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
Rough temperature after inflation?
Very cold - 10^-15 K
If a second order phase transition caused inflation, would there be a problem with baryogengesis?
Yes
Breaking up of EM and weak forces is 2nd order phase transition, but also when we think baryon asymmetry is created.
The fact we’re here is due to baryogengesis, this has to happen after inflation otherwise products would be inflated away
What is the fundamental condition that gives rise to inflation?
Acceleration
What equation of state parameter is required for inflation?
w < -1/3
(could be cosmology constant w = -1/3)
How does nucleosynthesis occur?
T«1GeV so n and p are non-relativistic (MB stats)
n and p are kept in balance by weak reactions
Universe cools - at 1MeV, weak interactions freeze out, equilibrium between p and ns stops
free neutrons decay, creating protons, electrons and neutrinos (time dependent)
deuterium can form as photons are lower energy so won’t break it apart
What do weak interactions vs neutron decay depend on?
Weak interactions temp dependent
Neutron decay time dependent
What does the ratio of light elements constrain?
Number of relativistic neutrinos
Density of baryons
Can baryons make up DM?
No
Why can’t DM be ordinary neutrinos?
Light neutrinos remain relativistic until late, smearing out the forming galaxies
Very heavy neutrinos as common make early universe matter dominated
To avoid making the early universe matter-dominated, what does DM need to be?
Need a non-relativistic (cold) particle that is either low mass or rare
What are the 3 main problems with big bang theory?
Flatness – why is Ω0 ~ 1?
Horizon – why is CMB smooth across the sky?
Relics from the past – where are the heavy relic
particles?
Why is it a problem that CMB temperature is the same across the entire sky? And how is this solved?
CMB looks same in all directions and formed at decoupling when horizon ~ 1-2 degrees now on sky
So patches larger than 1-2 degrees should not be in
thermal equilibrium
Solve this if horizon size increases more slowly than expansion rate a
Why is the lack of presence of heavy relics an issue?
All unification models for particle physics predict the
presence of heavy particles that should be present in
the early universe
Would make universe matter dominated in very early stages
How is the problem of lack of heavy relic particles solved?
If Universe expands greatly after these particles are last formed (can’t form after inflation as not energetically favourable)
What is inflation?
Any point when the universe is accelerating
Condition for inflation?
ä > 0
What happens at the end of inflation?
Reheating (x+xbar - 2photons)
Why does reheating tell us about matter - antimatter imbalance?
Can happen no earlier than inflation or it would be inflated away
Cause of inflation?
Symmetry breaking in particle physics
When does inflation happen?
At energy of grand unification (10^15GeV)
Does a phase 2 transition give inflation?
No
Only phase 1 - this gives a lambda like term - this gives acceleration and so inflation
What type of expansion does cosmological constant give?
Exponential
What happens if expansion in inflation goes on too long?
Reheating happens later - less energy - we’re not here
Or eternal inflation
Remaining problems with expansion?
No working GUT yet
Does not explain where matter excess comes from
Gives us expansion but does not explain origin
Where might excess matter come from in inflation?
Type 2 phase transition
What is grand unification?
Unification of EM, weak and strong force
Why can deuterium appear to be less abundant than it actually is?
It can get locked into dust grains
What type of phase transition is the energy gap associated with?
1sr order phase transition - inflation
How do we get a 1st order phase transition?
Strong force breaking away
Features of a second order phase transition?
No energy gap so no inflation
Caused by EM and weak transitions
In the matter / radiation dominated eras, does horizon size or scale factor increase more quickly?
Horizon size
What creates the hot big bang?
End of inflation - reheating
Does the horizon expand during inflation?
Barely
Does the big bang model work?
Mostly yes, after nucleosynthesis
Everything before that is speculative and needs better physical models at very high energies
When did the CMB form?
Decoupling
