Cosmology

Question 1

Doppler effect

Answer 1

The wavelength and frequency of waves from a moving source are changed

Question 2

Requirement for the doppler effect equations

Answer 2

Velocity of the source is much less than the velocity of the wave

Question 3

Binary star system

Answer 3

Two stars that circulate around each other about a common axis

Question 4

Why do graphs of intensity against time for binary stars have two different size increases in m?

Answer 4

One star may be dimmer than the other (so there is a smaller increase when this star is blocked)

Question 5

How can the velocity of binary stars be calculated?

Answer 5

Doppler shift

Question 6

Exoplanets

Answer 6

Planets which are not in our solar system

Question 7

Why are exoplanets hard to detect by direct observation?

Answer 7

• Don’t reflect much light and are drowned out by light from their star
• Small so telescopes can’t distinguish them form their stars

Question 8

Which exoplanets can be detected by direct observation?

Answer 8

Large planets a long way away from their star

Question 9

Methods for detecting exoplanets

Answer 9

• Direct observation
• Doppler shift
• Transit method

Question 10

How can exoplanets be detected by Doppler shift?

Answer 10

• Exoplanets and the star orbit around their COM
• COM is much closer to the star so the star ‘wobbles’ around this point
• Light from the star is Doppler shifted
• This suggests presence of a planet, and the planets minimum mass

Question 11

Which exoplanets can be detected by the Doppler shift method?

Answer 11

• Movement of the planet is aligned with the line of sight of the observer
• Planets close to their star produce larger ‘wobble’ in the star

Question 12

How can exoplanets be detected by the transit method?

Answer 12

• When exoplanets pass in front of the star, there is a small decrease in brightness
• If three of these dips are seen it suggests there is a planet orbiting the star

Question 13

Which exoplanets can be detected by the transit method?

Answer 13

• Movement of the planet is aligned with the line of sight of the observer
• Planets close to the star block the star more frequently and for greater lengths of time

Question 14

What information can be gained about the exoplanet from the transit method of detection?

Answer 14

• Diameter - from timing max to min brightness / range of brightness
• Elliptical orbits if the dip is not symmetrical

Question 15

Problems with the transit method for detecting exoplanets

Answer 15

• Unlikely for planet’s orbit to be aligned with the line of sight of observer
• Star is blocked for a very small fraction of T (hard to spot)
• Sunspots / asteroids / other stars / variable intensities also explain dips in brightness

Question 16

Hubble’s law

Answer 16

Red shift of distant galaxies is proportional to the distance of the galaxy from our own

Question 17

Units for v in Hubble’s law

Answer 17

km/s

Question 18

What is a type 1a supernova?

Answer 18

• Explosion that occurs when a white dwarf reaches 1.4 solar masses
• Silicon lines in absorption spectrum
• White dwarf gains mass from pulling material from the other star in a binary system

Question 19

Graph for type 1a supernova

Answer 19

• M against t
• Peak M at t=0
• t range from -20 to 40 days
• M range from -16 to -20

Question 20

M for type 1a supernova

Answer 20

Always around -19.6
(can be used as standard candles for large distances)

Question 21

How can the age of the universe be etimated?

Answer 21

Velocity of a distant galaxy = Distance moved since Big Bang / Age of universe
This can be combined with Hubble’s law to show age = 1/H

Question 22

How old is the universe, and what assumption is made to calculate this?

Answer 22

Around 15 billion years
Assuming that H is constant

Question 23

Big Bang model

Answer 23

Universe stared as an infinitely hot, infinitely dense point and has been expanding and cooling ever since

Question 24

Hydrogen to helium ratio as evidence for the Big Bang

Answer 24

• There was only a short period of time where temperature was right for hydrogen to fuse to helium
• Ratio of H to He predicted by the Big Bang theory matches observations
  (Gas clouds far from stars are observed to avoid elements from supernova)

Question 25

How can the temperature of the universe be calculated?

Answer 25

- Find wavelength with maximum intensity on cosmic microwave background radiation spectrum - Use Wien's law to calculate T - T = 2.7K

Question 26

Cosmic microwave background radiation

Answer 26

- From all parts of the universe - Comes from point in time where universe became transparent - Gamma rays red shifted by expansion of the universe - Temperature from CMBR spectrum agrees closely with Big Bang theory

Question 27

Evidence for the big bang

Answer 27

- Red shift of distant galaxies in all directions - Cosmic microwave background radiation - Percentage of hydrogen fused to helium

Question 28

Features of a quasar

Answer 28

- Very large red shift in Balmer lines (very far away) - Very powerful radio source - Smaller than a galaxy

Question 29

What are quasars?

Answer 29

Material falling into supermassive black holes

Question 30

Evidence for dark matter

Answer 30

1. There is not enough mass in a galaxy to keep the outer stars rotating at the speed they are observed at 2. The amount that light is bent by gravitational lensing as it travels through a galaxy shows there is more mass than can be accounted for my the stars

Question 31

Effects of dark energy

Answer 31

- Causes space to be 'flat' (caused also by matter and dark matter) - Accelerates the expansion of the universe

Question 32

Evidence for dark energy (and the accelerated expansion of the universe)

Answer 32

- Distance to type 1a supernova can be calculated through red shift and Hubble's law - Distance can also be calculated through M is this is know - Hubble's law suggests that the supernovae should be closer to us than they actually are - Suggests that expansion has been accelerated