Questions

Review Questions

Question 15.1

What is Olbers’s paradox? How can it be resolved?

Question 15.2

What does it mean when astronomers say that we live in an expanding universe? What is actually expanding?

Question 15.3

Describe how the expansion of the universe explains Hubble’s law.

Question 15.4

If a galaxy was discovered to be the most distant ever observed, what would astronomers notice about its spectra and speed?

Question 15.5

What does it mean to say that the universe is homogeneous? That it is isotropic?

Question 15.6

How is the expanding universe similar to a baking chocolate chip cake?

Question 15.7

Where is the center of the universe?

Question 15.8

Imagine an astronomer living in a galaxy a billion light-years away. Is the observable universe for that astronomer the same as for an astronomer on Earth? Why or why not?

Question 15.9

What is the cosmic microwave background?

Question 15.10

What was the era of recombination? What significant events occurred in the universe during this era? Was the universe matter-dominated or radiation-dominated during this era?

Question 15.11

What is dark energy? Describe two ways that we can infer its presence.

380

Web Chat Questions

Question 15.1

How can astronomers be certain that the cosmic microwave background fills the entire cosmos, not just the vicinity of Earth?

Question 15.2

How does the evidence for the Big Bang confirm or conflict with religious or spiritual views of the beginning of time?

Question 15.3

Some GUTs predict that the proton is unstable, although with a half-life far longer than the present age of the universe. What would it be like to live at a time when protons were decaying in large numbers?

Collaborative Exercises

Question 15.1

As a group, create a four- to six-panel cartoon strip showing a discussion between two individuals describing why the sky is dark at night.

Question 15.2

Imagine your firm, Creative Cosmologists Coalition, has been hired to create a three-panel, folded brochure describing the principal observations that astronomers use to infer the existence of a Big Bang. Create this brochure on an 8½ × 11 piece of paper. Be sure each member of your group supervises the development of a different portion of the brochure and that the small print acknowledges who in your group was primarily responsible for which portion.

Question 15.3

The three potential geometries of the universe are shown in Figure 15-17. To demonstrate this, ask one member of your group to hold a piece of paper in one of the positions while another member draws two parallel lines that never change in one geometry, eventually cross in another geometry, and eventually diverge in another.

Question 15.4

The four fundamental forces of nature are the strong force, the weak force, the gravitational force, and the electromagnetic force. List four things at your school that rely on one of these fundamental forces, and explain how each thing is dependent on one of the fundamental forces.

Question 15.5

Consider the following hypothetical scenario adapted from a daytime cable television talk show. Chris states that Pat borrowed Chris’s telescope without permission. Tyler purchased balloons and a new telescope eyepiece without telling Chris. Sean borrowed star maps from the library, with the library’s permission, but without telling Pat. Eventually, when the four met on Sunday evening, Chris was crying and speechless. Can you create a “grand unified theory” that explains this entire situation?

Observing Questions

Question 15.1

In an attempt to explore the far reaches of the universe, the Hubble Space Telescope (HST) took long-exposure images of very dark regions of space that appear to contain no bright stars or galaxies. These images, known as the Hubble Deep Field and Hubble Ultra Deep Field images, reveal very rich fields of faint and very distant galaxies. The light now arriving at Earth from some of these galaxies has traveled for more than 13 billion years and was collected by the HST at a rate of a few photons per minute! This light was emitted very early in the life of the universe, only a few 100 million years after the Big Bang. You can examine and measure these two images.

  • a) In Starry Night™, open the Options pane and ensure that the Hubble Images option is checked in the Deep Space layer. Open the Find pane, ensure that the search edit box is empty, and click on the icon in this box to display a list of image sources. Click on Hubble Images and double-click on Hubble Deep Field to center the view on this dark region of space. Note its position with respect to the Big Dipper. (Note: If you cannot identify this region of the northern sky, click on View > Constellations > Asterisms and View > Constellations > Labels. Remove these indicators after you have identified the region.) Zoom in to a field of view about 3° wide and note that the region still appears to be devoid of objects. Zoom in again until the Hubble Deep Field (HDF) fills the view. One-quarter of the full HDF, with dimensions of 1.15′ × 1.15′, is displayed in Starry Night™. The bright object with spikes radiating from it is a star in our own Galaxy (the spikes are caused by diffraction by the supports for Hubble’s secondary mirror), but all of the other objects that appear on this long-exposure image are galaxies containing millions of stars. Examine this image carefully and attempt to identify some examples of each kind of galaxy—spiral, barred spiral, elliptical, and irregular—in this field. Choose 5 or 6 of the largest galaxies in this field, record their shapes and galaxy types, and use the angular separation tool to measure carefully and record their angular dimensions.

  • b) Click the Zoom panel in the toolbar and select 90° from the dropdown menu. Return to the Find pane and the list of Hubble Images and double-click on Hubble Ultra Deep Field (HUDF) to center the view of this “dark” region of the sky. Zoom in on this region and note that, even at a field of view as small as 2°, no objects can be seen in the position of this long-exposure image. Zoom in further until the full HUDF, with dimensions of 3.3′ × 3.3′, fills the field of view to see this rich field of faint and very distant galaxies. Examine this image carefully and attempt to identify each kind of galaxy—spiral, barred spiral, elliptical, and irregular—in this field. Again, select 5 or 6 of the largest galaxies in this field, record their shapes and galaxy types, and use the angular separation tool to measure their dimensions.

  • c) Consider the mix of different kinds of galaxies and assess whether the proportions of different kinds are the same in these two images. Compare the angular sizes of the largest galaxies in these two images.

Question 15.2

Use the Starry Night™ program to examine a very distant pair of quasars. Most quasars are starlike in appearance but their spectra show very large redshifts. If these redshifts are cosmological (i.e., caused by the expansion of the universe), then quasars obey Hubble’s law, and their large redshifts show that they must be very far away. Since they appear bright in our sky, they must be very luminous, with outputs equivalent to that of billions and even trillions of Suns. They are part of a group of objects known as Active Galactic Nuclei. The source of energy output of these very luminous objects is probably a supermassive black hole at the object’s center. Select Favourites > Explorations > Quasar Pair and Zoom in to a field of view about 30 arcseconds wide. Open the Info pane for this pair of quasars and note the Distance from observer as given in the Position in Space layer.

  • a) What is the distance from Earth of these quasars?
  • b) Using a Hubble constant of 73 km/s/Mpc, calculate the expected recessional velocity of this quasar pair. (Note: 1 pc − 3.26 ly.)
  • c) What is the ratio of the recessional velocity of these quasars to the speed of light?