non-relativistic cold particle which is either low mass and rare
(this is to avoid making the universe matter dominated)
