# Cosmology

Questions in this category have been asked by MT, PJ, RS, SR, JW

• Describe the evolution of the Universe backwards in time from now to the beginning. Include the relative abundances of ALL particles (bosons & fermions) in the description.
• What is Omega_DarkMatter?
• Can it be neutrinos & how do we know?
• What is the flatness problem?
• How can you use inflation to explain it? (using Freedman eqs)
• What are other importances of inflation?
• What is CMB anisotropy?
• What is the value and the position of the first acoustic peak and its physics?
• What do you know about the CMB spectrum?
• Why do we need BBN to account for the abundance of He?
• Why can't BBN produce heavier elements but a star can?
• What do you know about dark energy?
• What is the Lyman-alpha forest?
• What priors go into measuring cosmological parameters from the CMB?
• (Hint: The "omegas" don't just depend on the energy densities. What else do they depend on?)
• In the SNe Ia measurements of cosmic acceleration, how can we be sure our measurement hasn't been messed up by dust?
• Helping question: Sketch the plot that comes out of these measurements, including both the measured curves and the curves that one would get if there were no cosmic acceleration but simply dust extinction.
• What methods of measuring H_0 do not depend on the cosmic distance ladder?
• Describe how the SZ effect allows one to measure distances.
• Follow-up: What if the cluster were REALLY close to us? Would this method work well? (Hint: What would the angular extent of the cluster be? How would that compare to a typical scale of the "background" that's used in the SZ effect).
• How does one use quasars to measure H_0? Walk through exactly what's done (i.e. what do I do with my telescope, what do I plot etc.)

The following was asked by MT

• At what temperature did recombination happen?
• why is this temperature different from 10,000K for which ionization happens in stars?
• What is the SZ effect? Can the photon lose energy to the electron? What is the use of the SZ effect in cosmology? What is the temp of the gas around a cluster where SZ effect is happening? What is the cross over frequency for it to occur?
• go back in time, when is the next change in the "chemistry" of the

universe?

• how much helium is in the universe?
• Write down the Friedman equation. What are the numerical values of the various quantities? How will they change with time?
• How old was the universe when recombination happened? Can you calculate it from the redshift?
• Draw a log-log diagram with X-axis to be scale factor of the universe and Y-axis to be the density. Plot the density evolution of the universe on the diagram.
• Write down the Friedmann equation (in terms of Omegas). (SR)
• Interpreting this as an energy equation, what terms correspond to total energy, potential energy, and kinetic energy?
• How does the potential energy vary with the scale factor (draw a plot)?
• Calculate the scale factor at the maximum potential energy.
• Show where we are today on the plot.
• What does the l=1 multipole of the CMB power spectrum correspond to? (SH)
• What is the speed of the Earth around the sun? The Sun around the Galactic center? The Local Group with respect to the CMB?
• What sets the scale of the 1st Acoustic Peak in the CMB power spectrum? What's the sound speed in the photon baryon fluid? Derive the sound speed. Why is $P=1/3\rho$ for photons? Where does the 1/3 come from? (MT)
• WMAP often quotes an "optical depth to reionization." What is meant by this? Calculate this optical depth in terms of an integral over redshift. (MT)
• What is the difference between the density perturbations at matter-radiation equality and those at recombination? (MT)
• Given an equation of state parameter w, how does the density depend on scale factor?
• For what values of the equation of state parameter w will a substance cause the universe to accelerate? What is w for the cosmological constant? (MT)
• Explain how you can get matter-radiation equality?
• What time and z is this? (SR)
• Derive when the universe starts to accelerate. What is this value? (MT)
• What determines if the universe expands forever or collapses?
• What happens if Lambda is negative?
• What are three ways to get Omega baryons?
• When is/isn't surface brightness conserved?