"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
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
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
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
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
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