3.9.3 - Cosmology

Question 1

What is changed during the Doppler effect?

Answer 1

The wavelength and frequency of the sound heard.

Question 2

As something moves away how is the sound heard?

Answer 2

Lower-pitched, waves spread out.

Question 3

As something moves towards you how is the sound heard?

Answer 3

Higher-pitched, waves are bunched together.

Question 4

What does the sound depend on with the Doppler effect?

Answer 4

The velocity of the object.

Question 5

What does the Doppler effect occur in?

Answer 5

All waves, including electromagnetic radiation.

Question 6

What happens when a light source moves away from us?

Answer 6

The wavelength of the light reaching us becomes longer and the frequencies become lower. This shifts the light that we receive towards the red end of the EM spectrum and is called a red shift.

Question 7

How us the light we receive from a star moving away from us different to actual light emitted?

Answer 7

It is towards the red end of the EM spectrum and so is redder.

Question 8

What happens when a light source moves towards us?

Answer 8

The light undergoes blue shift, the star looks bluer than it actually is.

Question 9

If a star is red shifted what will the value of z be?

Answer 9

Positive.

Question 10

If a star is blue shifted what will the value of z be?

Answer 10

Negative.

Question 11

What is cosmological red shift and what does it show?

Answer 11

All distant galaxies show red shift and so are moving away from us. This shows space is actually expanding and the light waves are being stretched along with it.

Question 12

What are the cosmological principles?

Answer 12

On a large scale the universe is homogenous (every part is the same as every other part) and isotropic (everything looks the same in every direction).

Question 13

What is the recessional velocity?

Answer 13

The amount of red shift given by a galaxy.

Question 14

What did Hubble realise?

Answer 14

The speed that galaxies moved away from us at depended in how far away they were.

Question 15

In v = Hd what is v in?

Answer 15

kms-1

Question 16

In v = Hd what is d in?

Answer 16

Mpc

Question 17

What is the Big Bang theory?

Answer 17

The universe started off very hot and very dense and has been expanding ever since.

Question 18

How is red shift evidence for the Big Bang theory?

Answer 18

The spectra from all galaxies show red shift, so they are moving apart.

Question 19

How can you estimate the age of the universe?

Answer 19

t = d/v = 1/H

Question 20

Why is there a limit on the observable size of the universe?

Answer 20

Because we can only see light that has had time to travel to us.

Question 21

Why is the observable size of the universe just an estimate?

Answer 21

The rate of expansion of the universe hasn’t been constant.

Question 22

What is dark energy and what does it explain?

Answer 22

All mass in the universe is attracted together by gravity, which would suggest the rate of expansion show be slowing down, however it is accelerating. This acceleration is being explained by dark energy - a type of energy that fills the whole of space.

Question 23

What does the Big Bang model predict?

Answer 23

Lots of EM radiation was produced in the very early universe.

Question 24

What has happened to the radiation produced in the Big Bang?

Answer 24

Because the universe has expanded, the wavelengths of this cosmic background radiation are in the microwave region - so is known as cosmic microwave background radiation.

Question 25

What does CMBR show?

Answer 25

The Doppler effect, it agrees with the cosmological principle and has a perfect black body spectrum.

Question 26

How does the Big Bang theory explain the large abundance of helium in the universe?

Answer 26

The early universe was very hot, so must gave been hot enough for hydrogen to fuse.

Question 27

What are spectroscopic binary stars?

Answer 27

2 stars that orbit each other, and are too far away to be resolved with telescopes.

Question 28

What is an eclipsing binary system?

Answer 28

One whose orbital plane lies almost in our line of sight, so the stars eclipse each other as they orbit.

Question 29

How can orbital period be calculated from an eclipsing binary system?

Answer 29

By observing absorption lines in the spectrum - as stars orbit each other, the separation of the lines increases from zero to a maximum and then back to zero in half a period.

Question 30

How can a graph of apparent magnitude and time be used in a binary star system?

Answer 30

As the stars eclipse each other, the apparent magnitude drops because some of the light is blocked out. It drops more as the dimmer star passes in front of the brighter star.

Question 31

What are quasars?

Answer 31

The most distant objects seems and shoot out jets of material - very radioactive sources.

Question 32

What do quasars produce?

Answer 32

A continuous spectrum of the Balmer series of hydrogen but red-shifted enormously.

Question 33

What does the huge red shift of quasars suggest?

Answer 33

They’re a huge distance away.

Question 34

Quasars are very powerful and so what is it suggested that they are?

Answer 34

Very powerful galactic nuclei centred around a huge black hole

Question 35

What is an active galaxy?

Answer 35

One in which the black hole at the centre ejects huge amounts of material from their nucleus.

Question 36

What is the black hole in an active galactic nucleus surrounded by?

Answer 36

A doughnut shaped mass of whirling gas falling into it, which emits matter and radiation.

Question 37

What do the magnetic fields produce in an active galactic nucleus?

Answer 37

Jets of matter and radiation streaming out from the poles.

Question 38

What are exoplanets?

Answer 38

Any planet not in our solar system.

Question 39

Why are exoplanets difficulty to find?

Answer 39

• They’re orbiting stars much brighter than them, so the bright light from the stars they’re orbiting drowns out any light from the exoplanet.
• They’re too small to distinguish from nearby stars.
• Only a few of the largest and hottest exoplanets that are furthest away from their stars can be seen directly using specially built telescopes.

Question 40

What two ways can exoplanets be detected?

Answer 40

Doppler shift, the transit method.

Question 41

What else is the Doppler shift method called?

Answer 41

The radial velocity method.

Question 42

What does the Doppler shift method do?

Answer 42

Measures how much the emissions from stars have been red or blue shifted.

Question 43

Why does the Doppler shift method work?

Answer 43

An exoplanet orbiting a star has a small effect on the star’s orbit. It causes tiny variations in the stars orbit as the star and exoplanet are actually orbiting around the centre of mass between them - but as the star is so much bigger than the exoplanet, the centre is much close to the centre of the star. This wobble causes tiny red and blue shift’s in the star’s emission which can be detected.

Question 44

What is the problem with the Doppler shift method?

Answer 44

The movement needs to be aligned with the observer’s line of sight - if the planet orbits the star perpendicular to the line of sight then there won’t be any detectable shift in the light from the star.

Question 45

What can be calculated using the Doppler shift method?

Answer 45

The minimum mass of the exoplanet.

Question 46

What does the transit method measure?

Answer 46

The change in apparent magnitude as an exoplanet travels in front of a star.

Question 47

Why does the transit method work?

Answer 47

As an exoplanet orbits a star, if it passes in front of the star then it blocks out some of the star’s light, causing a change in apparent magnitude. This can be viewed as a dip in the star’s light curve.

Question 48

How and what can be calculated with the transit method?

Answer 48

The radius of the exoplanet and the amount that the star is dimmed depends on the size of the orbiting planet.

Question 49

What are the limitations of the transit method?

Answer 49

Even if aligned wit the observer, the transit is likely to last only a tiny fraction of its whole orbital period - so transits can be few and far between, making them easy to miss. Therefore you can only use this method to confirm already observed exoplanets.