Chapter 20- Cosmology

Chapter 20- Cosmology

1. Measurements

   • Astronomical unit (AU)
     • "The average distance from the Earth to the Sun"
     • 1.5*10^11m
   • The light-year (ly)
     • "The light- year is the distance travelled by light in a vacuum in a time of one year
     • 9.46*10^15m
       • Speed of light * Seconds in a year
   • The parsec (pc)
     • "The parsec is defined as the distance at which a radius of 1 AU subtends an angle of 1 arcsecond"
       • 1 arcsecond=(1/3600)°
     • 3.1*10^16m

2. Stellar parallax

   • This is the apparent shift of position of any nearby star against the background of distant objects
   • Distance to star=1/parallax angle
     • Distance is in parsecs
     • Angle in archseconds

3. The Dopler shift

   • Whenever a wave source moves relative to an observer, the frequency & wavelength of the waves recieved by the observer change
     • Waves in front of source are compressed
     • Waves behind source are stretched
   • If galaxy is moving towards Earth=Absoprtion lines will be blue-shifted because wavelengths appear shorter
   • If galaxy is moving away from Earth=Absorption lines will be red-shifted because wavelengths appear stretched

4. Hubble's law

   • 1) Light from the vast majority of galaxies was red-shifted (They had a relative velocity away from the Earth)
   • 2) In general the further away the galaxy was the greater the observed red-shift and so the faster the galaxy was moving
   • This shows that recessional speed is proportional to the distance from us
   • Hubble's law is key evidence for the Big Bang theory
     • Since light from nearly all the galaxies is red-shifted it states the fabric of space is expanding in all directions

5. The cosmological principle assumes that- "When viewed on a large enough scale, the Universe is homogeneous and isotropic, and the laws of physics are universal

6. The Big Bang theory

   • There are 2 main pieces of evidence for the Big Bang theory
     • 1) Hubble's law
       • Shows the Universe is expanding in all dimensions- hence the expansion could be traced back to a single, denser and hotter point
     • 2) Microwave background radiation
       • The Big Bang model predicts that loads of gamma radiation was produced in the hot early universe and should still be observed today
         • Due to the Universe expanding, the wavelengths of the Cosmic radiation are now in the microwave region
   • Age of the Universe ≈ 1/Hubble's constant
     • The Universe is roughly 14 billion years old

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