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# 7 Quiz

Answer the following questions in order to test your understanding of the key ideas that you have been learning about.

## Question 1

The galaxy NGC1234 has a redshift of z = 0.0113. Assuming the Hubble constant is 67.7 km s-1 Mpc-1, what is the distance to NGC1234 (to 2 significant figures)?

0.23 Mpc
5.0 Mpc
21 Mpc
34 Mpc
50 Mpc
68 Mpc

### Answer

The Hubble constant implies that the apparent recession speed of galaxies increases by 67.7 km s-1 for every increase in distance by 1 Mpc. Using Equation (1), , so D = 50 Mpc.

## Question 2

Which of the following is the correct description for the expansion of the Universe?

The expansion rate of the Universe has been constant since soon after the big bang.
The expansion rate of the Universe has constantly decelerated since soon after the big bang.
The expansion rate of the Universe has constantly accelerated since soon after the big bang.
The expansion rate of the Universe initially decelerated soon after the big bang but has accelerated more recently.
The expansion rate of the Universe initially accelerated soon after the big bang but has decelerated more recently.
The expansion rate of the Universe cannot be measured.

### Answer

Soon after the big bang, the expansion rate of the Universe decelerated because of the dominance of matter. About 6 billion years ago, dark energy came to dominate the Universe causing the expansion rate to accelerate.

## Question 3

Which one of the following statements about the CMB is true?

The CMB radiation currently has a spectrum that is characteristic of a temperature of about 3000 K.False
The CMB radiation is perfectly uniform in temperature and intensity across the entire sky.False
The CMB radiation is today detected mainly in the infrared part of the electromagnetic spectrum.False
The fluctuations in the CMB radiation across the sky imply that the geometry of the Universe has positive curvature.False
The fluctuations in the CMB radiation across the sky imply that the geometry of the Universe has negative curvature.False
The CMB radiation is a relic of the decoupling of photons from atoms that happened 380,000 years after the big bang.True

### Answer

Only the last statement is true.

## Question 4

Which of the following statements about baryonic and non-baryonic matter are true?

Baryonic matter makes up about 5% of the matter-energy density of the Universe.True
Non-baryonic matter makes up about 25% of the matter-energy density of the Universe.True
Baryonic matter comprises the familiar protons, neutrons and electrons.True
Non-baryonic matter may be composed of WIMPS and/or axions.True
Baryonic and non-baryonic matter together cannot account for all of the critical density of the Universe. True
Baryonic and non-baryonic matter exceed the critical density of the Universe.False

### Answer

All statements are true except the last one.

## Question 5

Which is the correct order of contributions to the overall matter-energy density of the Universe from smallest to largest?

baryonic matter, dark energy, dark matter
dark matter, dark energy, baryonic matter
baryonic matter, dark matter, dark energy
dark matter, baryonic matter, dark energy
dark energy, baryonic matter, dark matter
dark energy, dark matter, baryonic matter

### Answer

The correct order is baryonic matter (5%), dark matter (25%), dark energy (70%).

## Question 6

Which values of the equation of state parameter are associated with each of the following types of fluid?

baryonic matter

radiation

cosmological constant

quintessence

Using the following two lists, match each numbered item with the correct letter.

1. 1

   baryonic matter

2. 2

   radiation

3. 3

   cosmological constant

4. 4

   quintessence

The correct answers are:

- 1 =
- 2 =
- 3 =
- 4 =

### Answer

The equation of state parameter is zero for matter, minus one for a cosmological constant, +1/3 for radiation, and a function of the scale parameter for quintessence.

## Question 7

Which of the following problems in cosmology cannot be solved by inflation?

The monopole problem
The flatness problem
The horizon problem
The problem of the accelerating expansion of the Universe
The problem of dark energy
The problem of dark matter

## Question 8

What type of curvature does the universe have for each of the following combinations of matter density (Ωm) and dark energy density (ΩΛ)? (Assume that only matter and dark energy are present in these hypothetical universes.)

flat

Ωm = 0.25, ΩΛ = 0.75

negative

Ωm = 0.05, ΩΛ = 0.50

positive

Ωm = 0.65, ΩΛ = 0.65

Using the following two lists, match each numbered item with the correct letter.

1. flat

2. negative

3. positive

1. 1

   Ωm = 0.25, ΩΛ = 0.75

2. 2

   Ωm = 0.05, ΩΛ = 0.50

3. 3

   Ωm = 0.65, ΩΛ = 0.65

The correct answers are:

- 1 =
- 2 =
- 3 =

### Answer

If Ωm + ΩΛ > 1, the universe has positive curvature; if Ωm + ΩΛ < 1, the universe has negative curvature; and if Ωm + ΩΛ = 1, the universe is flat.

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OpenLearn - Unsolved problems in cosmology
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