What is the
Universe Made
of?
Scales in The Universe
• If the Earth were the size of a golf ball,
how far away would the Sun be?
• How far away would Neptune be?
• How far away would the nearest star be?
Kepler’s Laws
1. All planets orbit in ellipses with the Sun
at one focus
2. The orbit draws out equal areas in equal
times.
3. The square of the period is proportional
to the cube of the radius
These apply to all orbiting bodies, not just
planets...
Circular Orbits
• Centripetal acceleration
• Gravitational force
• Newton’s 2nd Law
2
v
a
r
GMm
F 2
r
F  ma
Moon
Radius of Orbit
Period of
Orbit
(km: all nearly
circular)
(days)
Io
4.22 x 105
1.77
Europa
6.71 x 105
3.55
Ganymede
1.07 x 106
7.15
Callisto
1.88 x 106
16.69
GM
v 
r
2
Finding Dark Matter
• Use Kepler’s 3rd Law to predict the
velocities of stars around the centre of a
galaxy:
–
–
–
–
Mass of Galaxy = 28 billion solar masses
30
1 solar mass = 2x10 kg
-11
3 -2
-1
G = 6.67x10 m s kg
16
1 light year = 10 m
Measured velocities
• Here are the measured velocities
– Do they look like what you predicted?
• Use Kepler’s 3rd Law to estimate the mass of the
galaxy
What is Dark Matter?
• It’s matter that’s not visible, but can only
be detected by its gravitational effects
– Best theory is that it is an exotic particle
– If it is what we think it is, we might be close
to finding it!
Here are some other examples…
Expansion of the Universe
• In 1929 Edwin Hubble found link between
distances to galaxies and their radial
velocities
• Plot Hubble's data
Object
Distance (Mpc) Velocity
(km/s)
Object
Distance Velocity
(Mpc)
(km/s)
Hubble's
Data
0.032
170
3627
0.9
650
L.
Mag.
0.034
290
4826
0.9
150
N.G.C.6
822
0.214
-130
5236
0.9
500
598
0.263
-70
1068
1
920
221
0.275
-185
5055
1.1
450
224
0.275
-220
7331
1.1
500
5457
0.45
200
4258
1.4
500
4736
0.5
290
4151
1.7
960
5194
0.5
270
4382
2
500
4449
0.63
200
4472
2
850
4214
0.8
300
4486
2
800
3031
0.9
-30
4649
2
1090
S.
Mag.
The Hubble Space Telescope
The Hubble Space Telescope
The Cosmic Microwave
Background
The contents of the Universe
• We currently think the Universe is
composed of three constituents:
– Normal matter (“atoms”)
– Dark Matter
– Dark Energy
• To find out how much of each, we need to
look at the evidence
Heavens’ Kitchen
• Choose your ingredients
– Total Matter, Normal Matter, Dark Energy
• How does the Cosmic Microwave
Background Compare?
• When did Galaxies form?
• How to the galaxies compare?
What is the Universe made of?
• We think the Universe
is made of:
– 5% Normal matter
– 27% Dark Matter
– 68% Dark Energy
The Planck Satellite
•
•
•
•
•
•
4x4x4m
2 tonnes on launch
2m mirror
Cooled to 0.1K
2 instruments
300-10000 microns
All sky (visible)
DSS
www.chromoscope.net
All-sky (microwave)
Planck