Exam 15: Galaxies and the Foundation of Modern Cosmology

Which of the following is not a piece of evidence supporting the conclusion that active galactic nuclei are powered by accretion disks around massive black holes?

Based on counting the number of galaxies in a small patch of the sky and multiplying by the number of such patches needed to cover the entire sky, the total number of galaxies in the observable universe is estimated to be approximately

How did Edwin Hubble measure the distance to the Andromeda Galaxy?

The larger the value of Hubble's constant, the more rapid the expansion of the universe and the smaller (and hence younger) our estimate for the age of the universe.

Explain how we can estimate that there are about 50-100 billion galaxies in the observable universe.

According to current understanding, what is a quasar?

What makes white-dwarf supernovae very good standard candles for distance measurements?

Does It Matter What the Universe Expands Into? A natural question of anyone who meets Hubble's Law for the first time is to ask what exactly the universe is expanding into. This is often further motivated by the analogy of an inflating balloon universe, where two-dimensional galaxies ride along on the surface of the balloon. Imagine, that you live in one of these balloon-borne galaxies. Your universe is the surface of the balloon. You can move along the surface, but you can never exist above or below it. Now one of your two-dimensional friends asks what your (balloon) space is expanding into. Do you need to answer this question, or can the expansion you experience be completely described without reference to an unseen, third-dimension? Imagine someone argues that question "what is the universe expanding into?" does not even need an answer! Would you agree or disagree? Explain. If you disagree, would there be any way to gather scientific evidence to answer this question?

Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) ________.

Which of the following statements is not an assumption used in models of galaxy formation?

Where are the X-rays produced that are emitted by quasars and other active galactic nuclei?

Which of the following is true about irregular galaxies?

Using the technique of main-sequence fitting to determine the distance to a star cluster requires that ________.

Which of the following is an important starting assumption in models of galaxy formation?

How do observations of galaxies at different distances help us learn about galaxy evolution?

Why should galaxy collisions have been more common in the past than they are today?

Based on the number of galaxies visible in the Hubble Deep Field (Figure 15.1 in your textbook), the estimated number of galaxies in our observable universe is about ________.

If we say that a galaxy has a lookback time of 1 billion years, we mean that ________.

Briefly explain white-dwarf supernovae are useful for measuring cosmic distances.

Recall that Hubble's law is written v = H₀d, where v is the recession velocity of a galaxy located a distance d away from us, and H₀ is Hubble's constant. Suppose H₀ = 20 km/s/Mly. How fast would a galaxy 1000 Mly distant be receding from us?