Our Dynamic Universe Part 2

Question 1

Name the quantities shown in this formula.

Answer 1

v = recession velocity of galaxy

H₀ = Hubble’s constant

d = distance to galaxy

Question 2

What is the unit for frequency?

Answer 2

The unit for frequency is hertz (Hz).

Question 3

What key idea about the universe is shown in this graph?

Answer 3

FARTHER = FASTER

The further away a galaxy is, the faster it is moving (relative to us).

Question 4

What is meant by ‘time dilation’?

Answer 4

Time dilation is the difference in a time interval as measured by an object moving relative to a stationary observer.

Question 5

Explain what this diagram shows.

Answer 5

The diagram shows redshift.

Redshift is when the wavelength of light from a source (usually a galaxy or star) is increased, based on the velocity at which it is moving away from us.

The overall spectrum for the source is the same, but shifted towards a longer wavelength.

The further away a galaxy is, the faster it is moving, and so the more the light from that galaxy has been redshifted.

Question 6

Name the quantities shown in this formula.

Answer 6

z = redshift

λ_observed = wavelength as measured by an observer (in a different frame of reference)

λ_rest = wavelength as measured when at rest

Question 7

What is the unit for redshift?

Answer 7

Redshift has NO UNITS.

It is simply a ratio of speeds (or of wavelengths).

Question 8

What is the unit for velocity?

Answer 8

The unit for velocity is metres per second (ms^-1).

Question 9

Name the quantities shown in this formula.

Answer 9

z = redshift

v = recession velocity of galaxy

c = speed of light

Question 10

Name the quantities shown in this formula.

Answer 10

t = time

t’ = “t prime” = time as measured by observer

(in a different frame of reference)

v = speed of object

c = speed of light

Question 11

What is the unit for time?

Answer 11

The unit for time is seconds (s).

Question 12

Explain what is being shown in each of the two diagrams:

Answer 12

In the first diagram, a source of sound waves (loudspeaker) is stationary. The frequency detected by an observer is the same as the frequency emitted by the source.

In the second diagram, a source of sound waves (ambulance) is moving. The frequency detected by an observer will be higher as the ambulance moves towards them, and lower as the ambulance moves away from them.

Question 13

Name the quantities shown in this formula.

Answer 13

f₀ = frequency as measured by an observer

f_s = frequency (of moving source)

v = speed (of sound waves)

v_s = speed (of moving source)

Question 14

What can be said about the temperature of these stars, based on their colour?

Answer 14

The hotter the star, the shorter the peak wavelength.

Therefore, Rigel (blue) has the highest temperature of the three stars.

Betelgeuse (red) has the lowest temperature.

Question 15

Explain what this graph shows.

Answer 15

• for a particular temperature of star, there is a characteristic curve called a blackbody radiation curve
• the hotter the star (higher temperature), the more radiation emitted per unit surface area
• the hotter the star, the shorter the peak wavelength (at which the maximum amount of radiation is emitted)

Question 16

The length of an object on Earth is measured by a stationary observer and by an observer moving at high speed (relative to the Earth).

Which length will be greater?

Answer 16

The length measured on Earth will be greater.

The length measured by the moving observer is l’ (“L prime”).

This is length contraction - to the moving observer, an object appears shorter.

Question 17

A time for a particular event on Earth is recorded by a stationary observer on Earth, and by an observer moving at high speed (relative to Earth).

Which time will be greater?

Answer 17

The time as recorded by the moving observer will be greater.

This is t’ (“t prime”).

This is time dilation - to the moving observer, time for an event appears longer.

Question 18

What does Einstein’s Theory of Special Relativity say about the speed of light?

Answer 18

The speed of light in a vacuum is the same for all observers.

Question 19

What is meant by ‘length contraction’?

Answer 19

Length contraction is the difference in length (in the direction of motion) of an object moving relative to a stationary observer.

Question 20

What is the unit for wavelength?

Answer 20

The unit for wavelength is metres (m).

Question 21

Name the quantities shown in this formula.

Answer 21

l = length of object

l’ = “l prime” = length of object as measured by observer

(in a different frame of reference)

v = speed of object

c = speed of light

Question 22

Explain why relativistic effects are not noticed on a passenger flight from UK to USA, even though an observer would be moving relative to the Earth.

Answer 22

Relativistic effects (such as time dilation and length contraction) are only noticeable at speeds greater than 0.1c (10% of the speed of light).

Question 23

Explain what the term redshift means.

Answer 23

Redshift is when the wavelength of light from a source (usually a galaxy or star) is increased, due to the source moving away from us.