Midterm 3 Flashcards
Number the following events in order where 1=closest in time to the big bang
Recombination
Neutrino decoupling
The energy density of radiation and matter are equal
The peak of Deuterium formation
Inflation
Inflation
Neutrino decoupling
The peak of deuterium formation
The energy density of radiation and matter are equal
Recombination
What is the numerical value of the primordial Helium mass fraction?
Why isn’t it higher? What ultimately limits the formation of He?
Yp = 0.24
Deuterium bottleneck, lack of free neutrons
Why is it useful to measure the primordial abundances of D, 3He, 4He, and 7Li?
Constrains Omega baryon
What event causes proton-neutron “freeze-out” ?
The ration “freezes out” when the time for the neutrino-neutron interaction to occur becomes greater than the age of the Universe
If the era of nucleosynthesis had been delayed by 3 hours relative to the time of proton-neutron “freeze-out”what would the consequences be?
More time for neutrons to decay, so there are fewer neutrons to use when nucleosynthesis starts
Why do overdense regions smaller than the Jeans Length not collapse?
They have an outward pressue greater than/equal to the force of gravity collapsing in
Why does baryon-photon decoupling cause a sudden drop in the Jeans Length?
P - wE ; The EOS for the baryon-photon soup is w = 1/3, but when matter decoupled from photons, w=0 because matter is pressureless
What observation suggests that the Universe is filled with cold rather than hot dark matter?
Hierarchical galaxy formation
What is the best guess as to why the baryon-to-photon ratio has the value η ~ 6x10-10? How does it evolve with redshift?
Slight asymmetry (800,000,003 to 800,000,000) in the matter-antimatter annihilations, resulting in 1.6 billion photons per baryon. It does NOT evolve with redshift
Why, physically, can’t baryonic density fluctuations grow prior to decoupling?
Baryonic matter fluctuations with sizes smaller than the hubble distance cannot grow, they just oscillate like sound waves. This is because they were pressure-supported and could not collapse
Why does the CMB provide the best measure of the curvature Ωk? Explain how you would measure it.
The first peak of the plot gives us the curvature Ωk (flat). We measure this by large surveys (SDSS) and fit these to the plot to compare fit. Can also use the CMB to constrain Ωtotal
Describe the “horizon problem” and how inflation provides a solution.
We do not expect all parts of the observable universe to be so homogeneous or “causally connected” but the CMB shows us this is the case. Inflation solves this by stating that due to inflation, the universe WAS causally connected, explaining the homogeneity and solving the fine-tuning problem
What is the approximate value of the baryonic Jeans mass, MJ, just before and just after decoupling?
Before : Mj = 10^19 solar masses
After: Mj = 2x10^5 solar masses
Describe what the 2-point correlation function measures.
It is the “excess probability” of finding a galaxy at a distance r from some other galaxy relative to a purely random distribution
How is the 2-point correlation function related to the power spectrum?
It is a Fourier transform of the power spectrum
