14: Astrophysics - Cosmology

Question 1

What is the Doppler effect

Answer 1

the compression or spreading out of waves that are emitted or reflected by a moving source
- as the source is moving, wavelengths in front of it are compressed and wavelengths behind are spread out

Question 2

Example of the Doppler effect

Answer 2

the sound of a car moving past you

Question 3

what effect does the Doppler effect have on line spectra of distant objects

Answer 3

• shift towards the blue end of the visible spectrum when they move towards the Earth (blue-shift)
• shift towards the red end of the spectrum when they move away from the Earth (red-shift)

Question 4

what is the formula for red shift

Answer 4

z = v/c = Δf/f = -Δλ/λ

where v is receding velocity

can only be used when v is much smaller than c

Question 5

what is a binary star system

Answer 5

where two stars are orbiting a common centre of mass

Question 6

what can the Doppler effect be used to identify

Answer 6

binary star systems

Question 7

what are spectroscopic binaries

Answer 7

binary star systems in which the stars are too close to be resolved by a telescope, so the only way to resolve them is by using the Doppler shifts of each star

Question 8

(DE) as the stars eclipse each other…

Answer 8

they are travelling perpendicular to the line of sight from the observer, so there is no Doppler shift in their emitted radiation

Question 9

(DE) when one star is travelling away from the observer…

Answer 9

the other is travelling towards the observer
- this causes each spectral line to be split into two, where one is blue-shifted and the other is red-shifted

Question 10

what are eclipsing binaries

Answer 10

when the plane of the orbit of the stars is in the line of sight from Earth to the system, meaning that the stars cross in front of each other as they orbit

Question 11

what are quasars (rs)

Answer 11

objects which have very large red shifts, suggesting they are very far away, but they are also extremely bright
- using the inverse square law, you can show that the power output of a quasar must be around that of an entire galaxy

Question 12

what is Hubble’s law?

Answer 12

a galaxy’s recessional velocity is directly proportional to its distance from the Earth
- the universe is expanding from a common started point

Question 13

hubble formula

Answer 13

v = Hd

v is recessional velocity in km s^-1
H is Hubble constant 65 km/sMpc

Question 14

1/H =

Answer 14

time

Question 15

what does the big bang theory suggest

Answer 15

the universe began with a huge explosion from a singularity that was infinitely hot and small

Question 16

Evidence of the Big Bang 1

Answer 16

• when the big bang happened, it is thought that there was high-energy radiation everywhere, and as the universe expanded and cooled, this radiation would have lost energy and been red-shifted, which is what we call Cosmological Microwave Background Radiation (CMBR)

Question 17

evidence for the Big Bang 2

Answer 17

• during the early stages of the Big Bang, nuclear fusion converted hydrogen nuclei into helium nuclei
• this only lasted for a short period of time before the universe cooled too much and nuclear fusion stopped
• 1/4 of all existing hydrogen nuclei were fused into helium, resulting in a relative abundance ratio of H:He of 3:1
• the relative abundance by mass of different elements observed today is approx 73% hydrogen, 25% helium and 2% everything else

Question 18

what is a quasar

Answer 18

an active galactive nucleus - a supermassive black hole surrounded by a disc of matter which, as it falls into the black hole, causes jets of radiation to be emitted from the poles

Question 19

how are quasars characterised

Answer 19

• extremely large optical red-shifts
• very powerful light output
• their size being not much bigger than a star

Question 20

what are exoplanets

Answer 20

planets that are not within our solar system

Question 21

why are exoplanets hard to detect

Answer 21

they tend to be obscured by the light of their host stars

Question 22

what are the two methods of detecting exoplanets

Answer 22

• radial velocity method
• transit method

Question 23

radial velocity method

Answer 23

• star and planet orbit a common centre of mass, which causes the star to ‘wobble’ slightly
• this causes a Doppler shift in the light received from the star
• effect is most noticeable with high mass planets
• this shows that there is something else near the star that is exerting a gravitational force on it
• the time period (T) of the planet’s orbit is equal to the time period of the Doppler shift

Question 24

transit method

Answer 24

• involves observing the intensity of the light output of a star
• if a planet crosses in front of the star, the intensity dips slightly
• if the intensity dips regularly, that is a sign there is an exoplanet orbiting it
• if there are variations in the regularity of the dips, there may be several planets orbiting the same star which have a gravitational effect on the transiting planet