chapter 20 - cosmology Flashcards
astronomical unit (au)
average distance from the Earth to the sun
1.5*10^11 m
light year (ly)
distance travelled by light in vacuum in a time of one year
= 3*10^8 * 365 * 24 * 60 * 60
stellar parallax
used to find distance to nearby stars
- apparent shift in position of a relatively close star against more distant fixed stars
- limited to close stars only
- assumes distant stars are fixed
stellar parallax method
when Earth at one side star appears in lien with distant stars diagonally
when on other side (6 moths later) appears in line with stars on opposite sides
- know distance from sun to earth
- can measure angle
- so can find distance to star
stellar parallax calc
d = 1/p
d(parsecs)
p (arcseconds)
degree to arc seconds
1 degree = 60 arc minute = 3600 arc seconds
1 arc second
1/3600 degrees
parsec
distance at which a radius of 1AU subtends an angle of 1 arc second
1 pc = 1AU/tan(1 arc second) = 1AU/1 arc second = 1.510^11/ (1/3600) = 3.110^16 m
(small angle so tanθ = θ)
doppler effect
apparent change in freq of a wave caused by the relative motion between the source and observer
moving towards = shorter wavelength = blue shift
away =longer = red shift
binary stars - why they appear to move towards
- binary stars are moving round each other so at some points it appears as if one is moving away (red shift) and one is moving towards (blue shift)
- in reality the whole system is moving away but as they are circling each other one can appear to be moving towards us
doppler effect on light
analyse absorption spectra
spectral lines appear to shift due to their movement in comparison to source data (lab spectrum)
- distance between spectral lines also increases as lines move a % of their og distance
doppler effect eq
Δλ/λ = Δf/f = v/c
v = recessional velocity
c = speed of light
faster - greater change in wavelength / freq
2 observations that made hubbles law
- light from most galaxies is red shift - moving away from earth
- further away the galaxy greater the observed red shift - moving faster
hubbles law
recessional speed of a galaxy is almost directly proportional to its distance from earth
v ⍺ d
v = H0 d
model of the expanding universe
model states
- fabric of space and time is expanding in all directions
- any point in the Universe is moving away from every other point - further away the points the faster their relative motion ( expanding)
hubble constant
67.8kms^-1Mpc^-1
2.2*10^-18 s^-1
kms^-1Mpc^-1 to s^-1
110^3/ 10^63.1*10^16
convert km/s to m/s
convert Mpc to m
age of the universe
- assume universe has expanded uniformly since the big bang
- time since a galaxy (of distance d away moving at speed v) was next to ours = d/v = 1/H0
age of universe = 1/H0
cosmological principle
assumption that when viewed upon a large enough scale the Universe is homogeneous and isotropic and the laws of physics are universal
homogeneous
- matter is distributed uniformly across the universe
- for a very large volume the density of the universe is constant
- same type of structures seen everywhere eg galaxies
isotropic
- looks the same in all directions to every observer
- there is no centre or edge of the Universe
laws of physics are universal
- theories and models tested on Earth can be applied to everything within the Universe over all space and time
red shift - evidence for big bang
- light from most galaxies is redshifted - moving away
- further away the galaxy the more the shift - moving faster
- therefore universe is expanding
- must start at a single hot dense point
evidence for big bang
- red shift
- CMBR
