Astro Final Exam Study Guide - 80 MC Questions
● Olber’s Paradox
○ Why is the night sky dark? → The Universe is expanding… and We cannot see
an infinite Universe.
● Interpretation of Hubble Law: Hubble’s Law demonstrates that the Universe is expanding
in a systematic way: Hubble Parameter→ Rate of expansion today.
● Nature of Expansion
○ General Expansion of Spacetime
■ All observers in different galaxies see the same expansion around them.
■ No center - all observers appear to be at the center.
○ What is the recession velocity?
■ NOT motions through space…
■ Expansion of spacetime: galaxies carried along.
● The Big Bang: If we run the clock back far enough, eventually the size of the Universe
would be: zero size and therefore infinite density
○ Infinitely hot
○ This initial state must have existed at some finite time in the past.
○ We call this hot, dense initial state the “Big Bang”
■ In the beginning it must be very compact → The Big Bang Theory
■ • The Big Bang: the Universe is expanding from a hot, dense, initial state.
● As universe expands: recession velocities get larger; light waves get stretched and
redder; “Cosmological redshift” of light
○ All galaxies (with very few exceptions) are receding from us.
○ Recession is quantified in terms of the “cosmological redshift” of the galaxy, z
■ Not a Doppler shift: measures expansion of spacetime, not motions
○ Expansion of space also stretches light: wavelengths get
stretched=longer=redder; bigger distances = bigger stretching
■ Result: The redshift of an object gets larger with increasing distance.
● Big Bang naturally explains the observed Cosmological redshifts.
● The Hot Big Bang
○ What we see now: the Universe is cold and low density; as it expands, it cools;
matter (galaxies) gets further apart.
○ In the past: Universe was smaller, hotter, and denser
■ Is there any evidence of this early hot, dense phase in the past?
● Cosmic Background Radiation-• Prediction of the Big Bang: cosmic microwave
background radiation at T ∼ 3 K
○ The Universe is filled with diffuse, “relic” blackbody radiation.
○ As the Universe expands further: Blackbody photons redshift; spectrum peak
shifts to redder wavelengths, and hence cooler temperatures
■ Cosmological redshift: The stretching of wavelength of light as it travels
through the expanding universe.
■ By today, the spectrum is redshifted down to T~3K ○ Discovery→ 1965: Penzias and Wilson (Bell Labs)
■ Mapping sky at microwave wavelengths
■ Found a faint microwave background noise.
■ First thought it was equipment problems; finally determined it was cosmic
● Won the Nobel Prize in 1978 for discovering the Cosmic
● The Big Bang model makes very specific predictions:
○ The cosmic background radiation spectrum is a perfect blackbody
○ Characterized by a single temperature
○ 1965-1990: Experiments with balloons, rockets, & radio antennas show a rough
blackbody spectrum; temperature ~2.7 K
● Cosmic Background Explorer Satellite (COBE); Launched in Nov. 1989
○ Mapped the entire sky at Near-IR to microwave wavelengths
○ Searched for fluctuations in the background as evidence of early-large
● The COBE results confirm and greatly strengthen the Big Bang Model: The
cosmic background radiation has (as predicted)...
○ Perfect bla