Midterm 2 Flashcards
How is the luminosity distance, dL, defined?
dL = (L / 4 pi F)^(1/2)
How is the angular diameter distance, dA, defined?
da = l / dtheta
Why can’t we directly measure the proper distance, dp?
Inflation + we don’t have a long enough ruler
What is meant by a “standard candle” and a “standard ruler” ?
An object whose luminosity or size is known from well-known physics
List two standard candles currently used by astronomers.
Cepheids, Type 1a supernovae
Why is it important to observe standard candles at redshifts greater than z = 0.025 to obtain a precision measurement of H0?
You have to look at redshifts high enough to remove the noise of proper motion compared to H0
Why is it important to observe standard candles at redshifts greater than z = 0.25 to obtain a precision measurement of the deceleration parameter, q0 ?
At low redshifts, you cannot differentiate between cosmological models
Would the observed flux of a supernova Ia at z = 0.3 be brighter in a Universe with ΩΛ,0 = 0 or ΩΛ,0 = 0.7? (Assume Ωm,0 = 0.3 in both cases.) Show your work or describe your reasoning.
ΩΛ,0 = 0 would be brighter (~0.25 mag) as the ΩΛ,0 = 0.7 universe has expanded more due to Λ
How can astronomers use type Ia supernovae to measure distances when their brightness changes with time?
The brightness has a known period-luminosity relationship, i.e. the brighter Sne have a longer peak brightness, so correcting for this relation provides a (mostly) clean light curve to calculate distance
Name four observations that suggest the existence of dark matter
Spiral galaxy rotation curves, x-ray emission / SZ Effect (CMB photon scattering), Virial Thereom, Gravitational Lensing
How do we know that most dark matter can’t be made of baryonic matter, for instance, dark stars like brown dwarfs?
Surveys using microlensing searching for brown dwarfs in the galaxy have been done, and found they contribute AT MOST 20% to the Ωbaryon value
What is the largest difficulty faced by experiments trying to (i) directly or (ii) indirectly detect WIMPS?
(i) low incidence rate, high background noise
(ii) hard to rule out similar radiation from other sources
Explain how observations of the Bullet Cluster help prove the existence of dark matter. It may be useful to draw a diagram.
Two clusters collide
- Galaxies are far apart, simply fly past each other
- Intracluster gas is fluid, so collides
This gas is left behind as it collides
We know most baryonic mass is in the gas
However, we see that most mass is still concentrated in the cluster (from lensing)
Conclusion: non-collisional dark matter/mass in clusters
