Cosmology

• Astronomical unit (AU) --> the astronomical unit is the average distance from the Earth to the Sun, 150 million km.

  • The astronomical unit is most often used to express average distance between the Sun & other planets in the solar system.
• Light year (ly) --> the light year is the distance travelled by light in a vacuum in a time of 1 year

  • The light year is often used when expressing distances to stars or other galaxies
• • 60 arcminutes in 1 degree
• • 60 arcseconds in each arcminute
• • 1 arcsecond = 1/3600 degrees
• Parsec --> defined as the distance at which a radius of one AU subtends an angle of one arcsecond
• tan ( 1 arcsecond) = 1 AU/ 1pc
• stellar parallax --> a technique used to determine the distance to stars that are relatively close to the Earth, at distances less than 100pc.
• Parallax --> the apparent shift in the position of a relatively close star against the backdrop of much more distant stars as the Earth orbits the sun.

  • d = 1/p
• • as d increases the parallax angle decreases, eventually becoming too small to measure accurately
• Doppler effect --> used to determine the speed of moving objects
• The doppler effect:

  • whenever a wave source moves relative to an observer, the frequency & wavelength of the waves received by the observer change compared with what would be observed without relative motion
• Doppler shifts in starlight:

  • can be used to determine the relative velocity of a distant galaxy.
  • If the galaxy is moving towards the Earth the absorption lines will be blue shifted - they move towards the blue end of the spectrum, because the wavelength appears shorter
  • If the galaxy is moving away from the Earth the absorption lines will be red-shifted - they all move towards the red end of the spectrum because the wavelength appears stretched.
• Doppler equation:

  change in wavelength / wavelength = change in frequency / frequency = speed of sound / speed of recorded wavelength
• • the faster the source moves the greater the observed change in wavelength & frequency
• Hubble's Law:

  1. He confirmed earlier observations that light from vast majority of galaxies was red-shifted. They had a relative velocity away from Earth.
  2. He found in general the further away the galaxy was the greater the observed red shift & so the faster the galaxy was moving
• Hubble's Law --> the recessional speed v of a galaxy is almost directly proportional to its distance from the Earth
• velocity = Hubble's constant x distance
• The expanding universe:

  • The fabric of space, and time is expanding in all directions
  • It is not simply the galaxies moving away from each other, but the actual space itself expanding
  • Any point in any part of the Universe, is moving away from every other point in the universe, and the further the points are apart the faster their relative motion away from each other
• Cosmological principle --> the assumption that, when viewed on a large enough scale the universe is homogenous and isotropic, and the laws of physics are universal
• The cosmological principles:

  • The theories and models tested here on the Earth can be applied to everything within the universe over all time and space.
  • Homogenous --> matter is distributed uniformly across the universe. For a very large volume, the density of the universe is uniform
  • Isotropic --> the universe looks the same in all directions to every observer. It follows that there is no centre or edge to the universe
• Microwave background radiation:

  • When the universe was young & extremely hot, space was saturated with high energy photons
  • The expansion of the universe means that space itself was stretched over time.
  • This expansion stretched the wavelength of these high energy photons. So now we observe this primordial EM radiation as microwaves
  • The universe was extremely dense & hot --> expansion of space over billions of years has reduced that temperature to around 2.7K
  • The universe may be treated as a black-body radiator
• • Hubble's law shows that galaxies are receding from each other
  • The ratio d/v is equal to 1/H0
  • Age of universe t= 1/H0
  • H0 = 2.2 x 10^-18 s^-1
• Quasar --> a nucleus of an active galaxy, a supermassive black hole surrounded by a disc of matter
• • cosmological microwave background radiation (CMBR) is the heat signature left behind from the big bang
• Dark energy --> thought to be energy that has no overall repulsive effect throughout the universe