6. The Even Earlier Universe Flashcards
Why should only 1-2º patches of sky be in thermal equilibrium?
The size of the horizon at recombination was about 200Mpc
If we work back to recombination from now, only 1-2º patches should be in thermal equilibrium
Can be solved if horizon size increases more slowly than expansion rate, a
Why is the CMB the same temperature everywhere?
Horizon size increases more slowly than expansion rate, a
What happens to the light element ratios if total matter density increases, but baryon density remains constant?
Not much
Only baryons take part in nucleosynthesis
What happens to the light element ratios if total matter density is constant, but baryon increases?
More reactions - more heavier elements (He4) produced relative to the lightest
What happens to the light element ratios if radiation density increases?
T/T0 prop to rho^1/4 * t^1/2
So at a fixed temperature, time decreases
So element formation happens earlier (i.e., universe is younger) since T dependent
So neutron decay is later - so more 4He can be produced
In nucleosynthesis, what is time and temperature dependent?
Element formation is temperature dependent
Neutron decay is time dependent
What happens to the light element ratios if a neutrino of mass 1GeV (but rare) exists?
1GeV»_space; 0.1 MeV (when nucleosynthesis occurs)
so the neutrino is non-relativistic and acts like ordinary matter - no effect
Which neutrino gets involved in nucleosynthesis?
Electron neutrino - as it is the lightest
What happens to the light element ratios if the neutron half life is ten times larger?
More heavier elements get a chance to form
If its loads longer then neutron decay will happen after element formation is complete anyways (as density and temperature too low)
What happens to the light element ratios if the neutron half life is ten times shorter?
Fewer heavier elements have time to form
What happens to the light element ratios if weak reactions freeze-out after the EM ones rather than before?
Neutrinos would be hotter than the photons (extra energy)
So more energetic neutrinos around at the time of D2 formation
Potentially this would slow the neutron decay a bit, since there may be enough high energy anti-neutrinos to convert some of the free protons into neutrons
So may get a few more neutrons
What is the fundamental fusion reaction that underpins all other element formation processes?
Deuterium formation
n + p -> D2 + photon
Why does 2D formation set a timescale for nucleosynthesis to occur?
2D is unstable (low binding energy), so easily destroyed by high energy photons
So start time is set when universe is cool enough - so that number of high energy photons is less than the number of 2D ions
Which gives the greatest constraint on the primordial light elements?
Deuterium to helium-4 ratio
(diverging on graph)
Which is the most accurate observational measure of all the primordial light elements?
Deuterium - not formed in stars so all D is cosmological in origin (hard to measure)
But He is the easiest to measure - abundant, but easiest to make so hard to get primordial values
Why do we not use He3 as a constraint on light element formation?
Doesn’t change much in critical range and hard to see
Why do we not use Li as a constraint on light element formation?
Not well understood
Why do we not use Be as a constraint on light element formation?
Not that abundant in the first place, and formed in stars
Condition for freeze out?
H (expansion rate) > reaction rate
i.e. universe expanding too quickly for reactions to happen
In the reaction rate formula, what is number density proportional to (i.t.o. a, T)?
n prop to 1/V = 1/a^3
Since T0/T=a
n prop to T^3
For radiation, what is T prop to?
1/a
Can a second order phase transition give rise to inflation?
No - it has no latent heat