Becoming a PhD Candidate at MIT
(5/8/20) After taking a number of classes in the foundations of physics (graduate mechanics, E&M, quantum, and statistical physics), and survey courses in astrophysics, the final part of becoming a physics PhD candidate includes taking an oral exam, proctored by three members of the MIT astrophysics faculty. To get you going, they allow you to pick your first topic of discussion, which you typically discuss for 15-20 minutes or so. Then they move on, probing the breadth and depth your knowledge in other areas, as well as checking your intuition for astrophysics.
As a scientist, I really like making measurements and connecting them to the theory. I don't feel like I own the science if I don't really own and understand the data. So, for my prepared question, I chose to discuss the dispersion measure-redshift relation for fast radio bursts, which connects observables to interesting cosmological information.
"How does the dispersion measure of a fast radio burst relate to its redshift,
and how does that relation include cosmological information?"
- What fraction of the baryonic mass is within the IGM? How do we arrive at this number? (I knew it's 0.85, but I didn't know how to get the number). (MT)
- What are several components of the ISM? Is the ISM completely ionized? (talked about three phase model) (MV)
- What’s the average electron density of the Milky Way? (0.03 cm^-3) (MM)
- What’s the average dark matter density on Earth? (I didn't know this. Offered to calculate it given the mass of the MW and the DM halo radius. Turns out it's 0.1 GeV / cm^3) (MV)
- Is the IGM completely ionized? (I said yes) (MT)
- How do we detect the IGM? (Thermal bremsstrahlung emission, x-ray absorption lines) (MM)
Dark matter:
- What is the dark matter profile of the MW, and what is the power law exponent of the density as a function of radius at small and large radii? (MV)
- Why is Vera Rubin famous? What power law exponent corresponds to a flat rotation curve? (MT)
- Why are Navarro, Frenk, and White famous? (MT) (Profile explains dark matter only simulations)
- How do we know there’s a SMBH at the center of the Milky Way? What experiments measure the mass there? What’s the typical speed of a star going around the galactic center? (Had to calculate the last one, 10^3 km/s) (MT)
High Redshift Cosmology:
- How does the CMB constrain omega_curv? (MV)
- How large is the sound horizon at recombination? (MV)
- How does one constrain omega_matter, omega_baryon from the CMB power spectrum? (MV)
- What if reionization happened at a redshift of z = 50 instead of z = 7? (MT)
- How does that affect the observed power spectrum of the CMB? (mumbled something about the Sachs-Wolfe effect, the finite duration of recombination, and then said the optical depth to reionization, which was what they actually wanted) (MT)
- Draw the ionization fraction of the universe as a function of time (z = 1100 to 0) (MT)
- What other constraints on reionization exist? (GP trough in high-z quasars) (MT)
- What are some 21 cm experiments probing reionization? (MT) (said SKA, CHIME, HERA, EDGES, PAPER, ASKAP, MWA)
- What is the observable in EDGES? (MT)
- Who has measured the matter power spectrum? (MT) (nobody directly, a few studies in cross-correlation, including my advisor Kiyo Masui's thesis)
- Which of these experiments would you bet money on detecting reionization? (MT)
- What did they cross correlate the matter power spectrum with? (MT) (galaxy surveys)
- What’s the highest redshift quasar observed (I said z = 7.5) (MT)
- Draw the spectrum of a high redshift quasar (MT) (Ly alpha, GP trough, Si IV line)
Low Redshift Cosmology
- What is a Type 1a supernovae, what causes it, and what is the luminosity distance? (MT)
- Explain the cosmic distance latter from Cepheids to Sn1a. (MT)
- How do the value of Omega_lambda and Omega_matter affect the luminosity distance to, e.g., a type 1a supernova? (I did the integral for a matter dominated and DE dominated universe) (MT)
- Are they really standard candles? (No, need to calibrate by the decay time of the light curve) (MT)
Exoplanets/Stars
- What’s Kepler/TESS? How are they different? (MV)
- Different methods to search for exoplanets? (MV)
- What is the primary technical challenge in these four methods (Transit, RV, Direct imaging, microlensing)? (MM) (I said shot noise, inclination angle degeneracy, blocking out host star, and rarity of microlensed exoplanets)
- Draw the sun’s track on an HR diagram. How long will the sun stay on the main sequence? (MV)
- What’s the efficiency of the pp chain? (MV)
- How fast does the sun burn mass? (MV) (I said “a few hundred kg per year”, before realizing I was way way off. They really didn’t like this. The actual number is 400 metric tons per second.)
- How long would a 2 solar mass star live on the main sequence? (MV)
Rapid fire
- What is GAIA?
- What are upcoming missions to detect exoplanets? (JWST, WFIRST)
- Which of the four methods will JWST use to detect exoplanets? (Direct imaging) Why is the sky blue? (Rayleigh scattering) (MT)
- What’s the wavelength dependence? (lambda^-4) (MT)
- Why isn’t it violet? (Sun doesn’t produce that much UV)