Cosmology

Question 1

What is the Doppler Effect

Answer 1

• The change in wavelength and frequency of a wave as the source moves away or towards the observer
• As the source moves towards the observer, the waves are compressed and wavelength decreases
• As the source moves away from the observer, the waves spread out and the wavelength increases

Question 2

What is Red Shift

Answer 2

The shift in wavelength and frequency of waves towards the red end of the spectrum from a sound moving away from the Earth

Question 3

What is Blue Shift

Answer 3

The shift in wavelength and frequency of waves towards the blue end of the electromagnetic spectrum from a sound moving towards the Earth

Question 4

What is Red Shift evidence for

Answer 4

The expanding universe and the Big Bang

Question 5

What is the Red Shift Equation

Answer 5

z = Δf / f = - Δλ / λ = v /c

Red Shift = Change in Frequency / Original Frequency = -Change in Wavelength / Original Wavelength = Velcoity / Speed of Light in a Vacumm

Question 6

What are the Limits of the Red Shift Equation

Answer 6

• This formula can only be used when v is much smaller than c
• This is because the formula was derived without taking into account any relativistic effects, which occur when objects are moving close to the speed of light

Question 7

What are Binary Star Systems and Spectroscopic Binaries

Answer 7

• Binary star systems - where two stars are orbiting a common centre of mass
• Spectroscopic Binaries - binary star systems in which the stars are too close to be resolved by a telescope

Question 8

Explain how you can use the Doppler effect to distinguish between two Bianry Stars

Answer 8

• As the stars eclipse each other, they are travelling perpendicular to the line of sight from the observer, so there is no Doppler shift in their emitted radiation
• However, when one star is travelling away from the observer, 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 9

Describe the Typical light curve of Eclipsing Binaries

Answer 9

1. No Eclipse - brightness is maximum
2. Large star is in front of the small star - blocking all of the light
3. No Eclipse - brightness is maximum
4. Smaller star is front of the large star - blocking some of the light

Question 10

What is Hubble’s Law

Answer 10

A galaxy’s recessional velocity is directly proportional to its distance from the Earth

Question 11

State an Interpretation of Hubble’s Law

Answer 11

It essentially states that the universe is expanding from a common starting point

Question 12

What is Hubble’s Law Equation

Answer 12

v = Hd

Recessional velocity (km s-1) = Hubble constant x Distance from Earth (Measured in megaparsec)

Question 13

Use Hubble’s Law to Estimate the Age of the Universe

Answer 13

Time = Distance / Velocity = 1 / H (since v = Hd)

65 km s^-1 Mpc^-1 x 10³ = H = 65 000 m s^-1 Mpc^-1

1 Mpc = 3.08 x 10²² m

65 000 / 3.08 x 10²² = 2.11 x 10^-18 s^-1 = H

1 / H = 4.74 x 10¹⁷ s

Age (Convert to Years) = 4.74 x 10¹⁷ / 3600 / 24 / 365 = 1.5 x 10¹⁰ years or 15 billion years

Question 14

What are the Limitations of any Result found on the Hubble Constant

Answer 14

• Need data from lots of supernovas
• At large distances accelerating universe will effect graph
• Much variation in the data (random error)
• Value of apparent magnitude may be affected by what light passes through

Question 15

What is the Big Bang Theory

Answer 15

• As the universe is currently expanding, it would be reasonable to assume that the universe began from one point – a singularity that was infinitely small and infinitely hot
• The Big Bang Theory suggests that the universe began with a huge explosion from this point

Question 16

What is some of the evidence for The Big Bang Theory

Answer 16

Cosmological Microwave Background Radiation -

• 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
• The remains of this radiation is what we call Cosmological Microwave Background Radiation. This is microwave radiation that has been detected from all directions in space
• This provides evidence for the Big Bang

Relative abundance of hydrogen and helium -

• During the early stages of the Big Bang, nuclear fusion converted lotsof hydrogen nuclei into helium nuclei
• Approximately ¼ of all of the existing hydrogen nuclei were fused into helium
• The relative abundance by mass of different elements observed today is approximately 73% hydrogen, 25% helium and 2% everything else
• The Big Bang explains further explains why there is a larger abundance of helium in today’s universe
• This provides evidence for the Big Bang

Question 17

What is a Quasar

Answer 17

A nucleus of an active galaxy, a supermassive active black hole. As matter falls into the blackhole, jets of radiation are emitted from the poles of the quasar

Question 18

What suggested that Quasars are extremely distant objects

Answer 18

Quasars have large optical red shifts indicating that they are at the edge of the universe

Question 19

What are the properties of a Quasars

Answer 19

• Very powerful light output, much greater than the light output of a star
• Relatively small size, not much larger than a solar system
• Large red shift indicating that they are at the edge of the universe and travelling at speeds close to the speed of light

Question 20

What electromagnetic wave were quasars initially thought to be

Answer 20

They were initially thought to be powerful radio sources but with further telescopes we now know they emit all wavelengths of electromagnetic radiation

Question 21

What is the link between black holes, quasars and galaxies

Answer 21

• Quasars are produced by black holes
• These black holes are at the centre of active galaxies

Question 22

What are Exoplanets

Answer 22

Planets that are not within our solar system

Question 23

Why are exoplanets difficult to detect

Answer 23

They can be difficult to detect directly as they tend to be obscured by the light of their parent stars

Question 24

What are the two ways of detecting exoplanets

Answer 24

• Doppler Effect or Radial Velocity Method - a star and a planet will orbit their common centre of mass, this means that the star will have tiny variations in its distance from the Earth, shown by tiny red and blue shifts in its spectrum. This shows that there is something else near the star that is exerting a gravitational force on it – the exoplanet
• Transmit Method - as a planet moves between the star it orbits and the Earth, the star’s brightness appears to decrease slightly. The size and the orbital period of the planet can be determined from the amount that the intensity falls by and the duration of the dip

Question 25

What are the Problems with the Methods used to detect exoplanets

Answer 25

• Doppler Effect or Radial Velocity Method - only noticeable with high-mass planets since they have a greater gravitational pull on the star
• Transmit Method - only works if the line of sight to the star is in the plane of the planet’s orbit
• Also, other planets in the same system could cause confusion on the data

Question 26

Why is more likely for exoplanets with small orbits to be detected rather than exoplanets with large orbits

Answer 26

Most orbits are inclined (do not pass in front of the star when observed from the Earth), and smaller orbits mean that parts of the planet are more likely to cross in front of the star and block some of its light

Question 27

Describe the Typical Light Curve of a star with a Transiting Planet

Answer 27