• Home
  • Textbooks
  • Pathways to Astronomy
  • The Edges of the Universe

Pathways to Astronomy

Stephen E. Schneider

Chapter 80

The Edges of the Universe - all with Video Answers

Educators

Chapter Questions

01:12

Problem 1

The temperature of the universe needs to be less than $100 \mathrm{K}$ before stars can form. At what redshift does this occur?

Khoobchandra Agrawal
Khoobchandra Agrawal
Numerade Educator
00:55

Problem 2

When our Solar System formed about 4.5 billion years ago, approximately what would the temperature of the CMB radiation have been? Would the structure in the CMB have looked the same as today? Explain your answer.

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
01:01

Problem 3

Imagine that there were so many galaxies that 30 -K dust inside their interstellar clouds blocked our view in every direction. What would their redshift have to be for the dust to have a temperature like the CMB? What problems are there with this model for explaining the observed properties of the CMB?

Farhanul Hasan
Farhanul Hasan
Numerade Educator
02:05

Problem 4

At the time of the last scattering epoch (at a redshift $z=1000$ ),
a. what was the average density of normal matter, if today it is about $3 \times 10^{-31} \mathrm{kg} / \mathrm{L} ?$
b. how large a sphere would have contained $10^{11} M_{\odot}$, about as much normal matter as in the Milky Way Galaxy? Express your answer in parsecs.

Farhanul Hasan
Farhanul Hasan
Numerade Educator
01:46

Problem 5

If the average density of matter in the universe is $3 \times 10^{-31} \mathrm{kg} / \mathrm{L}$ what is the total mass of the universe within our current cosmic horizon? How many galaxies of $10^{11} M_{\odot}$ does this represent?

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
02:16

Problem 6

On a flat surface, the angles of a triangle always add up to $180^{\circ}$ but on a spherical surface they may add up to a larger number.
a. Find a triangle on a spherical surface that contains three $90^{\circ}$ angles. Illustrate your example.
b. What's the largest sum of the angles you can find for a triangle on a spherical surface?

Matthew Miranda
Matthew Miranda
Numerade Educator
01:43

Problem 7

Table $79.1,$ in Unit $79,$ shows that material we see at a redshift of 1000 was emitted when the material was 45 million ly away from us. This distance determines the angular size of regions at this redshift, as given by the angular size formula in Unit 10.4
a. Show that a region that is $1^{\circ}$ across at this redshift has a diameter of approximately 800,000 ly.
b. How big is that region today?

Farhanul Hasan
Farhanul Hasan
Numerade Educator