Cosmology Flashcards
What is the Doppler Effect
- 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
What is Red Shift
The shift in wavelength and frequency of waves towards the red end of the spectrum from a sound moving away from the Earth
What is Blue Shift
The shift in wavelength and frequency of waves towards the blue end of the electromagnetic spectrum from a sound moving towards the Earth
What is Red Shift evidence for
The expanding universe and the Big Bang
What is the Red Shift Equation
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
What are the Limits of the Red Shift Equation
- 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
What are Binary Star Systems and Spectroscopic Binaries
- 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
Explain how you can use the Doppler effect to distinguish between two Bianry Stars
- 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.
Describe the Typical light curve of Eclipsing Binaries
- No Eclipse - brightness is maximum
- Large star is in front of the small star - blocking all of the light
- No Eclipse - brightness is maximum
- Smaller star is front of the large star - blocking some of the light
What is Hubble’s Law
A galaxy’s recessional velocity is directly proportional to its distance from the Earth
State an Interpretation of Hubble’s Law
It essentially states that the universe is expanding from a common starting point
What is Hubble’s Law Equation
v = Hd
Recessional velocity (km s-1) = Hubble constant x Distance from Earth (Measured in megaparsec)
Use Hubble’s Law to Estimate the Age of the Universe
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
What are the Limitations of any Result found on the Hubble Constant
- 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
What is the Big Bang Theory
- 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
What is some of the evidence for The Big Bang Theory
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
What is a Quasar
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
What suggested that Quasars are extremely distant objects
Quasars have large optical red shifts indicating that they are at the edge of the universe
What are the properties of a Quasars
- 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
What electromagnetic wave were quasars initially thought to be
They were initially thought to be powerful radio sources but with further telescopes we now know they emit all wavelengths of electromagnetic radiation
What is the link between black holes, quasars and galaxies
- Quasars are produced by black holes
- These black holes are at the centre of active galaxies
What are Exoplanets
Planets that are not within our solar system
Why are exoplanets difficult to detect
They can be difficult to detect directly as they tend to be obscured by the light of their parent stars
What are the two ways of detecting exoplanets
- 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
What are the Problems with the Methods used to detect exoplanets
- 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
Why is more likely for exoplanets with small orbits to be detected rather than exoplanets with large orbits
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
Describe the Typical Light Curve of a star with a Transiting Planet
