Astrophysics (Cosmology)

Question 1

Describe the Doppler effect.

Answer 1

A change in frequency (or wavelength) due to relative motion between a source and an observer.

Question 2

If a light source is receding (moving away from us), describe what happens to its wavelength and frequency. Give the name for this.

Answer 2

Wavelength increases (wave stretch out).
Frequency decreases.
This is red shift.

Question 3

If a light source is moving towards us, describe what happens to its wavelength and frequency. Give the name for this.

Answer 3

Wavelength decreases (wave compress).
Frequency increases.
This is blue shift.

Question 4

Give the equation for red shift (z) in terms of recessional velocity (v) and speed of light (c). What do positive and negative values represent?

Answer 4

Positive z = light source moving away from us.
Negative z = light source moving towards us.

Question 5

How can red and blue shift be observed?

Answer 5

Measure the wavelength or frequency of known absorption lines in the observed spectrum.
Compare the observed values and the laboratory values.

Question 6

Give the equation for red shift (z) in terms of frequency. Define any terms.

Answer 6

z = Δf / f
Δf = f - f_obs
f = actual frequency emitted (given by laboratory value)
f_obs = observed frequency (from spectrum)

Question 7

Give the equation for red shift (z) in terms of wavelength. Define any terms.

Answer 7

z = -Δλ / λ
Δλ = λ - λ_obs
λ = actual wavelength emitted (given by laboratory value)
λ_obs = observed wavelength (from spectrum)

Question 8

Describe cosmological red shift.

Answer 8

Red shift from distant galaxies moving away from us due to the expansion of the Universe, as opposed to moving through space away from us.

Question 9

Describe and explain the relationship between the distance a galaxy is from Earth and the cosmological red shift.

Answer 9

The more distant the galaxy, the greater the cosmological red shift, as they are moving away from us faster.

Question 10

What is happening to the size of the Universe?

Answer 10

The Universe is expanding at an increasing rate.

Question 11

How did Edwin Hubble calculate the distance to distant galaxies?

Answer 11

Using Type 1a supernovae as standard candles

Question 12

How did Edwin Hubble calculate the recessional velocity of distant galaxies?

Answer 12

Measured red shift (z) of spectral lines.
Calculated recessional velocity using v = zc.

Question 13

What relationship was found between recessional velocity and distance from Earth of distant galaxies?

Answer 13

Recessional velocity is directly proportional to distance from Earth.

Question 14

Give the equation that represents Hubble’s law.

Answer 14

v = Hd

Question 15

Define the Hubble constant.

Answer 15

H = v/d, so H is the ratio of the recessional velocity of distant galaxies to their distance from Earth.

Question 16

Give the value for the Hubble constant from the data sheet including units.

Answer 16

H = 65 kms^-1Mpc^-1

Question 17

How can the Hubble constant be used to approximate the age of the Universe?

Answer 17

At the start of time, the distance between the galaxies was zero.
Age of Universe = time = distance/speed = d/v = 1/H

Question 18

Why does the calculation give an approximate age?

Answer 18

It assumes the expansion rate and the Hubble constant are constant over the lifetime of the Universe.

Question 19

What is happening to the rate of expansion of the Universe?

Answer 19

Rate of expansion is increasing (i.e. an accelerating expansion).

Question 20

What is dark energy?

Answer 20

An energy filling the whole of space that has been suggested to explain the accelerating expansion of the Universe.

Question 21

Describe the Big Bang Theory.

Answer 21

The universe started off from an extremely hot and dense point, and has been expanding ever since.

Question 22

Give three pieces of evidence for the Big Bang Theory.

Answer 22

Cosmological red shift of distant galaxies.
Cosmic microwaves background radiation.
Relative abundance of H and He.

Question 23

What is cosmic microwave background radiation (CMBR) and how does it support the Big Bang theory?

Answer 23

Big Bang theory predicts lots of electromagnetic radiation was produced in the very early universe.
This cosmic background radiation leftover from the Big Bang can still be observed today.
As the universe has expanded, the wavelengths of this cosmic background radiation have been red-shifted and are now in the microwave region.
Isotropic and homogeneous which agrees with the cosmological principle.
CMBR has a black body spectrum corresponding to a temperature of 2.7 K.

Question 24

What is the relative abundance of H and He in the Universe and how does it support the Big Bang theory?

Answer 24

Big Bang theory suggests that a very brief period of fusion occurred in the very early universe, resulting in the production of helium from fusing hydrogen.
Fusion stopped as the universe then expanded and cooled.
Therefore, relative abundance of hydrogen and helium in the ratio of 3:1

Question 25

What property of quasars led to their discovery?

Answer 25

They are very powerful radio sources.

Question 26

Why do quasars resemble stars?

Answer 26

They are point-like sources of light.

Question 27

Where are quasars found?

Answer 27

At the centre of active galaxies.

Question 28

Describe the structure of a quasar.

Answer 28

Produced by a supermassive black hole at its centre. Surrounded by an accretion disc of rotating matter that is drawn into it, causing a very high power output of radiation

Question 29

What do you know about the distance of quasars from Earth and what is the evidence for this?

Answer 29

Quasars are very distant. Evidence is the large red-shift in light from the galaxy.

Question 30

Why do quasars outshine their host galaxies?

Answer 30

Very bright absolute magnitude due to very large power output. Approximately 10⁴²W.

Question 31

Why is the power output of quasars so high?

Answer 31

As matter falls into the supermassive black hole, it forms an accretion disc. This is a rotating disc of matter which heats up due to friction and emits intense radiation. Some quasars also emit jets from their poles - making them very powerful radio sources.

Question 32

What do you know about the size of quasars and what is the evidence for this?

Answer 32

Quasars are small in size (approximately size of a solar system). Evidence is the rapid fluctuations in their power output.

Question 33

Why do quasars appear faint even though they have a very bright absolute magnitude?

Answer 33

Very distant from Earth. Inverse square law means that the intensity is greatly reduced.

Question 34

How do quasars form?

Answer 34

The merging of two galaxies leads to the formation of quasars. Matter moves towards the supermassive black hole. The supermassive black hole becomes active, drawing in nearby matter and emitting intense radiation.